Gas-Phase Negative Ion Chemistry: Photoelectron Spectroscopy,
Reactivity, and Thermochemical Studies

byStephanie M. Villano
B. S., University of Colorado at Denver, 2001

A thesis submitted to the Faculty of the Graduate Schoolof the University of Colorado in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry2008

This thesis entitled:
Gas-Phase Negative Ion Chemistry: Photoelectron Spectroscopy,
Reactivity, and Thermochemical Studies
Written by Stephanie M. Villano
has been approved for the
Department of Chemistry and BiochemistrybyDate: December 2, 2008
A final copy of this thesis has been examined by all signatories, and we find that boththe content and the form meet acceptable presentation standards of scholarly workin the above mentioned discipline.

iiVillano, Stephanie M. (Ph.D. Physical Chemistry)
Gas-Phase Negative Ion Chemistry: Photoelectron Spectroscopy, Reactivity, andThermochemical Studies
Thesis directed by Professors Veronica M. Bierbaum and W. Carl Lineberger
Abstract:
This thesis describes the reactive and spectroscopic characterization ofseveral anions. An overview of gas-phase ion-molecule reactivity and photoelectron
spectroscopy is given in Chapters I-IV.
In Chapter V the competition between nucleophilic substitution and baseinduced elimination for the reactions of BrO– and ClO– with a series of alkyl chlorides
is investigated. These results are in stark contrast to prior theoretical work, and they
indicate that the elimination channel becomes the dominant pathway as the neutral
reagent becomes more sterically hindered. Additionally, the highly debated
α-nucleophilicity of these two anions and of HO2– is examined; no enhanced
reactivity is displayed suggesting that the α-effect is not due to an intrinsic propertyof the anion.
The reactivity of simple halogen substituted carbene anions is investigatedwith a series of oxygen, sulfur, and halogen containing neutral reagents in Chapter
VI. These carbanions display diverse reactivity that includes substitution, elimination,
proton transfer, and electron transfer reactions. In addition to these classictransformations, isotopic labeling studies indicate that these carbanions can undergo
an insertion-elimination mechanism. Such reactivity has been observed in neutralcarbenes; however, this is the first report of such gas-phase anion reactivity.
Spectroscopic studies of SF6– and c-C4F8– are reported in Chapter VII. The
photoelectron spectra of both anions are consistent with a large geometry changeiii

between anionic and neutral species. The photoelectron spectrum of c-C4F8– iscomposed of combination bands of two symmetric ring-breathing modes. The
photoelectron spectrum of SF6– is dominated by an extended progression in the S−F
stretching mode as well as a strong overtone/combination progression.
In Chapter VIII we report the photoelectron spectra of the N-methyl-5pyrazolide and N-methyl-5-imidazolide anions, which provide electron affinities forthe corresponding radicals. The gas-phase acidities of N-methylpyrazole andN-methyimidazole are also measured, and these measurements are used todetermine the C-H bond strengths.
The thermochemistry of peroxyformic acid and peroxyacetic acid isinvestigated in Chapter IX. The electron affinities of the peroxyformyl andperoxyacetyl radicals are determined from the photoelectron spectra of thecorresponding anion. The gas-phase acidities of both acids are determined, andthese measurements are used to determine the O-H bond strengths.

iv

Dedicated to my parents:
Denise Thornton and Steve E. Villano.

v

Acknowledgements:
There are so many people who have supported me in this journey. I am
foremost grateful to both Veronica Bierbaum and Carl Lineberger for their guidance,patience, and commitment over the years. I am very appreciative for theopportunities that they have given me and for the scientific freedom to work on
projects that interested me. I especially need to thank Veronica for the enormous
amounts of time that she has given me and for putting up with me on a daily basis; it
has really meant a lot to me!
Barney Ellison and Chuck DePuy in many ways have been surrogate
advisors to me, and I am appreciative of that. Shuji Kato has also been of immense
help to me, and I am grateful for his time and patience. There have been many
visitors to CU during my time here, of these, John Stanton, Kent Ervin, and Steve
Blanksby standout for their excellent advice and suggestions. I also need to thank
Don David and the CIRES Machine Shop Crew for all of their damage control andhelp over the years.
I am grateful to Nicole Eyet, for the help that she has given me, dailycommiseration, and for her friendship. I need to thank Adam Gianola for teachingme about PES experiments and Scott Wren and Kristen Vogelhuber for their
excellent job in running the PES lab. There have been a number of wonderfulpeople who have in one way or another helped me: Brad Flowers, Tim D’Andrea,
A. J. Vasiliou, Al Maccarone, Django Andrews, Jeff Rathbone, Katy Plath, andNabilah Rontu-Carlon. I need to thank Jack Barbera and Heather Meylemans fortheir friendship and help. Lastly, I need to thank my husband, Todd Kreutzian, andmy family for their love and support.

viPublications:(1)
Villano, S. M.; Kato, S.; Bierbaum, V. M. Deuterium Kinetic Isotope
Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory,
Journal of the American Chemical Society, 2006, 128, 736-737.
(2)
Bopp, J. C.; Roscioli, J. R.; Johnson, M. A.; Miller, T. M.; Viggiano,
A. A.; Villano, S. M.; Wren, S. W.; Lineberger, W. C. Spectroscopic Characterizationof the Isolated SF6– and C4F8– Anions: Observation of a Very Long Harmonic
Progression in Symmetric Deformation Modes upon Photodetachment, Journal of
Physical Chemistry A, 2007,111, 1214-1221.
(3)
Eyet, N.; Villano, S. M.; Kato, S.; Bierbaum, V. M. Deuterium Kinetic
Isotope Effects in Microsolvated Gas-Phase E2 Reactions, Journal of the American
Society of Mass Spectrometry, 2007, 18, 1046-1051.
(4)
Villano, S. M.; Gianola, A. J.; Eyet, N.; Ichino, T.; Kato, S.; Bierbaum,
V. M.; Lineberger, W. C. Thermochemical Studies of N-methylpyrazole andN-methylimidazole, Journal of Physical Chemistry A, 2007, 111, 8579-8587.
(5)
Villano, S. M.; Eyet, N.; Lineberger, W. C.; Bierbaum V. M. GasPhase Carbene Radical Anions: New Mechanistic Insight, Journal of the American
Chemical Society, 2008, 130, 7214-7215.
(6)Eyet, N.; Villano, S. M.; Bierbaum, V. M. Deuterium Kinetic Isotope
Effects in Microsolvated E2 Reactions; Methanol and Ethanol as Solvents, Journal of
the American Society of Mass Spectrometry, 2008, 19, 1296-1302.
(7)
Martinez, O.; Betts, N. B.; Villano, S. M.; Eyet, N.; Snow, T. P.;
Bierbaum, V. M. Gas-Phase Study of C+ Reactions of Interstellar Relevance, The
Astrophysical Journal, 2008, 686, 1486-1492.
(8)
Villano, S. M.; Eyet, N.; Lineberger, W. C.; Bierbaum, V. M. GasPhase Reactions of Halogenated Radical Carbene Anions with Sulfur and Oxygen
Containing Species, International Journal of Mass Spectrometry, 2009, In Press.
(9)Eyet, N.; Villano, S. M.; Bierbaum, V. M. Anchoring the Gas-Phase
Acidity Scale: Formic Acid to Methanethiol, International Journal of Mass
Spectrometry, 2009, In Press.
(10) Villano, S. M.; Eyet, N.; Lineberger, W. C.; Bierbaum, V. M.
Reactions of α-Nucleophiles with Alkyl Chlorides: The Competition Between SN2
and E2 Mechanisms and the Gas-Phase α-Effect, Submitted to Journal of theAmerican Chemical Society.
(11) Villano, S. M.; Eyet, N.; Wren, S. W.; Ellison, G. B.; Bierbaum, V. M.;
Lineberger, W. C. Thermochemical Studies of the Peroxyformyl and Peroxyacetyl
Radicals, In Preparation.
(12) Eyet, N.; Villano, S. M.; Bierbaum, V. M. E2 Reactions ofMicrosolvated Fluoride Ion Using Deuterium Kinetic Isotope Effects, In Preparation.
vii

TABLE OF CONTENTS
Chapter I: Gas-Phase Anion Chemistry Overview

1

1.1 Introduction

1

1.2 Gas-Phase Ion-Molecule Potential Energy Surface

2

1.3 Collision Rate Theory

4

1.4 Kinetic Isotope Effects

9

1.5 Thermochemistry11

1.6 References

17

Chapter II: Anion Photoelectron Spectroscopy Overview

19

2.1 Introduction

19

2.2 Atomic Species

19

2.3 Molecular Species

20

2.4 Selection Rules

25

2.5 Photoelectron Angular Distributions

27

2.6 References

30

Chapter III: Flowing Afterglow-Selected Ion Flow Tube Triple Quadrupole
Mass Spectrometer: Instrument Description.

32

3.1 Introduction

32

3.2 Flowing Afterglow Ion Source

33

3.3 Ion Optics and Mass Selection

38

3.4 Reaction Flow Tube40

3.5 Triple Quadrupole Detection

43

3.6 Data Acquisition

45

3.7 Rate Constant Determination and Error Analysis

46

3.8 Branching Ratio Determinations

54

3.9 References

58

Chapter IV: Negative Ion Flowing Afterglow Photoelectron Spectrometer:
Instrument Description

60

4.1 Introduction

60

4.2 Flowing Afterglow Ion Source

63

4.3 Ion Optics and Mass Selection

66

4.4 Ultraviolet Laser System

70

viii

4.5 Photoelectron Kinetic Energy Analyzer

73

4.6 Data Acquisition and Analysis

76

4.7 Franck-Condon Simulations

78

4.8 References

80

Chapter V: Reactions of BrO– and ClO– with Alkyl Halides: The Competition
Between SN2 and E2 Mechanisms and the Gas-Phase α-Effect.

83

5.1 Introduction

83

5.2 Experimental Methods

87

5.3 Results and Discussion

89

5.3.1 Reaction Trend and Product Distributions

89

5.3.2 Kinetic Isotope Effects

94–

5.3.3 ClO with EtCl: Comparison of Experiment and Theory

100

5.3.4 α-Nucleophilic Character

102

5.4 Conclusions

106

5.5 Acknowledgments

106

5.6 References

108

Chapter VI: Gas-Phase Carbene Radical Anions: New Mechanistic Insights

114

6.1 Introduction

114

6.2 Experimental Methods

117

6.3 Results and Discussion

119

6.3.1 Reactions of Carbene Anions with Sulfur/Oxygen
Containing Species

119

6.3.2 Reactions of CHX•⎯ and CClX•⎯ (X=Cl and Br) with
Halogenated Species

134

6.4 Conclusions

148

6.5 Acknowledgments

148

6.6 References

150

Chapter VII: Spectroscopic Characterization of the c-C4F8– and SF6– Anions

159

7.1 Introduction

159

7.2 Experimental Methods

161

7.3 Results and Discussion

161

7.3.1

Photoelectron Spectrum of the c-C4F8– Anion

161

7.3.2

Photoelectron Spectrum of the SF6– Anion

168

7.4 Conclusions

175

ix

7.5 Acknowledgements

176

7.6 References

177

Chapter VIII: Thermochemical Studies of N-methylpyrazole andN-methylimidazole

182

8.1 Introduction

182

8.2 Experimental Methods

185

8.2.1 Negative Ion Photoelectron Spectroscopy

187

8.2.2 Flowing Afterglow-Selected Ion Flow Tube Measurements

187

8.3 Results and Discussion

189

8.3.1 Computational Results

189

8.3.2 Photoelectron Spectroscopy of the N-methyl-

189

5-pyrazolide Anion
8.3.3 Photoelectron Spectroscopy of the N-methyl-

199

5-imidazolide Anion
8.3.4 Gas-Phase Acidity Measurements

206

8.3.5 H/D Exchange of the N-methylimidazolide Anion

208

8.3.6 Thermochemistry209

8.4 Conclusions

213

8.5 Acknowledgements

214

8.6 References

215

Chapter IX: Thermochemical Studies of the Peroxyformyl andPeroxyacetyl Radicals

221

9.1 Introduction

221

9.2 Experimental Methods

224

9.2.1 Negative Ion Photoelectron Spectroscopy

224

9.2.2 Flowing Afterglow-Selected Ion Flow Tube Measurements

225

9.2.3 Electronic Structure Calculations

226

9.3 Results and Discussion

227

9.3.1 Computational Results

227

9.3.2 Photoelectron Spectroscopy of the Peroxyformate Ion

234

9.3.3 Photoelectron Spectroscopy of the Peroxyacetate Ion

241

9.3.4 Collision Induced Dissociation Studies

246

9.3.5 Gas-Phase Acidity Measurements

250

9.3.6 Thermochemistry256x9.4 Conclusions

260

9.5 Acknowledgements

261

9.6 References

262

Bibliography

268

xi

List of Tables
5.01

5.02
6.01
6.02
6.03
6.04
6.05
6.06

7.01

7.02

8.01
8.02
8.03
8.04

Reaction rate constants (10-10 cm3 s-1), kinetic isotope effects andproduct ion branching ratios for the reactions of 79BrO– and 35ClO–
with RCl (R = methyl, ethyl, i-propyl, and t-butyl).

91

Reaction exothermicities (kcal mol-1) for the reactions of BrO– and
ClO– with RCl (R = methyl, ethyl, i-propyl, and t-butyl).

95

Reaction rate constants (10-10 cm3 s-1) for the reactions of CXY•– with
CS2, COS, CO2, O2, CO, and N2O.

120

Product ion distributions for the reactions of CXY•– with CS2, COS,
CO2, and O2.

121

Calculated and experimental reaction exothermicities (kcal mol-1) for
the reactions of CHCl•–, CCl2•–, and CF2•–.

123

Reaction rate constants (10-10 cm3 s-1) for the reactions of CXY•⎯ with
CCl4, CHCl3, CH2Cl2, CH3Cl, CH3Br, and (CH3)3CBr.

135

Product ion branching fractions for the reactions of CXY•– with CCl4,
CHCl3, and CH2Cl2.

136

Reaction rate constants (10-10 cm3 s-1) and product ion branchingratios for the reactions of CH35Cl•⎯ with CHnCl4-n (n=0-3) and production isotope distributions for the reactions of CH35Cl•⎯ and CH37Cl•⎯
with CHnCl4-n (n=0-3).

137

Theoretical frequencies (cm-1), calculated using the MP2/TZVPP levelof theory for both c-C4F8 anion and neutral along with the experimental
values for neutral c-C4F8.

165

Theoretical frequencies (cm-1), calculated using the MBPT(2) level oftheory for SF6 anion and neutral and the experimental values forSF6 neutral.

173

B3LYP/6-311++G(d,p) Optimized Geometries for N-methylpyrazole,N-methyl-5-pyrazolide and N-methyl-5-pyrazolyl.

190

B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of N-methyl-5pyrazolide and N-methyl-5-pyrazolyl.

191

B3LYP/6-311++G(d,p) Optimized Geometries for N-methylimidazole,N-methyl-5-imidazolide and N-methyl-5-imidazolyl.

192

B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of N-methyl-5imidazolide and N-methyl-5-imidazolyl.

193

xii

B3LYP/6-311++G(d,p) ∆acidG298 and BDE of N-methylpyrazole andN-methylimidazole and the EA of the corresponding radicals.

194

8.06

Thermochemical Parameters.

211

9.01

Experimental and Calculated (G3MP2B3) Electron Affinities,
Enthalpies of Deprotonation and Bond Dissociation Enthalpies for
Alkyl Peroxides and Carboxylic Acids.

229

9.02

Relative Energies (kcal mol-1) of the cis- and trans-conformers.

230

9.03

B3LYP/6-311++G(d,p) Optimized Geometries for Peroxyformic Acid,
Peroxyformate Anion and Peroxyformyl Radical.

232

B3LYP/6-311++G(d,p) Optimized Geometries for Peroxyacetic Acid,
Peroxyacetate Anion and Peroxyacetyl Radical.

233

B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of Peroxyformate
Anion and Peroxyformyl Radicals. Experimental values are provided inparentheses.

237

B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of Peroxyacetate
Anion and Peroxyacetyl Radical. Experimental values are provided inparentheses.

243

Thermochemical Parameters.

258

8.05

9.04
9.05

9.06

9.07

xiii

LIST OF FIGURES
1.01

Representative reaction coordinate diagrams for the SN2 reaction of
BrO– + CH3Cl in the gas-phase and in solution.

3

1.02

Schematic diagram of ion-neutral molecule trajectories.

6

1.03

Schematic diagrams of potential energy surfaces for a vibration in thereactant and in the transition state for a) a loose transition state where
EH < ED and kH/kD > 1 and for b) a tight transition state where EH > ED
and kH/kD < 1.

12

Schematic diagram that shows the relationship of the thermochemicalquantities that compose the proton affinity cycle, negative ion cycle,
and the appearance energy cycle.

16

1.04

2.01

The 351.1 nm photoelectron spectrum of the O•– anion. The solid line isthe experimental spectrum while the sticks are the calculated transitions. 21

2.02

Schematic representation of the photodetachment from the groundstate of AB– to the ground state and first excited state of AB.

22

Photograph of the Flowing Afterglow-Selected Ion Flow Tube Mass
Spectrometer (FA-SIFT) at CU Boulder

34

Schematic of Flowing Afterglow-Selected Ion Flow Tube Mass
Spectrometer (FA-SIFT).

35

3.03

Schematic of the flowing afterglow source and mass selection region.

36

3.04

Schematic of triple quadrupole detection.

44

3.05

a) Spectrum of the SIFT-injected 79BrO– anion (m/z 95). b) Spectrumof the SN2 reaction of 79BrO– + CH3Cl.

47

79

Reactant ion depletion and product ion formation for the reaction
BrO– + CD3Cl.

49

3.07

Sample branching ratio determination for the reaction 79BrO– + CD3Cl.

56

4.01

Photograph of the JILA Negative Ion Flowing Afterglow Photoelectron
Spectrometer.

61

Schematic representation of the JILA Negative Ion Flowing Afterglow
Photoelectron Spectrometer.

62

4.03

Schematic representation of the flowing afterglow source.

65

4.04

Schematic representation of the ion optics and Wien filter.

68

4.05

Schematic of Ar-ion laser system and external buildup cavity.

71

3.01
3.02

3.06

4.02

xiv

4.06

Schematic representation of ion-photon interaction region and of theelectron kinetic energy analyzer.

74a) Reaction coordinate diagram for the reaction of ClO– with CH3Cl.
b) Transition state structures are provided.

96

Anion basicity versus reaction efficiency for the SN2 reactions of Nu–
with CH3Cl. The normal nucleophiles are indicated by the closed
circles and the α-nucleophiles are indicated by the open circles.

104

6.01

Reaction efficiency versus anion basicity.

122

6.02

Reaction efficiency versus the number of chlorine atoms in theneutral reagent.

144

The 351.1 nm photodetachment spectrum of c-C4F8– taken at themagic angle.

163

7.02

Optimized geometries of c-C4F8 neutral and anion.

164

7.03

Calculated photoelectron spectra of the c-C4F8– anion reproduced from
Borrelli, R.; Peluso, A. J. Chem. Phys. 2008, 128, 044303/1-044303/7
with permission copyright 2008, American Institute of Physics, wherea) only the ν5 and ν1 modes were considered and b) where all modes
were considered.

167

The 351.1 nm photodetachment spectrum of SF6– taken under cold
flow tube conditions at the magic angle.

170

Comparison of the a) 351.1 nm photodetachment spectrum of SF6–
reproduced from Figure 7.04, and (b and c) the SF6– photoelectron
spectrum taken by Mark A. Johnson’s group at Yale University using adouble focusing tandem time of flight pulsed spectrometer.

171

Calculated photoelectron spectra of the SF6– anion reproduced from
Borrelli, R. Chem. Phys. Lett. 2007, 445, 84-88 with permission
copyright 2008, Elsevier.

174

5.01
5.02

7.01

7.04
7.05

7.06

8.01

(a) The 351.1 nm magic-angle photoelectron spectrum of the N-methyl5-pyrazolide anion. (b) Simulations of the spectrum for the transitions
from X 1A′ N-methyl-5-pyrazolide anion to X 2A′ and A 2A′′ N-methyl-5pyrazolyl radical.
195

8.02

Relative atomic displacements in the activated normal modes of theX 2A′ state of N-methyl-5-pyrazolyl and their frequencies.

198

The 351.1 nm magic-angle photoelectron spectrum of the N-methylimidazolide anion.

201

8.03

xv

8.04

a) The 351.1 nm magic-angle photoelectron spectrum of the N-methylimidazolide anion, reproduced from Figure 8.03. Simulations of thespectrum for the transitions of b) X 1A′ N-methyl-5-imidazolide to X 2A′
N-methyl-5-imidazolyl, c) X 1A′ N-methyl-2-imidazolide to X 2A′ N-methyl2-imidazolyl, and d) X 1A N-imidazolylmethyl anion to X 2A′ N-imidazolylmethyl radical.
202

8.05

The 351.1 nm magic-angle photoelectron spectrum of the N-methylimidazolide anion (black), reproduced from Figure 8.03 and thesimulated 10 meV Gaussian convoluted and stick spectra, reproduced
from Figure 8.04b.

203

Relative atomic displacements in the activated normal modes for theX 2A′ state of N-methyl-5-imidazolyl.

204

Potential energy surface (E + Ezpt) calculated at the G3MP2B3 levelof theory for the cis- to trans-isomerization of the peroxyformate anion
and the peroxyformyl radical.

231a-b) The 351.1 nm magic-angle photoelectron spectrum of the peroxyformate anion reproduced from Figure 9.02. c-f) Franck-Condon
simulations for transitions from X 1A′ state of the trans-peroxyformate
anion to the X 2A′′ state of the trans-peroxyformyl radical.

238a) The 351.1 nm magic-angle photoelectron spectrum of the peroxyformate anion reproduced from Figure 9.02a. b) Franck-Condon
simulation for transitions from X 1A′ state of the cis-peroxyformate
anion to the X 2A′′ state of the cis-peroxyformyl radical.

239a) The 351.1 nm magic-angle photoelectron spectrum of the peroxyacetate anion. b) Simulations of the spectrum for transitions fromX 1A′ trans-peroxyacetate anion to X 2A′′ and A 2A′ trans-peroxyacetyl
radical.

244

8.06

9.01

9.02

9.03

9.04

9.05

a) The 351.1 nm magic-angle photoelectron spectrum of the peroxyacetate anion reproduced from Figure 9.04a. Franck-Condon simulation
based on the B3LYP/6-311++G(d,p) optimized geometries and harmonicfrequencies for transitions from b) X 1A′ cis-peroxyacetate anion to X 2A′′
cis-peroxyacetyl radical and c) X 1A′ state of the hydrogenperoxide
ethylenolate anion to X 2A′′ state of the hydrogenperoxide ethylenolyl
radical.
245

9.06

Collision Energy (ECOM) dependence of the relative yields of the parent
and daughter ions in the CID of a) the peroxyformate anion and b) the
peroxyacetate anion.

248a-d) Representative potential energy surfaces for the proton transfer
reaction of the trans-conformer of the peroxyacyl ion with a reference
acid.

254

9.07

xvi

CHAPTER IGAS-PHASE ANION CHEMISTRY OVERVIEW

1.1) Introduction
While anions are not as prevalent in nature as cations, their reactions areimportant in the atmosphere and other environments.1 The primary source of anions
in the atmosphere is due to electron attachment to molecular oxygen anddissociative electron attachment to ozone. These anions eventually go on to reactwith other trace gases to form a variety of anions. Additionally, while the chemistryof the interstellar medium is dominated by cation-atom/molecule chemistry, therecent discovery of carbon chain anions, CnH–, may indicate that anions also play animportant role in interstellar processes.2,3 The study of gas-phase anion chemistry isparticularly relevant to condensed-phase synthetic organic chemistry. Many
common synthetic organic schemes (such as substitutions, eliminations, Grignard
additions, aldol condensations, hydrolysis of esters, etc.) involve anions; in contrast
to cations, anions seldom undergo unimolecular rearrangement. Gas-phase studies
provide insight into the intrinsic nature of a reaction without the interference ofsolvent effects and the presence of counter ions.
Studying reactions in a solvent free environment provides insight into the roleof the solvent. For example, in the gas-phase the alcohols become more acidic as
the size of the alkyl group increases (t-C4H9OH > i-C3H7OH > C2H5OH > CH3OH >H2O), which is the opposite of the acidity ordering in protic solutions.4 Additionally, insolution polarizable nucleophiles are more reactive than nonpolarizable nucleophiles(CH3S– > CH3O– and I– > Br– > Cl– > F–). However, in the gas-phase this enhanced
nucleophilicity is not observed.5 In both cases the trends observed in solution do notreflect an intrinsic property of the molecules. Instead the ordering of the aliphatic
1alcohol acidity series in solution is due to the steric hindrance to solvation for thelarger alkoxides, while the enhanced reactivity of polar nucleophiles in solution stems
from their ability to better facilitate charge reorganization as the reaction proceedsfrom the reactants to the transition state.
Another stark difference between gas-phase studies and solution phasestudies is the rates at which reactions occur in the two media. For example, in thegas-phase the SN2 reaction of hydroxide with methyl bromide is sixteen orders ofmagnitude faster than in solution.6 The decrease in reactivity is attributed to thereorganization of the solvent, as well as the greater thermodynamic stability of thesolvated ion relative to the transition state. Stepwise hydration of the nucleophile
indicates that the solvent reorganization effects become evident with the attachmentof only two solvent molecules. In order to understand the differences between gasphase and condensed phase ion-molecule reactions it is important to understand theenergetic factors that influence gas-phase processes.
1.2) Gas-Phase Ion-Molecule Potential Energy Surface
The potential energy surface of a typical gas-phase ion-molecule reaction has
a double well shape, where the transition state is flanked by two stable ion-molecule
complexes.5 This is qualitatively shown for the SN2 reaction of BrO– + CH3Cl, in
Figure 1.01a. The collision of BrO– with CH3Cl, results in a reactant ion-dipole
complex that is stabilized (by ~10-20 kcal mol-1) relative to the separated reactants.
Initially, this energy is in the form of translational energy. However, as the reactantscome together the energy is equilibrated among the various rotational and vibrationalmodes, resulting in a relatively long-lived complex. Eventually this energy will returnto the translational degrees of freedom and the reactants will separate, or the energy
will concentrate along the reaction coordinate thereby allowing the reaction to2

BrO– + CH3 Cl[BrO-CH3-Cl]– ‡CH3OBr + Cl–a)

Potential Energy∆Hrxn∆Hsol (BrO–) +∆Hsol (CH3Cl)∆Hsol (TS)

b)
∆Hrxn

Reaction Coordinate

Figure 1.01: Representative reaction coordinate diagrams for the SN2 reaction of
BrO– + CH3Cl in a) the gas-phase and b) in solution.

3

overcome the activation barrier to form a product ion-dipole complex. Similarly theexcess energy contained in the product ion-dipole complex must concentrate into thetranslation modes for the complex to dissociate into the free products. While there is
ample energy to cross over the barrier, the transition state is an ordered structure
limiting the rate of the reaction. In other words, dissociation of the reactant ion-dipole
complex into the free reactants is entropically favored over crossing the barrier.
The gas-phase ion-molecule potential energy surface is qualitatively very
different from that in solution, shown in Figure 1.01a and b. The solvent stabilizes
the reactants, products, and transition state compared to those species in the gasphase. The enthalpy of solvation of the reactants is greater than that of the transition
state structure since the charge is more localized in the reactants than in thetransition state. Additionally, the solvent shields the attractive ion-dipole/ion- induced
dipole forces, therefore, the double well shape is absent in solution. In solution theactivation energy is positive, thus most collisions are unreactive. For a reaction toproceed, the reactants must come together with enough energy to surmount theactivation barrier, and therefore only the high-energy tail of the Maxwell-Boltzmannenergy distribution reacts. Since the reactants are in thermal equilibrium with thesolvent the reactants are activated through collisions with the surrounding solvent.
1.3) Collision Rate Theory
In the gas-phase, an ion and a neutral molecule approach one another on an
attractive potential surface, which results from the interaction of the ion charge withany permanent dipoles or multipoles and any induced dipoles or multipoles. Thispotential is represented by a sum of terms, one for each interaction. For relatively
simple potentials, the collision rate constant can easily be evaluated. However, as
the number of terms in the potential increases, it becomes increasingly by more
complicated to solve for the collision rate constant. Two common scenarios are4

discussed below: 1) the collision of an ion and a non-polar molecule where theattractive potential is due to an ion-induced dipole force and 2) the collision of an ion
and a molecule with a permanent dipole where the attractive potential is due both toion-induced dipole and ion-dipole forces.
In the simplest case, the ion is described by a point charge that interacts witha structureless, isotropically polarizable molecule that does not have a permanent
dipole. The potential between the two species results from the charge-induced
dipole that arises from the interaction of the neutral molecule with the electric field of
the ion, shown in eq 1.01.
V(r) =-q2 α
2r 4(1.01)

Here, q is the anion charge, α is the isotropic polarizability, and r is the distance
between the ion and the neutral species. The total energy of the system is given byeq 1.02, where the dot represents the derivative with respect to time, µ is the
reduced mass of the colliding pair, and L is the angular momentum.

E=

µr 2
L
+
+ V(r)
2 2µr 2

(1.02)

Because the potential depends only on the distance between the ion and the neutral,r, this motion is confined to a plane, shown in Figure 1.02, where v0 is the initial
velocity of the molecule and b is the distance between the ion and the trajectory of
the neutral molecule before it is influenced by the electric field of the ion; v0 and b are
perpendicular to one another and θ is the angle between r and v0. As the ion andmolecule approach one another the radial kinetic energy decreases, then increases
again as the ion and molecule move away from one another. At a certain value of b,
the radial kinetic energy becomes zero and the ion-induced dipole force is equal but

56

Figure 1.02: Schematic diagram of ion-neutral molecule trajectories. The critical impact
factor, bc, is shown. Trajectories where b > bc are scattered. Trajectories where b < bc are
captured.

opposite to the centrifugal force, where L=µvob. The critical impact factor, bc, can befound, eq 1.03.7
14⎛ 4q2 α ⎞bc = ⎜⎟⎝ µv 0 ⎠(1.03)At this critical impact factor, bc, the ion and molecule enter a metastable orbit. Any
trajectories where b > bc will be deflected by the attractive potential but not captured;for large values of b, the initial trajectory of the molecule remains unchanged. Any
trajectories where b < bc will spiral towards the ion and be captured. The collisioncross section is given by σc=πbc2. Using a classical cross section treatment andaveraging over the Maxwell-Boltzmann energy distribution gives the LangevinGioumousis-Stevenson equation,8 kL in eq 1.04.
12⎛α⎞kL = 2π q ⎜ ⎟⎝µ⎠(1.04)

This theory adequately predicts collision rate constants, when the neutral species
does not have a permanent dipole. For collision of an ion with a neutral moleculethat does has a permanent dipole, kL provides a lower limit to the true collision rate
constant.
The calculation of the collision rate constant is more complicated when an ion
interacts with a neutral molecule that has a permanent dipole. In this case thepotential is described by eq 1.05, where µD is the dipole moment of the neutral
molecule and θ is the angle between the dipole moment vector and r (shown in
Figure 1.02). The first term describes the ion-induced dipole potential and thesecond term describes the ion-dipole potential.

V(r) =-q2 α −qµD
+ 2 cos θ
2r 4
r(1.05)

7

In the same manner as above, the collision rate constant can be derived if one
assumes that the dipole is locked in the direction of the approaching ion.9 In thiscase θ=0, thereby giving a potential that depends only on r. The resulting rate
constant is given in eq 1.06.
12⎛ 2 ⎞ ⎤
⎛ 2πq ⎞ ⎡ 1 2
k = ⎜ 1 2 ⎟ ⎢ α + µD ⎜⎟ ⎥⎝ µ ⎠ ⎢⎣⎝ πk b T ⎠ ⎥⎦(1.06)

Neglecting the rotation of the dipole, however, leads to an overestimation of thecollision rate, since the complete alignment of the dipole is prevented by therotational energy and forbidden quantum mechanically. To compensate for this, aneffective-locking constant, c, is placed in front of the ion-dipole term in eq 1.06. This
constant is between 0 and 1 and is estimated from a plot of calculated c versusµD/α1/2; this approach is referred to as average dipole orientation (ADO) theory.10,11
Through trajectory calculations, Su and Chesnavich12 have derived anempirical function to determine the collision rate constant based on the ratio of thedipole moment of the neutral species to the square root of the polarizability. The
collision rate constant is determined from kc = kLKcap, where kL is found in eq 1.04
and Kcap is found from either eq 1.07 or eq 1.08, depending upon the value of x,
which is defined in eq 1.09.

K cap ( x ≥ 2 ) = 0.4767x + 0.6200( x + 0.5090 ) + 0.9754( x ≤ 2) =(1.07)

2

K cap

x=

10.526µD( 2αk T )12

(1.08)(1.09)b

This theory is referred to as parameterized trajectory theory. It is important to notethat at high temperatures or for molecules without a permanent dipole, the collision8

rate constant reduces to the Langevin-Gioumousis-Stevenson equation. This theory
has been shown to calculate the collision rate constant to within 3% of the true value.
The calculated collision rate constants provided in this thesis are calculatedusing parameterized trajectory theory.12 The ratio of the reaction rate constant to thecollision rate constant gives the reaction efficiency, Eff=krxn/kcol. This ratio expresses
the percentage of collisions that cross over the activation barrier. It is often more
appropriate to compare the reaction efficiencies of ion-molecule reactions, since thecollision rates may vary significantly for different reactions.
1.4) Kinetic Isotope Effects
For reactions whose rate is controlled by the crossing of an activation barrier,isotopic substitution of a deuterium for a hydrogen atom can change the reaction
rate.13 Deuterium kinetic isotope effects (KIE), defined as the ratio of perprotio toperdeuterio rate constants (KIE = kH/kD), can provide insight into the reaction
mechanism. A classic example is the competing nucleophilic substitution (SN2) andbase-induced elimination (E2) mechanisms. Since these two mechanisms have thesame ionic reactants and products, they are not readily distinguished using mass
spectrometric techniques. These mechanisms, however, are distinguishable by their
distinct KIE. For an SN2 mechanism, deuteration of the neutral reactant increases
the reaction rate giving an inverse KIE (kH/kD<1).14,15 For an E2 mechanism
deuteration of the neutral reactant decreases the reaction rate giving a normal KIE(kH/kD>1).
The total KIE can be factored into a collisional (ηcol), translational (ηtran),rotational (ηrot), and vibrational (ηvib) contribution, eq 1.10.

kH kD = ηcolη tranηrotη vib(1.10)The origin of the KIE for each component is given in eq 1.11-1.14.

9

12⎛ µD ⎞
ηcol = ⎜ H ⎟⎝µ ⎠⎛ µD ⎞
ηtran = ⎜ H ⎟⎝µ ⎠(1.11)
32(1.12)
12⎛ ΙH,TS ΙD,R ⎞ηrot = ⎜ D,TS H,R ⎟⎝Ι Ι ⎠(1.13)

q H,TS
q D,R
vib
vib
ηvib = D,TS H,Rq vib q vib(1.14)

The collisional and translational contributions (eq 1.11 and 1.12, respectively) aredetermined by the ratio of the reduced mass, µ, for the perdeuterio reactants(superscript D) to the perprotio reactants (superscript H). This ratio gives a normal
effect that is generally small. The rotational component (eq 1.13) is a spatial effect,
where I is the determinant of the moment of inertia tensor and the superscripts R and
TS indicate the reactants and the transition state, respectively. The ratio of IH,TS to
ID,TS gives an inverse effect while the ratio of ID,R to IH,R gives a normal effect. The
total rotational effect is normal since the change in the moments of inertia upondeuteration is greater for the reactants than for the transition state. The magnitudeof the rotational effect can range from small to large depending on the structure of
the transition state. The vibrational component (eq 1.14) is given by the ratio ofvibrational partition function, qvib, for the reactants and transition state. Depending
upon changes in vibrational frequencies in the reactants and in the transition state
upon deuteration, this effect can vary from inverse to normal and from small to large
in size.
The rotational and vibrational components provide insight into the reaction
mechanism since they probe the structure of the transition state. The vibrational

10

component is more sensitive, and typically provides the distinguishing factor in
determining a reaction mechanism since deuterataion results in either a normal or
inverse effect.16 Normal effects are observed when the transition state is loose (i.e.,the bonds are lengthened in the transition state) whereas inverse effects areobserved when the transition state is tight (i.e., the bonds are shortened in thetransition state). This is schematically shown in Figure 1.03a and 1.03b.
respectively. Deuteration lowers the vibrational frequencies in both the reactants
and transition states relative to the hydrogenated compounds. For a reaction
proceeding through a loose transition state a) the frequencies for the hydrogenatedspecies are lowered more substantially than for the deuterated species in thetransition state, thus the activation energy for the perprotio reaction is lower than thatof the perdeuterio reaction, EH‡ < ED‡. For a reaction proceeding through a tighttransition state b) the frequencies for the hydrogenated species are raised more
substantially than for the deuterated species in the transition state, thus theactivation energy for the perprotio reaction is greater than that of the perdeuterio
reaction, EH‡ > ED‡.
1.5) Thermochemistry
The gas-phase acidity of AH, (∆aG0(AH)), is the free-energy change for theheterolytic cleavage of AH → A– + H+. Typical gas-phase acidities are ~300-400 kcal
mol-1; thus, these quantities are difficult to measure directly and instead aremeasured relative to a reference acid (RH) of known acidity. Proton transfer
equilibrium experiments determine the free energy difference between an acid of
unknown acidity and a reference acid via eq 1.15.

∆( ∆ aG298 ) = −RTln K eq

(1.15)

11

12

Figure 1.03: Schematic diagrams of potential energy surfaces where R indicates a vibrationin the reactant and TS indicates the vibration in the transition state. The activation barriers forhydrogen and deuterium labeled compounds are shown for a) a loose transition state where
EH < ED and kH/kD > 1 and for b) a tight transition state where EH > ED and kH/kD < 1.

Here ∆(∆aG298) = ∆aG298(AH) - ∆aG298(RH), where ∆aG298(RH) is a known quantity.
The equilibrium constant can be directly determined by measuring the relative
intensities of two conjugate bases and the relative concentrations of the acids in an
equilibrium mixture. This technique requires sufficiently long residence times so thatequilibrium is established. A more common approach is to separately determine theforward and reverse proton transfer rate constants (kf and kr) for the reactions shownin eq 1.16; the rate constant ratio gives the proton transfer equilibrium constant,
Keq=kf/kr.
kf⎯⎯→ R- + AH
RH + A - ←⎯kr ⎯(1.16)

Gas-phase ion-molecule bracketing experiments are employed when the
more desirable equilibrium method is not feasible. Complications in measuring theproton transfer equilibrium constant arise when one of the conjugate bases cannot
readily be formed or when the neutral acid is not available in pure form. Ionmolecule bracketing experiments overcome these difficulties by only examining one
direction of the proton transfer equilibrium, either kf or kr. This method provides a
semiquantitative estimate of the gas-phase acidity by examining the occurrence or
non-occurrence of proton transfer with several reference acids, RH. For the case of
A– + RH, if rapid proton transfer is observed then the acidity of the reference acid is alower bound, while if proton transfer is not observed then the acidity of the referenceacid is an upper bound. Conversely, for the reaction of R– + AH, the occurrence of
rapid proton transfer sets an upper bound while the non-occurrence of protontransfer sets a lower bound.
The enthalpy of deprotonation of AH, (∆aH298(AH)), is determined from gasphase acidity measurements via eq 1.17, where ∆aS298(AH), is the entropy of
deprotonation.

13

∆ aH 298 (AH) = ∆ aG298 (AH) + T∆ aS298 (AH)

(1.17)The entropy of deprotonation can be determined by measuring the gas-phase acidity
as a function of temperature; however, more often this value is found from electronic
structure calculations. A small thermal correction (~1.5 kcal mol-1) is applied toconvert the 298 K enthalpy of deprotonation to 0 K (eq 1.18). Often, the integrated
heat capacities are not available in the literature and therefore these values are also
approximated from electronic structure calculations.
298∆ a H 0 (AH) = ∆ a H 298 (AH) − ∫ ⎡⎣Cp (A − ) + Cp (H+ ) − Cp (AH)⎤⎦ d T(1.18)

0The enthalpy of dissociation can be related to other important thermodynamic
quantities. Of particular importance, the bond dissociation energy (BDE, D0(AH)) is
the enthalpy required to homolytically cleave the AH bond, shown in eq 1.22. Since
the making and breaking of bonds is the basis of all chemical transformation,knowledge of BDEs provides insight into whether or not a given chemical process
can occur. BDEs can be determined from the enthalpy of deprotonation of AH(∆aH0(AH)), the electron affinity of the A radical (EA(A)), and the ionization energy ofthe hydrogen atom (IE(H)) via a negative ion thermochemical cycle:17,18
AH → A– + H+

∆aH0(AH)(1.19)

A– → A + e–

EA(A)(1.20)+ H+ + e– → H-IE(H)(1.21)

AH → A + H

D0(AH)(1.22)The sum of eq 1.19-1.21, gives the bond dissociation energy at 0 K, eq 1.23.
D0(AH) = ∆aH0(AH) + EA(A) - IE(H)(1.23)

Electron affinity measurements are discussed in Chapter 2 and the ionization energyof the hydrogen atom is well-known (13.59844 eV).19 If the heat of formation of AH is

14

known, then the heat of formation of the A radical, ∆fH0(A), can be determined fromthe BDE of the AH, using eq 1.24.

∆ f H0 (A) = D0 (AH) + ∆ f H0 (AH) − ∆ f H0 (H)(1.24)

Here, ∆fH0(H) is the heat of formation of hydrogen atom (52.103 ± 0.001 kcal mol-1)20
This relationship is significant since radicals are highly unstable species, and theirheats of formation cannot be determined directly from more traditional methods.
The thermodynamic quantities that make up the negative ion cycle areconnected to other important thermodynamic quantities. Figure 1.04 shows therelationship between the negative ion cycle, the appearance energy cycle, and theproton affinity cycle. Through the appearance energy cycle, the BDE of AH is foundfrom the difference of the ionization energy of A and the appearance energy of
A+/AH. The ionization energy of the A radical, IE(A), is the energy required toremove an electron from A to form A+ and a free electron. The appearance energy,AE(A+,AH) is the energy at which A+ is observed in the photodissociation of AH. In
the proton affinity cycle, the sum of EA(A) and ∆aH0(AH) equals the sum of theionization energy of AH and the proton affinity of A. The ionization energy of the AH,
IE(AH), is the energy required to remove an electron from AH to form AH+ and a freeelectron. The proton affinity of A, PA(A), is the energy required to remove a proton
from AH+ to form A and H+.

15

16

Figure 1.04: Schematic diagram that shows the relationship of the thermochemicalquantities that compose the proton affinity cycle, negative ion cycle, and theappearance energy cycle.

1.6) Chapter 1 References(1)

Wayne, R. P. Chemistry of Atmospheres; Oxford: Clarendon, 1991.

(2)
McCarthy, M. C.; Gottlieb, C. A.; Gupta, H.; Thaddeus, P. Laboratory
and Astronomical Identification of the Negative Molecular Ion C6H-. AstrophysicalJournal 2006, 652, L141-L144.
(3)
Cernicharo, J.; Guelin, M.; Agundez, M.; Kawaguchi, K.; McCarthy,M.; Thaddeus, P. Astronomical Detection of C4H-, the Second Interstellar Anion.
Astronomy & Astrophysics 2007, 467, L37-L40.
(4)
Brauman, J. I.; Blair, L. K. Gas-Phase Acidities of Alcohols. Journal of
the American Chemical Society 1970, 92, 5986-5992.
(5)
Olmstead, W. N.; Brauman, J. I. Gas-Phase Nucleophilic
Displacement-Reactions. Journal of the American Chemical Society 1977, 99, 42194228.
(6)
Mackay, G. I.; Bohme, D. K. Bridging the Gap between Gas-Phaseand Solution - Transition in Relative Acidity of Water and Methanol at 296 ± 2K.
Journal of the American Chemical Society 1978, 100, 327-329.
(7)Langevin, P. A Fundamental Formula of Kinetic Theory. Annales De
Chimie Et De Physique 1905, 5, 245-288.
(8)
Gioumousis, G.; Stevenson, D. P. Reactions of Gaseous Molecule
Ions with Gaseous Molecules .5. Theory. Journal of Chemical Physics 1958, 29, 294299.
(9)
Moran, T. F.; Hamill, W. H. Cross Sections of Ion-Permanent-Dipole
Reactions by Mass Spectrometry. Journal of Chemical Physics 1963, 39, 1413-1422.
(10) Su, T.; Bowers, M. T. Theory of Ion-Polar Molecule Collisions Comparison with Experimental Charge-Transfer Reactions of Rare-Gas Ions toGeometric Isomers of Difluorobenzene and Dichloroethylene. Journal of Chemical
Physics 1973, 58, 3027-3037.
(11) Bass, L.; Su, T.; Chesnavich, W. J.; Bowers, M. T. Ion-Polar Molecule
Collisions - Modification of Average Dipole Orientation Theory: The cosθ Model.
Chemical Physics Letters 1975, 34, 119-122.
(12) Su, T.; Chesnavich, W. J. Parametrization of the Ion-Polar Molecule
Collision Rate-Constant by Trajectory Calculations. Journal of Chemical Physics
1982, 76, 5183-5185.
(13) Melander, L.; Saunders, J. H., Jr. Reaction Rates of Isotopic
Molecules; John Wiley and Sons: New York, 1980.

