Ultraviolet Photoelectron Spectroscopyof Organic AnionsbyAdam J. Gianola

B.S., University of Nevada-Reno, 2000

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry2006This thesis entitled:
Ultraviolet Photoelectron Spectroscopyof Organic Anions
Written by Adam J. Gianola
has been approved for
The Department of Chemistry and Biochemistryby___________________________
W. Carl Lineberger___________________________
J. Mathias Weber

Date: ___________________

The final copy of this thesis has been examined by both the signatories, and we find that boththe content and the form meet acceptable presentation standards of scholarly work in theabove mentioned discipline.

iii
Gianola, Adam J. (Ph.D. Physical Chemistry)
Ultraviolet Photoelectron Spectroscopy of Organic Anions
Thesis directed by Professor W. Carl Lineberger

Ultraviolet photoelectron spectroscopy has been used to study several negative
organic ions and their corresponding neutrals: cyclopentadienide, pyrrolide, imidazolide,pyrazolide, vinyl diazomethyl anion, and 2-oxepinoxide. The photoelectron spectra obtained
for these systems gives structural, vibrational, and electronic information about thecorresponding neutral radicals.
Several of the ions listed above are related:

cyclopentadienide, pyrrolide,imidazolide and pyrazolide. Each is a 5-membered ring system, with 0, 1, or 2 nitrogenatoms in the ring. The cyclopentadienyl radical is a well known radical exhibiting JahnTeller distortion, and these effects are evident in the photoelectron spectrum of
cyclopentadienide. Replacement of one C—H group in cyclopentadienide by an N atom
gives the pyrrolide ion. Jahn-Teller effects are no longer present in the radical counterpart,
pyrrolyl; however, electronic interactions between the ground and first excited states areevident in the photoelectron spectrum.

Imidazolide and pyrazolide are isomers of one

another, and each has two N atoms in the ring. In imidazolide the N atoms are separated,while in pyrazolide they are adjacent. The corresponding radicals (imidazolyl and pyrazolyl)
also show interesting electronic interactions between their ground and first excited states. In
imidazolyl the interactions are weak, and the effects are not evident in the photoelectron
spectrum. In pyrazolyl they are significantly stronger, and unexpected vibrational bands
appear in the photoelectron spectrum as a result.
Also observed in the spectra of imidazolide and pyrazolide were photoelectron
signals from other isomers. These isomers are nearly structurally identical to the imidazolide
and pyrazolide ions, with the exception that a hydrogen atom is displaced from a carbon siteiv
to a nitrogen site.

The photoelectron spectra of these isomers appear very different.

Electronic interactions in the analogous radicals are not present, and the photoelectron spectra
appear very Franck-Condon like. Gas-Phase acidities for these ions were also obtained
through bracketing experiments. Combination of the electron affinities obtained from thephotoelectron spectra with these gas-phase acidities allows for determination of a C—H bonddissociation enthalpy of the parent molecules imidazole and pyrazole.
Although not a ring system, the vinyldiazomethyl anion is an isomer of imidazolide
and pyrazolide. It can be thought of as a ring opened pyrazolide ion (by breaking a C—Nbond). Its photoelectron spectrum is rather simple, appearing very Franck-Condon like.
Interestingly, it appears that the spectrum originates mostly from the E-vinyldiazomethyl
anion only; there is little indication of the presence of the Z-isomer.

Franck-Condon

simulations show each isomer would have a different spectral signature.
The 2-oxepinoxide anion is a seven-membered ring containing oxygen.

Thecorresponding neutral, the 2-oxepinoxy radical, is a proposed intermediate in the combustionof benzene. The phenyl radical is expected to be formed first in a benzene flame, and many
pathways for reaction of phenyl radical with O2 have been studied theoretically. The phenylperoxy radical is thought to isomerize into the 2-oxepinoxy radical as a first step to theformation of CO, CO2, and other combustion products. Production of the 2-oxepinoxide ion
and subsequent photodetachment to form the 2-oxepinoxy radical is one of the firstexperimental steps to be taken to investigate these combustion pathways.

v
Acknowledgements

My advisor, Professor Carl Lineberger, is the first of a long list of people I would liketo thank for assistance in my graduate career. Carl is a great advisor who has given me thechance to work in a fantastic place and interact with and meet many other people who share
his enthusiasm for science. His ability to identify trouble spots with experiments and his
physical intuition into the meanings of results still amaze me to this day. Somehow, Carl has
managed to organize and run a research group which does high quality and very interestingscience without sacrificing the comfort of an enjoyable work environment. Perhaps it is theexperiments or maybe Carl himself who attracts students and postdocs who share common
interests and similar attitudes.
It is also possible that these people are attracted by JILA.