17

(14) Kato, S.; Davico, G. E.; Lee, H. S.; DePuy, C. H.; Bierbaum, V. M.
Deuterium Kinetic Isotope Effects in Gas-Phase SN2 Reactions. International Journalof Mass Spectrometry 2001, 210, 223-229.
(15) Viggiano, A. A.; Morris, R. A.; Paschkewitz, J. S.; Paulsen, J. F.
Kinetics of the Gas-Phase Reactions of Chloride Anion, Cl- with CH3Br and CD3Br:
Experimental Evidence for Nonstatistical Behavior? Journal of the American
Chemical Society 1992, 114, 10477-10482.
(16) Poirier, R. A.; Wang, Y. L.; Westaway, K. C. A Theoretical-Study of
the Relationship between Secondary α-Deuterium Kinetic Isotope Effects and theStructure of SN2 Transition-States. Journal of the American Chemical Society 1994,
116, 2526-2533.
(17) Berkowitz, J.; Ellison, G. B.; Gutman, D. Three Methods to Measure
RH Bond-Energies. Journal of Physical Chemistry 1994, 98, 2744-2765.
(18) Blanksby, S. J.; Ellison, G. B. Bond Dissociation Energies of Organic
Molecules. Accounts of Chemical Research 2003, 36, 255-263.
(19) Chase Jr., M. W.; Davies, C. A.; Downey Jr., J. R.; Frurip, D. J.;
McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed. Journal of
Physical and Chemical Reference Data 1985, 14.
(20) NIST Chemistry WebBook, NIST Standard Reference Database
Number 69; Linstrom, P. J.; Mallard, W. G., Eds.; National Institute of Standards andTechnology: Gaithersburg MD, 20899 June 2005 (http://webbook.nist.gov).

18

CHAPTER IIANION PHOTOELECTRON SPECTROSCOPY OVERVIEW

2.1) Introduction
In negative ion photoelectron spectroscopy1-3 an anion beam (AB–) isbombarded with a photon beam (hν) of fixed frequency. If the energy of the photon
beam is greater than the electron binding energy, electrons will be ejected to yieldneutral molecules (AB) and free electrons (e–). This process is described by eq 2.01.
AB– (Etot) + hν Æ AB(Etot) + e– (eKE)(2.01)

Conservation of energy requires that the internal energy of the neutral molecule plus
the kinetic energy of the neutral molecule and of the ejected electron equals thephoton energy. Since the electron is so much lighter than the neutral molecule,nearly all of the kinetic energy is imparted to the electron. As a result, measurementof the electron kinetic energy (eKE) distribution gives the internal energy of theneutral molecule. As will be discussed below, anion photoelectron spectroscopy
provides thermodynamic information such as electron affinities (EA) and term
splitting energies (Te), allows for the measurement of vibrational frequencies andanharmonicities, and gives insight into the molecular structure of both the anion andthe neutral species. Furthermore, by selectively choosing the anionic species,
electron detachment yields unique or unstable radical and transition state species,
which are difficult to study by other techniques.4
2.2) Atomic Species
Electron detachment from an atomic species, to form the corresponding
neutral, results in purely electronic transitions. Consider for example, thephotoelectron spectrum of O•– collected at 351.1 nm (3.531 eV) photon energy

19

(shown in Figure 2.01). Photodetachment yields electrons with six different discrete
kinetic energies (peaks a-f), which represent transitions from the two lowest spinorbit states of O•– (2P3/2,1/2) to the three lowest states of O (3P2,1,0);5-7 thecorresponding energy diagram is included in the inset of Figure 2.01. The next
electronic state of atomic oxygen is 1.96 eV higher in energy than the 3P2 state andis not accessible with a 3.531 eV photon. Peak c corresponds to the EA of theoxygen atom (1.461112 eV), which is the transition from the ground state of theanion (2P3/2) to the ground state of the neutral (3P2). The difference of peak a and cgives a term energy splitting of 177.13 cm-1 for the anion, while the differences ofpeak c and e and of peak c and f represent the energy differences between the
ground state and the first and second excited states term energies of neutral oxygen(158.27 cm-1 and 226.98 cm-1, respectively). For atomic transitions, the line width of
the spectrum (fwhm) is determined by the instrument resolution (7 meV in Figure2.01) and the relative intensities are determined by the population of the various
levels of the anion.
2.3) Molecular Species
Electron detachment from molecular anions is more complicated since eachelectronic transition is composed of vibrational bands, which in turn are composed ofrotational bands, eq 2.02.1,2
AB– (Eele’ + Evib(v’) + Erot(J’)) + hν Æ AB(Eele’’ + Evib(v’’) + Erot(J’’)) + e– (eKE)(2.02)In this process, an anion, which has an internal energy distribution of electronic,
vibrational, and rotational energies (Eele’ + Evib(v’) + Erot(J’)) interacts with a photon toyield a neutral species with an internal energy distribution (Eele’’ + Evib(v’’) + Erot(J’’))and a free electron with discrete amounts of kinetic energy e–(eKE). Typically, using
this experimental method, rotational frequencies cannot be resolved and are20

Photoelectron Counts

3P
0
3P
1
3P
2

c

abc de f

2P
1/2
2P
3/2

e
f

1.62

1.60

1.58

d
1.56

b

a
1.54

1.52

Electron Binding Energy, (eV)Figure 2.01: The 351.1 nm photoelectron spectrum of the O•– anion. The solid lineis the experimental spectrum while the sticks are the calculated transitions. Peak cis the electron affinity of atomic oxygen. An energy diagram for the two lowest spinorbit states of the anion (bottom) to the three lowest states of the neutral atom (top)is provided in the inset.

21

hν
~

(C) A ABi

Energyhg~(B) X AB

f
e
d
c
b
a

Te(AB)~(A) X AB–

EA(AB)r (A-B)

Figure 2.02: Schematic representation of the photodetachment from the groundstate of AB– (curve A) to the ground state (curve B) and first excited state (curve C)of AB.

22

only considered as corrections8 to final energies (discussed below); therefore, for thefollowing discussion only vibrational transitions are considered, eq 2.03.
AB–(Eele’ + Evib(v’)) + hν Æ AB(Eele’’ + Evib(v’’)) + e–(eKE)(2.03)

Figure 2.02 is a schematic representation of photodetachment from ahypothetical system, AB–. This figure shows the potential energy curves andvibrational levels of the ground state of the anion (curve A) and the ground state(curve B) and first excited state (curve C) of the neutral AB. For a diatomic moleculethe x-axis represents the A-B stretching motion. For a polyatomic molecule with n
atoms (and 3n-5 or 3n-6 normal modes), the x-axis is a cut through one of the activemodes. The transition from the ground electronic and vibrational state of the anion to
the ground electronic and vibrational state of the neutral is the electron affinity (EA)of the neutral species (peak a). Vibrational levels of the neutral are seen as peaks at
higher binding energies than the EA or origin peak (peaks b-f), while the peaks atbinding energies lower than the origin represent vibrations of the anion; thesetransitions are referred to as hot bands (not shown in spectra on Figure 2.02). The
spacing between these peaks gives the vibrational frequencies and anharmonicities.
If the neutral molecule has a low-lying electronic state, it is also possible to observe atransition to this state. As in the ground state progression, the difference in thepeaks, observed at higher binding energies than the excited state band origin (peakg), gives vibrational frequencies and anharmonicities of the neutral excited state.
The difference between the electronic band origin of the ground state and the excitedstate (peaks a and g) gives the neutral term energy splitting (Te).
For molecular anions, the intensities of the various transitions are determinedby three factors: the intrinsic strength of the electronic transition, the population of
the various levels, and the square of the vibrational overlap integral (i.e., FranckCondon overlap). For a given electronic transition, the relative intensities of the23various vibrational levels can be explained by the Franck-Condon (FC) principle.
This principle is based on the idea that electronic transitions occur very fast (~10-15sec) relative to nuclear motion and, therefore, the nuclear positions do not change.
Quantitatively the Frank-Condon principle can be derived from the transition
dipole moment, shown in eq 2.04.

M ev = ψ e' v' µ ψ e'' v''(2.04)

where ψev is the wavefunction for the initial state of the anion (single prime) and forthe final state of the neutral plus a free electron (double prime) and µ is the dipole
moment operator. According to the Born-Oppenheimer approximation, the electronic
and nuclear motions of the molecule are separable and the wavefunction can be
written as a product of the two parts, ψev = ψeψv. Additionally, the dipole moment
operator, which is the sum over all the electrons (index i) and nuclei (index I) in themolecule, can be broken into an electronic part and a nuclear part, eq 2.05.

µ = −e∑ ri + e ∑ Z l R l = µ e + µ Ni(2.05)l

The transition dipole moment can be rewritten as:

M ev = ψ e' µ e ψ e'' ψ v' ψ v'' + ψ e' ψ e'' ψ v' µ N ψ v''(2.06)The second half of eq 2.06 is zero, since the electronic wavefunctions of two different
states are orthogonal; further simplification gives:

M ev = ψ e' µ e ψ e'' ψ v' ψ v'' = R e ψ v' ψ v''(2.07)The intensity of a given transition is proportional to the square of the transition
moment dipole9,10, I ∝⎪Mev⎪2. Therefore, for a given electronic transition the relative
intensities of the vibrational peaks are proportional to the square of the vibrational
overlap integral of the anion vibrational wavefunctions and the neutral species
vibrational wavefunctions, I ∝⎪〈ψv’⏐ψv’’〉⎪2, which are the Franck-Condon factors.

24

The intensity profile of the peaks within a given Franck-Condon envelope is a
result of the difference in geometry between the anion and neutral. This has
important implications, especially in our experimental setup, since typically the anion
is in its ground electronic and ground vibrational state. The removal of an electron is
rapid when compared to atomic nuclear motion, thus electron detachment results inthe projection of the anion geometry and hence the anion wavefunction onto theneutral molecule potential energy surface. In other words, electron detachment
results in a vertical transition, which corresponds to the most intense peak in theFranck-Condon envelope. For example in Figure 2.02, peak c is the most intense
peak in the ground state envelope, this indicates that there is a significant change in
the AB equilibrium bond length upon photodetachment thus, the transition from theground vibrational state of the anion to the v=2 vibrational level of the X state of AB
has the best Franck-Condon overlap. On the other hand, peak g is the most intense
peak in the excited electronic state manifold, indicating that there is only a modestchange in the AB equilibrium bond length upon photodetachment; thus, the transition
from the ground vibrational level of the anion to the ground vibrational level of the Astate of AB has the best Franck-Condon overlap.
2.4) Selection Rules
Equation 2.07 allows us to define a set of selection rules. The selection rules
for photodetachment are slightly different than those that govern the transitions in
optical electronic spectroscopy. These differences arise since optical electronic
spectroscopy involves a bound process, whereas photoelectron spectroscopy
involves an unbound process; the initial state wavefunction describes the anion while
the final state wavefunction describes the neutral molecule and the detachedelectron.

251) Electronic Selection Rule - Only single electron processes are allowed. Twoelectron processes occur when there is a detached electron with thesimultaneous promotion of another electron to an excited molecular orbital.
These processes are rare, but can occur if there is configuration mixing to
produce a weak transition.
2) Spin Selection Rule - The total spin of the system (molecule and unboundelectron) is conserved. Because the electron detaches with a spin ± ½ħ and only
one electron is ejected, the spin of the molecule must also change by the sameamount. This yields a selection rule for the molecule of ∆S= ± ½, which isdistinctly different from traditional electronic spectroscopy. Photodetachmentfrom an anion that is a doublet (2s+1=2) results in a neutral molecule that iseither a singlet (2s+1=1) or triplet (2s+1=3), allowing for direct determination of
singlet-triplet splitting energies.
3) Angular Momentum Selection Rule - The angular momentum of the system must
be conserved. The incident photon has an angular momentum of 1ħ, which is
imparted to the outgoing electron. For example, if an electron is detached froman atomic s-orbital, as in the case of H–, the electron will detach as a p-wave,
eq 2.08.
AB– (0ħ) + hν (1ħ) Æ AB (0ħ) + e– (1ħ)

(2.08)

If an electron is detached from an atomic p-orbital, as in the case of O•–, thephoton angular momentum adds or subtracts, and the electron will leave as an
d-type or s-type wave, respectively, eq 2.09 and 2.10.
AB– (1ħ) + hν (1ħ) Æ AB (0ħ) + e– (2ħ)(2.09)

AB– (1ħ) + hν (1ħ) Æ AB (0ħ) + e– (0ħ)

(2.10)

26

Thus, there is a selection rule for the system (molecule and unbound electron) of∆L= 0, ± 1. This rule is analogous to optical electronic spectroscopy, except thatin a bound process, absorption of a photon will promote an electron from ans-orbital to a p-orbital and from a p-orbital to either an s- or d-orbital.
4) Vibrational Selection Rule - Only vibrations that correspond to the changes ingeometries between the anion and the neutral molecule are activated in thephotoelectron spectra. In the simplest approximation, all 3n-5 (or 3n-6) normal
modes are separable and can be approximated as harmonic oscillators. Under
this simple approximation, only totally symmetric vibrational modes will yield anon-zero Franck-Condon factor, hence only totally symmetric stretches are activein the photoelectron spectrum for any ∆v. In practice, however, vibrations are notharmonic and non-totally symmetric modes can be active. Typically non-totallysymmetric modes only have significant Franck-Condon factors for ∆v = 0, unlessthere is a significant difference in the frequency between the anion and theneutral molecule. In this case, non-totally symmetric vibrations the FranckCondon factors are non-zero for even quanta, ∆v = 0, ± 2, ± 4… and zero for odd
quanta, ∆v = 1, ± 3, ± 5…
2.5) Photoelectron Angular Distributions
Photoelectrons are not ejected isotropically, instead the intensity isdependent upon the relative polarity of the incoming linearly polarized photon.11,12
For linearly polarized light, photoelectron intensity is proportional to the differential
photodetachment cross-section, which is described by eq 2.11.
I∝σ∂σ(E ) = tot ⎡⎢1+ β (E ) (3cos 2 θ − 1)⎤⎥
∂Ω
4π ⎣
2
⎦

(2.11)

Here, σtot is the total photodetachment cross-section, β is the asymmetry parameter(-1 ≤ β ≤ +2), and θ is the angle between the electric field vector of the incident
27photon and the photoelectron collection axis. If the spectrum is collected at the“magic angle” (θ = 54.7°), then 3cos2θ -1=0, and the photoelectron intensity is
proportional the photodetachment cross-section. Thus, the relative intensities of thevarious final vibrational states for a given initial vibrational state are proportional to
the Franck-Condon factors only.
The anisotropy parameter (β) can be estimated by relative intensity
measurements collected at laser polarizations perpendicular (θ=90°) and parallel(θ=0°) to the photoelectron collection axis as shown in eq 2.12.

β=I0° − I90°
I + I90°1
2 0°(2.12)

The anisotropy parameter (β) provides valuable information about the symmetry of
the orbital from which the electron was ejected. Consider for example, ejection of ans-orbital electron, as the case of H–, versus ejection of a p-orbital, as in the case ofO•–. Electron detachment from an s-orbital results in a p-wave, which is polarized in
the same direction as the photon. The angular distribution is cos2θ and β=2;additionally, the electron angular distribution is independent of the electron kinetic
energy. Ejection of an electron from a p-orbital results in both an s- and d-wave.
The photodetachment cross-sections for s- and d-waves are dependent upon theelectron kinetic energy. For slow electrons, ejected just above threshold, thephotodetachment cross-section of an s-wave is much larger than that of a d-wave,
thus the photoelectron angular distribution is isotropic (β=0). As the electron kinetic
energy increases, the d-wave cross-section becomes larger and the relative
contributions of the s- and d-waves change. The asymmetry parameter is a measureof the interference between the s- and d-wave and, depending on the amount ofinterference, β is between 0 and 1.

28

While detachment of an electron from a molecular orbital is more
complicated, β can still provide valuable information. It has been shown that β < 0for electron detachment from π-orbitals and β > 0 for electron detachment fromσ-orbitals.13,14 Additionally, dramatic changes in β are indicative of photodetachmentof an electron from a different molecular orbital and therefore β can be used to
identify different electronic states within the same spectrum.

292.6) Chapter 2 References(1)
Eland, J. H. D. Molecular Photoelectron Spectroscopy. Journal of
Physics E - Scientific Instruments 1978, 11, 969-977.
(2)
Ervin, K. M.; Lineberger, W. C. Photoelectron Spectroscopy of
Negative Ions. In Advances in Gas-Phase Ion Chemistry; N. G. Adams and L. M.
Babcock, Eds. Greenwich, 1992; pp 121-166.
(3)
Chen, P. Photoelectron Spectroscopy of Reactive Intermediates. In
Unimolecular and Bimolecular Reaction Dynamics; C. Y. Ng, T. Baer and I. Powis,
Eds.; John Wiley and Sons: New York, 1994; pp 371-425.
(4)Wenthold, P. G.; Lineberger, W. C. Negative Ion Photoelectron
Spectroscopy Studies of Organic Reactive Intermediates. Accounts of Chemical
Research 1999, 32, 597-604.
(5)
Moore, C. E. Atomic Energy Levels. NSRDS-NBS; Washington: US
GPO Circular No. 467, 1952.
(6)
Neumark, D. M. Laser Photodetachment Measurement of theElectron-Affinity of Atomic Oxygen. Physical Review A 1985, 32, 1890-1892.
(7)
Ervin, K. M.; Anusiewics, I.; Skurski, P.; Simon, J.; Lineberger, W. C.
The Only Stable State of O2− is the X 2Πg Ground State and It (Still!) Has an
Adiabatic Electron Detachment Energy of 0.45 eV. Journal of Physical Chemistry A
2003, 107, 8521-8529.
(8)
Engelking, P. C. Approximate Rotational Band Shifts. Journal of
Physical Chemistry 1986, 90, 4544-4545.
(9)
Bernath, P. F. Spectra of Atoms and Molecules; Oxford University
Press: New York, 1995.
(10) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy: An
Introduction to Vibrational and Electronic Spectroscopy; Oxford University Press:
New York, 1978.
(11) Cooper, J.; Zare, R. N. Angular Distributions of Photoelectrons.
Journal of Chemical Physics 1968, 48, 942-943.
(12) Hall, J. L.; Siegel, M. W. Angular Dependence of the Laser
Photodetachment of the Negative Ions of Carbon, Oxygen, and Hydrogen. Journal ofChemical Physics 1968, 48, 943-945.
(13) Gunion, R. F.; Gilles, M. K.; Polak, M. L.; Lineberger, W. C. Ultraviolet
Photoelectron-Spectroscopy of the Phenide, Benzyl and Phenoxide Anions, with Ab
Initio Calculations. International Journal of Mass Spectrometry and Ion Processes
1992, 117, 601-620.

30

(14) Wenthold, P. G.; Polak, M. L.; Lineberger, W. C. Photoelectron
Spectroscopy of the Allyl and 2-Methylallyl Anions. Journal of Physical Chemistry
1996, 100, 6920-6926.

31

CHAPTER III
FLOWING AFTERGLOW-SELECTED ION FLOW TUBE TRIPLEQUADRUPOLE MASS SPECTROMETER: INSTRUMENT DESCRIPTION

3.1) Introduction
The Flowing-Afterglow technique was developed by E. E. Ferguson, F. C.
Fehsenfeld, and A. L. Schmeltekopf at NOAA in 1963 as a means of studying
atmospherically relevant ion-molecule reactions.1 In this early technique, helium gas
was directed down a glass flow tube by a high capacity pump and ionized by adischarge. Trace amounts of neutral reagents were added to the He flow and ionswere observed with a quadrupole mass analyzer coupled to a particle multiplier.
Kinetic measurements were made by monitoring the falloff of ion intensity as afunction of neutral reagent concentration. Since its advent, several modifications
including temperature variability,2,3 drift capability,4 inclusion of an atomic and radical
species generator,5-8 and selected ion injection9 have been made. This powerful
technique provides several advantages for studying the kinetics andthermochemistry of gas-phase ion-molecule reactions: 1) ionic and neutral reactants
have a well-defined thermal energy distribution, 2) high ion densities are obtained, 3)
a variety of ions can be formed by coupling several different ionization techniques
and through multi step ion-molecule syntheses, 4) energy variability is possible, and5) the kinetic analysis is relatively straightforward. Over the past four decades, thistechnique has been applied to study atmospheric and interstellar chemistry, gasphase thermochemistry, fundamental and mechanistic physical organic chemistry,organometallic chemistry, and a variety of analytical problems.
The Flowing Afterglow-Selected Ion Flow Tube mass spectrometer (FA-SIFT)
at the University of Colorado Department of Chemistry incorporates selected ion

32injection, drift capabilities, and triple quadrupole detection. In this instrument ionsare formed in a flowing afterglow source through a series of ion-molecule reactions.
Ions of a certain polarity are extracted from the source region, mass selected by aquadrupole mass filter and injected into the reaction flow tube where they arethermalized to ~298 K through multiple collisions with He buffer gas. Measured
flows of neutral reagents are introduced into the reaction flow tube through a seriesof fixed inlets along the reaction flow tube and the reactant and product ions areanalyzed by a triple quadrupole mass filter coupled to an electron multiplier. Neutral
reactant flow rates are measured by monitoring the pressure change versus time in acalibrated volume system. Reaction rate constants are determined by changing theneutral addition inlet, thereby changing the reaction distance and time, while
monitoring the change in reactant ion intensity. Figure 3.01 is a photograph of the
CU chemistry FA-SIFT mass spectrometer and Figure 3.02 is a schematic diagramof this instrument. The following sections provide an overview of this instrument:
Flowing Afterglow Ion Source, Ion Optics and Mass Selection, Reaction Flow Tube,
Triple Quadrupole Detection, Data Acquisition, Rate Constant Determination andError Analysis, and Branching Ratio Determination. Several other sources havedescribed this technique in more detail.10-133.2) Flowing Afterglow Ion Source
Reactant ions are formed in a flowing afterglow source by passing helium
buffer gas, which has been purified by passage though a liquid nitrogen-cooled
molecular sieve trap, over a heated filament. The filament assembly, shown in
Figure 3.03, is composed of a repeller plate, a rhenium filament, and an acceleration
grid. The filament is typically biased at approximately ±70 V relative to the flow tube
and grid. One side of the filament is biased 5 V higher than the other side; thefilament is resistively heated and electrons are boiled off and accelerated towards
33

34

Figure 3.01: Photograph of the Flowing Afterglow-Selected Ion Flow Tube
mass spectrometer (FA-SIFT) at CU Boulder

35

Figure 3.02: Schematic of Flowing Afterglow-Selected Ion Flow Tube mass spectrometer (FA-SIFT).

36

Figure 3.03: Schematic of the flowing afterglow source and mass selection region.

the grid. The emission current is typically set to 60 µA and is regulated by a
feedback loop. The resulting helium plasma contains He 23S and 21S metastablespecies, He+, He2+, and free electrons. The addition of trace amounts of neutral
gases allows for a variety of ions to be produced from a wide range of mechanisms.
In this source, both positive and negative ions are produced; however, only themechanisms relevant to negative ion formation are discussed here. In general,
negative ions are more difficult to produce than positive ions. The conditions,however, in the flowing afterglow source are ideal for producing negative ions since
there are multiple collisions and an abundance of thermal energy electrons.
Equations 3.01a-d summarize several routes to anion formation: (a) collisional
electron attachment, (b) dissociative electron attachment, and (c and d) chemical
ionization processes.
SF6 + e– Æ SF6–

(3.01a)N2O + e– Æ O•– + N2

(3.01b)O•– + CH4 Æ OH– + CH3•

(3.01c)OH– + CH3OH Æ CH3O– + H2O(3.01d)Since both the filament and two neutral inlets are movable, it is possible to perform
an in situ ion synthesis through multiple, sequential ion-molecule reactions. For
example O•– is produced from dissociative electron attachment of N2O (eq 3.01b), a
subsequent H atom abstraction reaction with methane forms OH– (eq 3.01c). Further
downstream, introduction of an alcohol can lead to the formation of RO– from a
proton transfer reaction (eq 3.01d)
Since the plasma contains equal numbers of negatively and positively
charged species, high ion densities (~107 ions cm-3) are achieved due to shielding
effects. Ions are lost by recombination reactions and by ambipolar diffusion to the37walls of the flow tube. To minimize the diffusive loss of ions, the flow tube wasconstructed with a relatively large diameter (4.75 cm ID) and operated under highneutral gas flow rates and densities. The ions undergo multiple collisions (~106
collisions s-1) with the He buffer gas (~0.3 torr, 3-10 ms residence time). As a result,the ions are collisionally cooled to approximately room temperature. The He buffer
gas and any neutral precursor gases are pumped away by a high capacity Stokes
roots blower pump (190 L s-1) while anions (or cations) are extracted from the source
region into a low pressure region through a molybdenum nose cone with a 2 mmdiameter orifice.
3.3) Ion Optics and Mass Selection
The ions are extracted from the source flow tube by biasing the source nose
cone at an attractive potential relative to the source flow tube. A cone shaped
extractor with an aperture of 4 mm sits directly behind the nose cone. The extractor
is designed so that the electric field applied to it penetrates through the nose coneorifice into the source flow tube. Typically < 20 V is placed on this element, however,occasionally it is floated. The ion beam is focused into a SIFT quadrupole mass filterby a series of five lenses. These lenses are mounted on a common assembly withthe extractor. The voltage applied to each element can be independently adjusted totune the ion beam. The last three lenses in this assembly can operate as an Einzel
lens, which allows for ion focusing without acceleration. A 10-inch diffusion pump
maintains a pressure of approximately 10-4 torr in the lensing region. The SIFT
quadrupole is mounted in a separate chamber which is pumped by a 6-inch diffusion
pump to approximately 10-6 torr. A voltage is applied to the quadrupole case and toan entrance plate, which is mounted on the front of the quadrupole case. The SIFT
quadrupole (Extrel # 4-270-9) rods are 1.6 cm in diameter and 22 cm long, allowing
for transmission of ions up to 500 amu. The system is powered by an Extrel Power38

Supply # 011-1. For the work presented in this thesis, a 2.2 MHz High-Q head
(Extrel # 011-13) was employed, which allows for transmission of ions up to
180 amu.
A quadrupole consists of four parallel rods, and each set of opposite rods iselectrically connected. An equal but opposite polarity RF potential (Vcos(ωt)) is
applied between each set of rods causing the ions to oscillate. Superimposed on
the RF potential is an equal but opposite polarity DC voltage (U), shown in eq 3.02aand 3.02b.

Φ 0 = U − Vcos(ωt ) and Φ0 = −U + Vcos ( ωt )(3.02a-b)

In the absence of an RF voltage a negative ion will be attracted to the pair of positiverods, and a collision with the rods will neutralize the ion. Application of an RF
voltage offsets this movement. For lighter ions this offset will be greater than forheavier ions, hence this pair of rods acts as a high-mass filter. The other pair of rods
acts as a low-mass filter. In the absence of an RF voltage the negative pair of rods
repels a negative ion and therefore the ion remains near the center. Application ofan RF voltage will deflect the anions off the center axis. Lighter ions are displaced
more than heavier ions, eventually resulting in neutralization from a collision with therods. Only ions within a certain mass window will pass through the quadrupole.
Increasing both the RF and DC voltages simultaneously allows for the mass range tobe scanned. In these experiments, the SIFT quadrupole is set to pass an ion with a
specific mass to charge ratio, rather than to scan the mass range.
Following mass selection, the ion beam is focused through three electrostatic
lenses, which can also be used as an Einzel lens, into the reaction flow tube. These
lenses are mounted on a common assembly and the voltage on each element canbe applied separately. Injection of the ion beam from the source selection region into

39

the reaction flow tube requires the ions to travel from a low-pressure region into ahigh-pressure region. A Venturi inlet is employed for efficient injection of the ion
beam into the reaction flow tube and to minimize backstreaming of neutral gases into
the source selection region.9,14,15 This inlet is composed of an inner and outerannulus (0.007 cm2 and 0.6 cm2, respectively), which are positioned symmetrically
about the injection orifice. The Venturi effect results from forcing gas through thesmaller inner annulus by supplying a large backing pressure. The gas flow is
hypersonic and the gas is compressible. A missile-shaped jet is formed, where there
is high gas density at the edges and low gas-density in the interior, near the orifice.
3.4) Reaction Flow Tube
He buffer gas flows through the reaction flow tube at a rate of 12.0 SLPM
(maintained by Tylan flow controller). The pressure in the flow tube is approximately
0.5 torr and is maintained by a high capacity Stokes roots blower pump (755 L s-1).
The reaction flow tube is ~117 cm long and is composed of two distinct parts, anentry region and a reaction region. Introduction of He buffer gas through the Venturi
inlet results in turbulent flow. The entry region provides the necessary distance forthe gas flow to relax and for collisional cooling of the ions. The entry region iscomposed of a stainless steel cross and bellows. The cross is equipped with several
neutral gas inlets and a filament, which allows the instrument to operate in traditional
flow afterglow mode. This option is useful since higher ion densities are achieved.
The anions undergo multiple collisions with the He buffer gas (~0.5 torr, 104 cm s-1).
As a result the ions are vibrationally and rotationally relaxed to a room temperature
Boltzmann distribution; the excited electronic states of most negative ions in general
are repulsive and lead to dissociation.16
The reaction region is approximately 68 cm long. It is constructed frommultiple segments of 7.30 cm inner diameter stainless steel cylinders. Neutral40

reagents are introduced into the flow tube through a manifold containing seven fixedradial inlets that are spaced 9.7 cm from one another. Each radial inlet contains
seventeen (0.37 mm diameter) holes concentrically spaced around the flow tube
diameter. This design allows for efficient mixing of the neutral reagent into the buffer
gas. A capacitance manometer measures the pressure at approximately the centerof the reaction region.
The fluid properties of the He buffer gas in the reaction region are very welldefined. The flow of He gas is viscous (i.e., λ/d << 1, the mean free path of aparticle is much smaller than the flow tube diameter).17 Therefore, a given particle
will undergo more collisions with other particles than with the walls of the instrument.
Thus, within a given mean free path gas properties such as temperature, velocity,and density remain constant and the gas is described as a continuous medium. Gas
flowing through a cylindrical reaction vessel experiences resistance to flow and avelocity gradient is established across the cylinder radius. If the velocity gradient issmooth then the flow is laminar, while if the velocity profile is irregular then the flow is
turbulent.18 The flow is characterized by a dimensionless parameter called a
Reynolds number (Re),18 (eq 3.03), where ρ is the fluid density, v is the fluid velocity,d it the flow tube diameter, and η is the fluid viscosity.

Re =

ρ vd
η

(3.03)

Laminar flow is described by Re < 1200, while turbulent flow is described by
Re > 2200. In the entry region of the flow tube, the gas flow is turbulent; however,these instabilities are damped out in the first ~15 cm of the entry region and the flowin the reaction region is laminar (Re ~ 40). Additionally the flow in the reaction regionis Poiseuille,17 hence the gas is incompressible and the flow is stable. The velocity
gradient of a viscous, laminar medium flowing in a cylindrical reaction vessel is41parabolic. Equation 3.04 describes the parabolic velocity gradient (v(r)),18 where r is
the radial distance from the center, vo is the velocity at the center of the flow tube,
and R is the flow tube radius.

⎛r2 ⎞v (r ) = v 0 ⎜⎜1− 2 ⎟⎟⎝ R ⎠(3.04)At the chamber walls (r=R) the velocity is zero; at the center of the flow tube (r=0) thevelocity is the maximum velocity (vo). It can be shown that the average velocity (vave)
is one-half the maximum velocity.
The density of He in the flow tube is ~1016 particles cm-3. The density ofneutral reagents introduced into the flow tube is ~1011 molecules cm-3, thus theneutral reagents have the same velocity profile as the He buffer gas. On the other
hand, ionic species travel slightly faster than neutral species in the flow tube. Ions
that collide with the walls of the flow tube are neutralized resulting in a radialconcentration gradient. Fick’s law of diffusion, eq 3.05, describes the diffusive flux ofthe ions in the flow tube,

J = −D∇[A − ](3.05)

where ∇[A–] is the ion concentration gradient, D is the diffusion coefficient, and thenegative sign indicates that ions are transported from high to low concentration. The
concentration of ions is greatest at the center of the flow tube, where the He velocity
is at its maximum. As a result, the average ion velocity is larger than the average He
velocity (vions = αvave, where α is a correction factor between 1 and 2). For thissystem, the average ion velocity was experimentally determined to be 1.6 times
greater than the average He velocity.1,19-21 The well-defined velocity profile in thereaction region means that reaction distance can be related to the reaction time42

(v = z/t, where z is distance and t is time) allowing for rate constant determinations,as described in section 3.7.
3.5) Triple Quadrupole Detection
The reaction region of the flow tube ends ~4 cm before the detection nosecone; a thin molybdenum plate with a 0.5 mm orifice is mounted on a stainlesssteel carrier plate that separates the flow tube from the detection region. Anions
are gently extracted into the detection region (shown in Figure 3.04) by placing aslightly positive voltage (< 10 V) on the nose cone while neutral species are
pumped away. The carrier plate voltage is applied separately and is set to transmit
the maximum number of ions. The current on the nose cone is measured by anelectrometer; typical currents range between 0.5-30 pA in the SIFT mode ofoperation. The ion beam is focused into a triple quadrupole mass filter by a stackof five electrostatic lenses. The five lenses are mounted on a common assembly,
which sits on the front plate of the triple quadrupole case. The lens stack ishoused in the first differentially pumped chamber that is maintained at a pressureof ~10-5 torr by an Edwards 6-inch diffusion pump (805 L s-1). The voltage on eachlens can be controlled separately and is gradually increased to more attractive
voltages from the first lens to the third lens. The first lens is a cone shaped
extractor, which operates similarly to the source region extractor. The last threelenses are designed to operate as an Einzel lens. A voltage is also applied to thequadrupole entrance plate.
The triple quadrupole consists of a series of three identical stainless steel
quadrupoles that are housed in one common assembly. The triple quadrupole is partof the Extrel C50 TQMS system (1.2 MHz RF oscillator, 300 W DC Power level forQ1 and Q3, and 200 W Power level for Q2). The rods are 1.9 cm in diameter

43

44

Figure 3.04: Schematic of triple quadrupole detection.

allowing for the transmission of ions up to 500 amu. Each quadrupole is electricallyisolated from one another and from lenses (five total), which sit at the entrance andexit of each set of rods. The entire assembly spans the three differentially pumped
regions. The second and third chambers are maintained at ~10-6 and ~10-7 torr,
respectively, by two Edwards 10-inch diffusion pumps (2300 L s-1).
The triple quadrupole assembly can operate in two distinct modes, MS-MSand MS. In MS-MS mode both Q1 and Q3 are mass resolving while Q2 acts as anion pipe (RF voltage only). This arrangement allows for selection of a single massto-charge ratio ion by Q1, collision induced dissociation in Q2, followed by mass
analysis of the fragment ions by Q3. The fragment ions formed from CID canprovide structural information about the parent ion, thus MS-MS mode is used toelucidate the identity of an unknown ion. For the experiments described in thisthesis, the instrument was operated in MS mode only. In this setup, Q1 and Q2 areused as ion pipes, while Q3 is a mass analyzer. The mass selected ion beam
impinges on an electron multiplier that has a gain of 108.
3.6) Data Acquisition
The current collected by the electron multiplier is converted to pulses by acapacitor and passed through a pulse amplifier and discriminator. The TTL pulses
are fed into an Ortec ACE-MCS board that contains an array of 4096 channels. The
voltage output is sent to the Extrel quadrupole controller, which specifies the specificmass-to-charge ratio to transmit. As the ACE-MCS voltage is ramped, the channel
array is scanned and the Extrel quadrupole controller scans the RF and DC
quadrupole voltages. The data acquisition time is specified and the number of counts
per channel is collected. A plot of counts per second versus channel number
corresponds to ion intensity versus mass-to-charge ratio; hence a mass spectrum iscollected. Figure 3.05a is a mass spectrum of the SIFT-injected BrO– anion. A
45

portion of this anion fragments to form Br– upon injection into the reaction flow tube.
Figure 3.05b is a mass spectrum for the reaction of BrO– with CH3Cl.
Kinetics measurements are collected using a separate program that controls
the opening and closing of the neutral reagent solenoid values and a series ofvoltages, which are sent to the Extrel quad controller. For each neutral reagent inlet,
the program jumps through a series of voltages corresponding to different mass ionsand collects the number of counts per second. The user inputs certain experimental
parameters (temperature, flow tube pressure, and neutral regent flow) and theprogram calculates a reaction rate constant and product ion branching ratios as
described below.
3.7) Rate Constant Determination and Error Analysis
One of the benefits of this technique is that it affords a relatively straightforward kinetic analysis. Bimolecular reaction rate constants are measured underpseudo first order conditions (i.e., the concentration of one of the reactants is muchgreater than the concentration of the other reactant and therefore does notappreciably change over the course of the reaction). Under our experimental
conditions, the neutral reagent concentration is ~1011 molecules cm-3, while the ion
concentration is ~105 ions cm-3. For a given bimolecular reaction: A– + B Æ C– + D,
the loss of A– is described by eq 3.06, where kII is the bimolecular rate constant, [B] isthe neutral reagent concentration, [A–] is the ion concentration, and t is the reaction
time. Since there is a well-defined velocity profile, reaction time and distance are
related (t = z/αvHe, where z is the reaction distance, α is the ion velocity correction
factor of 1.6,1,19-21 and vHe is the average helium velocity).

d[ A − ]dt= α v Hed[ A − ]dz= −k II [ A − ][ B ](3.06)

46

Ion Counts Per Second

a)

b)

0

10

20

30

40

50

60

70

80

90

100

m/z

Figure 3.05: a) Spectrum of the SIFT-injected 79BrO– anion (m/z 95); the Br– anion(m/z 79) is also present due to collision induced dissociation upon SIFT injection of
the parent ion. b) Spectrum of the SN2 reaction of BrO– + CH3Cl; both Br– (m/z 79)
and Cl– (m/z 35 and m/z 37) are products of this reaction.

47

The integrated rate expression is given in eq 3.07, where [A–] 0 is the initial ion
concentration and [A–] z is the ion concentration at distance z.

⎡ − k II [B]z ⎤[A ]z = [A ]0 exp⎢⎥
⎣ αv He ⎦−−(3.07)

Ions injected into the reaction flow tube also undergo diffusive losses in addition toreactive losses. Incorporation of radial diffusion and of the parabolic radial velocity
profile leads to the solution shown in eq 3.08.

[A ] = [A ] exp ⎡⎢ − ⎛⎜⎜ ∂D + k [B ] ⎞⎟⎟ z ⎤⎥
II

−

−

z

0

⎣ ⎝R

2

α

⎠ v He ⎦(3.08)

where ∂ is a constant equal to 3.7,1,19-21 R is the flow tube radius, and D is the
diffusion coefficient of the reactant ion through He buffer gas; a detailed derivation ofthis solution can be found in reference 11. The ion density at any given axial position
can be found from this equation. The first term describes the diffusive loss while thesecond term describes the reactive loss. Diffusive loss occurs over the entire flow
tube length (L) and is independent of the neutral reagent concentration. Thus, thediffusive loss term is a constant for a given measurement and can be neglected
when measuring the reactive loss.
A plot of the logarithmic falloff of ion intensity versus reaction distance islinear; the slope is a result of the reactive loss and the intercept is a result of the
diffusion loss as shown in eq 3.09. A representative kinetics plot (i.e., reactant andproduct ion intensities versus reaction distance for given neutral reagent
concentration) is shown in Figure 3.06 for the reaction of BrO– + CD3Cl (discussed inchapter 5).
II⎛ [A− ] ⎞∂Dz k [B] zln ⎜ − z ⎟ = − 2 −⎜ [A ] ⎟R v Heα v He0 ⎠⎝(3.09)

48

Ion Counts Per Second

10000

1000

100
0

10

20

30

40

50

60

70

Reaction Distance, (cm)Figure 3.06: Reactant ion depletion and product ion formation for the reaction
BrO– + CD3Cl. Symbols: ♦ BrO–, ■ Cl–, and • Br–.

49

The reaction rate constants can be determined from the slope of the plot ofln([A–]z/[A–]0) versus z. The velocity of He buffer gas (vHe) is determined from theflow of He (FHe), the pressure of He (PHe), and the cross sectional area of the flow
tube (πR2), eq 3.10.

v He (cm s) =FHe (STP ⋅ cm3 s) 760(torr)
T(K)
×
×
2
2
πR (cm )PHe (torr)
atm
273.16(K)(3.10)

The concentration of B is determined using a calibrated volume technique. In thistechnique, the neutral reagent flow (FB) is determined by measuring a change inpressure (∆P) per unit time (∆t) for a given volume (V).

FB (atm ⋅ cm3 s)[B] (part. cm ) = πR (cm )v (cm s) ×
3

2

2

He

760torr
× 3.535 ⋅ 1016 part. cm3 ⋅ torr
atm

(3.11)

Equation 3.12 expresses the reaction rate constant in terms of measurable
experimental quantities.

k II (cm3 part. ⋅ s) =−dln [ A − ]α F (atm ⋅ cm s )T (K )
2

2

6

2

2

2

He

dz(cm) PHe2 (torr 2 )FB (atm ⋅ cm3 s) πR2 (cm2 )1760torr
××
16
3
3.535 ⋅ 10 part. cm ⋅ torr atm ⋅ 273.162 (K 2 )(3.12)The units of the bimolecular rate constant, kII, are the inverse of the product ofconcentration and time. When the flow is given in atm cm3 s-1, velocity in cm s-1,
temperature in K, and pressure in atmospheres, then kII is in cm3 s-1 particle-1.
Additionally, rate constants for three-body processes such as association reactions,can also be determined since the He buffer gas density is in excess, hence kII =[He]kIII. Given the above dimensions, termolecular rate constants, kIII, are given in
cm6 s-1 particle-2.

50

The systematic and random errors for a given rate constant measurement
must be considered. The random error is determined by propagating the error
associated with each individual measurement, as shown in eq 3.13.
σ ( k II )k II

1

⎡ σ 2 ( slope ) σ 2 ( α ) 4σ 2 (FHe ) 4σ 2 ( T ) 4σ 2 (PHe ) 4σ 2 (R ) σ 2 (FB ) ⎤ 2
=⎢
+
+
+
+
+
+
⎥ (3.13)
2
2
2
2
2
2
2
slope
α
F
T
P
R
F
He
He
B
⎣
⎦

σ (FB )FB1

⎡ σ 2 ( ∆P ) σ 2 ( V ) σ 2 ( ∆t ) σ 2 (T ) ⎤ 2
=⎢
+
+
+
⎥
2
2
V2
T 2 ⎦⎥(∆t )⎣⎢ ( ∆ P )(3.14)The error in the slope is the standard deviation determined by a least squares
analysis, typically 1-3%; the error in α is 12.5%; the error in the He buffer gas flow(FHe) is 0.9%, this is a combination of the precision of the Tylan flow controller andthe standard deviation determined by a least squares analysis for the Tylan flow
calibration curve; the error in the temperature (T) is conservatively estimated to be
0.4%, this encompasses a 2°C change in temperature over the course of ameasurement and an accuracy of ±0.5°C; the error in the He pressure (PHe) is 0.3%,
this is the accuracy of the capacitance manometer; and the error in the flow tube
radius (R) is 0.7%. The error in the neutral reactant flow (FB) encompasses
individual errors in time (t), volume (V), pressure (P), and temperature (T), as shownin eq 3.14. The error in time is the standard deviation of multiple measurements,
typically ~3%; the error in the volume is <0.9%; the error in the pressure
measurements is estimated to be 0.75%; and the error in the temperature is 0.4%.
Propagating each error results in a ±13.4% error bar for each rate constantmeasurement; if a rate constant is measured three separate times the error is
reduced to ±7.7%. Typically, a more conservative error bar of ±20% is reported.
This propagated error reflects the accuracy of each measurement. For the rate
constant measurements in this thesis, the reported error bar is one standard

51

deviation of at least three separate measurements. This error reflects the
reproducibility of each measurement and is usually smaller than the propagatederror.
In addition to random error, the systematic error must be evaluated. The
working equation, eq 3.09, embodies several important assumptions. The effect ofeach assumption has been rigorously evaluated in reference 11 and, therefore, will
only be briefly discussed here.
1) There are no other production or loss mechanisms for the ions other thandiffusive and reactive losses. Since reaction rate constants are measured underpseudo first order conditions, the concentration of neutral products is small and,
therefore, the back reaction to reform the reactant ion does not occur in any
significant amount. On the other hand, secondary reactions between the productions and the neutral reactant may occur. Secondary reactions of this sort do notaffect the rate constant measurement, but need to be accounted for in
determining a product branching ratio. One additional complication is theoccurrence of side reactions with trace contaminants contained in the neutral
reagent. If the reaction of the anion with the contaminant is fast compared to thatof the anion with the neutral reagent of interest then measured rate constants will
be larger than the true value.
2) The ion density is determined by diffusion rather than by reactive loss. When themagnitude of the reactive loss is comparable to diffusive loss, higher orderdiffusive terms must be considered. This results in a ~1% deviation from the true
rate constant.
3) For viscous, laminar gas flow through a cylinder, the velocity at the chamber
walls is assumed to be zero. Non-zero velocity at the chamber walls (termed slipflow) flattens the parabolic velocity profile, lowering the average ion velocity. A
52

more rigorous treatment of the velocity profile would result in a rate constant thatis 1-3% lower than the true value.
4) Ions are continually lost in the flow tube from diffusion. This radial loss is
described by Fick’s law of diffusion (eq 3.05), where the diffusion coefficientdepends on both the ion structure and the nature of the medium through which
the ion moves. This diffusive loss is assumed to be constant along the flow tube
and, therefore, is neglected when determining the rate constant. This
assumption is reasonable since the concentration of He is much greater than theconcentration of the neutral reagent and therefore the diffusion coefficient does
not significantly change upon introduction of neutral reagent into the flow tube.
However, for large neutral flows the diffusion coefficient may change. The
resulting error may be as large as a ~5% underestimation of the rate constant. In
addition to a radial density gradient, there is an axial density gradient. This axial
density gradient results from the continual radial loss of ions to the chamber walls
in combination with reactive loss of ions. The axial ion density gradient is small
and therefore neglected; however, consideration of axial diffusion would result ina rate constant that is ~1% higher than the true value.
5) The pressure drop across the reaction flow tube of ~12% is negligible. This axialpressure gradient results in an axial velocity gradient; thus, as an ion travels
down the flow tube its velocity increases. By measuring the flow tube pressure
at the center of the reaction region, the effects of the velocity gradient areminimized since the magnitude of error that results from evaluating the reaction
with the inlets upstream of the center position is approximately equal inmagnitude and opposite in direction to the error resulting form the inlets that are
downstream of the center position. In actuality, these errors do not quite cancel
one another, since the flow tube pressure is measured slightly upstream of the53

center position. This results in a slight overestimation of the rate constant by~1%. In practice, the error that results from the axial velocity gradient is lessimportant when rate constants are determined by changing the neutral flowrather than changing the neutral inlet position. However, comparison of rate
constant measurements collected by changing the neutral flow compare well withmeasurements collected by changing the reaction distance. However, it is morepractical to change the reaction distance, since the distance can be moreaccurately determined and since some neutral reagent flows are difficult tostabilize.
6) The introduction of neutral regents into the reaction flow tube is planar, uniform,
and occurs with instantaneous mixing. In our experimental design neutral

reagents are introduced into the instrument from the edges of the flow tube.
Introduction of neutral reagents is symmetric but not instantaneous nor uniform.
In general several centimeters are required for complete mixing to occur. The
seven radial inlets are designed to allow for efficient mixing. The effect of mixing
is minimized since each inlet is identical and, therefore, changes in reaction
distances are not affected.
3.8) Branching Ratio Determinations

The reaction of an ion with a neutral molecule may occur by multiple reaction
pathways. In this case, the measured reaction rate constant is the sum of thereaction rate constants for each channel, ktot = k1 + k2.
k1
A– + B ⎯⎯→C– + D(3.15a)k2⎯⎯⎯→ E– + F(3.15b)

The product branching ratios (BR) are determined from the ratio of concentration of agiven ionic product (C– or E–) to the total ionic products (C– + E–) or equivalently to54

the ratio of the individual reaction rate constant to the total rate constant, as shown in
eq 3.16a and 3.16b. The branching ratio is a measure of the fraction of A– thatreacts to form a given product.

[C ] = kC[ ] + [E ] k + k

and BR2 =

1

−

−

1

[E ] = k[C ] + [E ] k + k
−

−

BR1 =

2

(3.16a-b)

2

−

−

1

2

Branching ratios are experimentally determined by measuring the intensities of the
product ions for introduction of the neutral reactant at each inlet. A branching ratio is
determined at each inlet. A plot of branching ratio as a function of inlet distance isconstructed; the actual branching ratio is determined by extrapolating to “zeroreaction distance”, where there are no contributions from any secondary reactions.
Figure 3.07 shows the product ion branching fractions for the two product channels
for the reaction of BrO– + CD3Cl.
One major experimental complication in determining product branching ismass discrimination. Ideally, the sum of the product ion intensity plus the remaining
reactant ion intensity should equal the initial reactant ion intensity. This, however,rarely occurs since different mass ions are detected with different efficiency. Massdiscrimination is the result of differences in diffusion and transmission efficiency for
different mass ions. In the reaction flow tube ions of different masses or different
cross sections diffuse at different rates; typically heavier ions diffuse at a slower rate
than lighter ions. Additionally, different mass ions are transmitted through the ion
optics and quadrupole with different efficiencies; typically lighter ions are transmitted
more efficiently. The mass biases from diffusion and from transmission are opposite
to one another. However, under most experimental conditions some mass
discrimination still exists and must be considered. Typically the relative detection
sensitivity is estimated by examining a series of exothermic ion-molecule reactions

55

100

% Product Ion

80
60
40
20
0
0

10

20

30

40

50

60

70

Reaction Distance, (cm)Figure 3.07: Sample branching ratio determination for the reaction BrO– + CD3Cl.
Symbols: ■ Cl– and • Br–.

56

where only one ionic product is formed and the reactant and product ions are similar
in mass to the ions of interest. In this method, a reactant ion is SIFT-injected into thereaction flow tube and allowed to react with a neutral reagent. The reactant andproduct ion intensities were measured and a correction factor is applied to the
product ion so that the sum of the product and reactant ion intensities is equal to thatof the initial reactant ion intensity.

573.9) Chapter 3 References(1)
Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf, A. L. Flowing
Afterglow Measurements of Ion-Neutral Reactions. In Advances in Atomic andMolecular Physics; Academic Press, Inc: New York, 1969; Vol. 5; pp 1-56.
(2)
Dunkin, D. B.; Fehsenfeld, F. C.; Schmeltekopf, A. L.; Ferguson, E. E.
Ion-Molecule Reaction Studies from 300 to 600 K in a Temperature-Controlled
Flowing Afterglow System. Journal of Chemical Physics 1968, 49, 1365-1371.
(3)
Hierl, P. M.; Friedman, J. F.; Miller, T. M.; Dotan, I.; MenendezBarreto, M.; Seeley, J. V.; Williamson, J. S.; Dale, F.; Mundis, P. L.; Morris, R. A.;
Paulson, J. F.; Viggiano, A. A. Flowing Afterglow Apparatus for the Study of IonMolecule Reactions at High Temperatures. Review of Scientific Instruments 1996,
67, 2142-2148.
(4)
McFarland, M.; Albritton, D. L.; Fehsenfeld, F. C.; Ferguson, E. E.;
Schmeltekopf, A. L. Flow-Drift Technique for Ion Mobility and Ion-Molecule Reaction
Rate Constant Measurements. I Apparatus and Mobility Measurements. Journal ofChemical Physics 1973, 59, 6610-6619.
(5)
Fehsenfeld, F. C.; Albritton, D. L.; Bush, Y. A.; Fournier, P. G.;
Govers, T. R.; Fournier, J. Ion-Atom Interchange Reactions Using Isotopic-Species.
Journal of Chemical Physics 1974, 61, 2150-2155.
(6)
Fehsenfeld, F. C. Ion Reactions with Atomic Oxygen and Atomic
Nitrogen of Astrophysical Importance. Astrophysical Journal 1976, 209, 638-639.
(7)
Viggiano, A. A.; Howorka, F.; Albritton, D. L.; Fehsenfeld, F. C.;
Adams, N. G.; Smith, D. Laboratory Studies of Some Ion-Atom Reactions Related toInterstellar Molecular Synthesis. Astrophysical Journal 1980, 236, 492-497.
(8)
Zhang, X.; Kato, S.; Bierbaum, V. M.; Nimlos, M. R.; Ellison, G. B.
Use of a flowing afterglow SIFT apparatus to study the reactions of ions with organic
radicals. Journal of Physical Chemistry A 2004, 108, 9733-9741.
(9)
Adams, N. G.; Smith, D. Selected Ion Flow Tube (SIFT) - Techniquefor Studying Ion-Neutral Reactions. International Journal of Mass Spectrometry and
Ion Processes 1976, 21, 349-359.
(10) Van Doren, J. M.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V. M. The
Tandem Flowing Afterglow-SIFT-DRIFT. International Journal of Mass Spectrometry
and Ion Processes 1987, 81, 85-100.
(11) Van Doren, J. M. Developments and Applications to Gas-Phase Ion
Chemistry. Ph.D. Thesis, Department of Chemistry and Biochemistry, University ofColorado at Boulder, 1987.
(12) Grabowski, J. J. Studies of Gas Phase Ion-Molecule Reactions usinga Selected Ion Flow Tube. Ph.D. Thesis, Department of Chemistry and Biochemistry,
University of Colorado at Boulder 1983.

58

(13) Hankin, J. J. Gas-Phase Chemistry of Silicone and Boron Ions. Ph.D.
Thesis, Department of Chemistry and Biochemistry, University of Colorado at
Boulder 1995.
(14) Dupeyrat, G.; Rowe, B. R.; Fahey, D. W.; Albritton, D. L. Diagnostic
Studies of Venturi Inlets for Flow Reactors. International Journal of Mass
Spectrometry and Ion Processes 1982, 44, 1-18.
(15) Mackay, G. I.; Vlachos, G. D.; Bohme, D. K.; Schiff, H. I. Studies ofReactions Involving C2Hx+ Ions with HCN Using a Modified Selected Ion Flow Tube.
International Journal of Mass Spectrometry and Ion Physics 1980, 36, 259-270.
(16) Simon, J. Molecular Anions. Journal of Physical Chemistry A 2008,
112, 6401-6511.
(17) Dushman, S.; Lafferty, J. M. Scientific Foundations of Vacuum
Techniques; John Wiley and Sons: New York, 1962.
(18) Tritton, D. J. Physical Fluid Dynamics; Van Nostrand Reinhold
Company: New York, 1977.
(19) Stock, H. M. P. Theory of Flowing Afterglow. Journal of Physics B Atomic Molecular and Optical Physics 1973, 6, L86-L88.
(20) Huggins, R. W.; Cahn, J. H. Metastable Measurements in Flowing
Helium Afterglow. Journal of Applied Physics 1967, 38, 180-188.
(21) Bolden, R. C.; Hemsworth, R. S.; Shaw, M. J.; Twiddy, N. D.
Measurements of Thermal-Energy Ion-Neutral Reaction Rate Coefficients for RareGas Ions. Journal of Physics Part B - Atomic and Molecular Optical Physics 1970, 3,45-60.

59

CHAPTER IV
NEGATIVE ION FLOWING AFTERGLOW PHOTOELECTRON
SPECTROMETER: INSTRUMENT DESCRIPTION

4.1) Introduction
Over the past forty years, photoelectron spectroscopy has provided valuable
insight into the chemical physics of negative ions. Currently there are several
different types of photoelectron spectrometers, each with their own advantages anddisadvantages. In general, however, each one of these techniques involves theformation and mass selection an anion beam, which is then crossed with a photon
beam; the resulting photoelectrons are collected and energy analyzed.
The negative ion flowing afterglow photoelectron spectrometer (NIPES) at
JILA is pictorially shown in Figure 4.01 and schematically shown in Figure 4.02. In
this instrument, ions are created in a flowing afterglow source and extracted andfocused into a Wien velocity filter. The mass selected ion beam is refocused anddecelerated into an interaction region, where the ion beam is crossed with the
351 nm line of a cw Ar-ion laser in an external buildup cavity. Photoelectrons thatare ejected perpendicular to the ion and photon beams are collected, energy
analyzed by a hemispherical kinetic energy analyzer, and imaged onto a position
sensitive detector. Photoelectron spectra are obtained by measuring photoelectron
counts as a function of electron kinetic energy (eKE); the energy scale can beconverted to electron binding energy (eBE) by subtracting the eKE from the photon
energy. An overview of this technique is discussed in six sections below: flowing
afterglow ion source, ion optics and mass selection, ultraviolet laser system,
photoelectron kinetic energy analyzer, data acquisition and analysis, and Franck-

60

61

Figure 4.01: Photograph of the JILA Negative Ion Flowing Afterglow
Photoelectron Spectrometer.

62

Figure 4.02: Schematic representation of the JILA Negative Ion Flowing Afterglow
Photoelectron Spectrometer.

Condon simulations. A more detailed discussion of this instrument is provided inseveral other sources.1-44.2) Flowing Afterglow Ion Source
The flowing afterglow source is very similar to the flowing afterglow source in
the FA-SIFT instrument described in Chapter 3. Because these sources are sosimilar, these techniques can provide complementary information to one another; theunion of these techniques is discussed in Chapter 1. Despite the similarities, the twosources are not identical and these differences must be kept in mind whencomparing data between the two instruments.
The NIPES instrument flowing afterglow source is shown in Figure 4.03. The
work presented in this thesis utilizes a microwave discharge to initiate ionization.
Other types of sources such as a heated filament and a metal cathode discharge
have previously been used; however, since these sources were not utilized they are
not discussed here. The microwave discharge source consists of an Evenson-type
brass cavity5 and a one-inch diameter quartz tube. Helium gas, which has been
purified by passage though a molecular sieve trap immersed in liquid nitrogen, ispassed through the microwave discharge source at a rate of ~7-10 standard liters
per minute (maintained by a Tylan flow controller). The plasma is ignited by a sparkfrom a Tesla coil, which ionizes He to produce free electrons. These free electrons
absorb the 2.45 GHz microwave radiation at ~10-100 W of input power (provided by
an Opthos MPG 4 microwave power generator) to maintain the plasma.5
The properties of the NIPES source plasma are similar to the FA-SIFT source
plasma and, therefore, a detailed discussion is not provided. The discharge contains
free electrons, He 23S and 21S metastable species, He+, and He2+.6,7 Since theplasma contains equal amounts of negatively and positively charged species, highion densities (~108 ions cm-3) are achieved due to shielding effects and the diffusion63

is ambipolar. Trace amounts of neutral gases are either passed through or
introduced directly downstream of the discharge source. Oxygen anion is the mostcommonly produced ion since it can be formed in high densities and it displays
diverse ion chemistry that can be utilized to synthesize other target ions; moreoversince its electron binding energy is well-known,8 it is often therefore used to calibrate
the energy scale. Oxygen anion is produced from dissociative electron attachment(eq 4.01a) by introducing a controlled flow of O2 gas (7-10 cm3 per minute) throughthe discharge. Further downstream in the source flow tube, other precursor gases
can be introduced to synthesize a target anion. For example, hydroxide is produced
from an H atom abstraction reaction (eq 4.01b) by introducing methane into thesource downstream of the ion production region.
O2 + e– Æ O2– Æ O•– + O(4.01a)O•– + CH4 Æ OH– + CH3•

(4.01b)

The flowing afterglow source allows for a variety of ions to be produced, from a wide
range of ion-molecule reactions, some of which were outlined in Chapter 3. The flow
tube is constructed from multiple segments of 4.8 cm inner diameter stainless steel
so that its length can be changed from 25 – 100 cm. Additionally, two movable inlets
make it possible to introduce neutral precursor gases at various distances along thesource flow tube.
The ions undergo multiple collisions (~106 collisions s-1) with the He buffergas (~0.5 torr, 3-10 ms residence time). As a result the ions are vibrationally androtationally cooled to approximately room temperature. Vibrational and rotationalcooling is important, since anion hot bands can significantly congest and complicate
the resulting photoelectron spectrum. Surrounding the source flow tube with a liquid

64

65

Figure 4.03: Schematic representation of the flowing afterglow source.
Figure from Dr. Adam J. Gianola.

nitrogen jacket can further cool the ions. Prior experience with this arrangement
suggests that the resulting ions are prepared in the 150-200 K range;9 typically ion
densities in the source are also reduced. Comparison of photoelectron spectracollected both at room temperature and under liquid nitrogen conditions can help
identify anion hot bands and aid in the assignment of the origin peak. The He buffer
gas and any neutral precursor gases are pumped away by a high capacity Stokes
roots blow pump (755 L s-1) while anions are extracted from the source region into
the first differentially pumped region through a molybdenum nose cone with a 1 mm
diameter orifice.
4.3) Ion Optics and Mass Selection
It is important to gently extract the anions from the source region. The nose
cone is held at a small positive voltage (<+3 V). The first differentially pumped
region is maintained at a pressure of ~10-4 torr by a 6-inch diffusion pump (805 L s-1).
At these pressures an ion still may collide with a neutral background molecule; if theion is accelerated with too much energy it can be excited through ion-molecule
collisions.10 The ion beam is focused and gradually accelerated through a series of
ion optics, which are referred to as the afterglow lenses (L1-L6, shown in Figure4.04). The voltage of the first four lenses (L1-L4) can be independently adjusted totune the ion beam. The first and second deflectors (D1 and D2) electrostaticallymove the ion beam both vertically and horizontally; however, they float on the L3 andL4 voltages, respectively. The voltages of L5 and L6 are held at fixed potentials of
500 and 735 V, respectively. Lenses 1-3 and the first set of deflectors are mounted
on one assembly, while Lenses 4-6 and the second set of deflectors are on a
separate assembly mounted behind an aperture plate (A1).
After L6, the ion beam is steered around a 10° bend by a third set ofdeflectors (D3) into the second differentially pumped region, maintained at a66

pressure of ~10-6 Torr by a 8-inch diffusion pump (2000 L s-1); at this point any
neutral species or photons from the source region collide with the walls of the
instrument. The ion beam, now traveling at 735 V, is focused by a three-element
quadrupole Einzel lens (Q1).11 After Q1 the ion beam passes through a second
aperture (A2), where an electrometer measures the ion current (typical currents are
0.5-6 pA), and a fourth set of deflectors (D4). After D4 there is a gate valve, which
separates the front half of the instrument from the back half of the instrument; thisallows the back half of the instrument to remain under vacuum when the instrument
is not in operation and when performing routine maintenance and cleaning in thesource region. Following this gate valve there is a second three-element quadrupole
Einzel lens, which focuses the ion beam into a Wien velocity filter.
Up to this point, the ion beam is composed several different anions. A Wien
filter serves to select a narrow window of mass-to-charge ratio anions. A Wien filter
is composed of perpendicular electric and magnetic fields (E and B) that are also
perpendicular to the incoming ion beam (z-axis). The forces (F) that result from theelectric field (FE = qE) and from the magnetic field (FB = qv × B) are opposite to one
another; the net result is the deflection of the ion along the y-axis, where q is theparticle charge and v is the velocity of the ion. An ion will pass through the Wien
filter only if the forces from the electric and the magnetic fields are equal andopposite to each other, FE = –FB. Kinetic energy (KE) is related to the velocity andmass of an ion (KE = ½mv2), therefore ions of a different mass (hence velocity)require a different set of forces to pass through the filter. Ions that are too heavy or
too light will be deflected into the walls of the instrument. Different mass ions areallowed to pass through the filter either by holding the electric field constant andscanning the magnetic field or vice versa.

67

68

Figure 4.04: Schematic representation of the ion optics and Wien filter. Figure from Dr. Adam J. Gianola.

The magnetic field is established by a water-cooled electromagnet poweredby a Kepco regulated DC voltage supply. The electric field is produced by applying
equal but opposite polarity voltages (along the y-axis) on a series of ten electricallyisolated stainless steel shims. The voltage on each shim is symmetrically steppedup (along the x-axis) so that the outer shims are at the lowest voltage and the innershims are at the highest voltage; this arrangement minimizes the fringe fields at theentrance and exit of the Wien filter. The electric field is supplied by a voltage divider
circuit powered by a 0-40 mA / 0-5000 V Kepco power supply. The Wien filter
resolution (m/∆m) is approximately 40; therefore, as the mass scale is increased,ions that differ by 1-2 amu cannot be resolved from one another. Despite having
poor resolution at higher masses, use of a Wien filter is advantageous because of its
high ion transmission. The Wien filter sits in a common assembly with Q2 and Q3;
the pressure is maintained at 10-7 torr by a Varian turbomolecular pump (250 L s-1).
Following mass selection, the ion beam passes through several more ion
optics. First is another three-element quadrupole Einzel lens (Q3), which focuses
the ion beam through a short field-free region and a third aperture (A3). The ion
beam then passes through the fourth and last three-element quadrupole Einzel lens(Q4), which focuses the ions into a deceleration lens stack (DL1-4). The
deceleration stack is composed of four lenses, which gradually slow the ions to 35 V
and focus the ion beam into an ion-photon interaction region. A faraday cup collects
the ions that pass through the interaction region. The current is measured by asecond electrometer; typical ion currents range from 10-2000 pA. The pressure in
the interaction region is maintained at 10-8 torr by a Varian turbomolecular pump(280 L s-1).

694.4) Ultraviolet Laser System
The ion beam is perpendicularly crossed with either the 351.1 or the363.8 nm line of a Spectra Physics cw argon-ion laser coupled to an external buildup cavity producing ~100 W of circulating power; a diagram of this setup is provided
in Figure 4.05. A detailed description of the optical build-up cavity is provided
elsewhere.12 Additionally, during the fall of 2007, the Ar-ion laser was replaced with anewer model and some minor modifications were made to the optics table. Thesechanges do not change the fundamental operation of the laser or optical build-up
cavity; they are outlined in the thesis of Mr. Scott W. Wren.
The Ar-ion laser is composed of a plasma tube, a broadband high reflector
back mirror, an output coupler (either 351.1 nm or 363.8 nm), and an etalon. The
plasma tube is filled with argon gas; an electric discharge doubly ionizes the argon
atoms. The distance between the broadband high reflector back mirror and the front
output coupler define the length (Ll) of the resonant optical cavity. These two mirrors
select a single Doppler broadened lasing transition of Ar2+ and provide feedback to
the Ar medium. Only standing waves (or longitudinal modes) of νn = nc/2LA, where νn
is a frequency, n is an integer, c is the speed of light, and LA is the cavity length, will
oscillate within the cavity. An etalon selects one of these longitudinal modes (ν).
Output powers of ~1 W are typically obtained.
The output of the laser is a single monochromatic beam that is verticallypolarized. Several mirrors (M) and optical components direct the laser beam into an
external build-up cavity.12 M1 and M2 adjust the beam to the height of the optical
table and additionally rotate the laser polarization by 90°. The laser is directed to theback of the optics table by M3 where it hits a partial reflector, M4. From M4, thebeam passes through a telescope lens, and an acoustical-optical modulator (AOM).

70

71

Figure 4.05: Schematic of Ar-ion laser system and external buildup cavity.
Figure from Dr. Adam J. Gianola.

The telescope lens serves to minimize the wave front curvature of the beam entering
the build-up cavity; the purpose of the AOM will be discussed shortly. Prior toentering the build-up cavity through M7, the light is reflected off two additional
steering mirrors, M5 and M6, and passed through a half-wave plate, which allows thebeam polarization to be rotated. A small portion of light reflects off M7 back throughthe AOM and telescope lens onto M4. A fraction of this reflected light passes
through M4 into a reflected-light photodiode. The external build-up cavity iscomposed of two highly reflective curved mirrors (M7 = 99.6% and M8 = 99.8%)
mounted on either side of the vacuum chamber; a transmitted light photodiode ispositioned after M7. The external cavity operates on the same principle as the Ar-ion
laser cavity. For build-up to occur, the length of the external cavity (defined by the
distance between M7 and M8) must match the laser wavelength, LE = nc/2ν.
However, since the length of the cavity is affected by a variety of external sources
such as acoustic noise, air current, and temperature fluctuations, active steps must
be taken to maintain this condition.
An electronic servo amplifier system is employed to actively match theexternal build-up cavity length and the laser wavelength.12 To accomplish this, boththe laser cavity back mirror and the external build-up cavity front mirror are mounted
on a stack of piezoelectric transducers and an AOM is placed in the beam path. An
AOM is composed of a transducer that is connected to a quartz crystal. A voltage
applied to the crystal vibrates it back and forth, producing a standing wave, whichacts like a Braggs diffraction grating. The light passing through the crystal is shiftedby an amount equal to the acoustical frequency of the glass. The first orderdiffraction is shifted by ~55 MHz; this serves to frequency isolate the two cavities sothat any reflected light from the build-up cavity will not return to the laser cavity andinterfere. Additionally, a small 1 MHz frequency dither is placed on top of the ~55
72

MHz shift; this dither varies the frequency around the resonance frequency
producing an error function. When the two cavities are off resonance, the intensity oflight reflected from M7 increases. The servo system electronics monitor the RF
signal from the reflected light photodiode and respond by either changing the lengthof the laser cavity, changing the length of the build-up cavity, or by varying thefrequency of the AOM shift to regain the resonance condition. The piezo-mounted
external build-up cavity mirror corrects for noise at 0-100 Hz, the piezo-mounted
laser cavity mirror corrects for noise at 0.1-10 kHz, while the AOM frequency shiftcorrects for noise that is greater than 5 kHz. When resonance is achieved the laser
power is increased by a factor of 100.
4.5) Photoelectron Kinetic Energy Analyzer
Figure 4.06 schematically depicts the ion-photon interaction region, theanalyzer entrance and exit lenses, the hemispherical analyzer, and the position
sensitive detector.3 Photoelectrons are ejected over 4π steradians; however, only
the photoelectrons that are ejected perpendicular (along the y-axis) into a 5° halfangle acceptance cone are energy analyzed. Prior to entering the hemisphericalanalyzer the photoelectrons are passed through a series of lenses (V1-3 and VHC).
Lenses 1 and 2 serve to accelerate the electrons by a factor of 5-10 times their initial
kinetic energy. Acceleration is necessary since the kinetic energy of the ejected
photoelectrons is between 0-3.5 eV, while the transmission energy (TE) of thehemispherical analyzer is 4 eV. Lens 3 serves to focus the photoelectrons to thecenter of the hemispherical analyzer entrance, while VHC (Herzog corrector voltage)compensates for fringe fields, which are present at the entrance of the analyzer. The
hemispherical analyzer is composed of an inner and an outer hemisphere, which are
made of OFHC copper. The transmission energy of the analyzer is defined by

73

74

Figure 4.06: Schematic representation of ion-photon interaction region and of the electron
kinetic energy analyzer. Figure from Dr. Adam J. Gianola.

the difference in voltage between the inner and outer hemispheres. Only electrons
that are at 4 eV ± 2 % will pass through the analyzer. Photoelectrons with energy
lower than the transmission window will collide with the inner hemisphere, while
those with energy higher than the transmission window will collide with the outerhemisphere. The photoelectron energy scale (0-3.5 eV) is scanned by sweeping theacceleration voltages on the entrance lens stack.
After passing through the analyzer, the photoelectrons pass through an exit
lens stack prior to impacting a position sensitive detector. Lens 4 (V4) compensates
for fringe fields at the exit of the analyzer. Lenses 4-6 (V4-6) accelerate thephotoelectrons to 300 eV for efficient detection and to magnify their image so that theentire area of the detector is utilized. The detector is composed of a stack of five
microchannel plates coupled to a resistive anode encoder (Quantar Technology
3318A). Each microchannel plate has 106 channels per plate. The five
microchannel plates are connected through a series of resistors so that each plate isheld at a successively higher voltage; the last plate is held at ~3400 V. For every
one electron that impinges upon this detector, a gain of ~107 electrons is obtained.
The resistive anode is a curved rectangular shaped plate with a circular effective
detection area. Each corner of the plate (A,B,C, and D) is electrically connected to a
position sensitive analyzer (Quantar 2401B). When an electron cloud hits thedetector, a current is measured at each of the four detector corners; the detector
registers each electron cloud as a single event. The x-y position of the electron isdetermined from eq 4.02a and 4.02b.
x=

B+C
A +B+C+D

and y =A +B
A +B +C+D(4.02a-b)

Here the y-axis represents photoelectron counts per second and the x-axis
represents kinetic energy. The resolution of the detector is 5 meV; however, the75

limiting resolution of the instrument (typically 7-15 meV) is determined by the size ofthe ion-photon interaction region and its position relative to photoelectron extraction.
4.6) Data Acquisition and Analysis
The position sensitive analyzer outputs digitized pulses of x-y coordinates;
the x-position indicates electron kinetic energy, while the y-position indicates thenumber of counts for a given electron kinetic energy. This signal is sent to an
oscilloscope for real-time observation of the electron flux, and a 2412 B Histogram
Memory Card (HMC) coupled to a PC.13 The HMC alternates between datacollection and data writing (to the PC). The HMC has 12 bits, 8 bits for the x-axis
and 4 bits for the y axis; thus the detector area is divided into 28=256 channels along
the x-axis and 24=16 channels along the y-direction. Electrons hit the detector andthe number of events per channel is counted. After a given accumulation time the
HMC outputs this information to the computer. A LabView program (PESDAT)
controls the data collection process. Data are sampled over the entire energy range
in small overlapping segments, which are defined by their center eKE. The PESDAT
program sends the center eKE value and the corresponding V3 voltage of the first
segment to a voltage controller (JILA LC036), which sets the analyzer input lensstack voltages. Electrons are collected for 0.5 seconds; after this time the data aresent from the HMC to the computer. This collection process is repeated for eachsegment over the course of multiple scans. A more detailed description of the dataacquisition components is given in reference 13.
In order to convert the data file into counts versus electron kinetic energy, aseparate calibration spectrum is collected. The absolute energy scale is calibrated
by measuring the photoelectron spectrum of an atomic species with a well-known
electron affinity (typically oxygen atom14-16). This spectrum additionally provides themagnification (i.e., the energy width of each detector channel, meV/channel) of the76

energy scale. Since the energy scale is not linear, a small compression factor ( γ <1%) is measured (typically from the term energies of tungsten14). Lastly, since theions are traveling at 35 V through the interaction region, eq 4.03 converts theelectron kinetic energies, which are measured in the lab frame, to the center-of-massframe of the ion.
⎡
⎛ 1
1 ⎞⎤
+ ⎟⎥
eKE = hν − ⎢EA ref + γ ( eKE ref − eKE lab ) + W ionM e ⎜⎝ M ref M ⎠ ⎦⎥⎣⎢(4.03)

Here hν is the photon energy, EAref is the calibration ion electron affinity, Wion is theion beam kinetic energy, γ is the electron kinetic energy scale compression factor,
eKEref is the measured electron kinetic energy of the calibration ion origin peak in thelab frame, eKElab is the measured electron kinetic energy of the molecule of interestin the lab frame, Mref is the mass of the calibration ion, Me is the mass of an electron
and M is the mass of the molecule under investigation.
Each resolvable transition in the spectrum is fit with a Gaussian curve to findthe center of the peak. Since photoelectron spectroscopy cannot typically resolve
rotational transitions, the measured electron band origins (EAexp) are rotationallyuncertain and must be corrected (EAcor).
EAcor = EAexp + Erot(J’’) + Erot(J’) = EAexp + ∆rot

(4.04)

In eq 4.04, Erot(J’’) and Erot(J’) are the mean rotational energies of the anion andneutral, respectively. The difference in rotational energies (∆rot) for an asymmetricrotor can be approximated by eq 4.05, where A, B, and C are rotational constants forthe anion and neutral molecule.17 The single prime denotes the anion and the
double prime denotes the neutral species. These rotational constants are either
measured by a separate experimental technique or estimated from electronic
structure calculations. A typical rotational correction is < 0.005 eV.

77

B'
C' 3 ⎞ ⎛ B'' - B' ⎞
⎛ A'
+
+
- ⎟+⎜
∆ rot = k B T ⎜⎟⎝ 2A'' 2B'' 2C'' 2 ⎠ ⎝ 3 ⎠(4.05)The error associated with electronic band origin measurements is the sum of therotational correction, the energy scale compression factor, and the error associatedwith fitting the data with a Gaussian function. The error associated with a vibrationalband is the sum of the error for the electronic band origin and the error associatedwith fitting the vibrational band with a Gaussian function. Statistical errors can beassessed by collecting multiple spectra; an error bar is assigned from the standard
deviation of multiple measurements of the peak positions. In general, the statisticalerror is larger and, therefore, this value is typically reported.
4.7) Franck-Condon Simulation
Electronic structure calculations and spectral simulations are performed to aid
in the interpretation of the photoelectron spectra. Optimized geometries andharmonic vibrational frequencies for the neutral parent, anion, and radical are
determined from either density functional or ab initio calculations using the Gaussian
03 suite of programs.18 Electronic structure calculations can predict the electron
affinity and identify which normal modes will be active in the spectrum. For more
complicated systems, calculations can predict the lowest energy isomer and possible
excited states.
Spectral simulations are performed by calculating the Franck-Condon (FC)factors using the PESCAL program,19,20 using the calculated geometries, normal
modes, and electron binding energies for the anion and neutral species. A separate
Fortran program (FCFGAUS)21 extracts the necessary information from the Gaussian
03 output files for the anion and neutral species. FCFGAUS outputs the Duschinsky
rotation matrices, normal mode displacement vectors, geometries, and normal mode
frequencies in a usable format for the PESCAL program. The PESCAL program78

calculates the Franck-Condon (FC) factors by assuming that each normal mode is anindependent harmonic oscillator and by transforming the normal modes of the anion
into linear combinations of the normal modes of neutral species.19,22-25 The resulting
stick spectra can be convoluted with a 7-15 meV fwhm Gaussian function, which
represents the instrument resolution. For most systems that are free of nonadiabatic
effects, the PESCAL program provides excellent reproduction of the experimental
spectra.

794.8) Chapter 4 References(1)
Leopold, D. G.; Murray, K. K.; Lineberger, W. C. Laser PhotoelectronSpectroscopy of Vibrationally Relaxed CH2− - A Reinvestigation of the Singlet Triplet
Splitting in Methylene. Journal of Chemical Physics 1984, 81, 1048-1050.
(2)
Leopold, D. G.; Murray, K. K.; Miller, A. E. S.; Lineberger, W. C.
Methylene: A Study of the X 3B1 and a1A1 States by Photoelectron-Spectroscopy ofCH2− and CD2−. Journal of Chemical Physics 1985, 83, 4849-4865.
(3)
Feigerle, C. S. Laser Photoelectron Spectroscopy Studies ofTransition Metal Negative Ions. Ph.D. Thesis, Department of Chemical Physics,
University of Colorado at Boulder, 1983.
(4)
Ervin, K. M.; Lineberger, W. C. Photoelectron Spectroscopy of
Negative Ions. In Advances in Gas-Phase Ion Chemistry; N. G. Adams and L. M.
Babcock, Eds. Greenwich, 1992; pp 121-166.
(5)
Fehsenfeld, F. C.; Evenson, K. M.; Broida, H. P. Microwave Discharge
Cavities Operating at 2450 MHz. Review of Scientific Instruments 1965, 36, 294-298.
(6)
Fehsenfeld, F. C.; Schmeltekopf, A. L.; Goldan, H. I.; Schiff, H. I.;
Ferguson, E. E. Thermal Energy Ion - Neutral Reaction Rates. I. Some Reactions ofHelium Ions. Journal of Chemical Physics 1966, 44, 4087-4094.
(7)
Schmeltekopf, A. L.; Fehsenfeld, F. C. De-excitation Rate Constantsfor Helium Metastable Atoms with Several Atoms and Molecules. Journal ofChemical Physics 1970, 53, 3173-3177.
(8)
Neumark, D. M.; Lykke, K. R.; Andersen, T.; Lineberger, W. C. Laser
Photodetachment Measurement of the Electron-Affinity of Atomic Oxygen. PhysicalReview A 1985, 32, 1890-1892.
(9)
Wenthold, P. G.; Polak, M. L.; Lineberger, W. C. Photoelectron
Spectroscopy of the Allyl and 2-Methylallyl Anions. Journal of Physical Chemistry
1996, 100, 6920-6926.
(10) Duncan, M. A.; Bierbaum, V. M.; Ellison, G. B.; Leone, S. R. LaserInduced Fluorescence Studies of Ion Collisional Excitation in a Drift Field: Rotational
Excitation of N2+ in Helium. Journal of Chemical Physics 1983, 79, 5448-5456.
(11) Marynowski, M.; Franzen, W.; Elbatanouny, M. Analysis of theProperties of an Electrostatic Triplet Quadrupole Lens Used as an Electron-BeamTransport Device. Review of Scientific Instruments 1994, 65, 3718-3723.
(12) Ho, J. Negative Ion Laser Photoelectron Spectroscopy of Mass
Selected Small Metal Clusters. Ph.D. Thesis, Department of Physics, University ofColorado at Boulder, 2001.

80

(13) Ramond, T. Negative Ion Photoelectron Spectroscopy of Alkyl
Peroxides, Alkoxides, and Group VIII Transition Metal Oxides. Ph.D. Thesis,
Department of Physics, University of Colorado at Boulder, 2001.
(14) Moore, C. E. Atomic Energy Levels. NSRDS-NBS; Washington: US
GPO Circular No. 467, 1952.
(15) Ervin, K. M.; Anusiewics, I.; Skurski, P.; Simon, J.; Lineberger, W. C.
The Only Stable State of O2− is the X 2Πg Ground State and It (Still!) Has an
Adiabatic Electron Detachment Energy of 0.45 eV. Journal of Physical Chemistry A
2003, 107, 8521-8529.
(16) Neumark, D. M. Laser Photodetachment Measurement of theElectron-Affinity of Atomic Oxygen. Physical Review A 1985, 32, 1890-1892.
(17) Engelking, P. C. Approximate Rotational Band Shifts. Journal of
Physical Chemistry 1986, 90, 4544-4545.
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;
Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J.
V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision B.05; I. Gaussian, Ed. Pittsburgh, PA, 2003.
(19) Ervin, K. M.; Ramond, T. M.; Davico, G. E.; Schwartz, R. L.; Casey, S.
M.; Lineberger, W. C. Naphthyl radical: Negative Ion Photoelectron Spectroscopy,
Franck-Condon Simulation, and Thermochemistry. Journal of Physical Chemistry A
2001, 105, 10822-10831.
(20)

Ervin, K. M. PESCAL, Fortran program; University of Nevado, Reno,(21)

Ervin, K. M. FCFGAUS, Fortran program; University of Nevado, Reno,

2008.
2003.
(22) Chen, P. Photoelectron Spectroscopy of Reactive Intermediates. In
Unimolecular and Bimolecular Reaction Dynamics; C. Y. Ng, T. Baer and I. Powis,
Eds.; John Wiley and Sons: New York, 1994; pp 371-425.

81

(23) Ozkan, I. Franck-Condon Principle for Polyatomic-Molecules - AxisSwitching Effects and Transformation of Normal Coordinates. Journal of Molecular
Spectroscopy 1990, 139, 147-162.
(24) Botter, R.; Dibeler, V. H.; Walker, J. A.; Rosenstock, H. M.
Experimental and Theoretical Studies of Photoionization-Efficiency Curves for C2H2
and C2D2. Journal of Chemical Physics 1966, 44, 1298-1299.
(25) Sharp, T. E.; Rosenstock, H. M. Franck-Condon Factors for
Polyatomic Molecules. Journal of Chemical Physics 1964, 41, 3453-3463.

82

CHAPTER V
REACTIONS OF BrO– AND ClO– WITH ALKYL CHLORIDES: THE COMPETITIONBETWEEN SN2 AND E2 MECHANISMS AND THE GAS-PHASE α-EFFECT

5.1) Introduction
Gas-phase ion-molecule studies provide a route to understanding the intrinsic
factors that affect a reaction in an environment free from solvent effects. Thesestudies, therefore, provide important insight into the role of the solvent in a given
reaction. Several notable differences between the intrinsic gas-phase reactivity andthe reactivity in solution have been observed.1-4 This work focuses on two areas
where solvent effects are important: 1) the competition between nucleophilic
substitution (SN2) and base-induced elimination (E2) and 2) the α-effect or enhanced
reactivity due to a lone pair of electrons adjacent to the nucleophilic center.
It has been shown theoretically that the reaction of ClO– with ethyl chloride
proceeds by both an SN2 and an E2 reaction,5 as shown in Scheme 5.01.
Scheme 5.01:
H
SN2

δ

ClO

H
Cα

Cl

ClOCH2CH3 + Cl-

δ

(a)

CβH3

ClO- + CH3CH2Cl
E2

H
H

Cl
Cβ
H

δ

Cα

H
H

ClOH + C2H4 + Cl -

(b)

ClOδ

In the SN2 pathway (a), ClO– attacks the α-carbon of ethyl chloride, displacing
Cl–. Since the attack by the nucleophile occurs from the opposite direction of theleaving group, the transition state is characterized by an sp2-like hybridized83

α-carbon, where there is simultaneous formation of the Cα-OCl bond and breaking of
the Cα-Cl bond; the two α-hydrogen atoms lie approximately in the same plane as
the β-carbon. As the reaction proceeds the substituents around the α-carbon are
inverted. In the E2 pathway (b), ClO– abstracts a proton from the β-carbon of ethyl
chloride, forming a double bond, and simultaneously displacing Cl–. In the transition
state the two leaving groups are coplanar, with the lower energy antiperiplanartransition state structure being highly favored over the synperiplanar transition state
structure.
Since these two mechanisms generate the same ionic product and since thedetection of the distinct neutral products presents serious analytical challenges, gasphase experiments that address this competition are severely limited.6-18 One
distinguishing difference, however, is that SN2 mechanisms generally display inverse
deuterium kinetic isotope effects (KIE<1), while E2 mechanisms display normal
deuterium kinetic isotope effects (KIE>1).10,19,20 Deuterium kinetic isotope effects aredefined as the ratio of perprotio to perdeuterio rate constants (KIE=kH/kD).
Deuteration of the neutral reactant changes the rate of the reaction, providing insight
into the transition state structure and hence the reaction mechanism. The origin of
these effects is primarily the result of changes in vibrational frequencies as thereaction proceeds from the reactants to the transition state structure.21 In an SN2reaction (Scheme 5.01a), the Cα-H bonds are shortened in going from the sp3
hybridized orbitals in the reactants to the sp2-like hybridized orbitals in the transition
state, thus raising these frequencies relative to reactants. Since the frequencies forthe hydrogenated species are raised more substantially than for the deuterated
species in the transition state, the threshold energy for the hydrogenated reaction isgreater than that of the deuterated reaction, EH‡ > ED‡. In an E2 mechanism

84

(Scheme 5.01b), the Cβ-H bond is lengthened in going from the reactants totransition state, thus lowering these frequencies relative to reactants. Since thefrequencies for the hydrogenated species are lowered more substantially than for the
deuterated species in the transition state, the threshold energy for the hydrogenatedreaction is smaller than that of the deuterated reaction, EH‡ < ED‡. When bothreaction pathways are viable an overall KIE is measured, which provides qualitative
insight into the competition between these two mechanisms.
Hu and Truhlar5 have calculated reaction rate constants and deuterium KIEs
as a function of temperature for both the SN2 and E2 pathways of ClO– with ethyl
chloride using correlated electronic structure calculations (MP2/ADZP) and dual-level
direct dynamical calculations based on variational transition state theory and acompetitive canonical unified statistical model. Their work has provided us theunique opportunity to compare experiment and theory. Their computations predict
that, although the classical E2 barrier height is 3.2 kcal mol-1 higher than the SN2barrier, the vibrational zero point energy for the E2 reaction is 3.5 kcal mol-1 lower
than the SN2 vibrational zero point energy; thus, the zero point corrected barrierheights are approximately equal to each other. While both mechanisms are
predicted to occur, the E2 pathway is highly favored (by a factor of 10) over the SN2
pathway by entropic effects at room temperature.
Nucleophiles such as ClO– have been the subject of intense study for anotherreason. Nucleophiles of this type, where there is a lone pair of electrons on the atomthat is adjacent to the nucleophilic center, have been shown to display an enhanced
reactivity relative to that expected from a Bronsted-type correlation. This type of
nucleophile is known as an “α-nucleophile”, and the enhanced reactivity is termed an“α-effect”.22 In solution, this effect has been observed in several different types of85reactions including substitution reactions.23-29 However, in gas-phase reactions there
has been continuing controversy about whether or not the α-effect exists,27,30-38
prompting the question: is an α-effect really a solvent-induced effect?
Only a limited number of gas-phase experiments have addressed thisquestion. In an early study, DePuy et al.36 found that the reactivity of HO2– wassimilar to that of HO– in the reaction with methyl formate. These reactions proceed
via three channels: proton abstraction, BAC2 addition to the carbonyl center, and SN2
substitution at the methyl group. The product branching fractions for these anions
were found to be similar; thus, HO2– does not show an enhanced reactivity towards
addition or substitution. In a similar approach, McAnoy et al.30 have recently studied
the reactions of HO2– and CD3O– with dimethyl methylphosphonate. These reactions
proceed by proton abstraction and by SN2 substitution at a methyl group. The HO2–
anion reacts primarily via substitution while the CD3O– anion reacts preferentially via
proton abstraction. The difference in the reactivity of the two anions is attributed tothe α-nucleophiliciy of the HO2– anion. This conclusion is supported by computations(B3LYP/aug-cc-pVTZ// B3LYP/ 6-31+G(d)), which find that the SN2 barrier is lower(by ~1.9 kcal mol-1) for the reaction of HO2–. While these two studies come toconflicting conclusions, the later study may be more compelling since the basicity of
the HO2– and CD3O– anions are comparable.
Recent theoretical work predicts that the α-effect results from an intrinsic
property of a nucleophile and, therefore, this effect should be manifested in the gasphase. Ren and Yamataka31,32 have investigated a series of SN2 reactions, including
the reactions of ClO– and BrO– with methyl, ethyl, and i-propyl chloride, using G2(+)
calculations. In this work, the authors plot the barrier heights for the reactions ofnormal nucleophiles versus basicity to obtain a linear correlation curve. The barrier