The facilities andpersonnel in JILA are surely among the best in the world. I would particularly like to thankJames Fung-A-Fat from the electronics shop for his expertise with the electronics of thephotoelectron spectrometer. Without his help, it would have been nearly impossible tonavigate through the ‘dark ages’ and perform the experiments contained in this thesis. I think
he still has the old voltage programmer sitting on a shelf next to his desk. I also thank Hans
Green, Kim Hagen, and Todd Asnicar from the machine shop, who were always willing todrop whatever they were doing and give me a hand at a moments notice.
I would also like to thank Professors Barney Ellison and Veronica Bierbaum, whom I
have had many close interactions with. Barney and I have had many informal discussions,which usually start with me intending to get his opinion on an experiment or result andalways end with me being extremely interested in whatever unrelated topic we talked about
instead. Regardless of this, Barney’s enthusiasm for the research I have been working on hasconstantly helped me to maintain a positive attitude throughout graduate school. Veronica
Bierbaum has been one of the most helpful people I have encountered in graduate school.

vi
Several of the papers on my publication list have been in collaboration with her group, andwithout her careful attention to detail these papers would surely have numerous errors whichwere overlooked by several others including myself.
Two other professors who deserve my thanks are Professor Barry Carpenter andProfessor John Stanton. Professor Carpenter suggested the experiments which are containedin Chapter 8 to me. He also helped me to realize how skillful I actually am in the lab when I
put all my efforts forth. Professor Stanton was the person who suggested an elegant solution
to the issues with my results that I didn’t quite understand initially. He also has been ahelpful friend who I felt comfortable speaking with very openly and honestly, who offered
good advice to boot.
I want to thank all the members of the Lineberger, Ellison, and Bierbaum groups fortheir friendship and help over the years: Mark Taylor, Todd Sanford, Jack Barbera, Jeff
Rathbone, Stephanie Villano, Alan Maccarone, Xu Xhang, Tim D’Andrea, Shuji Kato,
Rebecca Hoenigman, and Nick Betts. I especially wish to thank Evan Jochnowitz, Django
Andrews, and Jon Goldwin for being great friends and helping me out through the extremely
difficult times of graduate school and life. There are also many other friends I met throughthe Chemistry Department who have entertained me at karaoke night and helped make mylife in graduate school a little more enjoyable and probably a little longer: Chris Downey,
Melissa Trainer, Aaron Miller, Michele Jacobson, Carrie Stoffel, Aaron Hieb and Jen Boots.
Coincidentally, many of these people at one time or another joined me on the ice rink in our
intramural ice hockey team, the Ice Buckets. Special thanks also go to Mary Ellen Flynn,
Amanda Daflos, and Katarina Miller for being there to not talk science.
I also thank my parents and sisters who have never fully realized what I was still
doing in school.

Many times I heard “How are classes going?” or “When are you

graduating?” when the answers to those question were usually “What classes?” and “I don’t

vii
know.” Still, they have always been supportive and interested in whatever I was working toaccomplish.
Finally, I wish to thank Danielle Blank for her love, support, and encouragement overthe past few years. Whether she understood them or not, she has always been willing to listen
to my problems (both scientific and otherwise) and I really have valued her opinion on
practical matters concerning life and other day to day stuff. She has helped me talk out anddeal with many problems which I’m sure other people were tired of hearing about. Without
her I’m sure I would be in the funny farm by now!

viii
Contents

1

Introduction.......................................................................................................................................1
1.1

Anion Photoelectron Spectroscopy ........................................................................................1

1.2

Photoelectron Intensities and Selection Rules........................................................................6

1.3

Nonadiabatic Effects ..............................................................................................................9

1.4

Photoelectron Angular Distributions....................................................................................12

1.4

Dissertation Outline .............................................................................................................13

Chapter 1 References ......................................................................................................................16
2

Methodology ...................................................................................................................................18
2.1