86heights for the reactions of the α-nucleophile were projected onto this plot and were
found to be smaller than predicted for a hypothetical normal nucleophile of similarbasicity, thus providing evidence for a gas-phase α-effect. The size of this α-effect is
proportional to the amount of the deviation from the correlation curve and scales withthe size of the neutral substrate and with the electronegativity of the α-atom.
In this work we report overall reaction rate constants and deuterium KIEs forthe reactions of BrO– and ClO– with RCl, where R = methyl, ethyl, i-propyl, andt-butyl.18,39 As the extent of substitution in the neutral reagent increases, the KIEs
become increasing more normal. This result indicates that the E2 pathway becomes
the dominant pathway as the neutral reagent becomes more sterically hindered. The
reactions of methyl chloride, ethyl chloride, and i-propyl chloride also proceed by anadditional reaction pathway besides SN2 substitution and elimination; two potential
reaction mechanisms are investigated using electronic structure calculations. The
overall reaction efficiency and KIE for the reaction of ClO– with ethyl chloride iscompared to the calculated results of Hu and Truhlar. Lastly, the α-nucleophilicity of
the ClO– and BrO– anions is investigated by comparing their reactivity to that of othernucleophiles including the HO2– anion.
5.2) Experimental Methods
The overall reaction rate constants and KIE for the reactions of BrO– and
ClO– with RCl were measured with a flowing afterglow-selected ion flow tube (FASIFT) mass spectrometer, which was described in Chapter 3. BrO– and ClO– areideal nucleophiles for this type of study since the rate constants for the entire neutral
series, methyl through t-butyl, are within the measurable kinetic window of 10-9 –10-13 cm3 s-1 and are all below the calculated collision rate so the KIEs are evident.
These two reactant ions were formed in the source flow tube from electron impact on

87

N2O to produce O•⎯, which was then allowed to react with CCl3Br or CCl4,
respectively. Ions of a single isotopomer, 79BrO–and 35ClO–, were mass selected bya quadrupole mass filter and injected into the reaction flow tube where they were
thermalized to 302 ± 2 K by collisions with He buffer gas (~0.5 torr, 104 cm s-1).
Despite injecting the reactant ions with minimal energy, 79Br– or 35Cl– ions produced
from collision-induced dissociation were also present in the reaction flow tube; thepresence of these additional ions was considered in the data analysis below.
Measured flows of neutral reagents are introduced into the reaction flow tube
through a series of fixed inlets along the reaction flow tube and the reactant andproduct ions are analyzed by a quadrupole mass filter coupled to an electron
multiplier. Neutral reactant flow rates were measured by monitoring the pressurechange versus time in a calibrated volume system. Reaction rate constants andproduct branching ratios were determined by changing the neutral addition inlet,
thereby changing the reaction distance and time, while monitoring the change inreactant ion intensity. The ratio of perprotio to perdeuterio rate constants gives theKIE (KIE = kH/kD). Product branching ratios were determined by plotting thepercentage of each ion at each neutral inlet; extrapolation to “zero distance” gives
the branching ratio. Efforts were made to minimize mass discrimination; however, it
was necessary to estimate the relative detection sensitivities when calculatingproduct branching ratios. The relative detection sensitivity was estimated byexamining a series of exothermic ion-molecule reactions where only one ionicproduct was formed.
The error reported for the reaction rate constants is one standard deviation ofat least three consecutive measurements. The systematic error is generally ±20%;
however, KIEs are more accurately determined because some of the systematic88

errors (pressure, temperature, He flow rate, etc.) cancel in the rate ratio. The
estimated error in the product branching fractions is ±25% of the smaller productchannel; this is mostly attributed to mass discrimination and to the fact that X– results
both from the collision-induced dissociation (CID) of the reactant ion and from thereaction. Neutral reagents were purchased from commercial sources and usedwithout further purification.40 However, it was verified that HCl was not a significant
contaminant by mass selecting 35Cl– and allowing it to react with the neutral
reagents. It has previously been shown that the reaction rates of Cl– with these alkyl
halides are below the detection limits of our instrument, while the Cl– + HCl exchange
reaction proceeds at approximately half of the collision rate.41 Thus, the absence of37

Cl– as a product ion demonstrates that HCl is not a significant contaminant. An HCl

impurity would complicate the rate measurements due to a rapid proton transfer
pathway, which could not be distinguished from the SN2 and E2 channels.
Optimized geometries and harmonic vibrational frequencies for the reactants,
products, and transition state structures were determined from electronic structure
calculations using the Gaussian 03 suite of programs42 at the MP2/aug-cc-pVDZ
level of theory.43,44 Transition state structures were identified to have only one
imaginary frequency and this frequency was animated to verify that its motion
corresponds to the transition state reaction coordinate.
5.3) Results and Discussion
5.3.1) Reaction Trend and Product Distributions
The experimental reaction rate constants, product branching fractions, anddeuterium kinetic isotope effects are reported in Table 5.01 for the reactions of BrO–
and ClO– with RCl (R = methyl, ethyl, i-propyl, t-butyl or abbreviated as Me, Et, i-Pr,
and t-Bu). For a given neutral reagent, the reactions of BrO– are slightly lessefficient than those of ClO– and the kinetic isotope effects are more pronounced.
89

This trend reflects the weaker basicity of the BrO– anion (353.5 kcal mol-1) relative to
the ClO– anion (355.6 kcal mol-1).
For the reactions of BrO– with RCl, the formation of Cl– is the major productchannel. The reactions of BrO– with MeCl, EtCl and i-PrCl also produce minor
amounts of Br–; this product was not observed for the reaction of BrO– with t-BuCl.
Additionally, the reactions of BrO– with i-PrCl and t-BuCl produce trace amounts(<1%) of an E2 product cluster, Cl–(BrOH) or Cl–(BrOD), and minor amounts of an
association product, BrO–(RCl). For the reaction with i-PrCl the amount ofassociation that occurs is negligibly small. For the reaction with t-BuCl the
association channel is less than 5% of the total reaction; this contribution is removed
from the rate constants and KIEs in Table 5.01. The reactions of ClO– with MeCl and
EtCl, exclusively produce the displaced halide, Cl–. For the reactions of ClO– with
i-PrCl and t-BuCl, trace amounts of an association cluster, ClO–(RCl), were observedin addition to Cl–; the occurrence of this association channel is negligible.
The formation of Br– in the reactions of BrO– with MeCl, EtCl and i-PrCl israther unexpected. The analogous pathway in the reactions of ClO– would produce
the same product as an SN2 or E2 mechanism, except that, in these classic
mechanisms, Cl– is produced with an isotope distribution that reflects the natural Cl
atom abundance in RCl (76:24). This is not the case, however, for the reactions of
ClO– with MeCl, EtCl and i-PrCl, where the Cl– isotope distribution is slightly skewed
towards the 35ClO– reactant ion isotopomers. For the reaction of ClO– with t-BuCl,
the Cl– isotope distribution reflects that of the natural abundance of Cl in t-BuCl. Thisadditional pathway is not readily obvious since CID of the 35ClO– reactant ion upon
SIFT-injection results in the formation of 35Cl–. The presence of this fragment ion
results in the appearance of a skewed 35Cl/37Cl isotope distribution. However, by
subtracting the amount of initial 35Cl– due to CID from the product distribution, we
90Table 5.01: Reaction rate constants (10-10 cm3 s-1), kinetic isotope effects, and product ion branching ratios
for the reactions of 79BrO– and 35ClO– with RCl (R = methyl, ethyl, i-propyl, and t-butyl).
Reaction
79

–

ka

KIE

Ionic products– 79

branching ratios–

BrO + CH3Cl
Cl , Br
0.82, 0.18
1.08 ± 0.03–
– 79–
BrO + CD3Cl
Cl , Br0.86, 0.14
1.31 ± 0.04
0.82 ± 0.03
79
–
– 79
–
BrO + CH3CH2Cl
Cl , Br
0.91, 0.09
1.07 ± 0.01
79
–
– 79
–
BrO + CD3CD2Cl
Cl , Br
0.92, 0.08
1.11 ± 0.03
0.96 ± 0.03
79
–
– 79
–
BrO + CH3CD2Cl
Cl , Br
0.91, 0.09
1.05 ± 0.04
1.02 ± 0.04
79
–
– 79
–
BrO + CD3CH2Cl
Cl , Br0.93, 0.07
1.12 ± 0.02
0.95 ± 0.02
79
–
b
– 79
–
–
79
Cl , Br , Cl (HO Br)
BrO + (CH3)2CHCl
0.97, 0.03, trace
0.934 ± 0.157
79
–
b
– 79
–
–
79
Cl , Br , Cl (DO Br)
BrO + (CD3)2CDCl
0.92, 0.08, trace
0.352 ± 0.137 2.65 ± 0.11
79
–
c
–
–
79
BrO + (CH3)3CCl
1.00, trace
Cl , Cl (HO Br)
1.93 ± 0.11
79
–
c
–
–
79
BrO + (CD3)3CCl
1.00, trace
Cl , Cl (DO Br)
0.638 ± 0.023 3.03 ± 0.22
35–
– 35 –
ClO + CH3Cl
Cl , Cl
0.73, 0.27
2.01 ± 0.01
35–
– 35 –
ClO + CD3Cl
Cl , Cl
0.78, 0.22
2.36 ± 0.01
0.85 ± 0.01
35–
– 35 –
ClO + CH3CH2Cl
Cl , Cl
0.90, 0.10
2.25 ± 0.01
35–
– 35 –
ClO + CD3CD2Cl
Cl , Cl
0.90, 0.10
2.27 ± 0.01
0.99 ± 0.01
35–
– 35 –
ClO + CH3CD2Cl
Cl , Cl
0.89, 0.11
2.17 ± 0.02
1.04 ± 0.01
35–
– 35 –
ClO + CD3CH2Cl
Cl , Cl
0.90, 0.10
2.35 ± 0.01
0.96 ± 0.0135–b
– 35 –
ClO + (CH3)2CHCl
0.94, 0.06
Cl , Cl
1.74 ± 0.0335–b
– 35 –
ClO + (CD3)2CDCl
0.90, 0.10
Cl , Cl
1.01 ± 0.02
1.71 ± 0.05
35–b–
ClO + (CH3)3CCl
1.00
Cl
2.33 ± 0.03
35–b–
ClO + (CD3)3CCl
1.00
Cl
1.01 ± 0.05
2.31 ± 0.12ab
The error is the standard deviation of at least three measurements. A negligible amount of an association
product was observed. c An association channel of 0.01 for the perprotio reaction and 0.04 for the perdeuterio
reaction was observed; this component was removed from the overall reaction rate constant.
79

91

91

were able to determine the product ion 35Cl/37Cl isotope distribution; the deviationfrom the Cl natural abundance gives the branching ratio for the two pathways(Table 5.01).
For both anions, the occurrence of this additional reaction pathway is mostsignificant for the reaction with MeCl and gradually diminishes as the size of theneutral substrate increases. This trend correlates with the anticipated falloff of theSN2 channel as steric factors become more important, indicating that this additional
product channel may result from a transformation within the SN2 product ion-dipole
complex. It is well established that SN2 reactions are characterized by a double wellpotential energy surface, where two stable ion-dipole complexes flank the transition
state.2 Depending on the thermodynamics of the system, the lifetime of thesecomplexes can be relatively long, allowing for additional chemistry to occur.45 The
complex retains all of its initial energy since there are no collisions during its lifetime;thus, endothermic processes can be driven by the complexation energy and as longas the overall process is exothermic, the complex can dissociate into products.
A possible reaction that would account for the formation of these products isshown in eq 5.01 for the reactions of ClO– with CH3Cl; the Cl atom that originatesfrom ClO– is indicated with an asterisk.
*ClO– + CH3Cl → CH3OCl + *Cl–

∆Hrxn = -20 kcal mol-1(5.01)

This reaction presumably is initiated by the SN2 reaction of *ClO– + CH3Cl, where*ClO– attacks the carbon atom of methyl chloride to displace Cl– (analogous to
Scheme 5.01a). A second substitution step within the [Cl– + CH3O*Cl] ion-dipole
complex, would scramble the chlorine atoms. This second substitution reaction
presumably would proceed in a concerted step, as shown Scheme 5.02, where Cl–
attacks the oxygen atom of CH3O*Cl displacing *Cl–. For the reaction of the ClO–
anion, this mechanism produces the same products as the traditional SN2 reaction,
92

however, the chlorine atom are exchanged in a thermoneutral process. For thereactions of the BrO– anion, a substitution step that exchanges Cl for Br is slightly
more exothermic. These exothermicities (SN2-induced substitution) are summarizedin Table 5.02.
Scheme 5.02:
Cl

Cl + CH3O*Cl

δ

O

*Cl δ

CH3OCl + *Cl

CH3OCl + *Cl

CH3

A second possible reaction that would account for the formation of theseproducts is shown in eq 5.02. These exothermicities (SN2-induced elimination) arealso summarized in Table 5.02.
*ClO– + CH3Cl → CH2O + HCl + *Cl–

∆Hrxn = -56 kcal mol-1(5.02)

This reaction is consistent with an elimination reaction that occurs following the initialsubstitution step, within the SN2 product ion-dipole complex as shown in Scheme
5.03. This second transformation likely occurs in a concerted step where Cl–
abstracts an H atom from CH3O*Cl, displacing *Cl– and forming a C-O double bond;similar elimination reactions have previously been reporteed.46 The transition state
for this reaction resembles that of a typical E2 transition state, where the two leavinggroups are antiperiplanar to one another.
Scheme 5.03:
H
H
Cl + CH3O*Cl

*Cl δ
C O
H

CH2O + HCl + *Cl

CH2O + HCl + *Cl

Cl δBecause we cannot detect the neutral products we are not able to distinguishbetween these two proposed mechanisms. Electronic structure calculations were93

employed to explore whether or not these two potential mechanisms arethermodynamically feasible. These results are summarized in Figure 5.01 for thereaction of ClO– with CH3Cl. For both potential reactions the highest energy point,
once the reactants come together, is the first SN2 barrier, TS1; passage over thisbarrier is the rate-limiting step. If the reaction occurs according to Scheme 5.02, thereaction proceeds through TS2 into a second product ion-dipole well. Passage overthe second substitution barrier requires more energy than for the initial SN2 products
to separate. If the reaction occurs according to Scheme 5.03, the reaction proceeds
through TS3. This elimination barrier is below the second substitution barrier (TS2)and below the energy required for the initial SN2 products to separate. While bothproposed mechanisms are thermodynamically viable, the SN2-induced elimination
channel (Scheme 5.03) is energetically more feasible and the overall reaction isconsiderably more exothermic.
A similar computational study was done for the reaction of BrO– with CH3Cl.
In this case, the relative energetics for the substitution-elimination pathway were
found to be similar to the corresponding reaction of ClO–. However, for this reaction
we were unable to locate a saddle point corresponding to the second substitution
transition state, TS2. This provides additional support for the occurrence of the SN2induced elimination mechanism rather than the double substitution mechanism.
However, without performing a more exhaustive computational investigation, which isbeyond the scope of this work, we cannot state with certainty which mechanism is
responsible for this additional channel.
5.3.2) Kinetic Isotope Effects
For a given neutral reagent, the reactions of BrO– with RCl are less efficientthan those of ClO– with RCl and, as the reaction efficiency decreases, the magnitude

94

Table 5.02: Reaction exothermicitiesa (kcal mol-1) for the reactions of BrO– and ClO–
with RCl (R = methyl, ethyl, i-propyl, and t-butyl).
Reaction

E2b

SN2c

SN2-induced
substitutiond

SN2-induced
eliminatione

BrO– + CH3Cl
BrO– + CH3CH2Cl
BrO– + (CH3)2CHCl
BrO– + (CH3)3CCl
ClO– + CH3Cl
ClO– + CH3CH2Cl
ClO– + (CH3)2CHCl
ClO– + (CH3)3CCl

-4
-5
-4
-6
-6
-6

-21f
-23f
-23f
-24f
-20
-22
-22
-23

-59
-69
-66
-56
-66
-69
-

-23
-25
-25
-26
-20
-22
-22
-23

a

Enthalpies of formation are determined from the thermochemical data on the NIST
WebBook47 or from the JANAF thermochemical tables.48 b See Scheme 5.01b in text
for example. The formation of the E2 cluster is not considered. c See Scheme 5.01ain text for example. d See Scheme 5.02 in text for example. e See Scheme 5.03 intext for example. f Enthalpy of formation of CH3OBr is calculated in reference 49; for
C2H5OBr, C3H7OBr, and C4H9OBr, enthalpies of formation are estimated.

95

0

a)
a)

[TS 1]
ClO + CH3Cl-20[TS 2]-1

E + Ezpt (kcal mol )ClO + CH3Cl

Cl + CH3OCl[TS 3]-40

Cl + CH3OCl

Cl + CH3OCl

Cl + CH2O + HCl

-60

-80

-100

Cl + CH2O + HCl

b)

Cl

H H
δ

ClO

C
H

[TS 3][TS 2][TS 1]

δ

Cl

δ

O

Cl

δ

H
H

CH3

Cl δ
C O
H

Cl δ

Figure 5.01: a) Reaction coordinate diagram for the reaction of ClO– with CH3Cl.
The relative energies (electronic + zero point energy) were calculated at theMP2/aug-cc-pVDZ level of theory; b) the transition state structures are provided.

96

(deviation from unity) of the KIE increases. This is consistent with slower reactions
having larger reaction barriers, hence larger KIEs. However, the efficiencies of thesereactions are all well below the collision-controlled limit, therefore, the KIE is
expected to accurately reflect the transition state. As a point of clarification, whenthe magnitude of a KIE is discussed as large or small, this refers to the absolutevalue of the displacement from unity. The terms inverse and normal indicate
whether the KIE is less than one or greater than one, respectively.
With both sets of reactions, the KIE becomes increasingly more normal as
the size of the alkyl substrate increases. These results indicate that the E2 pathway
becomes the dominant channel as the neutral reagent becomes more stericallyhindered. The reactions with t-BuCl are expected to exclusively occur via an E2
mechanism. Previous results have shown that the sterically bulky t-butyl groupinhibits the SN2 pathway.11 The normal effect observed for these reactions is
consistent with other E2 isotope effects reported in the literature.10
The reactions of CH3Cl proceed by two pathways; for the remainder of this
discussion, the major, protypical SN2 substitution channel will be referred to as the“direct substitution” channel and the minor channel will be referred to as the“substitution-induced displacement” channel. Since the rate-limiting step in both
pathways is crossing the initial SN2 barrier (Figure 5.01), the KIEs for these reactions
are exclusively attributed to the prototypical SN2 mechanism. The inverse KIEs
measured for these reactions are consistent with the KIEs of other SN2 reactions
reported in the literature.19,20 For a given anion, the substitution-induced
displacement product branching fraction is larger for the perdeuterio reaction than forthe perprotio reaction. The change in branching fraction upon deuteration is most
likely due to dynamical factors within the SN2 product ion-dipole complex. In general,heavier systems have more low-frequency vibrational modes that better facilitate97

vibrational coupling and increase the density of states. Thus, the lifetime of the SN2
product ion-dipole complex is longer for the perdeuterio reaction, increasing the
likelihood that the complexation energy can concentrate in the reaction coordinate of
the second transformation.
The KIEs measured for the reactions of EtCl and i-PrCl are larger than the
KIEs measured in the MeCl reactions but smaller than the KIEs measured for thet-BuCl reactions, indicating that these reactions proceed by competing SN2 and E2
mechanisms. For these reactions, the substitution-induced displacement channel issmall. However, the occurrence of this mechanism provides evidence that thesereactions proceed partially by an SN2 mechanism, since this channel results from asecond reaction step within the SN2 product ion-dipole complex. The reaction of
BrO– with i-PrCl additionally produces trace amounts of a clustered product, Cl–(HOBr), providing evidence that this reaction proceeds partially through anelimination mechanism since HOBr is a neutral product of this mechanism. This E2
cluster is also observed for the reaction of BrO– with t-BuCl. For these larger
systems the formation of this cluster is likely promoted by the lower reaction
efficiencies, hence a longer lifetime of the product ion-dipole complex.
To further verify that the EtCl systems proceed by both an SN2 and an E2
mechanism, we have measured kinetic isotope effects that arise from the selective
deuteration at the α and β positions of the EtCl (termed d2-KIE and d3-KIE,
respectively). The d2-KIEs are slightly more inverse than the d5-KIEs, consistent
with the occurrence of an SN2 mechanism. Deuteration at the α−position is expected
to be more sensitive to the SN2 pathway since the inverse effect is predominantly the
result of changes in the Cα-H umbrella bending motion. The d3-KIEs are slightly
more normal than the d5-KIEs, reflecting the occurrence of an E2 mechanism.

98

Deuteration at the β-position is expected to be more sensitive to the E2 pathway
since the normal effect is mainly the result of changes in the Cβ-H stretchingfrequency. These results indicate that both mechanisms do indeed occur. While thed2- and d3-effects provide a more sensitive probe of the SN2 and E2 pathways,
respectively, they are still an overall effect and do not reflect the absolute magnitudeof the KIEs for the respective pathways. For example, the KIE for the reaction of
BrO– + MeCl is 0.82. Since the reaction of BrO– with EtCl proceeds with approximately the same efficiency as the reaction with MeCl, we may expect that thetransition state for these two SN2 pathways is similar and, therefore, the SN2-KIE for
the EtCl system would be closer to 0.88, since this system contains two α-hydrogenatoms. Likewise, the E2-KIE is expected to be larger than d3-KIE, being closer inmagnitude to the KIE measured for the t-BuCl system.
For each anion series, the trend in the KIEs indicates that the E2 channel
becomes more important for larger systems, where steric effects inhibit the SN2channel. In a crude approximation, the E2 branching fraction (BR) for the reactionsof EtCl and i-PrCl can be estimated by assuming that the E2-KIE for these reactions
is approximately equal to the KIE observed in the t-BuCl systems and that the SN2KIE is approximately equal to the KIE observed for the MeCl system when scaled fornumber of α-hydrogen atoms, given that:
KIEtot = KIEE2 × BRE2 + KIESN2 × BRSN2(5.03)

BRE2 + BRSN2 = 1(5.04)

Here, BRSN2 is the combined substitution channels. The KIE for the t-BuCl system is
not scaled for the number of hydrogen atoms, since to a first order approximation,
only one Cβ-H bond is elongated in the E2 transition state (see Scheme 1b). This
analysis, of course, relies on many assumptions and is only intended to provide an99

estimate of which pathway is dominant. For the reaction of BrO– with EtCl the overall
KIE is 0.96. If the E2-KIE and SN2-KIE are taken to be 3.03 and 0.88, respectively,
then the E2 channel only accounts for ~10% of the reaction and the combined
substitution channels account for ~90% of the total reaction. Applying this same
analysis to the reaction of ClO– with EtCl, the E2 mechanism represents only ~10%of the reaction. About 80% of the reaction of BrO– with i-PrCl proceeds by the E2channel, while ~60% of the reaction of ClO– with i-PrCl proceeds by the E2 channel.
While this analysis is qualitative in nature, it does identify the major and minor
channels.
5.3.3) ClO– with EtCl: Comparison of Experiment and Theory
Hu and Truhlar5 have quantitatively evaluated the competition between SN2
and E2 pathways for ClO– with C2H5Cl and with C2D5Cl using dual-level generalized
transition state theory and statistical calculations based on high-level, correlated
electronic structure calculations using extended basis sets (MP2/ADZP). Theircomputations predict that, despite the nearly equal zero point corrected barrierheights, the E2 pathway is highly favored (by a factor of 10) over the SN2 pathway byentropic effects at room temperature; the reaction proceeds 93% of the time by an
E2 mechanism with an overall reaction efficiency of 0.28. This is in striking contrast
to experimental results in which the E2 channel is only estimated to occur ~10% of
the time. Additionally, Hu and Truhlar predict that the overall reaction has a KIE of
2.42, which disagrees with the experimentally determined KIE of 0.99.
It is difficult to account for the discrepancies between experiment and theory.
The treatment of the reaction between ClO– and ethyl chloride assumes that thereaction is statistical. Statistical theories are based on the assumption that energy is
randomized among different modes as the reaction proceeds from reactants to thetransition state. Non-statistical effects50 have been observed for SN2 reactions of100monatomic nucleophiles with methyl halides, and these effects have been
extensively documented in the literature.51-55 However, the use of statistical theories
has been successful for slightly larger systems.56-58 Furthermore, Hu and Truhlar
have previously calculated the KIE for the SN2 reaction of F–(H2O) with CH3Cl using
a statistical approach.59 Their results show excellent agreement with experimental
values of O’Hair et al.60 Thus the assumption that the ClO– + C2H5Cl reaction
behaves statistically is reasonable.
Hu and Truhlar’s calculations predict that the SN2 channel has a KIE of 0.60
while the E2 channel has a KIE of 3.1. These individual KIEs are combined with thetheoretically determined product branching fractions to give an overall KIE of 2.4.
However, it should be noted that these KIE effects are likely incorrect. The
calculated KIEs are based on the optimized geometries, rotational constants, andharmonic vibrational frequencies from MP2/ADZP calculations. For both C2H5Cl andC2D5Cl, ν18 was reported to be 279 cm-1. This mode corresponds to a methyl rocking
motion and should be lowered upon deuteration. Calculations performed here
(MP2/aug-cc-pVDZ) found that the frequency of ν18 is 271 cm-1 for C2H5Cl and197 cm-1 for C2D5Cl. If these newly calculated ν18 frequencies are used rather than
the suspect ν18 frequencies, the calculated SN2-KIE changes from 0.60 to 0.89. Thisvalue is very close to the experimental KIE of 0.85 for the SN2 reaction of ClO– +MeCl and the estimated SN2-KIE of 0.88 for ClO– + EtCl (see above). Additionallythe calculated E2-KIE would also become slightly more normal upon inclusion of thenew ν18 value. If the vibrational modes are treated as harmonic oscillators, then thecorrected E2-KIE is 4.6. Hu and Truhlar, however, treat the low frequency modes of
the E2 channel with a hindered rotor model and, therefore, this value is an upperbound.

101

While this error in the calculated individual KIEs certainly contributes to thediscrepancy between experiment and theory, it can be argued that the errors in the
branching ratio calculations are the primary cause for the large deviation from theexperimental results. It should be noted that the suspect value of ν18 in C2D5Cl does
not need to be considered in the branching ratios, since only the perprotio reactions
were evaluated. Likewise errors in the barrier heights do not affect the individual
KIEs since the contributions from the electronic energy cancel in the rate ratio.
Assuming that the branching ratios predicted by Hu and Truhlar are correct, then the
E2-KIE would need to be less than 1.0 or the SN2-KIE would need to be less than
0.13 to be consistent with the measured overall KIE. These values are notreasonable based on previous results. On the other hand assuming that theindividual KIEs are correct (provided that the corrected value of ν18 for C2D5Cl isused) and that the discrepancy lies in the calculation of the branching ratios, we canuse these individual KIEs (SN2-KIE = 0.89 and E2-KIE = 4.6) along with theexperimentally measured overall rate constant and KIE to predict the reaction
efficiencies for the SN2 and E2 channels. This analysis gives k = 2.12 x 10-10 cm3 s-1
and eff = 0.10 for the SN2 channel, and k = 0.13 x 10-10 cm3 s-1 and eff = 0.0062 for
the E2 channel for the perprotio reaction. Thus, the E2 channel represents 6% of theoverall reaction, very close to the above experimental estimates.
5.3.4) α-Nucleophilic Character
Both BrO– and ClO– are prototypical α-nucleophiles. It has been shown insolution that α-nucleophiles display an enhanced reactivity relative to that expectedfrom a Bronsted-type correlation. Figure 5.02 is a plot of reaction efficiency (k/kcol,
where kcol is calculated using parameterized trajectory theory61) as a function of anion
basicity over a 40 kcal mol-1 range for the SN2 reactions of various oxy anions with102

CH3Cl. As expected, the general trend displayed by the normal nucleophiles (opencircles) is that reaction efficiency increases with anion basicity. Unfortunately, there
are few oxygen anions in the lower basicity range of ClO– and BrO– (closed circles)
whose reactions with CH3Cl have been reported. However, by fitting a linear trendline to this data set, the efficiency of normal nucleophiles in this lower basicity region
can be estimated. The reaction efficiencies of the α-nucleophiles, ClO– and BrO–,
are consistent with the predictions of the basicity-reactivity correlation. In fact, thetrend line predicts the reaction efficiency for these anions within the error bars of
their measurement. This is in contrast to G2(+) calculations, which predict thebarriers for the reactions of BrO– and ClO– with CH3Cl to be 2.5 and 3.8 kcal mol-1
lower than those of normal nucleophiles of comparable basicity, respectively.31
Based on these results, we would expect the reaction efficiency of the BrO– and ClO–
anions to be enhanced by one to two orders of magnitude.
It perhaps can be argued that the BrO– and ClO– anions are poor
α-nucleophiles, since the halogen-oxygen bond is relatively long resulting in poor
orbital overlap of the lone pairs on the halogen and oxygen atoms. We, therefore,
have additionally studied the reaction of HO2– with CH3Cl (k = 1.23 ×10-9 cm3 s-1),
and this result is also included in Figure 5.02. The HO2– anion is an excellent
α-nucleophile whose reactivity has been extensively studied in solution. In one
study, HO2– was found to undergo SN2 substitution on a methyl group ten times
faster than HO–, despite the greater basicity of the HO– anion.29 In the gas-phase,
however, this anion does not display enhanced reactivity with CH3Cl. Again, thetrend line predicts the reaction efficiency for this anion within the error bars of themeasurement. In further support of these findings, the SN2 reaction of HO2– withMeF (k = 6.0 ×10-12 cm3 s-1; eff = 0.0025)62 is slower than that of HO– and CH3O–

103

1.0
HO-

Reaction Efficiency

0.8

C2H5Oi-C3H7OCH2FCH2O-

0.6

HO-

-

CHF2CH2O

HO2- CH3O-

0.4

t-C4H9O-

0.2
BrO-

ClO-

HO-(H2O)
CF3CH2O-

C2F5CH2O-

0.0
350

360

370

380

390

Anion Basicity (kcal mol-1)

Figure 5.02: Anion basicity versus reaction efficiency for the SN2 reactions of Nu–
with CH3Cl. The normal nucleophiles are indicated by the open circles and the
α-nucleophiles are indicated by the closed circles. The vertical error bars represent
the absolute uncertainty of ±20%. Reactivities: references 11, 63, and 64; Anionbasicities: reference 47.

104

(k = 2.5 ×10-11 cm3 s-1; eff = 0.0087 and k = 1.4 ×10-11 cm3 s-1; eff = 0.0058,
respectively).65 Since these reactions proceed at rates well below the collision rate,
the α-effect would likely be more apparent. However, the rates of these reactions
scale with anion basicity and are inconsistent with a gas-phase α-effect.
This result also contradicts theoretical results, which predict that the barrier
for the reaction of HO2– with MeCl is lower (by ~5.4 kcal mol-1) than that of a
nucleophile of similar basicity.66 It is difficult to account for the discrepancies
between experiment and theory. However, it is possible that these nucleophiles havedifferent entropic factors, which would influence their reactivity. Additionally, theseresults disagree with the experimental findings of McAnoy et al.,30 which find a larger
propensity for the reaction of HO2– with dimethyl methylphosphonate to proceed via
an SN2 substitution rather than proton transfer, whereas the corresponding reactionof CD3O– proceeds primarily by proton transfer. The difference in the productbranching fractions was attributed to the α-nucleophilicity of HO2–. In this study,
however, the overall reaction rate constants for HO2– and CD3O– with dimethyl
methylphosphonate could not be evaluated and, therefore, it is not known if theoverall reaction rates for the two reactions are comparable.
These results cast serious doubt on the existence of a gas-phase α-effect.
This implies that the α-effect is not due to an intrinsic property of the anion andinstead due to solvent effects. These findings are complemented by the solution
studies of Um and Buncel,37 which demonstrate the important and complicated roleof the solvent in the destabilization/stabilization of the α-nucleophile and thetransition state. However, as stated above, the calculations of Ren and Yamsataka31
predict that the α-effect in SN2 reactions becomes more pronounced as the size ofthe neutral substrate increases. Unfortunately, given the limitations of gas-phase105

experiments, a similar analysis for the SN2 reactions of larger alkyl halides (EtCl and
i-PrCl) is not feasible since the E2 pathway is competitive. Additionally, further
investigation of the gas-phase reactivity of α-nucleophiles with sp2-hybridized carbon
centers is a clear target for future studies.
5.4) Conclusions
The competition between SN2 and E2 channels for the reactions of BrO– and
ClO– with RCl (R = methyl, ethyl, i-propyl, and t-butyl) was explored throughdeuterium kinetic isotope effect measurements. These kinetic isotope effects
become increasingly more normal as the extent of substitution in the neutral reactant
increases. This indicates that the E2 channel becomes the dominant pathway as theneutral reagent becomes more sterically hindered. For the reactions of ethyl
chloride, elimination is estimated to be a minor channel, while for the reactions of ipropyl chloride, elimination is estimated to be the major channel. The reactions of
MeCl, EtCl, and i-PrCl proceed by an additional minor reaction pathway, which islikely due to a novel elimination step that occurs within the SN2 product ion-dipole
complex.
Additionally, the gas-phase substitution reactions of ClO– and BrO– with
CH3Cl occur as predicted by a basicity-reactivity relationship. The same result isobserved for the SN2 reaction of HO2– with CH3Cl and CH3F. The lack of enhanced
reactivity of these α-nucleophiles implies that the α-effect is not manifested in thegas-phase but instead is due to solvent effects.
5.5) Acknowledgements
This work benefited from the contributions of Ms. Nicole Eyet, Dr. Shuji Kato,
Prof. Veronica M. Bierbaum, and Prof. Charles H. DePuy. Additionally gratitude isexpressed towards Dr. Gustavo Davico, Prof. Stephen J. Blanksby, and Prof. W. Carl106

Lineberger. This work was supported by the AFOSR (FA9550-06-1-006) and NSF(CHE-0647088). The computational results are based upon work supported by the
National Science Foundation under the following NSF programs: Partnerships for
Advanced Computational Infrastructure, Distributed Terascale Facility (DTF) andTerascale Extensions: Enhancements to the Extensible Terascale Facility.

1075.6) Chapter 5 References(1)
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113

CHAPTER VI
GAS-PHASE CARBENE RADICAL ANIONS:NEW MECHANISTIC INSIGHTS

6.1) Introduction
Carbenes are highly reactive organic species that have the general structure:CXY, where the central carbon atom has two substituents (X and Y) and twoelectrons, which can either be paired in a singlet state or unpaired in a triplet state.
These molecules have been the focus of intense study, the results of which
demonstrate that the structure, thermochemical properties, and reactivity depend
upon the ground state multiplicity and the singlet-triplet energy gap.1-4 The groundstate spin multiplicity of a carbene depends on the nature of the substituents. For
example, electron-withdrawing groups stabilize singlet states, while electropositiveand sterically bulky groups stabilize triplet states.5-8

pπ

pπ

Y

pπ

Y

C

σ

X

C
X

Singlet

Y

σ

C

σ

X

TripletSimple halogen substituted carbenes (:CXY, where X=F, Cl, and Br and Y=H,
F, Cl, and Br) have singlet ground states.9-11 In solution, these molecules are highly
reactive and undergo characteristic reactions such as insertion into single bonds andcycloaddition to double bonds,1,2 making them useful synthetic organic chemistry
reagents. In the atmosphere, these compounds are likely photofragments ofchlorofluoro compounds and other halons;12-16 the role of halogenated compounds in114

ozone depletion has been well documented.17-20 In addition, the reactions of
halocarbenes are important to the plasma chemistry of halogenated compounds21
and to organometallic chemistry.22-24
There has been an extensive effort, both experimentally and theoretically,
aimed at determining the fundamental physical properties of halocarbenes. Thesestudies have been employed to determine the electronic and molecular structure of
the ground and first excited state, vibrational frequencies, ionization energies,
electron affinities, and the singlet-triplet splittings.9-11,25-52 Additionally, heats offormation, gas-phase acidities, and bond dissociation energies have been
determined either directly or indirectly through gas-phase ion-molecule bracketing
experiments and collision-induced dissociation threshold energy measurements.8,53-57
While halogen substituted neutral carbenes have been extensively
investigated, only a few studies have addressed the chemistry of the corresponding
anions. Addition of an electron to a halocarbene forms a π-radical anion.9-11 In
solution, radical anions are of interest since these species are often reactive
intermediates. In the gas-phase, the chemistry of radical anions has important
implications for reactions in the upper atmosphere,58 negative ion chemical ionization
mass spectrometry,59,60 and electron capture detectors.61
The majority of the studies involving CXY•⎯ anions have focused on electron
and proton transfer reactions;55-57 these studies have provided valuable
thermodynamic information about the corresponding neutral carbenes. Beyond this,Born, Ingemann, and Nibbering55,62 have investigated the reactivity of a series ofmono-halocarbene anions with methyl halides, organic esters, and aliphatic alcohols.
Their results show that reactions with methyl halides proceed solely by an SN2
mechanism, while the reactions with the esters proceed by competing SN2 and BAC2

115

mechanisms.62 The reactions with the alcohol series (ROH) indicate that, in addition
to proton transfer, an SN2 reaction to produce X– also occurs. This SN2 process mustoccur within the [RO⎯ + •CH2X] complex before the initial products separate.55
The first part of this chapter focuses on the gas-phase reactions of carbene
radical anions with a series of oxygen and sulfur containing neutral reagents (CS2,
COS, CO2, O2, CO, and N2O).63 In past studies from this laboratory, it has beenshown that these sulfur and oxygen containing neutral reagents can provide insight
into the structure of an anion as well as form interesting product ions from diverse
chemistry.64-72 Additionally, the reactivity of methylene anion, CH2•⎯, has been studied
with this neutral series,70 which allows for direct comparison to the results presented
here. Since there is very little thermochemical information available in the literature
for these systems, electronic structure calculations are employed to evaluate thereaction exothermicities for several proposed pathways. The second half of thechapter evaluates the gas-phase reactivity of carbene radical anions with a series of
halogenated neutral reagents (CCl4, CHCl3, CH2Cl2, CH3Cl, CH3Br, and (CH3)3CBr).
For the reactions of CHCl•⎯ with CCl4, CHCl3, CH2Cl2, and CH3Cl, careful quantitative
isotope labeling studies were performed; the results of these labeling studies provide
insight into the reaction mechanism.73 For the reactions of CCl2•– with CCl4, CHCl3,
CH2Cl2, and CH3Cl, isotope labeling studies only provide qualitative insight.
Additionally for the reactions with CH3Br and (CH3)3CBr, deuterium isotope effects
were measured. In this work we present reaction rate constants as well as production branching ratios. Additionally, we suggest reaction mechanisms, which accountfor the observed product ions.

1166.2) Experimental Methods
The reactivities of mono- and dihalocarbene anions (CHCl•⎯, CHBr•⎯, CF2•⎯,
CCl2•⎯, and CBrCl•⎯) were studied using a tandem flowing afterglow-selected ion flow
tube instrument (FA-SIFT), which was described in Chapter 3. Reactant ions wereformed in a flowing afterglow source from the H2•+ abstraction reactions of O•⎯: 74,75
O•⎯ + CH2XY

H2O + CXY•⎯(6.01)

Ions of a single isotopomer were mass-selected and injected into a reaction flow tube
where they were thermalized to 302 ± 2 K by collisions with He buffer gas (0.5 torr,~104 cm s-1). Despite injecting the reactant ions with minimal energy, X⎯ ionsproduced from collision-induced dissociation were also present in the reaction flow
tube; the presence of these additional ions was considered in the data analysis
below.
Measured flows of neutral reagents were introduced into the reaction flow
tube through a manifold of inlets and the reactant and product ions were analyzed bya quadrupole mass filter coupled to an electron multiplier. Reaction rate constants
were determined by changing the neutral reagent inlet position, thereby varying thereaction distance and time, while monitoring the change in reactant ion intensity.
Product branching fractions were measured at each neutral inlet and averaged
together; secondary reactions in general do not occur for these systems. Efforts
were made to minimize mass discrimination; however, it was necessary to estimate
the relative detection sensitivities when calculating product branching ratios. The
relative detection sensitivity was estimated by examining a series of exothermic ionmolecule reactions where only one ionic product was formed. For reactions of
CBrCl•⎯, we were unable to cleanly separate the CBrCl•⎯ anion from the CHBrCl⎯
anion in the injection process. The presence of this additional ion does not interfere

117with measurements of the overall reaction rate constant since CBrCl•– can be
resolved and monitored with the mass detection system. The presence of CHBrCl⎯,
however, does complicate the determination of the product branching ratios. For
this same reason, the CBr2•⎯ anion was not included in this study. Additionally the
CHF•⎯ anion was not studied since it occurs at the same mass as O2•⎯, which is alsopresent in the ion source.
For the perchloro and perbromo reactions, isotopic labeling studies were
performed by alternately SIFT-injecting different isotopomers of the reactant ion
while monitoring the product isotope distributions; these studies aided in theidentification of the product ions and of the reaction mechanism. For the reactions of
CHCl•⎯ with CCl4, CHCl3, CH2Cl2, and CH3Cl, careful, quantitative isotope labelingstudies were performed. For the reactions of CCl2•⎯ with CCl4, CHCl3, CH2Cl2, andCH3Cl and for the reactions of CHBr•⎯ with CH3Br and (CH3)3CBr, only qualitative
results were obtained.
The error reported for the reaction rate constants is one standard deviation ofat least three measurements. The uncertainty in the reaction rate constants due tosystematic error is ±20% and the uncertainty in the product branching ratios is ±30%.
Helium buffer gas (99.995%) was purified by passage through a liquid nitrogencooled molecular sieve trap. Neutral reagents were purchased from commercial
sources and used without further purification. The reactions of CXY•⎯ with COS
indicate that the neutral sample contains a trace amount of H2S impurity; as a result,the measured rate constants represent an upper bound value. The reported reaction
efficiencies are the measured rate constant divided by the calculated collision rate
constant (eff=k/kcol). Collision rate constants were calculated from parametrized
trajectory collision rate theory.76

118

Electronic structure calculations were performed using the G3 composite
technique77 provided in the Gaussian 03 suite of programs.78 The electronic energy,
harmonic frequencies, and rotational constants were determined for the optimized
geometries of the reactants and products of the fluorinated and chlorinated reactions.
These results were used to evaluate the exothermicities of the proposedmechanisms; a more detailed investigation to include the reaction intermediates andtransition states is beyond the scope of this work. The exothermicities for thebrominated reactions are not provided since the G3 method does not include fourth
row elements.
6.3) Results and Discussion
6.3.1) Reactions of Carbene Anions with Sulfur/Oxygen Containing Species
Table 6.01 displays the experimentally measured reaction rate constants forthe reactions of CXY•– with each neutral reagent; Table 6.02 presents product ion
branching fractions. Figure 6.01 is a plot of reaction efficiency as a function of anion
basicity for the reactions of CXY•– with CS2, COS, CO2, O2, and CO; N2O is excluded
from this plot since it only reacts with CF2•–. In this figure each trace represents a
different neutral reagent and the data points are connected to guide the eye only.
The reaction trends presented in this plot are not readily observed in Table 6.01
since the collision rate constants for the reactions of CO2 and CS2 are smaller thanfor COS. The CHCl•– and CHBr•– anions react with similar efficiencies and trends.
As expected, replacing a hydrogen atom with a chlorine atom to form CCl2•– and
CBrCl•– substantially decreases the reaction efficiency. The reaction efficiencies andtrends for the CCl2•– and CBrCl•– anions are similar to one another but differ fromthose of the CHCl•– and CHBr•– anions. The CF2•– anion displays unique reactivity.
This difference in reactivity is, in part, not surprising since the CF2•– anion has a119

Table 6.01: Reaction rate constants (10-10 cm3 s-1) for the reactions of CXY•– with CS2, COS, CO2, O2, CO,and N2O.
Neutral
CHCl•–
CHBr•–
CF2•–
CCl2•–
CBrCl•–areagent(384.8; 1.210)(380.7; 1.454)(377.4; 0.180)(364.2; 1.590)(361; 1.84)
CS2
9.14 ± 0.22
10.7 ± 0.3
0.400 ± 0.080
0.162 ± 0.009
10.5 ± 0.7b
COSc
6.87 ± 0.25
5.27 ± 0.14
5.02 ± 0.17
1.31 ± 0.03
0.850 ± 0.038
CO2<0.001<0.001
5.62 ± 0.17
5.13 ± 0.02
<0.070 ± 0.002
O2
2.11 ± 0.04
1.80 ± 0.05
2.65 ± 0.03
0.946 ± 0.025
0.748 ± 0.014
CO<0.001<0.001<0.0011.42 ± 0.25
0.943 ± 0.013
N2O<0.001<0.001<0.001<0.0010.178 ± 0.030
a
The enthalpy of protonation in kcal mol-1 and the electron binding energy in eV are given in parentheses;
see references 10, 11, 55, and 56. b Standard deviation of at least three measurements. c Reaction rate
constants represent an upper bound value, see experimental section.

120
120

Table 6.02: Product ion distributions for the reactions of CXY•– with CS2, COS, CO2,
and O2.
NeutralProduct
CH2•– b
CHBr•–
CF2•–
CCl2•–
CBrCl•– c
CHCl•–
reagent
ions(407.4;(384.8;(380.7;(377.4;(364.2;(361;
0.652)a1.210)
1.454)
0.180)
1.590)
1.84)
CS2
X–
0.07
0.50
1.00
Cl–, Br–
BrCl•–
X2•–
+
S•–
SH–
+
+
1.00
CS2•–
–
C2HS
+
0.02
0.01
+
C2S2•–
0.18
0.37
CXS•–
+
0.73
0.12
CYXS•–
–
e
+
COS

CO2

X–
X2•–
S•–
C2OS•–CXYS•–e–X–
CYXO•–
C2X2O2•–
e–

0.90

1.00

0.03
0.07

trace

0.94
0.06

1.00

0.94

Cl–, Br–
BrCl•–

+
+
+

1.00

0.06

0.20
0.80

+

X–
0.60
0.90
0.30
0.58
Cl–
•–
0.02
0.24
X2
BrCl•–
+
0.32
0.07
0.40
0.12
+
O•–
OH–
0.05
0.03
0.28
O2•–
–
OX
0.03
0.04
ClO–
0.02
CX2O•–•–The + sign indicates that the ion was formed. For the CH2 ion, X and Y=H; for the
CHCl•– and CHBr•– anions, X=halogen and Y=H; for the CF2•–, CCl2•–, and CBrCl•–anion both X and Y=halogen. a The enthalpy of protonation in kcal mol-1 and theelectron binding energy in eV are given in parentheses; see references 10, 11, 55,
and 56. b The product ions formed from the reactions of CH2•– with CS2, COS, CO2,
and O2 are taken from reference 70. c Product branching ratios are not determinedbecause the CHBrCl– ion was also present in the reaction flow tube.
O2

121

1.0
CHCl·- CHBr·-

CF2·-

CS2
COS
CO2
O2
CO

Reaction Efficiency

0.8

0.6

0.4
CCl2·- CBrCl·-

0.2

0.0
385

380

375

370

365

360

-1
Anion Basicity (kcal mol )Figure 6.01: Reaction efficiency versus anion basicity. (Lines are only intended to
guide eye.)122

Table 6.03: Calculated and experimental reaction exothermicities (kcal mol-1) for thereactions of CHCl•–, CCl2•–, and CF2•–.
expb
BRc
Reaction
calca
0
0.07
CHCl•– + CS2 → Cl– + C2HS2•-64
0.02→ C2S2•– + HCl
•–
-11
0.73
→ CHClS + CS
-6
0.18
→ CClS– + HCS•
CHCl•– + COS → Cl– + C2HOS•→ C2OS•– + HCl→ CHClS•– + CO
→ CClS– + HCO•

-48
-41
-42
-4

0.90
0.03
0.07
not obs.

CHCl•– + CO2 → Cl– + C2HO2•→C2O2•– + HCl→ CHClO•– + CO
→ CClO– + HCO•

-25
-10
-37
-29

-33

0.94
not obs.
0.06
not obs.

CHCl•– + O2

-66
-88
-134
-68

-91
-137
-70

0.32
0.05
0.60
0.03

CHCl•– + CO → Cl– + •C2HO→ C2O•– + HCl

-33
-7

-11
-10

>0.99
Trace

CHCl•– + N2O → CHClN– + NO•→ Cl– + HCN + NO•

-94
-71

-94
-72

not obs.
not obs.

CCl2•– + CS2 → Cl– + C2ClS2•→ Cl2•– + C2S2
→ CCl2S•– + CS
→ CClS– + CClS•

-26
-5
+5
+15

1.00
not obs.
not obs.
not obs.

CCl2•– + COS → Cl– + C2ClOS•→ Cl2•– + C2OS
→CCl2S•– + CO
→ CClS– + CClO•

-21
+13
-27
+11

0.94
not obs.
0.06
not obs.

CCl2•– + CO2 → Cl– + C2ClO2•→ Cl2•– + 2 CO
→ CCl2O•– + CO
→ CClO– + ClCO•

-6
-32
-29
-13

not obs.
not obs.
not obs.
not obs.

→ O•– + CHClO→ OH– + COCl•→ Cl– + CHO2•→ ClO– + CHO•

123

CCl2•– + O2 → O•– + CCl2O→ Cl– + CClO2•→ ClO– + CClO•→ CCl2O•– + O→ Cl2•– + CO2

-44
-116
-52
-39
-171

CCl2•– + CO → Cl– + •C2ClO→ Cl2•– + C2O-5+62

not obs.
not obs.

CCl2•– + N2O → CCl2N– + NO•→ Cl– + ClCN + NO•

-59
-40

not obs.
not obs.

CF2•– + CS2 → CS2•– + CF2→ F– + C2FS2•→ F2•– + C2S2
→CF2S•– + CS
→CFS– + FCS•

-5
+68
+52
-2
+25

CF2•– + COS → CF2S•– + CO
→ F– + C2FOS•→ F2•– + C2OS
→ CFS– + FCO•

-34
+74
+69
+20

1.00
not obs.
not obs.
not obs.

CF2•– + CO2 → F– + •C2FO2→ F2•– + 2 CO
→CF2O•– + CO
→ CFO– + CFO•
→ C2F2O2•–

+80
+24
-26
+6
-36

not obs.
not obs.
not obs.
not obs.
1.00

CF2•– + O2 → O•– + CF2O
→ F– + •CFO2→ O2•– + CF2→ F2•– + CO2

-68
-97
-5
-114

0.40
0.30
0.28
0.02

CF2•– + CO → F– + •C2FO→ F2•– + C2O

d
+118

-140
-63
-170

-40
-10

-7
-116

0.12
0.58
0.04
0.02
0.24

1.00
not obs.
not obs.
not obs.
not obs.

not obs.
not obs.

-43
<-4
0.90
CF2•– + N2O → CF2N– + NO•
-7
-10
0.10
→ F– + FCN + NO•a
G3 calculated reaction enthalpies. b Enthalpies of formation are determined fromthe thermochemical data on the NIST webbook79 or from the JANAF thermochemical tables.80 c This work. d Undetermined; the calculation for •C2FO would notconverge.

124significantly lower electron binding energy than the other ions studied here and, ingeneral, other fluoro-anions typically display distinct reactivity.81
As demonstrated in Figure 6.01 and by the formation of multiple product ions(Table 6.02), the reactivity of these carbene anions is complex. To be able toaddress these reactivity trends, we must first discuss the likely reaction mechanisms.
In the following sections the reactivity of the CHCl•–, CHBr•–, CCl2•–, and CBrCl•–anions will be discussed together while the reactivity of the CF2•– anion will be
discussed separately. Since our experimental setup does not allow for neutral
product detection, the neutral products are inferred from the ionic products, and thestructures of the neutral and ionic products are based on molecular stability and on
the occurrence of reasonable reaction mechanisms. Electronic structure calculations
are used as a guide in determining these structures and mechanisms. The G3
calculated exothermicities for the observed fluorinated and chlorinated reactions areprovided throughout the text and in Table 6.03. The calculated results are compared
to experimental values when possible.
Reactions of CHX•– and CClX•– (X=Cl and Br)
The reactions of CHX•– (X=Cl and Br) with CS2 produce X–, C2S2•–, CHXS•–,
and CXS–, as shown below for CHCl•– + CS2.
CHCl•– + CS2

Cl– + C2HS2•∆ H ~ 0 kcal mol-1(6.02a)C2S2•– + HCl∆ H ~ -61 kcal mol-1(6.02b)CHClS•– + CS

∆ H ~ -10 kcal mol-1(6.02c)CClS– + HCS• ∆ H ~ -6 kcal mol-1(6.02d)Reactions 6.02a and 6.02b presumably proceed by anion attack on the carbon atomof CS2, as shown in Scheme 6.01. These two reactions proceed through an

125

[HClC-CS2] •– intermediate followed by an intramolecular SN2 reaction, where thesulfur atom attacks the β carbon atom to displace Cl–. A trace amount of C2S2•– isdetected, which is the result of proton transfer before the initial products from 6.02a
separate. While proton transfer to form C2S2•– is thermodynamically favored, it is aminor product whose formation is likely influenced by kinetic or dynamic factors
within the product ion-molecule complex. Reactions 6.02c and 6.02d most likely
proceed by carbanion attack on the sulfur atom of CS2, as shown in Scheme 6.02.
Reaction 6.02c is the result of the displacement of neutral CS. The CClS– ion(6.02d) is formed from hydrogen atom transfer before the initial products of 6.02c
separate; G3 calculations predict that this ion is a [Cl–•S≡C] ion-dipole boundcomplex.
Scheme 6.01
H
Cl +
Cl
C

CHCl + CS2

H

S
C

C

S

C

C

S

S
C

Cl +
S

H

C

C

S

S
HCl + S

Scheme 6.02
Cl
Cl

Cl
C

CHCl + CS2
H

S

C

S

C

S

S

C +

+ CS

H
C

S

+ CS

H
H
Cl

C

The reactions of CHX•– (X= Cl or Br) with COS and CO2 proceed similarly to thecorresponding reactions with CS2; such parallels in reactivity of anions with thesethree neutral reactants have previously been reported in the literature.64,65 The
126

S

product ions formed are consistent with either attack on the neutral reagent carbon
atom, analogous to the mechanism shown in Scheme 6.01, or attack on neutral
reagent sulfur (or oxygen) atom, analogous to the mechanism shown in Scheme
6.02. As the neutral reagent is changed from CS2 to COS and CO2, sulfur/oxygen
atom attack becomes the minor pathway for the CHCl•– anion and is not observed for
the CHBr•– anion.
The product ions formed in the reactions of CCl2•– and CBrCl•– with CS2 areconsistent with a mechanism that is analogous to Scheme 6.01 only. For thereactions of CBrCl•– with CS2, either Cl– or Br– can be displaced by an intramolecular
SN2 reaction. The formation of BrCl•– is due to halogen atom abstraction, in contrast
to proton transfer, before the initial products separate; in some encounters, theelectron remains with C2S2. Both of these dihalo anions react with COS but do notreact with CO2 within our detection limits. The reaction of CCl2•– with COS primarily
proceeds by anion attack on the carbon atom of COS; additionally, a minor channelof anion attack on the sulfur atom of COS is also observed. The CBrCl•– anion
reacts with COS exclusively by attack on carbon.
An interesting observation is that the reaction efficiencies (Figure 6.01) for the
mono- and dihalocarbene anions do not mirror each other. For the CHCl•– andCHBr•– anions, the reaction with CS2 is most efficient, followed by CO2, then by COS.
In contrast for these three neutral reactants, the efficiency is greatest for thereactions of CCl2•– and CBrCl•– with COS, followed by CS2, while CO2 is below ourdetection limit. As established above, these carbene anions primarily react with thisneutral series by nucleophilic attack on the neutral reagent carbon atom; with the
exception of CHX•– with CS2, anion attack on the neutral reagent sulfur/oxygen atom
is at most a minor pathway. This mechanism (shown in Scheme 6.01) is complex

127

and multiple factors influence the observed reaction efficiency. If we consider the
first step of the reaction mechanism, we would expect the reactivity to increase as
the number of oxygen atoms in the neutral reagent increases, since in CO2 there is agreater partial positive charge on the carbon atom as compared to CS2. However,while oxygen is more electronegative, sulfur can expand its valence and therefore
more readily accept the negative charge. Following the anion attack on the carbon
atom of the neutral reagent, halide displacement occurs as a result of an intramolecular SN2 reaction. Here we must consider two further factors, the nature of theleaving group (Cl– or Br–) and of the nucleophile (S or O). It is known that Br– is abetter leaving group than Cl– and in the gas-phase the nucleophilicity of sulfur anions
are comparable to oxygen anions.82 Since multiple factors must be considered,predicting the reaction efficiency trend is not straightforward. Furthermore, it is likelythat the individual factors may have a different overall net effect for different anions,and hence the observed trends for the mono- and dihalocarbene anions differ fromone another.
Both CHCl•– and CHBr•– react with O2 with the same efficiency to produce
analogous products. The reaction of CHCl•– with O2 produces O•–, OH–, Cl–, andClO–.
CHCl•– + O2

O•– + CHClO∆ H ~ -66 kcal mol-1(6.03a)OH– + COCl•∆ H ~ -88 kcal mol-1(6.03b)

Cl– + CHO2•∆ H ~ -134 kcal mol-1(6.03c)ClO– + CHO•

∆H ~ -68 kcal mol-1(6.03d)Reaction 6.03a proceeds by an SN2 reaction with anion attack at the oxygen todisplace O•–. Before the product ion-molecule complex dissociates, O•– can abstracteither a hydrogen or chlorine atom to form OH– (6.03b) or ClO– (6.03d), respectively

128

(Scheme 6.03). The formation of Cl– in 6.03c is the major product. Analogous to theabove reactions with CS2, COS, and CO2, where the displaced halide is also
observed, this reaction most likely proceeds through an [HClC-O-O] •– intermediate
followed by an intra-molecular SN2 reaction where, in a concerted step, the terminal
O atom attacks the carbon to displace Cl– (Scheme 6.04).
Scheme 6.03
Cl

Cl
C

CHCl + O2

O + O

C

O + O

C

O + OHC

O + ClO

H

H

Cl

H

Scheme 6.04
O

Cl
C

CHCl + O2
H

O

O

Cl +

H

C
O

The dihalocarbanions, CCl2•– and CBrCl•–, react with O2 to form similar
product ions as the corresponding reactions of CHX•– (X= Cl and Br) with O2, themajority of which can be explained by the above mechanisms. The reaction ofCCl2•– with O2 additionally produces minor amounts of CCl2O•–, which is due toelectron transfer from O•– within the product ion-molecule complex. The reactions of
CCl2•– and CBrCl•– with O2 also produce the dihalide anions. These dihalide anions
could be formed from a mechanism similar to Scheme 6.04, where X atom abstraction occurs within the product ion-molecule complex before the initial products
separate to form XY•– and CO2. Interestingly, the product ions formed in the129reactions of CCl2•– and CBrCl•– with O2 are comparable to the carbene cation
reaction of CBr2•+ with O2, which has previously been studied by Cooks and coworkers.83 In that work, the authors speculate that this reaction proceeds through a
Br2CO2•+ intermediate, which can eliminate CO2 to form Br2•+. This intermediate canalso rearrange to form BrCO+ and BrO• and, in a competitive process, BrCO• andBrO+.
Both CHCl•– and CHBr•– react with CO to primarily produce the displaced
halide (6.04a). For the reaction of CHCl•–, a trace amount of C2O•– is also formed(6.04b), while for the reaction of CHBr•– a trace amount of C2HO– is detected.
CHCl•– + CO

[ClHC=CO]•–Cl– + •C2HO∆ H ~ -33 kcal mol-1(6.04a)HCl + C2O•–∆ H ~ -7 kcal mol-1(6.04b)

The formation of these products is consistent with a mechanism where the anion
attacks the carbon atom of CO. In an analogous manner to the other reactions
discussed thus far, Cl– could be displaced by an intramolecular SN2 mechanism
where the O atom attacks the carbon atom in the β position. However, if this wereindeed the case, the resulting neutral species is probably not stable. Instead, if theaddition of CHCl•– to CO is sufficiently exothermic, the C-Cl bond could break to
produce very stable products, Cl– and H-C≡C-O•. Alternatively, the initially formedintermediate, may undergo a retro-carbene reaction to make HCl and :C=C=O•–
directly; proton transfer within the product ion-molecule complex then would form Cl–
and HCCO•. In this case, both mechanisms are feasible; however, neither one likely
occurs for the other reactions discussed above that also produce the displaced130halide as less energy is required to form a C=O bond from a C≡O bond, than to forma C-O bond from a C=O bond. Neither CCl2•– nor CBrCl•– react with CO, consistent
with calculations, which predict the reaction of CCl2•– with CO to be endothermic.
Reactions of CF2•–
The reactivity of the CF2•– anion is different from that of other carbene anions,due in part, to its lower electron binding energy (eBE(CF2•–)=0.180 eV).10 As aresult, this anion can react by electron transfer. This is the case for the reaction of
CF2•– with CS2, where electron transfer is the only pathway observed, consistent withcalculations, which predict that electron transfer is the only exothermic pathway. The
reactions of CF2•– with COS, CO2, and O2 show some similarities and somedifferences to the other anions discussed above. For example, CF2•– reacts with
COS via anion attack on the sulfur atom of COS to exclusively produce CF2S–,
analogous to Scheme 6.02. A mechanism analogous to Scheme 6.01 to form F– ispredicted to be endothermic. The reaction of CF2•– with CO2 exclusively forms
C2F2O2•–. This is most likely the result of anion addition to the carbon atom of CO2,
similar to the addition reaction of OH– with CO2,84 rather than from a clustering
process. For the reaction of CF2•– with O2, electron transfer is exothermic and thispathway is viable as evident by the formation of O2•–. Additionally this reaction
produces O•–, F–, and F2•–, which are analogous to the products formed from CClX•–(X= Cl and Br) with O2.
It has previously been demonstrated that neither CHCl•– nor CHBr•– react with
N2O,55 even though the reaction of CHCl•– with N2O is predicted to be exothermic.
As would be expected, CCl2•– and CBrCl•– also do not react with N2O. CF2•–,
however, does react with N2O to produce CF2N– and F– in a 0.90 to 0.10 branching131

fraction, respectively. This reaction most likely proceeds by attack of the anion on
the terminal nitrogen atom of N2O, as shown in Scheme 6.05. The addition of CF2•–
is sufficiently exothermic (∆ rxnH ~ -43 kcal mol-1) such that F– can be displaced. In
this case, addition-dissociation is similar to the reaction of CHX•– with CO, where X–
is displaced. The CF2•– anion, however, does not react with CO; this reaction ispredicted to be endothermic. Similar nitrogen atom transfer reactions havepreviously been observed for reactions of radical anions and N2O.67,85
Scheme 6.05
FFC

CF2 + N2O
F

N

N

O

C

N + NOF
F + F

C

N + NO

So far we have discussed the differences in the reactivity of CF2•– in terms of
its lower electron binding energy. Several groups have attributed the stability of theneutral halocarbene singlet state (and destabilization of the triplet state) to two
factors, the electronegativity of the substituents and the ability of the halogen to
donate electron density to the carbon through π−backbonding.11,54 These two effects
are magnified in the case of neutral CF2, since fluorine is the most electronegative
halogen and since the C-F bond is shorter than the C-Cl and C-Br bonds and,
therefore, there is good overlap between the carbon and fluorine atom π−orbitals. As
a result, the electron binding energy of CF2•– (and CHF•–) is significantly lower thanthat of the chloro and bromo carbene anions (eBE(CHCl•–) =1.210 eV; eBE(CHBr•–)=1.454 eV; eBE(CCl2•–)=1.590 eV; eBE(CBrCl•–) =1.84 eV).10,11,56 While this
explanation accounts for its propensity to undergo electron transfer, it clearly does
not explain every difference among CF2•– and the other anions studied here.

132

In general, for these carbene anions, the reaction efficiency increases as theanion basicity increases. It is not surprising that CF2•– is considerably more basic
than CCl2•– and CBrCl•–; thus, due to its basicity and its electron binding energy, we
would expect CF2•– to be highly reactive; this expectation is experimentally observedin some instances. Additionally, CF2•– is a hard base due to its small size; typicallyhard bases are good nucleophiles. However, since fluorine is so electronegative, thefluorine substituents inductively withdraw electron density from the carbon σ−orbital,
decreasing the nucleophilicity of the anion. As a result, CF2•– reacts with COS fairly
efficiently by sulfur atom transfer but does not react with CO.
Comparison to the CH2 •– anion
The product ions formed in the reactions of CH2•– with CS2, COS, CO2, O2,
CO, and N2O have been previously documented.70 The ionic products formed in thereactions with CS2, COS, CO2, and O2 are included in Table 6.02. The reaction ofCH2•– with CS2 shows some similarities to the reactions of CXY•– studied here.
CH2•– + CS2

S•– + C2H2S(6.05a)SH– + C2HS•(6.05b)C2HS– + HS•(6.05c)CH2S•– + CS

(6.05d)CS2•– + CH2(6.05e)neutral products + e–

(6.05f)Evidence of anion addition to the carbon atom of CS2 is observed in 6.05a-c;
however, instead of the sulfur atom attacking the β carbon atom in an intramolecular
SN2 reaction, S•– is displaced. The product ions formed in 6.05b and c are the resultof hydrogen atom or proton transfer within the product ion-molecule complex of
6.05a before the initial products separate. In contrast, sulfur anion displacement is133not observed for the reactions of CHX•– and CClX•– (X= Cl and Br) with CS2 sincehalides are better leaving groups. Reaction 6.05d is the result of carbanion attack on
the sulfur atom of CS2 followed by loss of CS, analogous to Scheme 6.02.
Reaction 6.05e proceeds by electron transfer, which parallels the reaction of
CF2•– with CS2, again due to the low electron binding energy of CH2•–. The CH2•– andCF2•– anions show additional parallels in reactivity. For example, both anions reactwith N2O by attack on the terminal nitrogen. A notable difference in the reactivity ofCH2•– is that this anion has a propensity to react via associative or reactive electron
detachment. In fact, electron detachment is the major channel for the reaction ofCH2•– with CS2 (6.05f) and the sole channel for reactions with CO2 and CO. While
we cannot fully exclude the possibility of electron detachment in the present study ofCF2•–, it is at most a minor process.
6.3.2) Reactions of CHX•⎯ and CClX•⎯ (X=Cl and Br) with Halogenated Species
Table 6.04 displays the experimentally measured reaction rate constants forthe reactions of CXY•– with each halogenated neutral reagent. Table 6.05 presents
product ion branching fractions for the reactions of CXY•– with CCl4, CHCl3, andCH2Cl2; for the reactions of CXY•– with CH3Cl, CH3Br, and (CH3)3CBr the displaced
halide is the sole ionic product. As demonstrated in Table 6.05 by the number ofionic products formed, these anions display diverse chemistry. In order to elucidate
the likely reaction mechanisms, a detailed isotopic labeling study was performed forthe reactions of CHCl•⎯.
Reactions of CHCl•⎯ with CCl4, CHCl3, CH2Cl2, and CH3Cl
For the reactions of CHCl•⎯ with CCl4, CHCl3, CH2Cl2, and CH3Cl, both the
CH35Cl•⎯ and CH37Cl•⎯ isotopomers were separately mass selected and injected into

134

Table 6.04: Reaction rate constants (10-10 cm3 s-1) for the reactions of CXY•⎯ with CCl4, CHCl3, CH2Cl2, CH3Cl,
CH3Br, and (CH3)3CBr.
CHCl•⎯(384.8; 1.210)a

CHBr•⎯(380.7; 1.454)

CF2•⎯(377.4; 0.180)

CCl2•⎯(364.2; 1.59)

CBrCl•⎯(361; 1.84)

CCl4
7.98 ± 0.11
9.11 ± 0.14
5.62 ± 0.10
4.39 ± 0.18
9.26 ± 0.12b
CHCl3
11.3 ± 0.4
8.04 ± 0.11
9.60 ± 0.10
5.31 ± 0.07
3.82 ± 0.14
CH2Cl2<0.001<0.001
11.8 ± 0.4
8.01 ± 0.17
4.52 ± 0.07
CH3Cl
<0.0012.62 ± 0.10
0.933 ± 0.015
3.03 ± 0.02
0.003 ± 0.001
CH3Br
7.46 ± 0.06
4.98 ± 0.14
6.99 ± 0.09
0.665 ± 0.11
0.183 ± 0.002
d-KIE
0.99 ± 0.03
1.00 ± 0.04
1.01 ± 0.02
1.01 ± 0.02
1.01 ± 0.06
(CH3)3CBr<0.001<0.001
10.1 ± 0.1
5.78 ± 0.14
6.75 ± 0.02
d-KIE
1.14 ± 0.04
1.86 ± 0.08
1.37 ± 0.04a
-1
The anion basicity in kcal mol and the electron binding energy in eV are given in parentheses;
see references 10, 11, 55, and 56. b Standard deviation of at least three measurements.

135
135

Table 6.05: Product ion branching fractions for the reactions of CXY•– with CCl4,
CHCl3, and CH2Cl2.
neutralreagent

Product
ions

CCl4

Cl⎯
Cl2•⎯
CHCl2⎯CCl3⎯
CBrCl2⎯

CHCl3

Cl⎯
Cl2•⎯
CHCl2⎯
CCl3⎯
Br⎯
BrCl•⎯

CHCl•⎯(384.8;1.210)a

trace
1.00
trace
trace
0.21
0.79

CHBr•⎯(380.7;1.454)

1.00

1.00

CF2•⎯
(377.4;0.180)

CCl2•⎯(364.2;
1.59)

CBrCl•⎯ b
(361;
1.84)

0.97

0.06
0.04

+
+

0.03

0.90

+
+

0.43

0.23
0.03

+
+

0.57

0.74

+
+
+

CH2Cl2

Cl⎯
0.10
0.23
0.96
trace
0.03
Cl2•⎯
CHCl2⎯
0.90
0.67
0.01
0.08
Br⎯
•
0.02
BrCl ⎯a
The enthalpy of protonation in kcal mol-1 and the electron binding energy ineV are given in parentheses; see references 10, 11, 55, and 56. The + sign
indicates that the ion was formed. b Product branching ratios are not determinedbecause the CHBrCl– ion was also present in the reaction flow tube.

136

Table 6.06: Reaction rate constants (10-10 cm3 s-1) and product ion branching ratios
for the reactions of CH35Cl•⎯ with CHnCl4-n (n=0-3) and product ion isotope
distributions for the reactions of CH35Cl•⎯ and CH37Cl•⎯ with CHnCl4-n (n=0-3).
product ion isotope distributionsb
ionicbranching
injection of
injection of
CHnCl4-nkproducts
fractiona
CH35Cl•⎯CH37Cl•⎯CH3Cl
2.62 ± 0.10
Cl⎯
1.00
78:22
70:30
CH2Cl211.8 ± 0.4
Cl⎯
78:22
68:32
0.10
73:22:5
26:57:17
trace
Cl2•⎯
0.90
57:37:6
57:37:6
CHCl2⎯
59:41
81:19
0.05
CHCl3
11.3 ± 0.4
Cl⎯•28:55:17
trace
71:25:4
Cl2 ⎯49:44:7
60:35:5
0.15
CHCl2⎯
45:42:12:1
44:43:12:1
0.80
CCl3⎯
CCl4
9.26 ± 0.12
CHCl2⎯
trace
68:28:4
8:64:28
CCl3⎯
1.00
45:41:13:1
44:42:13:1
a
The errors for the product ion branching fractions are ± 30%. b The error
associated with the product ion isotope distributions is ± 0.05 for Cl⎯ and Cl2•⎯ and ±0.02 for CHCl2⎯ and CCl3⎯. The natural abundance isotope distributions are: Cl⎯(76:24); Cl2•⎯ (57:37:6); CHCl2⎯ (57:37:6); CCl3⎯ (44:42:13:1).

137

the reaction flow tube while monitoring the product ion isotope distributions.
Reaction rate constants, product ion branching fractions, and product ion isotope
distributions are provided in Table 6.06.
The CHCl•⎯ anion reacts with CCl4 to primarily produce CCl3⎯ and a trace
amount of CHCl2⎯, as shown in 6.06a-b.
CHCl•⎯ + CCl4

CHCl2• + CCl3⎯∆H~ -37 kcal mol-1(6.06a)CHCl2⎯ + CCl3•∆H~ -21 kcal mol-1(6.06b)

The formation of these products is consistent with attack on chlorine causing
displacement of CCl3⎯ or chlorine atom transfer to form CHCl2⎯; for both pathways,
electron transfer can occur before the initial products separate. Similar reactivity has
been reported for the reactions of other open and closed shell anions with CCl4.74,86,87
As predicted by this mechanism, the CCl3⎯ ion is produced with an isotope distribution that reflects the natural Cl atom abundance in CCl3 (44:42:13:1); hence this
distribution does not change for different reactant ion isotopomers. In contrast, theCHCl2⎯ product isotope distribution is skewed toward the reactant ion isotopomer.
The reactions of CHCl•⎯ with CHCl3 and CH2Cl2 primarily occur via protontransfer (6.07a and 6.08) to produce CCl3⎯ and CHCl2⎯, respectively. As expected,
the isotope distributions of these product ions are not altered upon changing thereactant ion isotopomer.
CHCl•⎯ + CHCl3

CHCl•⎯ + CH2Cl2

CH2Cl• + CCl3⎯∆H~ -25 kcal mol-1(6.07a)CHCl2• + CHCl2⎯∆H~ -16 kcal mol-1(6.07b)CH2Cl• + CHCl2⎯∆H~ -8 kcal mol-1(6.08)The reaction of CHCl•⎯ with CHCl3 (6.07b) additionally proceeds by anion
attack on Cl to produce CHCl2⎯ (analogous to 6.06a). The isotope distribution of thisproduct ion changes when the reactant ion isotopomer is changed, consistent with138

thermoneutral electron transfer within the product ion-molecule complex. An
intriguing result of these reactions is the formation of Cl2•⎯ and Cl⎯ ions, and thatchanging the CHCl•⎯ isotopomers affects the isotope distributions of both of theseproduct ions. For example, when CH37Cl•⎯ is allowed to react with CHCl3, all threeisotopomers of Cl2•⎯ are detected, however the isotope distribution is skewed toreflect incorporation of 37Cl. Likewise although both Cl⎯ isotopes are detected, 37Cl⎯
exceeds the natural abundance. These observations suggest that the products must
result from a mechanism that partially scrambles the reactant ion and neutral reagent
Cl atoms.
The formation of both Cl2•⎯ and Cl⎯ is consistent with an insertion-elimination
mechanism, shown below in Scheme 6.06 for the reaction of CHCl•⎯ with CHCl3.
Scheme 6.06

C· + CHCl3
H

·

Cl Cl

Cl

H

C

C

H

C2H2Cl2 + Cl2·C2H2Cl2 + Cl + Cl

Cl ClPresumably the first step of this reaction involves the formation of a σ–π complex,
where the singly occupied π–orbital of the anion overlaps with the C–Cl σ–bond of
the neutral species to transfer an electron. Neutral :CHCl then inserts into the Cl–C
bond in a concerted step, which proceeds through a three-centered transition state
where the Cl atom of CHCl3 attacks the carbene C atom and the carbene lone pairattacks the C atom of CHCl3. Insertion of :CHCl forms the [CHCl2CHCl2] •⎯ intermediate, where the extra electron is localized on a Cl atom in a low-lying σ-type
orbital. This intermediate is unstable, and eliminates Cl2•⎯; the overall reaction is
sufficiently exothermic (∆H~ -80 kcal mol-1) such that Cl2•⎯ can dissociate into Cl•

139and Cl⎯. Insertion of :CH35Cl into a Cl–C bond results in a Cl⎯ (82:18) and Cl2•⎯(67:30:3) isotope distribution, while insertion of :CH37Cl results in a Cl⎯ (57:43) andCl2•⎯ (29:56:15) isotope distribution. These predictions are in agreement with themeasured Cl⎯ and Cl2•⎯ isotope distributions for the reaction of CHCl•⎯ with CHCl3.
The predictions fall slightly outside of the experimental error bars for the reaction of
CHCl•⎯ with CH2Cl2; however, the occurrence of minor amounts of SN2 substitution
would influence the Cl isotope distribution. Despite this, the predicted distributions
for an insertion-elimination mechanism provide the best agreement with experimentcompared to other possible mechanisms. Furthermore, these results indicate that
insertion occurs into a C–Cl bond rather than into a C–H bond. (Insertion into a C–H
bond results in a Cl⎯ (88:12) and Cl2•⎯ (76:24:0) isotope distribution for the reaction of
CH35Cl•⎯ and a Cl⎯ (38:62) and Cl2•⎯ (0:76:24) isotope distribution for the reaction of
CH37Cl•⎯.) In solution carbenes typically insert into C–H bonds; however, in the gasphase insertion into a C–Cl bond is favored since the addition of an extra electron
increases the nucleophilicity of the Cl atom.
The reaction of CHCl•⎯ with CH3Cl exclusively produces Cl⎯. These results
are in excellent agreement with the work of Nibbering and co-workers, which show aslightly skewed Cl⎯ isotope distribution for the reaction of CH35Cl•⎯ with CH3Cl (3%deviation from the 35Cl:37Cl natural abundance of 76:24);56 however, we propose analternate, energetically favorable mechanism to explain the skewed Cl isotope
distribution. The authors proposed that this reaction proceeds by two pathways, as
shown in 6.09a and 6.09b. We agree that the primary path (6.09a) is a traditional
SN2 reaction to produce Cl⎯ and the CH3CH35Cl• radical, where the displaced halide
ion is produced with an isotope distribution representative of the Cl atom natural
isotope abundance. Additionally, it was suggested that this reaction proceeds by a140

minor second path (6.09b), where the anion abstracts a proton from methyl chlorideprior to SN2 substitution; the result of this pathway is the formation of 35Cl⎯ and
ClCH2CH2•. Instead, we propose a mechanism (6.09c) in which the chlorine atoms
are scrambled by two proton exchanges. In this process, CH35Cl•⎯ and CH3Cl enter a
CH35Cl + CH3Cl

CH35Cl + CH3Cl

CH3CH35Cl + Cl

(6.09a)CH235Cl + CH2Cl

CH2CH2Cl + 35Cl

(6.09b)

CH235Cl + CH2Cl

CH335Cl + CHCl

CH3CHCl + 35Cl

(6.09c)long-lived ion-dipole complex which typically contains ~15-20 kcal mol-1 excessenergy.88 The first step of the exchange, the proton transfer reaction of CH35Cl•⎯ with
CH3Cl, is endothermic (~15 kcal mol-1) and consumes most of the complexationenergy; thus, it seems unlikely that the SN2 reaction of CH235Cl• with CH2Cl⎯ (6.09b)
could proceed past an SN2 reaction barrier despite the fact that the overall reaction is
exothermic. Instead, back transfer of a proton to form [CH335Cl + CHCl•⎯] restoresthe complexation energy, scrambles the Cl atoms, and allows for an SN2 reaction to
occur. Reaction 6.09c produces different isotopomers of the reactants and products
in 6.09a through an energetically favorable mechanism.
Reactions of CXY•⎯ with CCl4, CHCl3, CH2Cl2, and CH3Cl
Although the reactions of CCl2•⎯, CHBr•⎯, and CBrCl•⎯ have not been explicitly
discussed, all of the observed ionic products (Table 6.05) can be explained by thepreviously outlined mechanisms. Additionally, isotopic labeling studies for thereactions of CCl2•⎯ with this neutral series were performed by alternately SIFTinjecting the C35Cl35Cl•⎯ and C37Cl37Cl•⎯ isotopomers while monitoring the change in141the product ion isotope distributions. While only a qualitative study was performed,
these results are in agreement with the previously discussed mechanisms. Notably,
the reactions of CCl2•⎯ with CCl4 and CHCl3 produce minor amounts of Cl⎯ and Cl2•⎯,
analogous to the reactions of CHCl•⎯ with CHCl3 and CH2Cl2. Similarly, for bothreactant ions, the isotopic distributions for the Cl⎯ and Cl2•⎯ product ions differ whendifferent isotopomers of the reactant anion are allowed to react, reflecting theincorporation of the reactant anion chlorine atoms; this observation is consistent theoccurrence of an insertion-elimination mechanism.
One further interesting observation is that the reaction of CCl2•⎯ with CCl4
(eq 6.10) produces CCl3⎯ with an isotope distribution that reflects the natural Cl atom
abundance in CCl4.
CCl2•⎯ + CCl4

CCl3• + CCl3⎯∆H~ -57 kcal mol-1(6.10)

This reaction is analogous to reactions of CHCl•⎯ with CCl4 and CHCl3 (6.06a and6.07b); however, these results allow us to assign an SN2 type mechanism, where theCCl2•⎯ anion attacks a Cl atom of CCl4 to displace CCl3⎯ rather than a mechanism,
where the CCl2•⎯ anion abstracts a neutral Cl atom from CCl4. Additionally, incontrast to 6.06a and 6.07b, thermoneutral electron transfer does not occur within
the product ion-molecule complex; this may reflect the large geometry differences,and hence poor Franck-Condon overlap, between the CCl3 anion and radical.
The CF2•⎯ anion displays unique reactivity as apparent in the reported
reaction efficiencies and product ion distributions (Tables 6.04 and 6.05). As
previously discussed, this difference is partially due to the lower electron binding
energy of the CF2•⎯ anion. As a result, dissociative electron transfer to produce Cl⎯ isalso possible and is observed for the reactions of CF2•⎯ with CCl4 and CHCl3.
Additionally the CF2•⎯ anion displays increased nucleophilicity as demonstrated in the142reaction of CF2•⎯ with methyl chloride. The CF2•⎯ anion is ~7.5 kcal mol-1 less basic
than the CHCl•⎯ anion, however, both anions react at approximately the same
reaction efficiency; typically nucleophilicity scales with anion basicity. This isconsistent with CF2•⎯ being a hard base and hence a good nucleophile. This idea,
however, is in contrast to the fact that fluorine atoms are electronegative, and thus
the fluorine substituents on CF2•⎯ inductively withdraw electron density from thecarbon σ−orbital, decreasing the nucleophilic character (as observed in the above
section for the reactions of CF2•⎯ with sulfur and oxygen containing species). Thesestudies illuminate the complexity of the CF2•⎯ anion nucleophilicity and, due to theimportance of SN2 reactions, this aspect remains a clear target for further
experimental and theoretical study.
Figure 6.02 is a plot of reaction efficiency versus the number of chlorine
atoms in the neutral reagent series, CCl4, CHCl3, CH2Cl2, and CH3Cl. In general thereaction efficiency increases as the number of chlorine atoms in the neutral substrateincreases. This trend is not apparent upon inspection of the overall reaction rate
constants in Table 6.04, since the reactions with nonpolar CCl4 have a smaller
collisional component than the other reactions studied here. In contrast, the reaction
efficiency decreases as the number of chlorine atoms in the anion increases asevident for the reactions of CHCl•⎯, CHBr•⎯, CCl2•⎯, and CBrCl•⎯. This trend simply
reflects the relative stability of the reactant anions.
The reactions of CXY•⎯ with CH3Cl are the least efficient of the series studied
here. These reactions exclusively proceed via an SN2 reaction on carbon to displaceCl⎯. Substitution reactions of this type have been the focus of intense study.89-91 It is
well established that the rates of these reactions are controlled by the crossing of acentral barrier; as a result, exothermic reactions can proceed at a rate slower than

143

1.0
CHCl·CHBr-

Reaction Efficiency

0.8

CF2·CCl2·CBrCl·-

0.6

0.4

0.2

0.0
CCl4

CHCl3

CH2Cl2

CH3Cl

Figure 6.02: Reaction efficiency versus the number of chlorine atoms in the neutral
reagent. (Lines are only intended to guide eye.)144the collision rate. The reactions of CXY•⎯ with CCl4 are the most efficient of theseries studied here. With the exception of the reaction of CF2•⎯ with CCl4, thesereactions primarily proceed by an SN2 reaction on chlorine. It is expected that thesereactions do not have a large central barrier since there are fewer steric restrictions
due the absence of an inversion center in the transition state and the presence offour chlorine atoms in CCl4. As the number of chlorine atoms in the neutral reagent
increases, the likelihood that an SN2 reaction on carbon occurs decreases, while thelikelihood that an SN2 reaction on chlorine increases. For the reactions of CXY•– with
CHCl3 and CH2Cl2 it is unclear if a carbon centered SN2 reaction occurs; however, if
this reaction does occur, it is in minor amounts. Additionally for these reactions,chlorine centered SN2 substitution is only observed for the reaction of CHCl•– with
CHCl3. Instead the reactions of CXY•⎯ with CHCl3 and CH2Cl2 primarily proceed byproton transfer, and as expected the reactions with CHCl3 are more efficient thanthose with CH2Cl2, since CHCl3 is more acidic than CH2Cl2. Proton transfer is notobserved in the reactions of CCl2•⎯ and CBrCl•⎯ with CH2Cl2 since this pathway isendothermic.
Reactions of CXY•⎯with CH3Br and (CH3)3CBr
Reaction rate constants and deuterium kinetic isotope effects for thereactions of CXY•⎯ with CH3Br and with (CH3)3CBr are given in Table 6.04; for thesereactions, the halide (Br⎯) is the sole ionic product. Deuterium kinetic isotope effects
are the ratio of perprotio to perdeuterio reaction rate constants (KIE = kH/kD). For
reactions that proceed below the collision rate, kinetic isotope effects can provide asensitive probe of the transition state structure. For SN2 reactions inverse KIEs
(kH/kD<1) are observed, while for E2 reactions normal KIEs (kH/kD>1) are observed.
A detailed explanation of the origin of these effects is given in Chapter 5.

145

It has previously been established that reactions of CXY•– with methyl halides
proceed via an SN2 mechanism. Analogous to the above reaction of CHCl•– with
CH3Cl, our results are consistent with those of Nibbering and co-workers, which
report a slightly skewed Br⎯ isotope distribution for the reaction of CH79Br•– with
CH3Br (2% deviation from the 79Br:81Br natural abundance of 49:51).56 As explained
above, we attribute this skewed isotope distribution to H atom transfer within thereactant ion-molecule complex, rather than the previously suggested mechanism of
proton transfer followed by an SN2 reaction (see the analogous reactions 6.09a-c).
Typically deuterium kinetic isotope effects for SN2 reactions are less than one;however, for the reactions measured here, deuteration of the neutral reagent does
not change the reaction rate. The absence of an isotope effect is most likely due tothe large size of the nucleophile, resulting in a “loose” transition state structure.
Similarly, kinetic isotope effects are typically not observed for SN2 reactions of sulfur
anion nucleophiles.92
The reaction of CHBr•⎯ with (CH3)3CBr proceeds via an E2 reaction82 where
the displaced halide is the sole ionic product of the reaction, as shown in eq 6.11.
CH79Br•⎯ + (CH3)3CBr•

CH279Br + (CH3)2CCH2 + Br⎯(6.11)In this reaction, the isotope distribution of Br⎯ is the bromine atom naturalabundance, consistent with a pure E2 mechanism. Additionally, the measured
deuterium kinetic isotope effects are greater than one; similar effects have beenreported for E2 reactions in the literature.93,94 For CHCl•⎯, CHBr•⎯, and CF2•⎯
elimination occurs fairly efficiently while for CCl2•⎯ and CBrCl•⎯, no reaction occurs.
This is most likely because the CCl2•⎯ and CBrCl•⎯ anions are too weakly basic.

146

Comparison of O•⎯ and CH2 •⎯ with halogenated neutral reagents.
Mayhew and co-workers87 and Nibbering and co-workers74 have previously
investigated the reactions of O•⎯ with chloromethanes; both studies report theformation of Cl⎯ and Cl2•⎯. In light of the results presented here, these two products
may be formed from an analogous insertion-elimination mechanism, as shown in
Scheme 6.07 for the reaction of O•⎯ with CCl4.
Scheme 6.07
Cl
O + CCl4

Cl

O C
Cl

O

Cl

O

+ Cl2

C
Cl

Cl

C
Cl

+ Cl + ClClIn a similar manner to Scheme 6.06, the first step of the mechanism involves theformation of a σ–π complex, where the singly occupied π–orbital of O•⎯ overlaps with
the C–Cl σ–bond of the neutral species to transfer an electron. Neutral oxygen atomthen insertion into the Cl–C bond then occurs forming the [ClOCCl3] •⎯ intermediate,
which is unstable and eliminates Cl2•⎯ or Cl• and Cl⎯. Insertion reactions of oxygen
atoms into single bonds have previously been reported in the literature.95
The halogenated carbene anions studied here exclusively react withmethylhalides via SN2 substitution to displace a halide ion. The reactions of CH2•–and O•– also react with methyl halides via SN2 substitution, as shown for the reaction
with CH3Cl (6.12a and 6.13a) in addition to proton transfer (6.12b and 6.13b).70,74,96
CH2•⎯ + CH3Cl

O•⎯ + CH3Cl

CH3CH2• + Cl⎯(6.12a)CH3• + CH2Cl⎯(6.12b)CH3O• + Cl⎯(6.13a)OH• + CH2Cl⎯(6.13b)H2O + CHCl•⎯(6.13c)147

In addition, the reaction of O•⎯ forms CHCl•⎯ via an H2•+ abstraction (6.13c).74 This
unique reaction pathway, not exhibited by CH2•⎯ or the carbene anions studied here,reflects the high stability of the H2O product molecule.
6.4) Conclusions
The carbene radical anions studied here display diverse reactivity as
demonstrated by the reaction trends and the number of product ions formed. The
reactions of CXY•⎯ with CS2, COS, and CO2 proceed by two main pathways. The
first is carbanion attack on the neutral reactant carbon atom to displace the halide ion
while the second is carbanion attack on the neutral reactant sulfur/oxygen atom
resulting in sulfur/oxygen atom transfer. The reactions of O2 occur by carbanion
attack on the oxygen to yield O•⎯, the halide ion, and other minor product ions. The
reactions of CO solely produce the halide ion. The CF2•⎯ anion displays remarkably
different reactivity; these differences are discussed in terms of its lower electron
binding energy and the effect of the electronegative fluorine substituents.
The reactivity studies of CXY•⎯ with halogenated species show that thesecarbene anions can act as prototypical anions in that they undergo proton transfer,
substitution, and elimination reactions. In addition to these classic transformations,the product ion isotope distributions for the reaction of CHCl•⎯ with CHCl3 and CH2Cl2
provide convincing support for the occurrence of an insertion-elimination mechanism.
Neutral and cationic carbenes are known to insert into single bonds;1,2,97-99 however,
the results presented here are the first report of such gas-phase anion reactivity.
6.5) Acknowledgments
This work would not have been possible without the contributions of
Ms. Nicole Eyet, Prof. Veronica M. Bierbaum, and Prof. Charles H. DePuy.
Additionally, gratitude is expressed to Prof. W. Carl Lineberger and Prof. Tarek

148

Sammakia. Financial support is provided by AFOSR (FA9550-06-1-006) and NSF(CHE-0647088). The computational results are based upon work supported by the
NSF under the following NSF programs: Partnerships for Advanced Computational
Infrastructure, Distributed Terascale Facility (DTF) and Terascale Extensions:
Enhancements to the Extensible Terascale Facility.

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Jones, M.; Moss, R. A. Carbenes; John Wiley and Sons: New York,

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158

CHAPTER VII
SPECTROSCOPIC CHARACTERIZARATION OF THE
c-C4F8– AND SF6– ANIONS

7.1) Introduction
Both c-C4F8 and SF6 have industrial applications as reagents in etchingplasmas and as insulating gases for high voltage devices. In addition, both
molecules have long lifetimes in the upper atmosphere and are consideredgreenhouse gases whose atmospheric lifetimes are, in part, controlled by electron
attachment.1 These two perfluorinated molecules are of fundamental interest since
they have a propensity to nondissociatively attach low energy electrons, indicatingthat the corresponding anions have a very long lifetime with respect toautodetachment.2,3 One of the reasons for this slow autodetachment undoubtedly
results from the change in structure of the molecular framework upon anion
formation. For these reasons, both molecules have been the subject of many studies
that have focused on the characterization of the neutral and the formation andreactivity of the anion;4-10 however, very few spectroscopic studies have addressed
the structural aspects of the ions.
Various spectroscopic studies have shown that the geometry of neutral
c-C4F8 is bent with D2d symmetry and these studies have measured several
vibrational frequencies.11-18 High level CCSD(T)/CBS calculations were employed tomap the double well potential surface to the low-frequency ring-puckering motion; thetwo minima were found to be separated by 132 cm-1 barrier. Analysis of the c-C4F8electron density suggests that the puckering barrier arises principally from theσcc σcf* hyper-conjugative interactions that are more strongly stabilizing in thepuckered than in the planar form.18 Far less is known about the anion structure.

159

Recent ESR studies have shown that the anion structure is planar with D4hsymmetry19 and computational results predict that the added electron is delocalized
in a ‘p-like’ orbital extending over the entire molecule, strengthening the four C−C
bonds via π-bonding interactions, and weakening the eight C−F bonds via σ-antibonding interactions.19,20
As with c-C4F8, there is a large geometry change upon attachment of anelectron to SF6. This molecule has been the focus of intense study.21-32 The neutral
SF6 structure has been well characterized by several Raman and infrared studies.33
Only two reports, however, have addressed the structural aspects of the ion throughspectroscopy.34,35 An early study commented only on the lack of observed photodetachment in the near ultraviolet,34 far above the adiabatic threshold, while a matrix
isolation study identified a single weak vibrational transition near 600 cm-1.35 Morerecently, computational results predict that the Oh symmetry of SF6 is retained upon
attachment of an electron; however, the S−F bonds are elongated by about 0.2 Å.36
The present study37 investigates electron attachment to the closed shell
perfluorinated molecules c-C4F8 and SF6. Spectroscopic studies of the SF6− andc-C4F8− anions are reported in order to provide experimental benchmarks fortheoretical predictions of their structures and electron binding energies. The work
presented here was done in collaboration with Prof. Mark A. Johnson’s group at Yale
University and Thomas M. Miller and A. A. Viggiano at Hanscom Air Force ResearchLaboratory. The photoelectron spectra of both the c-C4F8– anion and the “cooled”
SF6– anion were collected at JILA. The photoelectron spectra of the SF6– anion was
also collected at Yale University. The photoelectron spectra of both the c-C4F8– andSF6– anions are compared to the calculated results of Borrelli,38,39 which provide
important insights into the observed spectroscopic character.

1607.2) Experimental Methods
The 351.1 nm photoelectron spectra of both the c-C4F8– and SF6– anions werecollected with a flowing afterglow-negative ion photoelectron spectrometer that was
described in Chapter 4. The target anions were prepared by flowing He buffer gas
seeded with a trace amount of SF6 or c-C4F8 gas through a microwave dischargesource. The SF6– anion was cooled by surrounding the flow tube with a liquid
nitrogen jacket; prior experience with this arrangement suggests that the resulting
ions are prepared in the 150–200 K range.40 Negative ions were gently extracted
from the source region and passed though a Wien filter for mass selection. The
mass selected ion beam was then crossed with a 351.1 nm (3.531 eV) line from a cw
argon-ion laser in an external build up cavity producing ~100 W of circulating power.
Photoelectrons that are ejected perpendicular to the plane of the ion-photon
interaction region are energy analyzed by a hemispherical kinetic energy analyzer
coupled to position sensitive detector. The absolute energy scale is calibrated withthe electron affinity (EA) of atomic oxygen41 in the measurements of thephotoelectron spectrum, and a linear energy scale compression factor (< 1%), which
is determined from the photoelectron spectrum of tungsten anion.42
Optimized geometries and harmonic vibrational frequencies (not scaled) for
the neutral molecules and corresponding anions are determined from density
functional theory (DFT) calculations (B3LYP/6-31+G(3df))43-45 using the Gaussian 03
suite of programs.46
7.3) Results and Discussion
7.3.1) Photoelectron Spectrum of the c-C4F8–Anion
Figure 7.01 shows the 351.1 nm photoelectron spectrum of the c-C4F8– anion.
In this spectrum, the c-C4F8– anion has a room temperature vibrational and rotational
distribution from collisions with He buffer gas (~ 0.5 torr, 3 –10 ms residence time)161

prior to leaving the source region.47 The arrow in Figure 7.01 indicates the value of
the EA(c-C4F8), which was determined by flowing afterglow Langmuir probe studies,
at 0.63 ± 0.05 eV;9 two other EA values have also been reported and will beelaborated on later.21,48 The absence of electron detachment at this energy indicatesa large geometry change upon photodetachment of the c-C4F8– anion. The observed
spectrum consists of an extremely long progression composed of over fifty resolved
peaks that are spaced by 355 cm-1 and extend almost 2 eV into the vibrational
manifold of the ground electronic state of c-C4F8. Based on the size of the moleculeand the large geometry change between the anion and neutral, one might expect tosee an extended Franck-Condon profile with virtually no resolved features.
However, what is so striking is that the spectrum appears to display a single modecharacter that is rather ‘diatomic-like’ and remains harmonic across the entire
spectrum.
It is known from electron diffraction and infrared studies that the carbon
framework of the neutral is bent with D2d symmetry,11-17 while recent ESR studies
have shown that the anion framework is planar with D4h symmetry.19 Optimized
structures (B3LYP/6-31+G(3df)) of the neutral and the anion are displayed in
Figure 7.02. Detachment of an electron elongates the C−C bonds, shortens the C−F
bonds, and increases the non-bonded F−F distance. This increase in symmetry forthe anion has been predicted by computations which also show that the negative
charge in the anion is a ‘p-like’ orbital that is delocalized over the entire molecule.19,20
This deformation corresponds to the ν5 ring-breathing mode, calculated to be 360
cm-1 at the MP2/TZVPP level of theory (Table 7.01). Thus, we expect this mode to
be active in the photoelectron spectrum, and based on a cursory evaluation of thespectrum, the long progression appears to correspond to excitation of this mode.

162

Photoelectron Counts

ν5

1.7

1.6

1.5

EA(c-C4F8)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Electron Binding Energy, (eV)Figure 7.01: The 351.1 nm photodetachment spectrum of c-C4F8− taken at the magic
angle. The EA(c-C4F8) determined by Miller9 is indicated by a vertical arrow.

163

Figure 7.02: Calculated structures (B3LYP/6-31+G(3df)) of c-C4F8 neutral (top) andanion (bottom). The neutral structure is bent out of plane by 8.0°; however, thefigure is exaggerated for clarity. For c-C4F8, R(CC) = 1.579 Å, R(CF) = 1.337Å, nonbonded R(FF) = 2.189 Å, ∠(FCF) = 109.0°, ∠(CCC) = 89.7°, D(CCCC) = 8.0°. For cC4F8−, R(CC) = 1.505 Å, R(CF) = 1.406 Å, non-bonded R(FF) = 2.170 Å, ∠(FCF) =100.9°, ∠(CCC) = 90.0°, D(CCCC) = 0.0°.

164

Table 7.01: Theoretical frequencies (cm-1), calculated using the MP2/TZVPP level oftheory, for both c-C4F8 anion and neutral along with the experimental values forneutral c-C4F8.
c-C4F8–Vibrational Modec-C4F8
Theory39
Exp.
Theory39
ν1

a1

ν2

a1

ν3

a1

ν4

a1

1448

1433a

a1g

1499

1315

1413

a

b2u

963

715

700

a

a1g

668

607

a

a1g

489

359

a

b2u

391

618

ν5

a1

370

ν6

a1

58

b2u

47

ν7

a2

893

a2g

889

ν8

a2

220

b1u

216

b2g

1203

b2g

253

ν9

b1

1047

ν10

b1

283

ν11

b1

251

ν12

b2

ν13

b2

ν14

b2

ν15

b2

ν16

b2

ν17

e

ν18

e

ν19

e

ν20

e

ν21

e

ν22
ν23
a

1010

a

a1u

206

1292

b

a2u

872

1264

1240

b

b1g

1042

669

659

b

b1g

631

348

b

b1g

358

195

b

a2u

190

1343

b

eg

1249

1246

1223

b

eu

847

987

964

b

eu

835

569

b

eu

559

437

b

eg

425

283

b

eg

271

eu

200

1323

360
197
1380

581
444

e

289

e

190
b

From reference 17 From reference 18

165

This simple explanation, however, requires this mode to behave as a harmonic
oscillator even at very high levels of vibrational excitation (up to fifty quanta)
indicating a weak coupling of this mode to the rather large density of backgroundstates. Furthermore, a vibrational progression of fifty quanta would require a large,
physical unrealistic displacement from the equilibrium geometry. Thus, a single
mode explanation would be quite unusual and, therefore, this issue requires further
investigation.
Borrelli and Peluso39 have analyzed the photodetachment of the c-C4F8– anionin terms of the Franck-Condon factors between the ground state of the anion andneutral species. Their results show that excitation of only the ν5 mode results in amuch narrower spectral profile (~5,000 cm-1) than experimentally observed, with the
maximum intensity corresponding to the 504 transition. Instead, consideration of boththe ν5 and the ν1 ring-breathing modes results in a spectral profile in qualitative
agreement with the experiment. This simulation (shown in Figure 7.03a) is
composed of ~60 peaks that are spaced by approximately 370 cm-1, resulting in an
overall bandwidth of ~23,000 cm-1. The spectrum is composed of a series ofcombination band progressions of the type 10m 50n , where n and m are integers.
Because the ratio of these two frequencies (ω1 ≈ 4ω5) is very close to an integer(Table 7.01), transitions of the type 10m 50n and 10m −150n + 4 are not resolvable from one
another; because of this accidental degeneracy all the peaks in the spectrum areequally spaced. Figure 7.03b displays the room temperature Franck-Condon
simulation where all modes have been considered. The excellent quality of thesimulation is readily apparent and the underlying continuum in the spectrum is due tothe low-frequency ring-puckering mode (ν6), which in the neutral species exhibits adouble well potential energy surface. Minor contributions from the other totally

166

a)

b)

Figure 7.03: Calculated photoelectron spectra of the c-C4F8− anion reproduced fromreference 39, with permission copyright 2008, American Institute of Physics, wherea) only the ν5 and ν1 modes were considered and b) where all modes wereconsidered. The Franck-Condon simulations are based on the equilibriumgeometries and frequencies for the c-C4F8− anion and neutral calculated at theMP2/TZVPP level of theory. For both spectra, the raw Franck-Condon factors wereconvoluted with a 25 cm-1 fwhm Gaussian function. Note that in this figure theenergy axis is reversed from Figure 7.01.

167

symmetric modes contribute to the underlying shape and to a complex fine structure
shown in the inset of Figure 7.03b; these peaks are not resolvable within the
instrument resolution.
As previously mentioned, other EA values have been reported in theliterature.21,48 Although the photoelectron spectrum of c-C4F8– does not give a direct
determination of this value, it does provide an upper bound of ~0.75 eV. Hiraoka andcoworkers measured EA(c-C4F8) relative to that of SF6, using a pulsed electron beamhigh pressure mass spectrometer.48 They reported a value of 1.05 eV, which isincompatible with the photoelectron spectrum reported here. Hiraoka interpreted his
observations to suggest the presence of an excited c-C4F8– isomer, which would form
at elevated temperatures (above 350 K) and undergo electron detachment. The EAof this other isomer was reported to be 0.5 eV, based upon electron transfer
equilibrium with O2. The coexistence of a second isomer, however, is not supportedby photoelectron angular distribution measurements, which give a smooth minor
progression of the anisotropy parameter (β) across the spectrum, or by FranckCondon calculations, which effectively reproduce the observed spectrum by only
considering a single isomer of the c-C4F8– anion. While these results do not provide
a definitive EA determination, they do allow for a definitive choice between the twomost reliable prior determinations. Thus we conclude that an EA(c-C4F8) of 0.63 eV9
is both consistent with the data presented here, and the most reliable value
available. A detailed discussion of all of the earlier determinations of EA(c-C4F8) has
been recently given by Miller et al.9
7.3.2) Photoelectron Spectrum of the SF6– Anion
Figure 7.04 presents the photoelectron spectrum of the SF6– ion collectedunder cold flow tube conditions where the anion temperature is estimated to be in the150-200 K temperature range.40 The most striking aspect of this spectrum is that,168

despite the fact that the adiabatic electron affinity is reported to be 1.05 ± 0.1 eV,49
significant photoelectron activity does not begin until almost 2 eV. The 3 eV locationof the band maximum (vertical electron detachment energy, VDE), however, isqualitatively consistent with calculations (CCSD(T)/6-311+G(2df)) of the anion
structure,36 which predict that the anion retains the Oh symmetry of the neutral andthat S−F bonds are elongated by ~0.2 Å. The spectrum is composed of two different
progressions spaced by a constant frequency as indicated by solid lines and dashedlines in the inset of Figure 7.04. This spectrum is compared to the photoelectron
spectrum of the bare SF6– anion and SF6– tagged with one argon atom collected using
a double focusing tandem time of flight pulsed spectrometer at Yale University50 in
Figure 7.05. In these experiments the bare SF6– anion was generated by entraining
trace SF6 in an expansion of N2 at a stagnation pressure of 5 atm. For both of thebare SF6 spectra (traces a and b), discernable vibrational fine structure appears withsimilar depth of modulation and location of the maxima, thus reflecting a robust
property of the ion that is independent of the method of preparation. The spectrumof SF6–·Ar (trace c) displays significantly sharper peaks than either of the bare ion
spectra (traces a and b), and the pattern appears uniformly shifted by ~52 meV
towards higher binding energy. The tagged spectrum clearly displays the second
series of interloper peaks with similar peak intensities as those evident in the bareion. Thus, the Ar atom does not seem to significantly perturb the structure, but rather
further quenches the SF6– subsystem into its ground vibrational state.
The main progression is remarkably harmonic and has a constant spacing of
750 ± 20 cm-1, which is assigned to the symmetric a1g S−F stretching mode (ν1) inneutral SF6. Note that at lower levels of vibrational excitation accessible by directabsorption from the ground state neutral, high resolution spectroscopic analysis

169

Photoelectron Counts

3.0

2.9

3.0
2.5
Electron Binding Energy, (eV)

2.8

2.7

2.0

Figure 7.04: The 351.1 nm photodetachment spectrum of SF6– taken under cold flow
tube conditions at the magic angle. A regular harmonic progression is present (solidline) with spacing corresponding to ν1 frequency of 750 ± 20 cm-1 of the neutral
molecule. A second progression is present (dashed lines) with the same character
spacing; however, this second progression is displaced by 440 ± 10 cm-1 relative tothe main features.

170

Photoelectron Counts

a)

b)

c)
2.5
3.0
Electron Binding Energy, (eV)2.0

Figure 7.05: Comparison of the (a) 351.1 nm photodetachment spectrum of SF6–
reproduced from Figure 7.04, and (b and c) the SF6– photoelectron spectrum taken
by Mark A. Johnson’s group at Yale University using a double focusing tandem timeof flight pulsed spectrometer, where SF6– was generated by entraining trace SF6 inan expansion of N2 at a stagnation pressure of 5 atm. Trace b is a spectrum of thebare SF6– ion while trace c is a spectrum of SF6–·Ar.

171

indicates that this mode indeed exhibits only rather small coupling constants in theanharmonic Hamiltonian. Inspection of the calculated spectrum suggests that thepeak of the vibrational progression occurs at v~27. This is consistent with the slightly
smaller (by 26 cm-1) observed spacing relative to the fundamental transition since
X11 = -0.896 cm-1.33
Assignment of the second active mode in the spectrum is less obvious.
Since the spacing of the interloper peaks remains constant throughout the spectrum,
it is unlikely that two different modes are activated upon photodetachment to give twodifferent Gaussian-like progressions that originate from the same initial state of theanion. Additionally, it is unlikely that the interloper peaks are due to contributions
from residual vibrational hot bands. As shown in Table 7.02, the calculated SF6–
frequencies vary from 237 to 722 cm-1, implying that there is plausible excitation of
ν4, ν5, and ν6 in the thermal ensemble; however, none of these frequencies matches
the observed energy gap. Furthermore, as discussed above, the consistency of thespectra in different ion sources indicates that the second mode does not arise fromexcited vibrational levels of the anion ground electronic state. This second mode issystematically displaced by about 440 ± 10 cm-1 relative to the main features. Thisfrequency does not correspond to any of the fundamental frequencies in neutral SF6(Table 7.02); thus, this second progression is not likely due to a combination bandassignment based on one quantum of another vibrational mode. One further
possibility is that the second progression is due to an overtone/combination band.
The closest match to the observed splitting is 2ν4 (~1220 cm-1), which is a non-totallysymmetric, IR active mode; therefore, we would only expect even quanta to be

172

Table 7.02: Theoretical frequencies (cm-1), calculated using the MBPT(2) level oftheory for SF6 anion and neutral and the experimental values for SF6 neutral.

Theory36

Exp.51SF6–Theory36

Vibrational Mode

SF6

ν1

a1g

779

775

626

ν2

tg

655

643

447

ν3

t1u

965

948

722

ν4

t1u

611

615

306

ν5

t2g

519

524

336

ν6

t2u

346

348

237

173

Figure 7.06: Calculated photoelectron spectra of the SF6− anion reproduced fromreference 38 with permission copyright 2008, Elsevier. In this simulation theequilibrium geometries of the SF6− anion and neutral are determined from
MP2/6-311G(2df) calculations. The raw Franck-Condon factors were convoluted
with a 70 cm-1 Lorentzian function. Note that in this figure the energy axis is reversed
from Figures 7.04 and 7.05.

174

populated in photodetachment. Based on this assignment, the ν4 frequency would
be 660 ± 10 cm-1.
This overtone/combination band assignment is confirmed by the FranckCondon calculation of Borrelli (shown in Figure 7.06).38 As expected, the calculated
spectrum displays an extended 1n0 progression, where n is an integer, with the
maximum intensity corresponding to 1028 , in excellent agreement with experimental
predictions. The interloper peaks are due to a progression of 1n0 402 . Thus for any
given 10x peak (where n=x), the interloper peak at 440 cm-1 higher energy
corresponds a 10x −1402 transition. The 0 Æ 2 transition of the ν4 mode derives its
intensity from the large change in vibrational frequencies upon photo-detachment.
As revealed by Borrelli’s calculations, the t1u ν4 mode is triply degenerate and mixes
with the t1u ν3 mode; this mixing is observed in the Duschinsky matrix.
7.4) Conclusions
The photoelectron spectra of SF6– and c-C4F8– both display extended
progressions in the ground electronic state manifold of the corresponding neutral. In
both cases, there is no observable signal near the measured EA, which is consistent
with a large change in geometry between the anionic and neutral species. The
observed spectral features in c-C4F8– are primarily due to combination bands of twosymmetric ring-breathing modes, ν1 and ν5. Since the frequency ratio of these twomodes is very close to an integer value, a very simple progression emerges that
resembles a single-mode harmonic progression. The photoelectron spectrum ofSF6− is dominated by a long progression in the ν1 S−F stretching mode, with anenvelope consistent with theoretical predictions that the anion preserves the Oh
symmetry of the neutral, but has a longer S−F bond length. This main progression

175

occurs with an unexpectedly strong ν1 and ν4 overtone/combination band
progression.
7.5) Acknowledgements
This work would not have been possible without the contributions from Mr.
Scott W. Wren, Dr. Adam J. Gianola, and Prof. W. Carl Lineberger at JILA, Mr.
Joseph C. Bopp, Mr. Joseph R. Roscioli, and Prof. Mark A. Johnson at Yale
University, and Dr. Thomas M. Miller and Dr. A. A. Viggiano at Hanscom Air Force
Research Laboratory. Furthermore, I wish to acknowledge the work of Raffaelle
Borrelli and Andrea Peluso, whose Franck-Condon calculations revealed flaws in our
original interpretation of these data. This work was supported by the Air Force Officeof Scientific Research (FA9550-06-1-006).

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181

CHAPTER VIII
THERMOCHEMICAL STUDIES OF N-METHYLPYRAZOLEAND N-METHYLIMIDAZOLE

8.1) Introduction
Azoles are nitrogen-containing five-membered ring heteroaromaticcompounds whose structures are analogous to cyclopentadiene, C5H6, where an N
atom has replaced one or more C-H groups. These compounds constitute animportant class of heterocycles whose structures appear as fundamental units inbiomolecules,1 pharmaceuticals,2,3 ionic liquids,4,5 dyes,6,7 explosives,8 and fuels.9-11
Sound knowledge of the thermochemistry, especially the bond dissociation energies(BDEs), of these compounds is important to fields such as combustion chemistry12
and transition metal coordination chemistry.13-16
The vast majority of information in the literature about azoles has focused on
pyrrole, C4NH5. It is postulated that the thermal decomposition of pyrrole, along withother N-heterocycles that are contained in coal, leads to the formation of NOx
species in the atmosphere.17,18 The thermal decomposition of pyrrole has been
investigated by two groups using shock tube pyrolysis.19,20 The initial decomposition
step proceeds by a hydrogen-atom transfer from the nitrogen to the adjacent carbon(C2 carbon) to form pyrrolenine, followed by the cleavage of the C2-N bond to
produce an acyclic structure.19,21 This acyclic structure further decomposes by C-C
bond cleavage. In contrast, there is evidence that thermal decomposition of pyridine,
the six-membered heterocycle that contains one nitrogen atom, is initiated by thecleavage of the C2-H bond.22 The difference between these two initiation steps, inpart, may be attributed to the difference in C-H bond strengths of the five- and sixmembered heterocycles. The C2-H and N-H bond strengths of pyrrole were182

measured by Lee et al. 23 using photofragment translational spectroscopy to be
112.5 ± 1 kcal mol-1 and 88 ± 2 kcal mol-1, respectively. Later experiments,24,25
however, have not observed a C-H bond dissociation process in the photochemistryof pyrrole and find the N-H BDE to be higher, 93.92 ± 0.11 kcal mol-1. In contrast,
the corresponding C2-H BDE of pyridine is reported to be much lower at 105 kcal
mol-1; for the C-H bonds that do not have an adjacent nitrogen atom (C3 and C4
carbons), the BDEs increase to 112 kcal mol-1.26 In contrast to this extensive
knowledge of these mononitrogen heterocycles, much less is known about thenitrogen heterocycles that contain additional nitrogen atoms in the ring.
Our research program has utilized negative ion spectroscopy along with gasphase ion chemistry to study several azoles.27-31 Negative ion spectroscopy provides
thermodynamic information such as electron affinities and term energies, allows forthe measurement of vibrational frequencies, and gives insight into the molecular
structure of both the anion and radical species.32 Gas-phase ion chemistry studies
include the measurement of the forward and reverse proton transfer reaction rate
constants in order to determine the gas-phase acidity relative to a reference acid.
These techniques are complementary since the electron affinity (EA) of the neutral
radical can be combined with the gas-phase acidity (∆aG298) to determine the bonddissociation energy (BDE) using a negative ion thermochemical cycle.33,34
Using these techniques we have previously investigated the thermodynamic
properties of pyrazole (I)29,30 and imidazole (II).28 In that work, the 1-azolide anion
was formed by deprotonation of the azole by hydroxide at the most acidic site, theN-H bond. Photoelectron spectra of the 1-azolide anions were reported along with a
discussion of the electronic structure of the corresponding radical. The gas-phase
acidity of the N-H bond was measured using proton transfer equilibrium183

measurements and the N-H BDE was obtained. In addition to deprotonation at thenitrogen, deprotonation also occurred at the next acidic site, the carbon adjacent to
the N-H group (C5 carbon), allowing for the assignment of the electron affinity of the
C5 radical. In both the pyrazolide and the imidazolide anion spectra, C5deprotonation was only observed when the azolide anion was prepared by protontransfer to hydroxide. When the azole anion was prepared by proton transfer to O•⎯,
only the 1-azolide anion spectra were seen. The acidity (∆aG298) of the C5-H bondwas bracketed between that of water and the hydroxy radical at 380 ± 4 kcal mol-1.
The C5-H BDE was then determined to be 121 ± 4 kcal mol-1 for pyrazole and 119 ±4 kcal mol-1 for imidazole. The large error bars reflect the acidity gap between waterand hydroxy radical. However, even when suitable reference acids are available,
quantitative determination of the gas-phase acidities and C-H bond dissociationenergies is difficult when there are multiple acidic sites in the molecule.
H
H

N1
N2

5C

H

N1
5C

C3

4C
H

H
C2

4C
H

(I)

CH3

H
H

N1
N2

5C

H
(II)

H

N1
C2

4C
H

H

5C

C3

4C

N3

H

CH3

N3

H

(III)

(IV)

Measurements of gas-phase proton transfer reactions generally determine
the gas-phase acidity (and hence the BDE) of only the most acidic site of ahydrocarbon. This limitation, in part, is due to the inherent difficulty in independently
preparing the more basic anion from neutral precursors. One commonly usedmethod to probe the acidity of the less acidic site is fluorodesilylation of substituted
trimethylsilanes.35-40 This method allows for the preparation of regiospecificcarbanions whose acidity can then be bracketed between two reference acids.

184

A second technique that allows for acidity measurements on molecules withmultiple acidic sites uses a dual-cell Fourier transform mass spectrometer.41,42 In
this method a mixture of isomeric anions is formed in the source region. To prevent
catalyzed isomerization of the more basic anion to the less basic species, the ion
mixture is extracted into a second chamber, which is free of neutral precursors.
Using this technique, a lower bound acidity can be found for the less acidic proton.
This method has recently been applied to determine the acidities of the N1 and N3
sites of uracil41 and the N9 and N10 sites of adenine and adenine derivatives.42 This
technique, however, requires a sufficient difference in the acidity of the two sites sothat bracketing experiments can be used; moreover, the timescale for ion extraction
must be shorter than for ion isomerization.
The principal motivation of this work is to improve the precision of the C5-H
BDE of pyrazole and imidazole through the investigation of the corresponding
N-methylated species (III and IV). N-methylation of pyrazole and imidazole blocks
the nitrogen site and makes the C5 position the most acidic site, without significantly
changing the C-H bond energetics (as discussed below). Here we report thephotoelectron spectra of N-methylpyrazolide anion and N-methylimidazolide anion,
as well as electronic structure calculations, which aid in the interpretation andassignment of the spectra. The gas-phase acidities of the C5 positions in N-methylpyrazole and N-methyimidazole were measured, and these measurements are used
to determine the C5-H BDE.
8.2) Experimental Methods
8.2.1) Negative Ion Photoelectron Spectroscopy
The 351.1 nm photoelectron spectra of the N-methylpyrazolide anion and the
N-methylimidazolide anion were obtained with a flowing afterglow-negative ion
photoelectron spectrometer that was described in Chapter 4. The N-methyl185

pyrazolide and N-methylimidazolide anions are prepared by proton abstraction fromN-methylpyrazole and N-methylimidazole, respectively, using hydroxide (OH⎯) as abase, downstream of a microwave discharge source under room temperature andliquid nitrogen conditions. Negative ions were gently extracted from the source
region and passed though a Wien filter for mass selection. The mass selected ion
beam was then crossed with a 351.1 nm (3.531 eV) line from a cw argon-ion laser inan external build up cavity producing ~100 W of circulating power. Photoelectrons
that are ejected perpendicular to the plane of the ion-photon interaction region areenergy analyzed by a hemispherical kinetic energy analyzer coupled to a position
sensitive detector. The absolute energy scale is calibrated with the electron affinity(EA) of atomic oxygen43 in the measurements of the photoelectron spectra, and alinear energy scale compression factor (< 1%), which is measured from thephotoelectron spectra of reference ions S•⎯ ,44 I⎯, 45 and O•⎯ using the EAs of thecorresponding atoms.
Optimized geometries and harmonic vibrational frequencies (not scaled) for
the neutral parent, anion, and radical are determined from density functional theory(DFT) calculations (B3LYP/6-311++G(d,p))46-48 using the Gaussian 03 suite ofprograms.49 Simulated photoelectron spectra were obtained by calculating theFranck-Condon (FC) factors at a vibrational temperature of 300 K with the PESCAL
program, 50 using calculated geometries, normal modes, and electron binding
energies for the anion and radical. The methyl torsional mode is excluded from thesimulation because of the incompatibility with the harmonic assumption used in thesimulation.51

186

8.2.2) Flowing Afterglow-Selected Ion Flow Tube (FA-SIFT) Measurements
Gas-phase acidity (∆aG298) measurements of N-methylpyrazole andN-methylimidazole were obtained using a tandem flowing afterglow-selected ion flow
tube instrument, FA-SIFT, described in Chapter 3. The acidity of N-methylpyrazoleis determined relative to methanol [∆aG298(CH3OH)= 375.5 ± 0.6 kcal mol-1],52 shownin reaction 8.01, while the acidity of N-methylimidazole was determined relative towater [∆aG298(H2O)= 383.68 ± 0.02 kcal mol-1]52, shown in reaction 8.02. The
photoelectron spectra collected for both isomers suggest that the C5 position is themost acidic site and therefore the acidity measurements represent the acidity of the
C5-H bond (see discussion below).
CH3

CH3
N
C

N
C

N
C

kr

H

C

kf

+ CH3OH

C

H

N

H

H
CH3

N
C
C
H

N

(8.01)

H

CH3
C

C

+ CH3O

H
+ H2O

kf

H

N
C

C

H
+ HO

C

kr

(8.02)NH

The reactant ions, N-methyl-5-pyrazolide, N-methyl-5-imidazolide and CH3O⎯,
were formed by proton abstraction from the corresponding neutrals by OH⎯, mass
selection, and injection into the reaction flow tube with minimal energy to reducefragmentation from collision-induced dissociation. For all of the reactions studied,with the exception of the reverse reaction 8.02, neutral reagent flows were
determined by measuring a change in pressure versus time in a calibrated volume;
reagents were added to the flow tube through a manifold of inlets. This technique187

was not used for the reaction of hydroxide with N-methylimidazole due to its low
volatility. Instead a regulated but unknown flow of N-methylimidazole was introduced
into the reaction flow tube at one position. A relative reaction rate was measured byalternately injecting NH2⎯ and OH⎯ for reaction with the constant reactant flow. The
neutral reagent concentration and the reaction rate constant were extracted from therelative rate measurement27,28 by assuming that the highly exothermic proton transfer
reaction of NH2⎯ with N-methylimidazole proceeds at 90 ±10% of the calculated
collision rate53 (kcol = 5.7 ⋅10-9 cm3 s-1).
The forward direction of reaction 8.01 and the reverse direction of reaction
8.02 are exothermic and proton transfer is the only observed pathway. For thecorresponding endothermic reactions, adduct formation is observed in addition to
proton transfer. The rate constant for proton transfer is determined from the overall
rate constant and the proton transfer branching ratio. An experimental complication
in measuring the proton transfer branching ratio is the occurrence of a secondary
switching reaction between the adduct and water.54 This reaction does not affect theoverall rate constant but must be considered in determining the proton transfer
branching ratio. This ratio was measured by monitoring the branching ratio atvarious reaction times and extrapolating to zero time, where there is no depletion of
the adduct due to secondary reaction. Efforts were made to minimize mass
discrimination, however, it was necessary to estimate the relative detection
sensitivities when calculating the proton transfer branching ratio. The relative
detection sensitivity was estimated by examining a series of exothermic ion-molecule
reactions where only one ionic product was formed and the reactant and productions were similar in mass to the ions of interest.

188

The absolute uncertainty of each rate constant measurement is generally±20%, but ±50% for reactions where branching ratios must be evaluated. The
absolute error in the gas-phase acidity is smaller because some systematic errors(pressure, temperature, He flow rate, etc.) cancel in the rate ratio. Collision rate
constants were calculated using parametrized trajectory collision rate theory.53
8.3) Results and Discussion
8.3.1) Computational Results
The B3LYP/6-311++G(d,p) optimized geometries of N-methylpyrazole,N-methyl-5-pyrazolide anion, N-methyl-5-pyrazolyl radical, N-methylimidazole,N-methyl-5-imidazolide anion, and N-methyl-5-imidazolyl radical along with theharmonic vibrational frequencies for the above anions and radicals are presented inTables 8.01-8.04. Examination of the DFT optimized geometries shows that, for both
N-methyl-5-pyrazolide anion and N-methyl-5-imidazolide anion, there are significant
geometry differences between the anion and the 2A′ state of the radical. Similar ring
distortions are predicted for both isomers; the most dramatic changes are seen in the
shortening of the C4-C5 and N1-C5 bonds and in the decrease of the N1-C5-C4
bond angle. It is therefore expected that vibrational modes, which contain significant
ring distortion along these coordinates, will be activated upon photodetachment.
8.3.2) Photoelectron Spectra of N-methyl-5-pyrazolide Anion
The 351.1 nm magic angle photoelectron spectrum of the N-methylpyrazolide
anion, generated from the reaction of OH⎯ with N-methylpyrazole, is shown in
Figure 8.01a. Electronic structure calculations were used to evaluate the acidity ofN-methylpyrazole; these results are summarized in Table 8.05. DFT calculations
predict that the C5 carbon is the most acidic site and therefore it is most likely that
deprotonation by OH⎯ will occur at this site. Peak a is assigned as the electronic

189

Table 8.01: B3LYP/6-311++G(d,p) Optimized Geometries for N-methylpyrazole,N-methyl-5-pyrazolide and N-methyl-5-pyrazolyl.
H'

H
C6

H

N1

H

C5
C4
H

N2
C3
H

N-methylpyrazole

N-methyl-5pyrazolide

N-methyl-5-pyrazolyl( X 1A′)( X 2A′)( X 1A′)
1.348
1.380
1.362
N1−N2
1.332
1.335
1.335
N2−C3
1.410
1.412
1.413
C3−C4
1.382
1.416
1.377
C4−C5
1.358
1.374
1.340
C5−N1
1.451
1.442
1.451
N1−C6
1.090
1.089
1.089
C6−H’
1.091
1.098
1.091
C6−H
C5-H
1.078
1.080
1.08591.0803C3−H
1.078
1.08351.0759
C4−H
105.0
102.4
105.1
∠N1−N2−C3
111.7
111.5
112.6
∠N2−C3−C4
104.3
108.3
101.9
∠C3−C4−C5
106.8
100.7
110.5
∠C4−C5−N1
112.2
117.1
109.9
∠C5−N1−N2
119.7
125.8
129.4
∠C5−N1−C6
108.8
107.5
108.4
∠N1−C6−H’
121.6
111.2
109.9
∠N1−C6−H
121.6∠N1−C5−H
119.6
118.9
119.2
∠N2−C3−H
127.3
125.6
128.6
∠C5−C4−H
Bond lengths are in Angstroms and bond angles are in degrees.

( A 2A′′)
1.450
1.313
1.418
1.490
1.318
1.453
1.087
1.092
1.081
1.081
103.6
110.8
108.3
100.3
116.9
128.0
107.6
109.9
120.0
124.8

190

Table 8.02: B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of N-methyl-5pyrazolide and N-methyl-5-pyrazolyl.
symmetryN-methyl-5N-methyl-5N-methyl-5ν
pyrazolide
pyrazolyl
pyrazolyl( X 1A′)( X 2A′)( A 2A′′)
1
A′
3176
3276
3238
2
3145
3227
3223
3
3121
3145
3166
4
2976
3050
3045
5
1504
1530
1518
6
1464
1508
1476
7
1420
1470
1439
8
1380
1444
1417
9
1368
1377
1321
10
1249
1299
1208
11
1232
1257
1194
12
1153
1205
1084
13
1062
1081
1008
14
999
1020
979
15
935
964
916
16
911
899
680
17
707
685
553
18
388
360
377
19
A′′
3007
3116
3106
20
1477
1478
1481
21
1136
1145
1156
22
836
877
898
23
749
754
759
24
654
659
671
25
615
523
499
26
222
199
233
27
81
34
145

191

Table 8.03: B3LYP/6-311++G(d,p) Optimized Geometries for N-methylimidazole,N-methyl-5-imidazolide and N-methyl-5-imidazolyl.
H

H'

C6
H
N1

H

C5
C4
H

C2
N3

H

N-methylimidazole

N-methyl-5imidazolide

N-methyl-5-imidazolyl( X 1A′) (TS)( X 2A′)( X 1A′)
1.367
1.378
1.368
N1−C2
1.314
1.316
1.318
C2−N3
1.375
1.395
1.382
N3−C4
1.372
1.398
1.365
C4−C5
1.380
1.402
1.370
C5−N1
1.454
1.444
1.455
N1−C6
1.090
1.097
1.090
C6−H’
1.092
1.095
1.092
C6−H
1.078
C5−H
1.080
1.084
1.080
C2−H
1.079
1.084
1.076
C4−H
112.3
111.0
111.9
∠N1−C2−N3
105.2
103.6
106.9
∠C2−N3−C4
110.5
114.8
107.3
∠N3−C4−C5
105.7
99.6
109.1
∠C4−C5−N1
106.4
110.9
104.9
∠C5−N1−C2
126.7
124.5
127.0
∠C5−N1−C6
108.9
110.9
108.7
∠N1−C6−H’
110.7
109.9
110.5
∠N1−C6−H
121.8
123.1
122.6
∠N1−C2−H
122.0∠N1−C5−H
128.0
126.3
130.1
∠C5−C4−H
Bond lengths are in Angstroms and bond angles are in degrees.

( A 2A′′)
1.390
1.330
1.345
1.487
1.362
1.461
1.087
1.093
1.083
1.082
112.1
103.4
114.5
98.9
111.1
123.9
107.3
110.6
123.2
124.8

192

Table 8.04: B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of N-methyl-5imidazolide and N-methyl-5-imidazolyl.
symmetryN-methyl-5N-methyl-5N-methyl-5ν
imidazolide
imidazolyl
imidazolyl( X 1A′)( X 2A′)( A 2A′′)
1
A′
3166
3273
3218
2
3159
3233
3212
3
3051
3134
3160
4
2991
3045
3039
5
1515
1519
1511
6
1501
1508
1490
7
1434
1473
1441
8
1404
1436
1373
9
1331
1365
1279
10
1272
1348
1264
11
1222
1261
1182
12
1188
1239
1169
13
1095
1101
1068
14
1025
1067
1015
15
971
1028
974
16
915
865
823
17
687
672
673
18
366
344
365
19
A′′
3041
3108
3100
20
1466
1485
1499
21
1133
1144
1225
22
788
811
1124
23
679
780
831
24
673
663
706
25
616
488
550
26
215
201
237
27
57i
60
80

193

Table 8.05: B3LYP/6-311++G(d,p) ∆aG298 and BDE of N-methylpyrazole andN-methylimidazole and the EA of the corresponding radicals.
D0
EA∆aG298(kcal mol-1)(eV)(kcal mol-1)
C6H3

H

N-methylpyrazole

N1
N2

5C
4C

C3

H

H

N1
5C
4C
H

C2
N3

394.3

1.254

115.5

C4-H

390.9

1.544

118.8

C5-H

377.6

2.045

117.5

C6-H

384.0

0.635

90.9

N-methylimidazole

C6H3
H

C3-H

H

C2-H

381.7

1.751

114.8

C4-H

397.9

1.058

115.1

C5-H

380.8

1.965

118.1

C6-H

381.0

0.785

91.4

194

a)
d
c

a

Photoelectron Counts

e
m l
b

b)

CH3
N
C
N
H

C
2

C

2

A'

H

A"

3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8
Electron Binding Energy, (eV)Figure 8.01: a) The 351.1 nm magic-angle photoelectron spectrum of the N-methyl5-pyrazolide anion. b) Simulations of the spectrum based on optimized geometries
and normal modes from B3LYP/6-311++G(d,p) calculations. The sticks (blue) are
the raw Franck-Condon factors and the solid line is the 10 meV fwhm Gaussianconvolutions for transitions from X 1A′ N-methyl-5-pyrazolide anion to X 2A′ andA 2A′′ N-methyl-5-pyrazolyl radical. In the simulated spectra the positions andintensities were set to match those observed in the experimental spectrum.

195

band origin at 2.054 ± 0.006 eV, which corresponds to the EA of the N-methyl-5pyrazolyl radical. The peaks observed at higher eBE correspond to excited
vibrational levels of the N-methyl-5-pyrazolyl radical. The photoelectron angular
distribution was measured and the anisotropy parameter (β ) is ~0.5 at the origin but
gradually decreases to ~0.1 at 2.8 eV. Beyond 2.8 eV, β rapidly decreases and is-0.4 for peaks l and m. DFT calculations find minima with Cs symmetry for both theanion and radical. The in-plane C-H bond of the methyl group is trans to the N1-N2
bond for both states. The electronic ground state of the N-methyl-5-pyrazolide anion
is predicted to be X 1A′, while that of the N-methyl-5-pyrazolyl radical is predicted to
be X 2A′. The calculated EA of the N-methyl-5-pyrazolyl radical is 2.045 eV, in good
agreement with the assignment of the origin above. DFT calculations predict that
there is an excited state, A 2A′′, with a term energy (Te) of 0.768 eV.

~

FC factors were computed for the transition from the X 1A′ state of the

N-methyl-5-pyrazolide anion to both the X 2A′ and A 2A′′ states of the N-methyl-5pyrazolyl radical. The stick spectra and the spectrum resulting from convoluting the
stick spectrum with a 10 meV fwhm Gaussian function, which represents the
instrument resolution, are shown in Figure 8.01b. The binding energies for bothradical states have been slightly shifted (< 40 meV) and the intensities normalized tomatch the experimental spectrum. The simulated spectrum is in excellent agreementwith the experimental spectrum, and it confirms that the spectrum is solely due to the
C5 deprotonated anion.
Using the FC simulation, several vibrations of the X 2A′ state of the N-methyl5-pyrazolyl radical can be assigned. The low intensity peak b is the fundamental of
an in-plane methyl rocking with a frequency of 345 ± 45 cm-1. Peaks c and e are thefundamental and first overtone of an in-plane a′ N2-N1-C5 bend with a frequency of196680 ± 35 cm-1. The three in-plane ring distortion modes (ring bending, N1-N2
stretch, and N1-C5 stretch) contribute to peak d and cannot be resolved within our
instrumental resolution. A large number of combination bands of these modes
contribute to the spectral features at higher eBE. The atomic displacements forthese normal modes, as well as the positions of all the peaks observed in the experimental spectrum, are provided in Figure 8.02. The simulation tends to underestimate the intensity toward the higher eBE, which may be a result of neglecting themethyl torsion levels (see the above section). The EA of the N-methyl-5-pyrazolyl
radical is very close to that of the 5-pyrazolyl radical, 2.104 ± 0.005 eV.29 Introductionof the methyl group at the N1 position leads to more complication and congestion in
the vibronic features, compared to the rather simple, extensive vibrational
progression for a ring bending mode observed in the 5-pyrazolide anion spectrum.29
The sharp change in β at 2.8 eV is consistent with the presence of anotherelectronic state, seen in the spectrum peaks l and m. Both peaks l and m have anegative β value, which is distinct from the feature for the X 2A′ state where positive

β values are found. Positive β values are often observed for low kinetic energy
photoelectrons detached from the in-plane orbitals of aromatic systems.29,55 This isconsistent with electron detachment from the C5-centered in-plane σ−orbital of theX 1A′ anion to form the X 2A′ radical. In contrast, negative β values are often
signatures of electrons detached from aromatic π orbitals.27-29,55 The negative β value
for peaks l and m may suggest that the detachment takes place from a π orbital toform the A 2A′′ state. This observation is consistent with DFT calculations, whichfind an 2A′′ excited state of the N-methyl-5-pyrazolyl radical with a calculated Te of
0.768 eV. The simulated spectrum (shown in Figure 8.01b at eBE > 2.8 eV)
qualitatively resembles the experimental spectrum, however the observed spectrum

197

a)

H

b)

H
C

H

H
C

H

N

N

C

N
C

C

C
H

d)

C

e)

H
C

H

H

H

C

N

C

H

H

ν17=680 ± 35 cm-1(exp)

H

H

N
N
C

C

H

ν18=345 ± 45 cm-1 (exp)
H

N
C

H

c) H

H

N

C

C

N
C

H

ν16=899 cm-1 (calc)

H

H

C

C

N
C

H

ν15=964 cm-1 (calc)

HCH

ν13=1081 cm-1 (calc)

Figure 8.02: Relative atomic displacements in the activated normal modes of theX 2A′ state of N-methyl-5-pyrazolyl and their frequencies. (Frequencies a and b are
experimentally determined while frequencies c-e are calculated.)198

is considerably broader than predicted. We attribute the observed broad feature ofpeaks l and m to the vibronic coupling between the X 2A′ and A 2A′′ states. Vibronic
coupling can generate a large number of vibronic levels among which transition
intensities are distributed, resulting in broadening.56 Such broadening has been
observed, for instance, in the photoionization of pyrrole.57 Thus, we conclude thatpeaks l and m represent the A 2A′′ vibronic levels, and the Te is determined to be
0.807 ± 0.009 eV. The EA for the A 2A′′ state of the N-methyl-5-pyrazolyl radical is2.861 ± 0.007 eV, which is close to the EA of the 1-pyrazolyl radical, 2.938 ±0.005 eV, where photodetachment takes place from a π orbital of the 1-pyrazolide
ion.29 It should be noted that vibronic coupling can also lend some intensity to thehighly excited vibronic levels of the X 2A′ state, which may explain some of thediscrepancy between the observed spectrum and the simulation for the X 2A′ state
shown in Figure 8.01b. The observed gradual decrease of the β value toward thehigher eBE in the X 2A′ spectrum may reflect the nonadiabatic effects.
8.3.3) Photoelectron Spectra of N-methylimidazolide Anion
The 351.1 nm magic angle photoelectron spectrum of the

N-methylimidazolide anion is shown in Figure 8.03. This ion was formed from thereaction of OH⎯ with N-methylimidazole. Analogous to N-methylpyrazole, DFT
calculations (Table 8.05) predict that the C5 position is the most acidic site.
However, deprotonation by OH⎯ at the methyl carbon and at the C2 carbon is also
exothermic and therefore, we might expect to see all three isomers in theexperimental spectrum. Instead, there appears to be one dominant FC envelope
beginning with peak a at 1.987 ± 0.008 eV, an electronic band origin. The
photoelectron angular distribution was measured and β is approximately 0.5 at theorigin and smoothly decreases to -0.2 at 3.0 eV. At energies lower than the199electronic band origin there are several peaks and a broad band which extends~1 eV. When the spectrum is collected under cold flow tube conditions (not shown)these lower binding energy peaks are absent, supporting the assignment of theorigin.
To aid in the assignment of the experimental spectrum, FC simulations were
performed for all three isomeric anions (C5, C2 and methyl anions) and theircorresponding ground state neutral radicals. The unadjusted stick spectra and thespectra that result from convoluting the stick spectra with a 10 meV fwhm Gaussianfunction are shown in Figure 8.04b-8.04d, along with the experimental spectrum(8.04a). It is clear that the spectral features for N-imidazolylmethyl anion and the
methyl-2-imidazolide anion are not observed in the experimental spectrum.
However, the EA, spectral profile, and even the individual resolved transitions for the

N-methyl-5-imidazolide anion simulated spectrum are in excellent agreement withthe experimental spectrum. Figure 8.05 shows the lower eBE portion of the C5anion FC simulation overlaid on the experimental data. In Figure 8.05 the origin of
the simulated spectrum (red trace) has been slightly shifted (< 25 meV) to match theexperimental data (black trace). This figure further demonstrates that within
300 meV from the origin, where the simulation is most reliable, the predicted relative
intensities match the experimental spectrum, confirming that the signal in this region
is primarily due to one species. Thus, we conclude that under our experimental
conditions deprotonation of N-methylimidazole by OH⎯ almost exclusively occurs at
the C5 position.
The calculated EA of the N-methyl-5-imidazolyl radical is 1.965 eV, which is
close to the assigned origin (peak a). Using the FC simulation, four vibrations of theX 2A′ state of the N-methyl-5-imidazolyl radical can be assigned. The small intensity

200

Photoelectron Counts

e
d

x
y

3.2

a

c
b

a
2.2

2.1

2.0

2.8
2.4
2.0
1.6
1.2
Electron Binding Energy, (eV)Figure 8.03: The 351.1 nm magic-angle photoelectron spectrum of the
N-methylimidazolide anion. A portion of the spectrum near peak a is expanded in
the inset.

201

a)

CH3

b)

N

H

Photoelectron Counts

C

C
C

N

H

c)

CH3

X Data

N

H

C

C
C

N

H

d)

CH2
N

H

H

C

C
C

N

H

3.0

2.5

2.0

1.5

1.0

0.5

Electron Binding Energy, (eV)Figure 8.04: a) The 351.1 nm magic-angle photoelectron spectrum of the
N-methylimidazolide anion, reproduced from Figure 8.03. Simulations of thespectrum based on optimized geometries and normal modes from B3LYP/6311++G(d,p) calculations. The sticks (blue) are the raw Franck-Condon factors andthe solid line is the 10 meV fwhm Gaussian convolutions for transitions of b) X 1A′
N-methyl-5-imidazolide to X 2A′ N-methyl-5-imidazolyl, c) X 1A′ N-methyl-2imidazolide to X 2A′ N-methyl-2-imidazolyl, and d) X 1A N-imidazolylmethyl anion toX 2A′ N-imidazolylmethyl radical.

202

Photoelectron Counts

e
d
a

c

b

2.25

2.20

2.15

2.10

2.05

2.00

1.95

Electron Binding Energy, (eV)Figure 8.05: The 351.1 nm magic-angle photoelectron spectrum of the
N-methylimidazolide anion (black), reproduced from Figure 8.03 and the simulated
10 meV Gaussian convoluted (red) and stick (blue) spectra, reproduced from
Figure 8.04b. The eBE of the origin for the simulated spectra was set to match theeBE of peak a in the experimental spectrum.

203

a)

H

b) H

H
C

C

H

N
C

H

N

H

C
C

H

C
C

N

H

C
N

H

H

ν18=345 ± 30 cm-1 (exp)ν17=675 ± 25 cm-1 (exp)

c)

d)

H

H
C

H
C

H

H

N

N
C

C
C

H

H

C
C

N

H

ν16=865 ± 35 cm-1 (exp)

C

H

N

H

ν15=1015 ± 25 cm-1 (exp)

Figure 8.06: Relative atomic displacements in the activated normal modes for theX 2A′ state of N-methyl-5-imidazolyl.

204

peak b is assigned to a methyl rocking mode with a frequency of 345 ± 30 cm-1.
Peaks c, d, and e are partially resolved fundamentals of in-plane ring distortion
modes. Peak c corresponds to a C2-N1-C5 bend with a frequency of 675 ± 25 cm-1,
peak d to a C2-N3-C4 bend with a frequency of 865 ± 35 cm-1, and peak e toC4-C5-N1 bend with a frequency of 1015 ± 25 cm-1. Combination bands of thesemodes are observed in the larger eBE region. The atomic displacements for these
normal modes are shown in Figure 8.06. The same bending modes are active in thephotoelectron spectrum of the 5-imidazolide anion.
Following the analysis of the N-methyl-5-pyrazolide anion spectrum, weconsider photodetachment to the A 2A′′ state of the N-methyl-5-imidazolyl radical.
DFT calculations predict the Te to be 0.452 eV. This Te corresponds to the broadband x in Figure 8.03. Analogous to the N-methyl-5-pyrazolyl radical system,
vibronic coupling between the X 2A′ and A 2A′′ states is expected for the N-methyl-5imidazolyl radical, which can broaden the spectral profile. In the previous section,
we noted the similarity between the eBE for the detachment to the A 2A′′ state of

N-methyl-5-pyrazolyl and the EA of the 1-pyrazolyl radical. In this sense, because
the EA of the 1-imidazolyl radical is 2.613 ± 0.006 eV,27 the broad band y in Figure8.03 seems to correspond to the vibrational origin of the A 2A′′ N-methyl-5-imidazolyl
radical. Rigorous assignment is impossible in the present study, but it is most likelythat some of the spectral features at eBE > 2.4 eV represents the A 2A′′ state of theradical. Gradual decrease of the β value toward the higher eBE is consistent withthis conclusion. The broad background at eBE > 2.0 eV may be a manifestation of
the nonadiabatic effects as well.
As mentioned above, deprotonation of N-methylimidazole at the methyl andC2 sites by hydroxide is predicted to be exothermic. Nonetheless the spectral

205

features for these two anions are absent in the experimental spectrum, suggestingthat these isomers are not formed in any significant amounts. This notion, however,should be taken with caution, because the identity of the low-intensity, broad featureobserved at eBE < 2.0 eV is uncertain. This feature is absent when the ions aresynthesized in a flow tube cooled with liquid nitrogen. There may be an
isomerization process (i.e., ring-opening) when OH⎯ interacts with a H atom at C2,
C4, or the methyl group.
8.3.4) Gas-Phase Acidity Measurements
The gas-phase acidity of N-methylpyrazole was measured relative tomethanol52 (shown in reaction 8.01). For the forward reaction, the N-methylpyrazolide anion is prepared from a proton transfer reaction of N-methylpyrazole withOH⎯ in the source. The photoelectron spectrum of N-methylpyrazolide, where theanion was prepared from proton transfer to hydroxide, shows that deprotonation only
occurs on the C5 site. The forward reaction of the N-methyl-5-pyrazolide anion with
CH3OH is exothermic and the reaction proceeds exclusively by proton transfer. The
rate constant is measured to be 1.3 ± 0.1 × 10-9 cm3 s-1 (0.68 efficiency). The
reverse reaction of CH3O⎯ with N-methylpyrazole is endothermic, and significant
adduct formation is observed in addition to proton transfer. The overall rate constantis 5.5 ± 0.4 × 10-10 cm3 s-1 (0.19 efficiency). The proton transfer rate constant wasdetermined from the overall rate constant and the proton transfer branching ratio tobe 1.2 ± 0.3 × 10-10 cm3 s-1.
Combining the forward and reverse proton transfer rate constants, the gasphase-acidity (∆aG298) of N-methylpyrazole is determined to be 376.9 ± 0.7 kcal
mol-1, which is in exact agreement with the calculated value of 376.9 kcal mol-1. The

206

DFT entropy of deprotonation, ∆aS298 = 23.7 cal mol-1 K-1, is used with ∆aG298 to
calculate the enthalpy of deprotonation (∆aH298) to be 384.0 ± 0.7 kcal mol-1.
The gas-phase acidity of N-methylimidazole was measured relative to water(shown in reaction 8.02). In both the forward and reverse reactions the

N-methylimidazolide anion is formed from deprotonation of the neutral by OH⎯. (Forthe forward reaction, this step occurs in the source.) As discussed in the analysis of
the N-methylimidazolide anion photoelectron spectrum, deprotonation of

N-methylimidazole by OH⎯ almost exclusively occurs at the C5 site. If the methyl andC2 isomers are present they are in minor amounts and therefore would not
significantly affect the experimental gas-phase acidity value.
The forward reaction of the N-methylimidazolide anion with H2O isendothermic, and significant adduct formation is observed in addition to protontransfer. The overall rate constant is found to be 1.0 ± 0.3 × 10-10 cm3 s-1 (0.04
efficiency). Kinetic plots for this reaction were measured as a function of SIFT
injection potential (over a 50 V range) and are observed to be linear, which supports
the conclusion that other isomeric ions are not present in significant amounts. The
proton transfer rate constant is determined from the overall rate constant and theproton transfer branching ratio to be 1.3 ± 0.3 × 10-11 cm3 s-1. The reverse reaction ofOH⎯ with N-methylimidazole is exothermic and proton transfer is the only reaction
pathway observed. The proton transfer reaction rate constant was determined
relative to that for the reaction of N-methylimidazole with NH2⎯, which is highly
exothermic, assuming that this reaction proceeds at 90 ±10% of the collision rate.
This assumption is validated by measurements of the proton transfer rate constantfor the reaction of NH2– with N-methylpyrazole, which is slightly more exothermic and207

found to occur at the collision rate. The proton transfer rate constant for N-methylimidazole with OH⎯ is determined to be 4.6 ± 0.5 × 10-9 cm3 s-1 (eff = 0.82).
Combining the forward and reverse proton transfer rate constants, the gasphase-acidity (∆aG298) of N-methylimidazole is determined to be 380.2 ± 1.0 kcal
mol-1. Although the gas-phase acidity of H2O is known to within ± 0.02 kcal mol-1, we
have assigned more conservative error bars to the gas-phase acidity ofN-methylimidazole to account for the possibility that another, more basic isomer may
be present.58 The experimental gas-phase acidity is in good agreement with thecalculated value of 380.8 kcal mol-1. The DFT calculated ∆aS298 = 26.5 cal
mol-1 K-1; therefore, ∆aH298 is determined to be 388.1 ± 1.0 kcal mol-1.
8.3.5) H/D Exchange of N-methylimidazolide Anion
The observation that deprotonation of N-methylimidazole by OH⎯ results inalmost exclusively the C5 anion has prompted us to investigate the H/D exchange
reaction of the N-methylimidazolide anion with D2O and ND3. H/D exchange
experiments have proven to be a useful probe for determining anion structures andfor studies of the structure and dynamics of ion-dipole complexes. 59 In this process,the ion and deuterated reagent enter a long-lived complex, which typically contains~15-20 kcal mol-1 excess energy.59 During the lifetime of this ion-dipole complex,
proton-deuteron scrambling can occur before the ion and neutral separate.
D2O and ND3 are useful exchange reagents in that they can both exchange
more than one deuteron in an encounter. These two exchange reagents differ fromone another in that ion-molecule complexes with ND3 typically contain less energy
than ion-molecule complexes with D2O.60 When N-methylimidazolide anion isallowed to react with ND3 four exchanges are observed. Assuming that the initial
anionic site is at the C5 position, three exchanges are due to the methyl group and208

one exchange is from a ring proton (either C2 or C4 protons). The first step of theexchange, the deuteron transfer reaction of N-methyl-5-imidazolide anion with ND3,
is sufficiently endothermic (~15 kcal mol-1) and consumes almost all of thecomplexation energy. Using the DFT calculated acidities as a guide (Table 8.05),
back transfer of a proton to ND2⎯ from the methyl or C2 position is energeticallyfavored, while proton transfer from the C4 position would consume an additional~2 kcal mol-1 of the complexation energy. Any subsequent exchange, therefore, ismost likely to occur at the methyl or C2 position. When D2O is used as an exchange
reagent four exchanges are also observed. Despite the fact that ion-molecule
complexes with D2O have larger excess energy than complexes with ND3 and thatthe initial deuteron transfer is less endothermic, exchange at the C4 position is notobserved. These results are consistent with computations, which predict that the C4
proton is significantly less acidic than other sites in the molecule. Furthermore, theseresults indicate that the methyl and C2 positions are accessible.
8.3.6) Thermochemistry
The C5-H BDE (D0) of N-methylpyrazole and N-methylimidazole can be
determined from the electron affinity measurements and the enthalpy of
deprotonation using a negative ion thermochemical cycle,33,34 as discussed in
Chapter 1. In N-methylpyrazole, where the two nitrogen atoms are adjacent, theD0(C5-H) is determined to be 116.4 ± 0.7 kcal mol-1. In N-methylimidazole, where
the two nitrogen atoms are separated by one carbon atom, the D0(C5-H) isdetermined to be 119.0 ± 1.0 kcal mol-1. The uncertainty in the BDE measurements
is largely due to the error in the gas-phase acidity measurements. Table 8.06
summarizes these BDEs as well as other thermodynamic values reported in thiswork. The BDE is the energy required to break the C-H bond to form the radical and209

an H atom. The difference in the BDE of the two isomers is 2.6 kcal mol-1, indicatingthat the location of the remote (β position) nitrogen atom in the ring does not
significantly change the strength of the C5-H bond. This subtle difference, however,does indicate that the N-methyl-5-pyrazolyl radical is slightly more stable than the

N-methyl-5-imidazolyl radical relative to their respective neutral azoles. Similarly,
gas-phase acidity measurements indicate that the N-methyl-5-pyrazolide anion ismore stable than the N-methyl-5-imidazolide anion when compared to thecorresponding neutral azoles.
The heat of formation of both N-methylazoles, ∆fH298(AH), has previously
been determined (∆fH298(N-methylpyrazole) = 37.4 ± 0.5 kcal mol-1 and∆fH298(N-methylimidazole) = 32.9 ± 1.0 kcal mol-1). Using the BDEs reported here for
the N-methylazoles, the heat of formation of the N-methylazolyl radicals, ∆fH298(A),can be determined (also discussed in Chapter 1). The heat of formation isdetermined to be 103.1 ± 1.5 kcal mol-1 for the N-methyl-5-pyrazolyl radical and101.2 ± 3.1 kcal mol-1 for the N-methyl-5-imidazolyl radical.
If the methyl group does not significantly change the C-H bond energetics,
the thermochemical properties measured here should be consistent with those ofpyrazole and imidazole. For imidazole, the previously measured C5-H bond strength(119 ± 4 kcal mol-1) is similar to the value reported here for N-methylimidazole(119.0 ± 1.0 kcal mol-1), which has much greater precision. In addition, the EA andgas-phase acidity reported here are also similar to those for imidazole (∆aG298 (C5-H,
imidazole) = 380 ± 4 kcal mol-1, EA(5-imidazolyl) = 1.992 ± 0.010 eV).27 The
previously measured bond strength of pyrazole (121 ± 4 kcal mol-1), on the other
hand, is 4.6 kcal mol-1 higher than the value reported here for N-methylpyrazole.

210

Table 8.06: Thermochemical Parameters.

N-methylpyrazole and N-methyl-5-pyrazolyl radical
EA(N-methyl-5-pyrazolyl)aTe(N-methyl-5-pyrazolyl)a∆aG298(C5-H, N-methylpyrazole)a∆aS298(C5-H, N-methylpyrazole)b∆aH298(C5-H, N-methylpyrazole)c∆aH0(C5-H, N-methylpyrazole) d
D0(C5-H, N-methylpyrazole)e∆fH298(N-methylpyrazole)f∆fH298(N-methyl-5-pyrazolyl)g2.054 ± 0.006 eV
0.807 ± 0.009 eV
376.9 ± 0.7 kcal mol-1
23.7 cal mol-1 K-1
384.0 ± 0.7 kcal mol-1
382.6 ± 0.7 kcal mol-1
116.4 ± 0.7 kcal mol-1
37.4 ± 0.5 kcal mol-1
103.1 ± 1.5 kcal mol-1

N-methylimidazole and N-methyl-5-imidazolyl radical
EA(N-methyl-5-imidazolyl)a1.987 ± 0.008 eVa∆aG298(C5-H, N-methylimidazole)
380.2 ± 1.0 kcal mol-1
26.5 cal mol-1 K-1∆aS298(C5-H, N-methylimidazole)bc∆aH298(C5-H, N-methylimidazole)
388.1 ± 1.0 kcal mol-1d∆aH0(C5-H, N-methylimidazole)
386.8 ± 1.1 kcal mol-1
D0(C5-H, N-methylimidazole)e
119.0 ± 1.0 kcal mol-1
f∆fH298(N-methylimidazole)
32.9 ± 1.0 kcal mol-1∆fH298(N-methyl-5-imidazolyl)g101.2 ± 3.1 kcal mol-1abExperimentally determined in this study. Determined from B3LYP/ 6-311++G(d,p)
calculations. c Calculated from ∆aG298 and ∆aS298. d Calculated from ∆aH298.
eCalculated using the EA and ∆aH0 in addition to the IE(H). f Value taken fromreference 64 g Calculated from ∆fH298(H), ∆fH298(RH) and D298(RH).

211

The difference in bond strength is primarily due to the difference in the measured
acidity, 4 kcal mol-1 (∆aG298 (C5-H, pyrazole) = 380 ± 4 kcal mol-1, EA(5-pyrazolyl) =2.104 ± 0.005 eV). It seems unlikely that the substitution of a methyl group on
pyrazole would significantly perturb the energetics of the C-H bonds, especially since
there is no significant effect for N-methylimidazole. More likely it seems that thedifference in BDEs of pyrazole and N-methylpyrazole reflects the difficulties indetermining the C5-H acidity of pyrazole.61 Thus, the C5-H BDE values reported inthis work, at the present time, represent the most accurate C-H BDE values for thefamily of azoles.
The C5-H BDEs of N-methylpyrazole and N-methylimidazole determined inthis work are significantly larger than the C-H BDE of benzene (D0(C-H) = 112.0 ±0.6 kcal mol-1), a benchmark C-H bond strength for aromatic compounds.62
Generally, C-H BDEs in five-membered ring aromatic compounds are predicted
theoretically to be larger than those in six-membered aromatics; this has been
discussed with respect to the difference in the molecular structure.12 The inherently
smaller interior angle in a five-membered ring makes the corresponding C-H bondstronger because the 2s orbital of the C atom contributes more to the C-H bonding
orbital. While our experimental findings are consistent with this argument, thereported C-H BDE of another five-membered ring compound, the cyclopentadienyl
radical (C5H5),63 D298 = 105 ± 3.4 kcal mol-1, is much lower than the C5-H BDEs of
the N-methylazoles and the C-H BDE of benzene. In this case, however, thecyclopentadienyl radical is not a 4n+2 Hϋckel system.64 Following C-H bond fission,
the resultant cyclopentadienylidene would maintain an unpaired electron in the π (b1)orbital in the electronic ground state (3B1), inducing a static Jahn-Teller stabilization,
which may explain the low C-H BDE of the cyclopentadienyl radical.

212

In order to extend the knowledge of the C-H BDEs of azoles, we have
performed preliminary experiments on N-methylpyrrole. It is difficult, however, todetermine the C2-H BDE (which is equivalent to the C5-H BDE by symmetry) using
the same experimental technique employed in this study, because N-methylation ofpyrrole does not make the C2 carbon the most acidic location.65 The C2-H BDE ofpyrrole has been reported to be 112.5 ± 1 kcal mol-1 according to a photofragment
translational spectroscopic study.23 It should be mentioned, however, that more
recent studies24,25 have not observed a C-H bond dissociation process in thephotochemistry of pyrrole. In light of the discrepancy in the N-H BDE of pyrrole,
there are some questions about the reported C2-H BDE of pyrrole in the
aforementioned study. It should also be noted that a photofragment velocity map
imaging technique has been ineffective to explore the C-H BDE of N-methylpyrrole.24
In light of the C5-H BDEs determined here for N-methylpyrazole and N-methylimidazole, it seems likely that the photofragment translational spectroscopic study
underestimates the C2-H BDE of N-methylpyrrole.
8.4) Conclusions
The 351.1 nm photoelectron spectrum of the N-methyl-5-pyrazolide anion
was measured and is in excellent agreement with the calculated FC factors for thetransition from the X 1A′ state of the anion to the X 2A′ and A 2A′′ states of the

N-methyl-5-pyrazolyl radical. The EA of the radical is 2.054 ± 0.006 eV and the Te of
the A 2A′′ state is 0.807 ± 0.009 eV. Fundamental vibrational frequencies of 345 ±45 cm-1 and 680 ± 35 cm-1 are found for an in-plane methyl rocking and C5-N1-N2
bending modes of the X 2A′ state of the N-methyl-5-pyrazolyl radical, respectively.
The gas-phase acidity of N-methylpyrazole was measured relative to methanol,∆aG298(N-methylpyrazole) = 376.9 ± 0.7 kcal mol-1 and ∆aH298(N-methylpyrazole) =213

384.0 ± 0.7 kcal mol-1. Using a negative ion thermodynamic cycle the C5-H BDE
was determined to be D0(N-methylpyrazole) = 116.4 ± 0.7 kcal mol-l and∆fH298(N-methylpyrazole) = 103.1 ± 1.5 kcal mol-1.
The 351.1 nm photoelectron spectrum of the N-methyl-5-imidazolide anion
was measured and is in good agreement with the calculated FC factors for the X 1A′
state of the anion to the X 2A′ state of the N-methyl-5-imidazolyl radical. The EA of
the radical is 1.987 ± 0.008 eV. Fundamental frequencies of four in-plane modes are
345 ± 30 cm-1 (methyl rocking), 675 ± 25 cm-1 (C5-N1-C2), 865 ± 35 cm-1
(C2-N3-C4), and 1015 ± 25 cm-1 (C4-C5-N1) for the X 2A′ state of the N-methyl-5imidazolyl radical. The gas-phase acidity of N-methylimidazole was measured
relative to water, ∆aG298(N-methylimidazole) = 380.2 ± 1.0 kcal mol-1 and∆aH298(N-methylimidazole) = 388.1 ± 1.0 kcal mol-1. Using a negative ion
thermodynamic cycle the C5-H BDE and the heat of formation of the N-methyl-5imidazolyl radical were determined to be 119.0 ± 1.0 kcal mol-l and 101.2 ± 3.1
kcal mol-1, respectively.
8.5) Acknowledgements
This work would not have been possible without the help of Dr. Adam J.
Gianola, Ms. Nicole Eyet, Dr. Takatoshi Ichino, Dr. Shuji Kato, Prof. Veronica M.
Bierbaum, and Prof. W. Carl Lineberger. In addition, thanks is given to Prof. Charles
H. DePuy for his useful comments, Mr. Todd B. Kreutzian for determining the watercontent of the neutral reagents, and Prof. Paul G. Wenthold and Prof. Jeehiun K. Lee
for sharing their unpublished work with us. This work was funded by the AFOSR(FA9550-06-1-006) and NSF (CHE0512188, PHY0551010, and CHE0349937).

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(39) Wenthold, P. G.; Squires, R. R. Biradical Thermochemistry from
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Monoanion of Acetic-Acid by Proton Abstraction. Journal of the American Chemical
Society 1989, 111, 3106-3108.
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to Solution: The Coalescence of the N1 and N3 Sites and Implications for Biological
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and Biological Implications. A Computational and Experimental Gas-Phase Study.
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(44) Hotop, H.; Lineberger, W. C. Binding-Energies in Atomic NegativeIons II. Journal of Physical and Chemical Reference Data 1985, 14, 731-750.
(45) Hanstorp, D.; Gustafsson, M. Determination of the Electron-Affinity of
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(48) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent
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Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
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Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
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Laham, M. A.; C. Y. Peng; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
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revision B.05; I. Gaussian, Ed. Pittsburgh, PA, 2003.
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(51) The B3LYP/6-311++G(d,p) calculations predict that the three-foldbarriers for the methyl torsion are 247 and 105 cm-1 for N-methyl-5-pyrazolide and Nmethyl-5-pyrazolyl, respectively. They are 47 and 111 cm-1 for N-methyl-5imidazolide and N-methyl-5-imidazolyl, respectively.
(52) Ervin, K. M.; DeTuro, V. F. Anchoring the Gas-Phase Acidity Scale.
Journal of Physical Chemistry A 2002, 106, 9947-9956.
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(54) For the reverse direction in reaction 2, water was present as trace
amounts in the N-methylpyrazole. The amount of water contained in the neutral
reagent was determined by Carl Fisher titration to be 1.48 wt. %, however thepercent of water in the vapor phase is estimated to be higher than in the liquid
phase. It is unlikely that a direct association between methoxide and water isoccurring because this reaction is known to be slow (kII= 1.0 ×10-11 cm3 s-1 at 0.5 torr
He). [Kato, S.; Dang, T. T.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V. M. Gas-phaseassociation reactions of hydroxide, methoxide, ethoxide, and their deuterated
analogs, International Journal of Mass Spectrometry 2000, 196, 625-638.] For theforward direction in reaction 6, water is present as the neutral reagent ([N]~5 ×1012
molecules cm-3). Again it is unlikely that direct association of hydroxide with water isthe major contribution to the hydroxide-water adduct formation, because the rate
constant for this process is known to be small (kII= 1.2 ×10-12 cm3 s-1 in 0.5 torr of
He). [see above Kato et al.].
(55) Gunion, R. F.; Gilles, M. K.; Polak, M. L.; Lineberger, W. C. Ultraviolet
Photoelectron-Spectroscopy of the Phenide, Benzyl and Phenoxide Anions, with abinitio Calculations. International Journal of Mass Spectrometry and Ion Processes
1992, 117, 601-620.
(56) Koppel, H.; Domcke, W.; Cederbaum, L. S. Multimode MolecularDynamics Beyond the Born-Oppenheimer Approximation. Advances in Chemical
Physics 1984, 57, 59-246.
(57) Trofimov, A. B.; Koppel, H.; Schirmer, J. Vibronic Structure of the
Valence π-Photoelectron Bands in Furan, Pyrrole, and Thiophene. Journal ofChemical Physics 1998, 109, 1025-1040.
(58) The largest source of experimental error is due to the measurement of
kr. The presence of a more basic anion would artificially raise the rate constant and219

proton transfer branching ratio for the reaction of N-methylimidazolide with water andtherefore, the reported proton transfer rate constant represents an upper boundmeasurement.
(59) DePuy, C. H.; Kato, S. H/D Exchange Reactions. In The Encyclopediaof Mass Spectrometry: Theory and Ion Chemistry; P. B. Armentrout, Ed.; Elsevier:
Amsterdam, 2003; Vol. 1; pp 670-674.
(60) NIST Chemistry WebBook, NIST Standard Reference Database
Number 69; Linstrom, P. J.; Mallard, W. G., Eds.; National Institute of Standards andTechnology: Gaithersburg MD, 20899 June 2005 (http://webbook.nist.gov).
(61) B3LYP/6-311++G(d,p) calculations predict that ∆acidG298(C5-H,
pyrazole) = 376.8 kcal mol-1 and D0(C5-H, pyrazole) = 118.1 kcal mol-1. These
values are close to the measured values of N-methylpyrazole reported here.
(62) Davico, G. E.; Bierbaum, V. M.; DePuy, C. H.; Ellison, G. B.; Squires,
R. R. The C-H Bond-Energy of Benzene. Journal of the American Chemical Society
1995, 117, 2590-2599.
(63) McDonald, R. N.; Bianchina, E. J.; Tung, C. C. Electron
Photodetachment of Cyclopentadienylidene Anion Radical in a Flowing Afterglow
Apparatus - EA and ∆Hf° of Cyclopentadienylidene. Journal of the American
Chemical Society 1991, 113, 7115-7121.
(64) The cyclopentadienyl radical is stabilized by delocalization but the
species lacks one electron in the π system for aromaticity under the 4n+2 Huckel's
rule.
(65) DePuy, C. H.; Kass, S. R.; Bean, G. P. Formation and Reactions of
Heteroaromatic Anions in the Gas-Phase. Journal of Organic Chemistry 1988, 53,
4427-4433.

220

CHAPTER IXTHERMOCHEMICAL STUDIES OF THE PEROXYFORMYL AND
PEROXYACETYL RADICALS

9.1) Introduction
Organic peroxy radicals (RO2) play an integral role in the chemistry of thetroposphere.1-3 These radicals are formed as a result of OH-initiated oxidation ofvolatile organic compounds (RH) emitted from biogenic and anthropogenic sources;
this process efficiently removes hydrocarbons from the atmosphere and serves as animportant sink for the OH radical. Once formed alkyl radicals, R, rapidly combinewith atmospheric O2 via a three-body reaction to produces organic peroxy radicals.
RH + OH → R + H2O(9.01)

R + O2 + M → RO2 + M(9.02)

Peroxy radicals react with the NOx and HOx radical families. In urban areas,
reactions with NOx species (NO and NO2) dominate. By oxidizing NO to NO2,
peroxyl radicals directly influence tropospheric ozone formation, since the photolysis
of NO2 forms O(3P), which rapidly reacts with O2 to form O3 (eq 9.03-9.05).4,5
RO2 + NO → RO + NO2(9.03)

NO2 + hν → NO + O(3P)(9.04)

O(3P) + O2 + M → O3 + M(9.05)In areas of low NOx, organic peroxy radicals primarily react with other peroxy radicals(mainly HO2), propagating chain reactions that eventually lead to the removal ofradicals from the atmosphere.
Due to their significance in the atmosphere, the structure and thermochemistry of these radicals is of great interest. This work focuses on two very
important peroxy radicals, the peroxyformyl radical (HC(O)OO) and the peroxyacetyl
221

radical (CH3C(O)OO). In particular, the latter species is one of the most abundant
peroxy radicals in the atmosphere. In polluted environments, this radical reacts with
NO2 to form peroxyacetyl nitrate (PAN).6,7⎯⎯→ CH3C(O)O2NO2 + M
CH3C(O)O2 + NO2 + M ←⎯⎯(9.06)

PAN is a key component in photochemical smog, it is a respiratory and eye irritant,and can damage vegetation. Due to its stability relative to other peroxynitrates, PAN
acts as a temporary reservoir for NOX species and can undergo long-range transport
from areas of high pollution to areas of low pollution, where subsequent thermal
decomposition releases the reactive radicals. In addition to their atmospheric
relevance, these two species are of fundamental structural interest since they have
two distinct conformations (cis and trans), which differ dramatically in their reactivity
and thermochemical properties.
The peroxyacetyl radical has been the focus of many spectroscopic andkinetic studies, the vast majority of which probe the strong UV B - X transition.8-12
However, due to the repulsive nature of the B state, this transition is broad andfeatureless, providing few structural details about the radical.1 In contrast, the lowlying A - X transition displays rotational and vibrational structure; however, due to its
relatively small absorption cross section this transition requires use of sensitive
spectroscopic techniques. Zalyubovsky et al.13 recorded the gas-phase absorption
spectrum of the peroxyacetyl radical produced from photolysis of (COCl)2 in a
CH3CHO/O2/N2 mixture using cavity ringdown spectroscopy. More recently,
Hu et al.14,15 employed near-IR/VUV ion enhancement spectroscopy to record thespectrum of peroxyacetyl radical produced from the thermal decomposition of PAN ina pyrolysis/supersonic pulsed nozzle. The results of these experiments are inexcellent agreement with one another. The vibronic features were found to222correspond to the trans-conformer of the radical, the A - X term energy splitting wasdetermined to be 5582.5 ± 0.5 cm-1, and several A state vibrational modes wereidentified.
The IR spectrum of the peroxyacetyl radical has been reported in a noble gas
matrix from the flash thermolysis of PAN.16 The primary decomposition pathway of
PAN was shown to occur by O-N bond fission yielding the peroxyacetyl radical. The
resulting IR features were attributed to both the cis- and trans-conformers of theperoxyacetyl radical for which several IR and Raman active vibrational modes wereidentified. The peroxyformyl radical has also been observed using matrix isolation
IR-spectroscopy. In an early study, this radical was produced by the photolysis offormaldehyde in a solid oxygen matrix.17,18 The assignment of the spectral features,
however, was complicated by the presence of the HO2 radical, which was likely
present in the photolysis cage. More recently, the isolated trans-peroxyformyl radical
was studied by the discharge production and deposition of the formyl radical onto asolid argon matrix followed by an association reaction with oxygen.19 The spectral
assignments were confirmed by 18O isotopic labeling and by electronic structure
calculations.
In this work we report the negative ion photoelectron spectrum of theperoxyformate (HC(O)OO–) and peroxyacetate anions (CH3C(O)OO–). This
technique provides access to the two lowest electronic states of the acylperoxy
radical species through photodetachment of an electron from the corresponding
anion. Analysis of the kinetic energy of the detached electron allows for thedetermination of electron affinities, term energy splitting, and vibrational frequencies,and gives insight into the molecular structure of both the anion and radical species.
Additionally, the gas-phase acidities of peroxyformic acid and peroxyacetic acid were223

determined from ion-molecule bracketing experiments and the collision-induced
dissociation of the corresponding anions was investigated. Combining of the peracid
gas-phase acidity with the peroxy radical electron affinity (EA) in a negative ion
thermochemical cycle allows for the determination of the O-H bond dissociationenergy (BDE).
9.2) Experimental Methods
9.2.1) Negative Ion Photoelectron Spectroscopy (NIPES)A detailed description of the anion photoelectron spectrometer that was usedin these studies is provided in Chapter 4. Acylperoxy ions (RC(O)OO–, R = H andCH3) were synthesized in a flowing afterglow source from a series of ion-molecule
reactions (eq 9.07 and 9.08) that occur downstream of a microwave dischargesource.
OH– + H2O2 → H2O + HO2–

(9.07)

HO2– + RC(O)OR’ → RC(O)OO– + R’OH

(9.08)

Equation 9.08 is a gas-phase Baeyer-Villiger reaction, which was previously reported
by Bowie et al.20 Due to the number of collisions in the source region, ions areprepared with a ~298 K rotational and vibrational Maxwell-Boltzmann distribution.
Negative ions were gently extracted from the source region and passed though a
Wien filter for mass selection. The mass selected ion beam was then crossed with a351.1 nm (3.531 eV) line from a cw argon-ion laser in an external build up cavity
producing ~100 W of circulating power. Photoelectrons that are ejected
perpendicular to the plane of the ion-photon interaction region are energy analyzed
by a hemispherical kinetic energy analyzer coupled to a position sensitive detector.
The electron energy resolution ranges from 8-15 meV fwhm. The absoluteenergy scale is calibrated with the well-known electron affinity (EA) of the sulfur

224atom21 and a linear energy scale compression factor (< 1%), which is measured fromthe photoelectron spectra of reference ions S⎯,21 I⎯,22 and O⎯ 23 using the EAs of thecorresponding atoms. Angular distribution measurements were conducted bychanging the angle (θ) between the electric field vector of the laser beam and thephotoelectron collection axis.
9.2.2) Flowing Afterglow-Selected Ion Flow Tube (FA-SIFT) Measurements
Collision-induced dissociation (CID) studies and gas-phase acidity (∆aG)
measurements were conducted using a tandem flowing afterglow-selected ion flow
tube instrument (FA-SIFT), which was described in Chapter 3. Acylperoxy ions wereformed in a flowing afterglow source using the same synthesis scheme as in theabove photodetachment experiments (eq 9.07 and 9.08). Peroxy ions wereextracted from the source flow tube, focused into a quadrupole for mass selection,and injected into a second flow tube where the flow of He buffer gas is maintained at
12 SLPM and a pressure of ~0.5 torr. Ions injected into the flow tube undergo
multiple collisions with the He buffer gas and a percentage of the ions fragment due
to CID in the vicinity of the injection orifice that connects the quadrupole region to thesecond tube. Further downstream from the injection orifice, the ions are collisionallycooled to a ~298 K rotational and vibrational Maxwell-Boltzmann distribution.
Measured flows of neutral reagents were introduced into the reaction flow tube
through a manifold of inlets and the reactant and product ions were analyzed using aquadrupole mass filter coupled to an electron multiplier.
SIFT-CID24,25 studies were performed by changing the injection potential (Elab)while monitoring the parent and daughter ion intensities. The SIFT-injection potential
is defined as the voltage differential between the source flow tube and the second
flow tube injector. The center of mass energy (Ecom) is calculated from eq 9.09:

225

Ecom =Elab ×MHe
MHe + Mion

(9.09)

where MHe is the mass of helium and Mion is the mass of the parent ion. Since CID
occurs under multiple collision conditions, large ions tend to store energy andfragment at a lower energy than the true threshold energy. Thus, the fragmentationenergies determined in this technique are lower bounds to the true threshold energy.
The gas-phase acidities of both acyl peroxides were obtained using a ionmolecule bracketing technique, where the acyl peroxy ion was SIFT-injected andallowed to react with several reference acids of known acidity. If rapid protontransfer is observed then the acidity of the reference acid is a lower bound to theunknown acidity. If proton transfer is not observed then the acidity of the referenceacid is an upper bound to the unknown acidity. One experimental complication inthese bracketing experiments is that, despite injecting these peroxy ions with minimalenergy, fragment ions due to CID were also present in the second flow tube. Sincesome of these fragment ions are more basic than the acyl peroxy ion, the M-H peakof the reference was always observed even when its acidity is below that of the acyl
peroxide. Additionally, depletion of the parent acyl peroxy ion was also generally
observed since it reacts by neutral O atom loss. In order to determine whether
proton transfer between acyl peroxy ion and the reference acid occurs, the intensitiesof all ions were monitored as a function of time and the contribution from thefragment ions to the reference acid M-H signal was removed.
9.2.3) Electronic Structure Calculations
Two different theoretical models, both provided in the Gaussian 03 suite ofprograms,26 are used to evaluate structural and energetic aspects of the alkyl peroxyions. DFT calculations (B3LYP/6-311++G(d,p))27-29 were used to calculate optimized
geometries, harmonic vibrational frequencies (not scaled), and the rotational226

constants. This level of theory was chosen since it is known to provide reliablemolecular structures. The G3MP2B3 composite technique30 was used to investigate
various thermodynamic properties of these species. In this technique geometries
and zero point energies are calculated at the B3LYP/6-31G(d) level of theory
followed by a series of well-defined ab-initio single-point energy calculations. Thismethod has been tested,30 and the average deviation from experiment is 1.25 kcal
mol-1. The accuracy of this method was independently evaluated in this work with atest group of molecules that was chosen based on their similarity to the compounds
studied here; the average deviation from experiment for the test group was 1.42 kcal
mol-1 (see Table 9.01). Simulated photoelectron spectra were obtained by
calculating the Franck-Condon (FC) factors at a vibrational temperature of 300 K with
the PESCAL program.31 These Franck-Condon simulations are based on optimized
geometries and normal modes from the DFT calculations and electron binding
energies from the G3MP2B3 calculations.
9.3) Results and Discussion
9.3.1) Computational Results
It has been shown both experimentally32,33 and theoretically34 that the lowest
energy structure of these peroxyacids is the cis-planar conformer (E,E) where theperoxy hydrogen atom is hydrogen bonded to the carbonyl oxygen atom. At theG3MP2B3 level of theory this hydrogen bond is calculated to be 1.92 Å inperoxyformic acid and 1.86 Å in peroxyacetic acid. Rotation by 180° about the
OCOO dihedral angle gives a higher energy trans-conformation, where the peroxyhydrogen atom points out of the plane of symmetry. In peroxyacetic acid, rotation
about the OCOO dihedral angle coincides with the rotation of the methyl group; inthe cis-structure the in-plane methyl hydrogen points towards the peroxy group, whilein the trans-structure it points towards the carbonyl oxygen atom. In peroxyformic
227acid rotation by 180° about the COOH dihedral angle gives one further conformer, a(E,Z) cis-planar form, which is the highest energy conformer.
O

O

C
R

(E,E) cis

O

H
O
O

C

C
O

R

trans

R

O
O

H

(E,Z) cisO
H

The potential energy curves for rotation about the OCOO dihedral angle,
which interconvert the trans- and (E,E) cis-structures of peroxyformic acid andperoxyacetic acid, were calculated using G3MP2B3 theory. This method was shown
to reproduce the thermochemical quantities relevant to this study reasonably well fora test group of molecules (see Table 9.01). For peroxyformic acid, the transconformer is ~3.4 kcal mol-1 higher in energy than the cis-conformer; these two
minima are connected by a ~11.1 kcal mol-1 (trans to cis) zero point energy correctedbarrier. The difference in energies of the cis- and trans-conformers reflects thestrength of the internal hydrogen bond. For peroxyacetic acid, the energy difference
between the trans- and cis-conformer increases to ~5.5 kcal mol-1 and a ~8.6 kcal
mol-1 (trans to cis) barrier connects the two minima. These results are summarized inTable 9.02 and the DFT optimized geometries are provided in Tables 9.03 and 9.04.
There are also two stable conformers for the acylperoxy anion and radical
defined by the OCOO dihedral angle. In the trans-configuration the in-plane O-O
bond is trans to the C=O bond with respect to the C-O bond. Rotation by 180° about
the OCOO dihedral angle results in the cis configuration, where the in-plane O-O
bond is cis to the C=O group. In the peroxyformate anion, the trans-structure isstabilized relative to the cis-structure by ~5.0 kcal mol-1; the trans-structure isconnected to the cis-structure by a ~18.3 kcal mol-1 barrier as shown in Figure 9.01.
In the formylradical the trans-structure is only ~2.3 kcal mol-1 lower in energy than228

Table 9.01: Experimental and Calculated (G3MP2B3) Electron Affinities, Enthalpies
of Deprotonation and Bond Dissociation Enthalpies for Alkyl Peroxides andCarboxylic Acids. The calculated values are in parentheses.
R’OH

EA(R’O)eV∆aH0(R’O-H)
kcal mol-1

D0(R’O-H)
kcal mol-1

HOOHa
CH3OOHbC2H5OOHb

1.076 ± 0.006 (1.061)
1.161 ± 0.004 (1.168)
1.186 ± 0.005 (1.175)

375.3 ± 0.8 (377.1)
372.6 ± 1.0 (373.2)
369.9 ± 2.2 (372.8)

86.7 ± 0.8 (88.0)
86.9 ± 1.0 (86.6)
83.9 ± 2.2 (86.4)

HC(O)OHc
3.498 ± 0.015 (3.575) 345.5 ± 0.5 (342.8) 112.8 ± 1.2 (113.1)
CH3C(O)OHd 3.449 ± 0.010 (3.296) 348.9 ± 0.5 (348.0)
112 ± 3 (110.4)abcdReference 35. Reference 36. Reference 37 and 38. Reference 39 and 38.

229

Table 9.02: Relative Energies (kcal mol-1) of the trans- and cis-conformers (goingfrom the trans- to the cis-structure) calculated at the G3MP2B3 level of theory.

Peroxyformic Acid
Peroxyformate Anion
Peroxyformyl Radical
Peroxyacetic Acid
Peroxyacetate Anion
Peroxyacetyl Radical

∆0E
-3.1
5.1
2.4
-5.2
5.7
0.7

∆298H
-3.4
5.0
2.3
-5.5
5.8
0.8

∆298G
-2.8
5.1
2.3
-5.3
5.5
0.3

E‡ (E+Ezpt)
11.1
18.3
8.9
8.6
17.0
5.9

230

O

80

O

C

O

O

H
C
O

H

O

-1

E+Ezpt (kcal mol )

60

C

O

O
O

H

8.9

2.484 eV2.364 eV

40

O
C

O

20

O

O

C
O

H

O

H

H

C

O
O

C
H

O

O
O

18.3

O

0
180

160

140

120

100

80

60

40

20

0

OCOO dihedral (θ)

Figure 9.01: Potential energy surface (E + Ezpt) calculated at the G3MP2B3 level oftheory for the tans- to cis-isomerization of the peroxyformate anion (bottom) and theformylperoxy radical (top)231

Table 9.03: B3LYP/6-311++G(d,p) Optimized Geometries for Peroxyformic Acid, Peroxyformate Anion andPeroxyformyl Radical.
Peroxyformic
Peroxyformate
Peroxyformyl
Acid
Anion
Radical1
12( X A′)( X A′)( X A′′)( A 2A′)
cis
trans
cis
trans
cis
trans
cis
trans
C-H
1.096
1.099
1.119
1.105
1.097
1.099
1.096
1.095
C=O
1.202
1.190
1.222
1.227
1.178
1.179
1.185
1.184
C-O
1.341
1.368
1.308
1.304
1.422
1.427
1.387
1.397
O-O
1.441
1.455
1.455
1.480
1.326
1.328
1.365
1.369
O-H
0.982
0.969
127.3
127.9
122.1
124.0
129.2
130.4
129.0
128.1
∠H-C=O
108.2
112.1
106.6
111.6
105.5
109.7
104.8
112.1
∠H-C-O
124.4
120.1
131.3
124.4
125.3
119.9
125.3
119.8
∠O=C-O
111.8
108.8
117.7
112.5
114.1
111.1
114.1
116.6
∠C-O-O
101.6
99.7∠O-O-H
Bond lengths are in Angstroms and bond angles are in degrees.

232
231

Table 9.04: B3LYP/6-311++G(d,p) Optimized Geometries for Peroxyacetic Acid, Peroxyacetate Anion andPeroxyacetyl Radical.
Peroxyacetic
Peroxyacetate
Peroxyacetyl
Acid
Anion
Radical1
12( X A′)( X A′)( X A′′)( A 2A′)
cis
trans
cis
trans
cis
trans
cis
trans
C-H’
1.099
1.088
1.092
1.092
1.088
1.088
1.088
1.087
C-H
1.092
1.092
1.095
1.092
1.092
1.091
1.092
1.093
C-C
1.500
1.501
1.513
1.513
1.496
1.495
1.503
1.497
C=O
1.209
1.198
1.223
1.234
1.182
1.856
1.189
1.187
C-O
1.355
1.380
1.313
1.314
1.458
1.454
1.406
1.420
O-O
1.440
1.462
1.452
1.471
1.320
1.322
1.364
1.366
O-H
0.984
0.968
111.8
108.0
112.1
108.9
109.2
107.9
112.2
109.5
∠H’-C-C
108.6
110.5
109.1
109.6
109.9
110.4
108.3
109.3
∠H’-C-C
127.2
127.0
121.2
122.6
130.6
129.5
128.8
129.1
∠C-C=O
111.0
117.6
109.7
116.7
107.5
115.8
108.1
113.9
∠C-C-O
121.8
115.4
129.0
120.6
121.9
114.6
123.2
117.0
∠O=C-O
111.6
117.8
117.6
115.5
113.8
114.6
117.1
117.0
∠C-O-O
100.9
98.5∠O-O-H
Bond lengths are in Angstroms and bond angles are in degrees.

233

232

the cis-structure and the isomerization barrier (trans to cis) is ~8.9 kcal mol-1. In theperoxyacetate anion, the trans-structure is stabilized relative to the cis-structure by~5.8 kcal mol-1. Rotation about the OCOO dihedral angle coincides with the rotationof the methyl group and a barrier of ~17.0 kcal mol-1 (trans to cis) separates the two
conformers. In the radical the cis- and trans-structures lie within 1 kcal mol-1 of eachother, with the trans-structure being slightly more stable; a barrier of 5.9 kcal mol-1
separates the two structures. These results are also summarized in Table 9.02 andthe DFT optimized geometries are provided in Tables 9.03 and 9.04.
For both the peroxyformate and peroxyacetate anions, there is a shallow well(<1.6 kcal mol-1) along the isomerization path, which corresponds to a quasi-stabledioxirane intermediate. Under our experimental conditions, it is expected that thisspecies does not exist in any appreciable concentration. However, it is interesting tonote that this observation is consistent with the condensed-phase work of Porter etal.,40 where the base-promoted interchange of the oxygen atoms of peroxybenzoate
anion (C6H5C(O)OO–) indicates that the corresponding dioxirane structure isaccessible from the anion.
9.3.2) Photoelectron Spectra of the Peroxyformate Ion
The 351.1 nm magic angle photoelectron spectrum of the m/z~61 ion
produced from the reaction of HO2⎯ with ethylformate is shown in Figure 9.02a. The
same spectral profile is observed when the m/z~61 ion is produced from the reactionof HO2⎯ with methylformate. In agreement with the ion-chemistry studies of Bowie etal.20, the bicarbonate anion is not the likely product of this reaction, since the EA of
the bicarbonate radical is ~3.9 eV,41 which is outside the range of the laser photon
energy and, therefore, cannot contribute to the observed spectrum. The
photoelectron angular distribution was measured and the anisotropy parameter (β) is234

negative across the entire spectrum, consistent with photodetachment from a π-type
orbital.35,36 The most striking feature of the spectrum is the series of intense peaks(b, e, g, h, i, j, k, and m), which are spaced by ~990 cm-1. With the exception ofpeak b, the width of these peaks is ~50 meV (fwhm), substantially greater than the
instrument resolution (12-14 meV in Figure 9.02a), indicating that each band iscomposed of multiple transitions. To confirm this, the same spectrum was collectedwith a slightly better resolution of 8-9 meV but lower signal to noise ratio; the lower
binding energy portion of this spectrum is shown in Figure 9.02b. With this slightly
higher resolution, a subtle splitting of peaks e, g, and h is observed.
As mentioned above, the trans-conformer of the peroxyformate anion ispredicted to lie ~5 kcal mol-1 lower in energy than the cis-conformer. The BaeyerVilliger reaction of HO2⎯ and ethylformate is predicted to be sufficiently exothermic
such that the cis to trans isomerization barrier lies below the total energy of thereaction. Based on this, the thermal population of peroxyformate anions in oursource is expected to overwhelmingly consist of the trans-configuration; thus, photodetachment should result in the trans-form of the radical. In accordance with this,the calculated EA of the trans-form of the peroxyformyl radical is 2.484 eV, very
close to the position of peak b. In contrast the EA of the cis-form of the peroxyformyl
radial is predicted to be 2.364 eV, which is lower in energy than the observed
spectral features. Using the DFT optimized geometries and harmonic frequencies(Tables 9.03 and 9.05, respectively) Franck-Condon simulations were performed forphotodetachment from the X 1A′ state of the trans-peroxyformate anion to the X 2A′′
state of the trans-peroxyformyl radical. Figures 9.02c and 9.02d show the simulated
stick spectra in blue while the red traces are the result of convoluting the stick235

spectra with a 13 meV or a 9 meV fwhm Gaussian function, respectively. The
simulation adequately predicts the main features of the experimental spectrum. Thatis, the simulation is composed of a series of clustered transitions, which correspondto the positions and intensities of the broad series of peaks (b, e, g, h, i, j, k, and m).
While the simulation predicts that there are more resolvable transitions than are
actually observed, it captures the essence of the spectral features including therelative intensities of the partially resolved peaks at lower electron binding energies(Figure 9.02b). Thus, the spectrum shown in Figures 9.02a and 9.02b is attributed tophotodetachment of the trans-form of the peroxyformate anion. It should be notedthat the spectrum for the cis-conformer was also simulated (Figure 9.03b) and isinconsistent with the experimental data.
Using the FC simulation as a guide, the vibronic features of the X 2A′′ state of
the peroxyformyl radical are assigned. Peak b at 2.493 ± 0.006 eV is the electronic
band origin, which corresponds to the EA of the peroxyformyl radical. The partiallyresolved peaks e1 and e2 are due to the fundamental of the in-plane OCOO
backbone deformation (ν5) and to the fundamental of an O-O stretch (ν4),
respectively. The first and second overtones of ν5 appear in the spectrum as peaks
g1 and h1. Peaks g2 and h2 are due to one and two quanta of ν5 on the ν4fundamental (ν5 + ν4 and 2ν5 + ν4, respectively), while peak h3 is due to two quantaof ν4 on the ν5 fundamental (2ν4 + ν5). The extended progressions in these twomodes enable a more accurate determination of fundamental vibrational frequencies,
as well as crude measurements of the anharmonicity of each mode. Using eq 9.10
we can estimate the harmonic frequencies (ω) and anharmonicity constants (χ) for
these two modes; here, g(n)-g(0) is the energy difference between n and 0 quanta ofa given mode.

236

Table 9.05: B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of Peroxyformate
Anion and Peroxyformyl Radical. Experimental values are provided in parentheses.
symmetry Peroxyformate
Peroxyformylν
Anion
Radical12( X A′)( X A′′)( A 2A′)
1
A′
2952
3053
3080
2
1690
1889 (1821.5a)
1851
3
1335
1335
1379
4
1286
1146 (1078b)
1016
5
866
954 (957.3a; 966b)
954
6
607
586 (574b)
586
7
370
404
333
8
A′′
1043
1009
997
9
220
175
195
a
b
Ar Matrix: reference 19. Gas-phase: this work.

237

a)

i

h1

g

j

l
m

b) h2

h

e

k

g1
e1
e2
b

b

f

Photoelectron Counts

h3

g2

dc

c)

a
d)

O
C
O

H

O-

e)

f)

O
C
H

O
O-

3.2

3.0

2.8

2.6

2.4

2.9

2.8

2.7

2.6

2.5

2.4

Electron Binding Energy, (eV)Figure 9.02: a-b) The 351.1 nm magic-angle photoelectron spectrum of theperoxyformate anion. c-f) Franck-Condon simulations for transitions from X 1A′ stateof the trans-peroxyformate anion to the X 2A′′ state of the trans-peroxyformyl radical.
The sticks are the raw Franck-Condon factors and the solid lines are the 13 meV(left) and 9 meV (right) fwhm Gaussian convolutions. In c-d) the simulation is basedon the B3LYP/6-311++G(d,p) optimized geometries and harmonic frequencies andthe origin position is determined by G3MP2B3 calculations. In e-f) the simulation isbased on the above optimized geometries and harmonic frequencies with the
exception of ν4, ν5, and ν6 in the radical where the experimental frequencies andanharmonicities were used; the calculated EA was slightly shifted (<10 meV) to
match the experiment.

238

h

i

a)

g

j

Photoelectron Counts

l

e

k

m

f

b
dc

a

2.6

2.4

b)
O
C

O
O

H

3.2

3.0

2.8

2.2

Electron Binding Energy, (eV)Figure 9.03: a) The 351.1 nm magic-angle photoelectron spectrum of theperoxyformate anion reproduced from Figure 9.02a. b) Franck-Condon simulation
based on the B3LYP/6-311++G(d,p) optimized geometries and harmonic frequencies
for transitions from X 1A′ state of the cis-peroxyformate anion to the X 2A′′ state of
the cis-peroxyformyl radical. The sticks are the raw Franck-Condon factors and thesolid lines are the 13 meV fwhm Gaussian convolutions. The position of the origin isdetermined by G3MP2B3 calculations.

239

g(n) - g(0) = ω(n + 12 ) − χ (n + 12 )2 − 12 ω + 14 χ

(9.10)The harmonic frequency and anharmonicity constant for the OCOO backbone
deformation (ν5) were found using the 5n0 (n=0-3) and 5n0 410 (n=0-2) progressions tobe ω5 = 973 ± 20 cm-1 and χ5 = 4 ± 3 cm-1. The harmonic frequency andanharmonicity term for the O-O stretch (ν4) were found using the 510 4n0 (n=0-2)
progression to be ω4 =1098 ± 20 cm-1 and χ4 =10 ± 5 cm-1. Using these parameters,the ν5 and ν4 fundamental frequencies were determined to be 966 ± 20 cm-1 and 1078
± 20 cm-1, respectively. The low intensity peak d is the fundamental of the ν6 OCO

bend at 574 ± 35 cm-1, while peaks f and h are due to combination bands of one
quanta of the ν6 OCO bend on the ν5 progression. Peaks a and c are due to excitedanion vibrations. Peak a is the fundamental of the anion ν6 OCO bend at 587 ± 30
cm-1, while peak c is a sequence band of one quanta of the anion ν6 OCO bend withthe radical ν5 OCOO backbone deformation. Combination bands of these threemodes contribute to the intensity at higher binding energies. The intense, broadseries of peaks primarily derives its intensity from an extended progression in the ν5
OCOO backbone deformation 5n0 , along with two combination band progressions,
5n0 410 and 5n0 4 02 . At higher binding energies higher order combination bands ( 5n0 4m0 )congest the spectrum.
Following the analysis of the X 2A′′ state vibronic features, it appears that thediscrepancy between the simulated and the experimental spectrum largely stems
from the overestimate of the O-O stretching frequency. In actuality, the frequenciesof O-O stretch and OCOO backbone deformation are closer to one another than
predicted and, therefore, these two modes are only partially resolved. Additionally,at higher binding energies anharmonic effects are manifested and the harmonic240

simulation overestimates the peak positions. Using the experimentally determinedfrequencies and anharmonicity constants in the Franck-Condon simulation (shown in
Figure 9.02e and 9.02f) greatly improves its quality and the main series emerges.
Additionally, the calculated binding energy was slightly shifted (< 10 meV) to matchthe experimental spectrum. Peak l is notably absent in the simulation. This peak islikely due to photodetachment to a low-lying electronic state of the radical. The
position of this peak relative to the X 2A′′ origin (peak b) is consistent with G3MP2B3
calculations, which find an A 2A′ excited state of the peroxyformyl radical with acalculated Te of 0.791 eV, which is comparable to the X - A energy splittings of otherperoxy radicals.13,24,35,42-44 Based on the simulated Franck-Condon profile forphotodetachment from the X 1A′ state of the anion to the A 2A′ state of the radical(not shown), this feature is either the excited state electronic band origin or the ν7
fundamental; thus, the Te of the peroxyformyl radical is determined to be
0.784 ± 0.045 eV.
9.3.3) Photoelectron Spectra of the Peroxyacetate Ion

The 351.1 nm magic angle photoelectron spectrum of the m/z ~75 ion
produced from the reaction of HO2⎯ with ethylacetate is shown in Figure 9.04. The
same spectral profile is observed when the m/z~75 ion is produced from the reactionof HO2⎯ with methylacetate and with 2,2-difluoroethylacetate. The anisotropy
parameter (β) was measured and is negative across the entire spectrum, consistent
with photodetachment from a π-type orbital.35,36 Since this ion is produced from areaction analogous to that for peroxyformate anion formation, it is likely that the m/z~75 ion is the peroxyacetate anion. To confirm this, Franck-Condon factors for thetransition from the X 1A′ state of the peroxyacetate anion to the X 2A′′ and A 2A′
states of the peroxyacetyl radical were computed. These results are shown in Figure2419.04b and the DFT optimized geometries and harmonic frequencies are provided inTables 9.04 and 9.06. The calculated binding energies for both radical states havebeen slightly shifted (< 20 meV) and the intensities normalized to match theexperimental spectrum. The simulation reproduces the observed spectral features
reasonably well, especially considering the complicated nature of the spectrum.
From this, we conclude that the observed spectral features are due to the transconformer of the peroxyacetate anion. It should be mentioned that the FranckCondon factors for the cis-conformer of the peroxyacetate anion were also simulated(Figure 9.05b) and are inconsistent with the observed spectral features. Additionally,
the possible formation of the hydrogenperoxide ethylenolate anion (CH2C(O⎯)OOH),
which would result from deprotonation of peroxyacetic acid at the methyl group, wasalso investigated. While the deprotonation at the O-H site is favored thermodynamically (by ~5 kcal mol-1), such lack of selectivity has previously been reported
in the deprotonation of acetic acid.45 The presence of this ion was also excluded by
Franck-Condon simulations (Figure 9.05c), which were inconsistent with theexperimental data.
Several features of the X 2A′′ and A 2A′ states of the peroxyacetyl radical canbe assigned with assistance from the Franck-Condon simulation. Peak a at 2.381 ±0.007 eV is the X 1A′ electronic band origin, which corresponds to the EA of theperoxyacetyl radical. The peaks observed at higher binding energies represent
excited vibrational levels of the peroxyacetyl radical. Introduction of the methyl groupat the carbonyl center leads to more complication and congestion in the vibronic
features, compared to the rather simple peroxyformate spectrum. The vibronic
features in the peroxyacetate spectrum are attributed to five modes: an OO
stretching mode (ν7), OCOO backbone deformation (ν8), CCOO backbone242

Table 9.06: B3LYP/6-311++G(d,p) Harmonic Frequencies (cm-1) of Peroxyacetate
Anion and Peroxyacetyl Radical. Experimental values are provided in parentheses.
symmetry Peroxyacetate
Peroxyacetylν
Anion
Radical12( X A′)( X A′′)( A 2A′)
3116
3164
3164
1
A′
2
3033
3061
3043
3
1676
1898 (1850a)
1883
a
4
1450
1460 (1420 )
1460
5
1382
1399 (1367a)
1391
1141 (1143.8b)
61325
1184 (1153a)a
7
1010
1121 (1099 )1005 (928.4b; 929c; 916d)a8934
978 (972 )
984(841.5b)a
9792
726 (736 )
751(759.9b)a
10562
540 (545 )
541(538.1b; 539c)
11
500
505
476
(473.4b; 473c)a12327
322 (348 )
272(269.8b)
13
A′′
3097
3124
3105
14
1443
1463 (1425a)
1471
15
1048
1045 (1029a)
1062
a
556
16
569
540 (500 )
17
205
145
180
18
146
135
141
a
Ne or Ar Matrix: reference 16. b Gas-phase: reference 15. c Gas-phase:reference 13. d Gas-phase: this work.

243

b
c

Photoelectron Counts

d

a

O
C
H3C

O
O

3.2

3.0

2.8

2.6

2.4

2.2

Electron Binding Energy, (eV)Figure 9.04: a) The 351.1 nm magic-angle photoelectron spectrum of theperoxyacetate anion. b) Franck-Condon simulations based on optimized geometries
and normal modes from B3LYP/6-311++G(d,p) calculations for transitions from X 1A′
trans-peroxyacetate anion to X 2A′′ and A 2A′ trans-peroxyacetyl radical. The sticks
are the raw Franck-Condon factors and the solid line is the 13 meV fwhm Gaussianconvolutions. The origin of each simulation was shifted (< 20 meV) and intensities
normalized to correspond to the experimental spectrum.

244

b

a) c

d

Photoelectron Counts

a
b)

O
C
H3C

O
O

c)
O

H
O

C
H2C

3.2

O

3.0

2.8

2.6

2.4

2.2

Electron Binding Energy, (eV)Figure 9.05: a) The 351.1 nm magic-angle photoelectron spectrum of theperoxyacetate anion reproduced from Figure 9.04a. Franck-Condon simulation
based on the B3LYP/6-311++G(d,p) optimized geometries and harmonic frequencies
for transitions from b) the X 1A′ cis-peroxyacetate anion to the X 2A′′ cis-peroxyacetyl
radical and c) from the X 1A′ state of the hydrogenperoxide ethylenolate anion to theX 2A′′ state of the hydrogenperoxide ethylenolyl radical. The sticks are the raw
Franck-Condon factors and the solid lines are the 13 meV fwhm Gaussianconvolutions. The position of the origin is determined by G3MP2B3 calculations.

245

deformation (ν9), OCO bend (ν10), and CCOO breathing (ν12). Combination bands ofthese five modes give rise to a multitude of transitions, which lead to congestion in
the spectrum. The main spectral carriers are the OCOO and CCOO back bone
deformations along with the O-O stretch. The frequencies of these modes havepreviously been determined using matrix isolation IR spectroscopy (see Table
9.06).16 Similar to the above peroxyformate spectrum, the O-O stretch and the
OCOO frequencies are within 130 cm-1 of each other and, therefore, can only bepartially resolved. This fact, combined with the extensive combination band
progressions in the CCOO back bone deformation and O-O stretch, adds to thecomplexity of the spectrum.
The intense narrow peaks appearing at higher electron binding energies are
due to photodetachment to the low-lying A 2A′ electronic state of the radical. Peak b
is the A 2A′ electronic band origin, giving a Te of 0.691 ± 0.009 eV. Peaks c and d
are the fundamental and first overtone of the A 2A′ state OO stretch with a frequencyof 916 ± 45 cm-1. These peak assignments are confirmed by two prior independentmeasurements of the A 2A′ ← X 2A′ transition.13-15
9.3.4) Collision-Induced Dissociation Studies

Measurement of ambient peroxide concentrations can provide insight into theextent of photochemical processing in a given air mass.46 Mass spectrometry has
become an increasingly important tool in the detection and quantification of trace
species in the atmosphere. Since peroxy anions are easily formed from protontransfer to a moderate base, negative ion chemistry presents an alternative to more
traditional positive ion chemical ionization methods. Furthermore, previous studies
on decomposition pathways of organic peroxy anions indicate that collisional246

activation of these ions gives characteristic fragment ions, which can assist in theiranalytical identification.24
Figure 9.06a shows the relative yields of the fragment ions that result fromthe CID of the peroxyformate ion as a function of SIFT-injection energy. The primary
dissociation pathway is the loss of an oxygen atom to form the formate ion. Minor
amounts of OH⎯ and HO2⎯ ions are also observed. The formation of these products
is consistent with an intramolecular hydrogen atom transfer within the activatedanion. The energetics of the various dissociation pathways were evaluated using
electronic structure calculations (G3MP2B3) in eq 9.11a-c; the differences between
these calculated exothermicities are in excellent agreement with the differences in
the known heats of formation of the products.
∆H ~ +29 kcal mol-1(9.11a)→ HO2⎯ + CO

∆H ~ +34 kcal mol-1(9.11b)→ HO⎯ + CO2∆H ~ -46 kcal mol-1(9.11c)HC(O)OO⎯ → HC(O)O⎯ + O

The decomposition of peroxyformic acid by means of an intramolecularhydrogen atom transfer is analogous to the decomposition of the peroxyformyl
radical. In the troposphere there is evidence that the association reaction of theformyl radical with oxygen forms a highly excited peroxyformyl radical intermediate,whose subsequent decomposition forms HO2 + CO and HO + CO2.17-19,47 Several
potential decomposition mechanisms have been suggested including hydrogen atomtransfer to the terminal oxygen atom of the peroxy group via a cyclic transition
state.47-49 Electronic structure calculations (G3MP2B3) were employed to investigate
the unimolecular rearrangement of the peroxyformate ion and a similar mechanism
has been found, as shown in Scheme 9.01, where hydrogen atom transfer and C-O
or O-O bond cleavage occur concomitantly. Due to the strained nature of these

247

1.0
a)
0.8
0.6

HC(O)O2HCO2HO2OH-

0.4

Relative Yield

0.2
0.0
b)
0.8

CH3C(O)O2CH2CO2OH-

0.6

0.4

CH2HCO2CH3CO2-

0.2

0.0
0.0

0.4

0.8

1.2

1.6

2.0

ECOM, (eV)Figure 9.06: Collision Energy (ECOM) dependence of the relative yields of the parent
and daughter ions in the CID of a) the peroxyformate anion and b) the peroxyacetateanion.

248

cyclic transition states, passage over the activation barrier is hindered by entropic
effects. In path a, the ∠HCO is 67.9° and ∠COO is 103.9° in the transition state(compared to 110.7° and 112.5° in peroxyformic acid, respectively) while the CObond increases 0.11 Å. In path b the ∠HCO is 75.9° and ∠COO is 96.9° in thetransition state while the OO bond increases 0.25 Å. The barrier heights for the twochannels are approximately equal, therefore, despite the difference in the reaction
exothermicities, the branching fractions are approximately equal.
Scheme 9.01
O

O

C
H

C
O

O

H

(a)

CO2 + HO

(b)

O

O

O

O

C
H

CO + HO2

C
O
O

O

H
O

The relative yields of the fragment ions that result from the CID of theperoxyacetate ion (eq 9.12a-e) are shown in Figure 9.06b. In analogy to the CID of
the peroxy-formate ion, one would expect that the major fragmentation pathway
would be O atom loss to form the acetate ion. While this pathway is observed (path0.12a), it is a very minor process. Instead, the major ionic products formed are HO⎯
and C2H2O2⎯ (m/z 58), both of which result from an intramolecular hydrogen atomtransfer. As the injection voltage is increased, the C2H2O2⎯ signal peaks around ECOM
= 1.4 eV then decreases. The falloff of the C2H2O2⎯ anion correlates with theformation of CH2⎯, which is likely due to the elimination of CO2 from the nascent249

C2H2O2⎯ species. A minor amount of m/z 45 is observed, which presumably is theformate ion. The mechanism responsible for the formation of this ion is uncertain;
however, one possibility is that the peroxyacetate ion rearranges to form thecorresponding methyldioxiranol ion, which eliminates formaldehyde.
The calculated energetics of the various pathways are shown below:
CH3C(O)OO⎯ → CH3C(O)O⎯ + O∆H ~ +31 kcal mol-1(9.12a)→ C2H2O2⎯ + HO

∆H ~ +25 kcal mol-1(9.12b)→ HC(O)O⎯ + CH2O

∆H ~ -47 kcal mol-1(9.12c)→ HO⎯ + C2H2O2∆H ~ +18 kcal mol-1(9.12d)→ CH2⎯ + OH + CO2∆H ~ +84 kcal mol-1(9.12e)In contrast to the CID of the peroxyformate ion, the decomposition ofperoxyacetate ion primarily occurs by unimolecular rearrangement rather than bydirect dissociation. In this case, intramolecular hydrogen atom transfer to theterminal oxygen atom of the peroxy group proceeds via a five-membered ring where
there is considerably less strain than in the analogous four-membered cyclicperoxyformate transition state. When entropic effects are considered, both cyclicperoxyformate transition states are higher in energy than the direct dissociation
channel. In contrast, the cyclic-peroxyacetate transition state is lower in energy thanthe direct dissociation channel, promoting the hydrogen atom transfer induced
dissociation of the activated anion.
9.3.5) Gas-Phase Acidity Measurements

The gas-phase acidities of peroxyformic acid and peroxyacetic acid were
determined by ion-molecule bracketing experiments. This method is employed when
proton transfer equilibrium constant measurements are not feasible; in this case, theperacid neutral reagent is not available in pure form. In the ion-molecule bracketing250

technique, the acyl peroxy anion is allowed to react with a series of reference acids
and the occurrence or nonoccurrence of proton transfer sets a lower or upper boundon the unknown acidity, respectively. For these bracketing experiments, theacylperoxy anions were formed in the same manner as described above.
The gas-phase acidity of peroxyformic acid was bracketed against fivereference acids: formic acid (338.6 ± 0.5 kcal mol−1), acetic acid (341.2 ± 0.5 kcal
mol−1), hydrogen sulfide (344.89 ± 0.02 kcal mol−1), tert-butylthiol (344.3 ± 3.3 kcal
mol−1), and iso-propylthiol (348.4 ± 0.5 kcal mol−1).38 For the reactions of peroxyformate anion with tert-butylthiol and iso-propylthiol, proton transfer did not occur;
instead these reactions proceed exclusively by neutral O atom loss to form theformate anion. Proton transfer was the only pathway observed in the reactions ofperoxyformate anion with formic acid and acetic acid. For the reaction with formic
acid, proton transfer was confirmed by evaluating the reaction with formic acid-d1,
DCO2H. In this case, an observed increase in m/z 46 is due to proton transfer,whereas the absence of m/z 45 indicates that neutral O atom loss from the peroxyformate anion does not occur. The reaction with hydrogen sulfide primarily proceeds
by O atom loss to form the formate ion. Only a minor amount of the proton transfer
product, HS⎯, was observed, indicating that proton transfer pathway is slightly
endothermic. Based on these results, the acidity of peroxyformic acid is found to be∆aG298 = 344.3 ± 3.3 kcal mol−1, between that of acetic acid and tert-butylthiol.

The gas-phase acidity of peroxyacetic acid was also bracketed against fivereference acids: formic acid, acetic acid, hydrogen sulfide, tert-butylthiol, andethanethiol (348.4 ± 0.5 kcal mol−1).38 Proton transfer was not observed in thereactions of peroxyacetate anion with tert-butylthiol and ethanethiol; instead thesereactions proceed via neutral O atom loss to form the acetate anion. Proton transfer
was observed in the reactions of formic acid and acetic acid. For the latter reaction,
251

the occurrence of proton transfer was confirmed by evaluating the reaction withacetic acid-d3, CD3CO2H, where the formation of m/z 62, instead of m/z 59, isconsistent with proton transfer. The reaction with hydrogen sulfide produces small
amounts of the proton transfer product in addition to the O atom transfer product,suggesting that this reaction is slightly endothermic. Similar to the acidity ofperoxyformic acid, the acidity of peroxyacetic acid is determined to be between thatof acetic acid and tert-butylthiol at ∆aG298 = 344.3 ± 3.3 kcal mol−1.
While these acidity bracketing experiments are simple in their implementation, the interpretation of these results is complicated by the significant structuraldifferences between the peroxy anion and the corresponding acid. As discussed
above, the lowest energy structure of the peroxyacids is a cis-planar conformer
where a strong intramolecular hydrogen bond exists between the peroxy hydrogen
atom and the carbonyl oxygen atom.32-34 Rotation about the OCOO dihedral angle
by 180° gives a higher energy trans-conformation, where the peroxy hydrogen atom
points out of the plane of symmetry. In contrast, the trans-conformer of the anion is
the lower energy structure, as was spectroscopically confirmed in the experiments
described above. Thus, protonation of the anion initially forms the higher energy
trans-conformation of the acid. Since a significant (trans to cis) isomerization barrier
exists in these peroxyacids systems, the reaction may not contain enough energy forthe acid to isomerize to the lower energy cis-conformer. Thus, it is not immediately
clear whether the above gas-phase acidity measurements probe the cis- or transform of the acid, eq 9.13a and b, respectively.

252

O
C

O

R
+

+ H

C
R

H
O

(9.13a)

O
O

O
O

-

C
R

(9.13b)O
O
H

Whether proton transfer leads to the formation of the cis- or trans-form of theacid depends on the size of the isomerization barrier and on the overall energetics ofthe proton transfer reaction; this is qualitatively demonstrated in Figure 9.07. For
each potential energy surface the total energy of the reaction is given by the dottedline and the enthalpies of reaction for the trans- and cis-product channels areindicated. As the strength of the reference acid decreases in going from a to d in
Figure 9.07, the reaction exothermicity decreases; however, the relative energeticsof the two product channels remain constant. If proton transfer is sufficiently
exothermic (a), then the trans-form of the acid can traverse the isomerization barrier
to form the lower energy cis-structure. However, as the strength of the referenceacid decreases (b), the reaction complex does not contain sufficient energy and theacid cannot surmount the isomerization barrier. In this scenario, proton transfer
exclusively leads to the formation of the trans-form of the acid, even though the cisproduct channel is more exothermic. Similarly, as the strength of the reference acidis further decreased such that the formation of the trans-form of the acid is endothermic (c), the reaction does not occur even though the cis-product channel is still
exothermic. Of course, the reaction will also not occur if both product channels areendothermic (d).

253

a)

transcis-b)transcis-transc)cis-transcisd)

Figure 9.07: Representative potential energy surfaces for the proton transfer
reaction of the trans-conformer of the acylperoxy ion with a reference acid where a)
the reaction is exothermic and both the trans- and cis-peroxyacids are formed, b) thereaction is exothermic and only the trans-peroxyacid is formed, c) formation of thecis-peroxyacid is exothermic, however, no reaction occurs, and d) the reaction isendothermic.

254

For peroxyformic acid, the trans-conformer is 3.4 kcal mol-1 higher in energy
than the cis-conformer; these two minima are connected by a ~11.1 kcal mol-1 (transto cis) barrier (see Table 9.02). For peroxyacetic acid, the energy difference
between the trans- and cis-conformer increases to 5.5 kcal mol-1 and a ~8.6 kcal
mol-1 barrier connects the two minima. Since the difference in the acidities of theupper and lower bound reference acids is smaller than the isomerization barrier, theabove acidity measurements of ∆aG298 = 344.3 ± 3.3 kcal mol−1 correspond to thetrans-form of the acids. Thus, these bracketing experiments probe the gas-phase
acidity of the higher energy conformer of the acylperoxy acids. This is consistent
with computations, which find that the gas-phase acidity of trans-peroxyformic acid is342.2 kcal mol-1 while trans-peroxyacetic acid is 343.8 kcal mol-1. The calculated
acidities of the cis-conformers are 345.0 kcal mol-1 and 349.1 kcal mol-1, respectively,
which are inconsistent with the experimental data.
It should be noted that a similar scenario would exist for the reverse gasphase acidity bracket, where the pure neutral acylperoxy acid is reacted with a seriesof reference anions of known basicity. In this case, the acylperoxy acid isoverwhelmingly in the cis-conformation.32,33 Proton abstraction would initially result in
the formation of the cis-conformer of the anion, which then would have to surmount asignificant isomerization barrier (Table 9.02) to form the lower energy transconformer. This acidity bracket would, therefore, probe the gas-phase basicity of thecis-conformer of the acylperoxy anion.
Given the complex nature of these acylperoxide systems it seems that themost feasible approach to determine the gas-phase acidity of the lower energy
conformer of the cis-peroxy acid is through a calculated correction factor that reflects
the relative stability of the cis- and trans-conformers (Table 9.02). In this case,computations are expected to accurately predict this relative stability since the255systematic errors from the ab initio method are likely similar for both conformers and,
therefore, cancel out in the energy difference. Based on this, the gas-phase acidity
is determined to be 347.1 ± 3.3 kcal mol-1 for cis-peroxyformic acid and 349.6 ± 3.3
kcal mol-1 for cis-peroxyacetic acid. Combining these results with the calculated
entropies of deprotonation (∆aS298) allows for the determination of the enthalpy of
deprotonation: ∆aH298(cis-HC(O)OOH) = 354.7 ± 3.3 kcal mol−1; ∆aH298(transHC(O)OOH) = 351.3 ± 3.3 kcal mol−1; ∆aH298(cis-CH3C(O)OOH) = 356.5 ± 3.3 kcal
mol−1; ∆aH298(trans-CH3C(O)OOH) = 351.0 ± 3.3 kcal mol−1.
9.3.6) ThermochemistrySound knowledge of the peroxide O-H bond dissociation energies (BDE, D0)
is important to atmospheric and low-temperature combustion processes.50,51 The
O-H BDE of peroxyformic acid and peroxyacetic acid can be determined from theabove electron affinity and enthalpy of deprotonation measurements using anegative ion thermochemical cycle,52,53 eq 9.14, as discussed in Chapter 1.

D0 (RC(O)OOH) = ∆ a H 0 (RC(O)OOH) + EA(RC(O)OO) − IE(H)(9.14)

Here, R = H and CH3, ∆aH0(RC(O)OOH) is the O-H deprotonation enthalpy of theperoxy acid at 0 K (a small conversion was applied to convert ∆aH0 to ∆aH298),54
EA(RC(O)OO) is the electron affinity of the peroxy radical, and IE(H) is theionization energy of the hydrogen atom (13.59844 eV).55 From the data determined
here, the BDEs of both the cis- and trans-conformers of the peoxyacids can be
determined. For both cases, cleavage of the O-H bond results in the formation of thetrans-form of the radical. The D0(O-H) for the cis-conformer of peroxyformic acid andperoxyacetic acid is determined to be 98.0 ± 3.3 kcal mol-1 and 97.5 ± 3.3 kcal mol-1,
respectively, while the BDEs for the trans-conformers were determined to be 95.7 ±3.3 kcal mol-1 and 93.0 ± 3.3 kcal mol-1, respectively. The uncertainty in the BDE

256measurements mainly reflects the error in the gas-phase acidity measurements.
These BDEs, as well as other thermochemical values reported in this work, aresummarized in Table 9.07.
The O-H BDEs of these peroxyacids are significantly larger than that ofhydrogen peroxide and of alkyl peroxides, which range from 84-87 kcal mol-1 (seeTable 9.01). Simple peroxy radicals such as the hydrogen peroxy and methyl
peroxy radicals have Cs symmetry. In the corresponding peroxide, the peroxyhydrogen atom points out of the ROO plane of symmetry, minimizing the repulsive
interaction between the O-H bonding orbital and lone pair of electrons on theneighboring oxygen atom. In contrast, the lowest energy of peroxyacids is planar,
due to the strong intramolecular hydrogen bond that is formed between the peroxyhydrogen atom and the carbonyl oxygen atom. The formation of this intramolecularhydrogen bond increases the O-H bond strength relative to that of alkyl peroxides, by~3-4 kcal mol-1; the difference between the cis and trans O-H BDEs reflects thestrength of this hydrogen bond. Furthermore, the acyl peroxide is stabilized by theelectron withdrawing effects of the carbonyl group. To illustrate this, consider aresonance structure, where there is a partial positive charge on the neighboring
oxygen atom (shown below), which reduces the unfavorable lone pair-bond pair
O
C
R

O

H
O
O

C
R

+

O

H
O
O

H

C
R

O
O
+

interaction. Cleavage of the O-H bond not only breaks the hydrogen bond but also
elongates the C-OO bond (by ~0.1 Å, see tables 9.03 and 9.04), reducing the C-OO
double character. Thus, the increased bond strength of these peroxyacids is due toback-bonding and hydrogen bonding effects, which stabilize the peroxyacids relative
to an alkyl peroxide. Similar back-bonding interaction stabilizes these acyl peroxy257

Table 9.07: Thermochemical Parameters.
Peroxyformyl Radical and Peroxyformic Acid
EA(HC(O)OO)a
2.493 ± 0.006 eV
a


0.784
± 0.045 eV
Te A - X (HC(O)OO)
b,c∆aG298(trans-HC(O)OO-H)
344.3 ± 3.3 kcal mol-1d,c∆aG298(cis-HC(O)OO-H)
347.1 ± 3.3 kcal mol-1
23.7 cal mol-1 K-1∆aS298(trans-HC(O)OO-H)e,c
25.5 cal mol-1 K-1∆aS298(cis-HC(O)OO-H)e,c∆aH298(trans-HC(O)OO-H)f,c
351.3 ± 3.3 kcal mol-1∆aH298(cis-HC(O)OO-H)f,c
354.7 ± 3.3 kcal mol-1g,cD0(trans-HC(O)OO-H)
95.7 ± 3.3 kcal mol-1
D0(cis-HC(O)OO-H)g,c
98.0 ± 3.3 kcal mol-1
Peroxyacetyl Radical and Peroxyacetic Acid
EA(CH3C(O)OO)a
2.381 ± 0.007 eV
0.69214 ± 0.00006 eV
Te A - X (CH3C(O)OO) a,h∆aG298(trans-CH3C(O)OO-H)b,c
344.3 ± 3.3 kcal mol-1d,c∆aG298(cis-CH3C(O)OO-H)
349.6 ± 3.3 kcal mol-1
22.4 cal mol-1 K-1∆aS298(trans-CH3C(O)OO-H)e,ce,c23.1 cal mol-1 K-1∆aS298(cis-CH3C(O)OO-H)∆aH298(trans-CH3C(O)OO-H)f,c
351.0 ± 3.3 kcal mol-1∆aH298(cis-CH3C(O)OO-H)f,c
356.5 ± 3.3 kcal mol-1g,cD0(trans-CH3C(O)OO-H)
93.0 ± 3.3 kcal mol-1
D0(cis-CH3C(O)OO-H)g,c
97.5 ± 3.3 kcal mol-1a
For trans-conformer of radical. b Bracketed between the acidity of acetic acid andtert-butylthiol. c Defined as the energy to go from the stated reactant to the lowest
energy trans-conformer of the anion or radical. d Determined from the transperoxyacid acidity and a the calculated energy difference between cis- and transconformers. e Computationally determined. f Calculated from the corresponding∆aG298 and ∆aS298. g Calculated using the EA, ∆aH298, IE(H), and a small thermochemical correction (see reference 54) to bring the enthalpy of deprotonation to 0 K.
hReference 13; Te A - X = 0.691 ± 0.009 eV, this work.

258

anions relative to that of an alkyl peroxy anion. As a result, the enthalpy of deprotonation of these acyl peroxides is less than that of an alkyl peroxide while the electron
affinity of these acyl peroxy radicals is higher than that of an alkyl peroxy radical.
It is surprising, given the importance of these peroxyacids, that their heats offormation have not been experimentally determined. Using G3MP2B3 calculations,
we have calculated the heats of formation of peroxyformic acid and peroxyacetic acidfrom the following isodesmic reaction:
RC(O)OOH + H2O → RC(O)OH + H2O2

(9.15)

In eq 9.15, the heat of formation of the peroxyacid is determined from the calculated
heat of reaction and the known heats of formation of the other three compounds.56
Since the bonding environments in the reactants and products are similar, thesystematic errors from the ab initio method cancel and, therefore, this method is
expected to produce reasonably good heats of formation for closed shell species.
Using this method, the heat of formation at 298 K of cis-peroxyformic acidwas determined to be -69.0 ± 1.2 kcal mol-1. This value is slightly less than the
CCSD(T)/CBS atomization energy calculation of Feller et al.;57 however, thedifference in these two values is equal to the relative stability of the cis- and transconformers and, therefore, it is unclear to which conformer Feller’s value applies.
The heat of formation of the cis-peroxyacetic acid was found to be -83.2 ± 1.2 kcal
mol-1, in good agreement with the calculations of Bozzelli et al.58 and Benassi et al.59
The heat of formation of the acyl peroxy radicals can be determined from thecalculated heats of formation of the acyl peroxides and the above O-H BDEs
measurements (see Chapter 1). The heat of formation of the formyl peroxy radical isdetermined to be -22.5 ± 4.5 kcal mol-1 while the acetyl peroxy radical is found to be-37.3 ± 4.5 kcal mol-1.

2599.4) Conclusions

The 351.1 nm photoelectron spectrum of the peroxyformate anion wasmeasured and the EA of the trans-peroxyformyl radical was determined to be 2.493± 0.006 eV and the Te of the A 2A′ state is 0.784 ± 0.045 eV. In the X 2A′′ state of

the peroxyformyl radical, the harmonic frequency and anharmonicity correction areω5 = 973 ± 20 cm-1 and χ5 = 4 ± 3 cm-1 for the ν5 OO stretch and ω4 = 1098 ± 20 cm-1
and χ4 = 10 ± 5 cm-1 for the ν4 OCOO backbone deformation. The fundamental
frequency of the in plane OCO bend is 574 ± 35 cm-1. The gas-phase acidity oftrans-pexoxyformic acid was bracketed between the acidity of acetic acid and tertbutylthiol, ∆aG298(trans-peroxyformic acid) = 344.3 ± 3.3 kcal mol-1 and ∆aH298(transperoxyformic acid) = 351.3 ± 3.3 kcal mol-1. The gas-phase acidity of cispexoxyformic acid was found from the trans-conformer acidity and a calculated
energy correction to be ∆aG298(cis-peroxyformic acid) = 347.1 ± 3.3 kcal mol-1 and∆aH298(cis-peroxyformic acid) = 354.7 ± 3.3 kcal mol-1. The O-H BDE for both thecis- and trans-conformers were determined using negative ion thermodynamic cycle
to be D0(trans-peroxyformic acid) = 95.7 ± 3.3 kcal mol-l and D0(cis-peroxyformicacid) = 98.0 ± 3.3 kcal mol-1.
The 351.1 nm photoelectron spectrum of the peroxyacetate anion wasmeasured and is in good agreement with the calculated FC factors for the X 1A′
state of the anion to the X 2A′′ and A 2A′ states of the peroxyacetyl radical. The EAof the radical is 2.381 ± 0.008 eV and the Te of the A 2A′ states is 0.691 ± 0.009 eV,
which is in excellent agreement with two prior values.13-15 The gas-phase acidity oftrans-peroxyacetic acid was bracketed between the acidity of acetic acid and tertbutylthiol, ∆aG298(trans-peroxyacetic acid) = 344.3 ± 3.3 kcal mol-1 and ∆aH298(transperoxyacetic acid) = 351.0 ± 3.3 kcal mol-1. The acidity of cis-peroxyacetic acid was260

found by adding a calculated energy correction to the acidity of trans-conformer;∆aG298(cis-peroxyacetic acid) = 349.6 ± 3.3 kcal mol-1 and ∆aH298(cis-peroxyaceticacid) = 356.5 ± 3.3 kcal mol-1. The O-H BDE for both conformers were determinedusing negative ion thermodynamic cycle to be D0(trans-peroxyacetic acid) = 93.0 ±3.3 kcal mol-l and D0(cis-peroxyacetic acid) = 97.5 ± 3.3 kcal mol-1.
9.5) Acknowledgements

This work benefited from the contributions of Ms. Nicole Eyet, Mr. Scott W.
Wren, Ms. Kristen M Vogelhuber, Prof. Veronica M. Bierbaum, Prof. W. Carl
Lineberger, and Prof. G. Barney Ellison. Additionally, I would like to thank Prof.
Charles H. DePuy, Prof. Kent Ervin, Prof. John F. Stanton, Prof. Stephen J.
Blanksby, Dr. Bradley A. Flowers, and Dr. Shuji Kato for providing useful comments.
This work was supported by the AFOSR (FA9550-06-1-006). The computational
results are based upon work supported by the National Science Foundation under
the following NSF programs: Partnerships for Advanced Computational
Infrastructure, Distributed Terascale Facility (DTF) and Terascale Extensions:
Enhancements to the Extensible Terascale Facility.

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