Overview..............................................................................................................................18

2.2

Ion Source ............................................................................................................................18

2.3

Ion Optics and Mass Selection .............................................................................................21

2.4

Ultraviolet Laser System......................................................................................................28

2.5

Photoelectron Energy Analysis and Detection .....................................................................32

2.6

Data Collection.....................................................................................................................33

2.7

Spectral Simulation and Assignment ...................................................................................36

2.8

Thermodynamics..................................................................................................................37

Chapter 2 References ......................................................................................................................40

3

Cyclopentadienide, C5H5− ...............................................................................................................42
3.1

Introduction..........................................................................................................................42

3.2

Experiment ...........................................................................................................................43

3.3

Results..................................................................................................................................44

3.4

Discussion ............................................................................................................................54

3.5

Conclusion ...........................................................................................................................56

Chapter 3 References ......................................................................................................................59

ix
4

Pyrrolide, C4H4N− ...........................................................................................................................62
4.1

Introduction..........................................................................................................................62

4.2

Experiment ...........................................................................................................................63

4.3

Results..................................................................................................................................63

4.4

Discussion ............................................................................................................................72

4.5

Conclusion ...........................................................................................................................76

Chapter 4 References ......................................................................................................................79

5

Imidazolide, C3H3N2− ......................................................................................................................81
5.1

Introduction..........................................................................................................................81

5.2

Experiment ...........................................................................................................................82

5.3

Results..................................................................................................................................82

5.4

Discussion ............................................................................................................................97

5.5

Conclusion .........................................................................................................................101

Chapter 5 References ....................................................................................................................103

6

Pyrazolide, C3H3N2− ......................................................................................................................105
6.1

Introduction........................................................................................................................105

6.2

Experiment .........................................................................................................................106

6.3

Results................................................................................................................................106

6.4

Discussion ..........................................................................................................................117

6.5

Conclusion .........................................................................................................................127

Chapter 6 References ....................................................................................................................130

7

Vinyldiazomethyl Anion, C3H3N2− ...............................................................................................132
7.1

Introduction........................................................................................................................132

7.2

Experiment .........................................................................................................................133

7.3

Results................................................................................................................................133

7.4

Discussion ..........................................................................................................................140x
7.5

Conclusion .........................................................................................................................144

Chapter 7 References ....................................................................................................................147

8

2-Oxepinoxide, C6H5O2− ...............................................................................................................148
8.1

Introduction........................................................................................................................148

8.2

Experiment .........................................................................................................................149

8.3

Results................................................................................................................................149

8.4

Discussion ..........................................................................................................................155

8.5

Conclusion .........................................................................................................................158

Chapter 8 References ....................................................................................................................161
9

Summary and Conclusions............................................................................................................162
9.1

Introduction........................................................................................................................162

9.2

Electron Affinities..............................................................................................................163

9.2

Bond Dissociation Energies and Heats of Formation.........................................................165

9.3

Electronic and Vibrational Structure..................................................................................167

9.4

Future Work .......................................................................................................................170

Chapter 9 References ....................................................................................................................171
Bibliography.........................................................................................................................................173

xi
Tables and Figures

Figure 1.1

363.8 nm (3.408 eV) photoelectron spectrum of atomic oxygen anion (O−). The
spectrum is composed of six transitions, from the O− (2P3/2,1/2) states to the O (3P2,1,0)
states.7 ...............................................................................................................................2

Figure 1.2

Schematic representation of photodetachment from molecular anion AB− to two
possible electronic states of neutral molecule AB. ...........................................................4

Figure 1.3

Harmonic oscillator potential energy curves showing a large geometry change upon
photodetachment. ...........................................................................................................11

Figure 2.1

Microwave Discharge Ion Source. A brass microwave cavity encloses a quartz
discharge cylinder. The microwave discharge is connected to the flow tube where ionsreact with neutral reagents introduced through movable ring inlets...............................19

Figure 2.2

Flow tube and afterglow ion optics. The nosecone is labeled N, acceleration lenses arelabeled L1-L6, deflectors are labeled D1 and D2, and the aperture is labeled A1. ........22

Figure 2.3

10 degree deflectors (D3), first 3-element quadrupole einzel lens (Q1), second aperture(A2) and fourth deflectors (D4)......................................................................................23

Table 2.1

Typical potentials for the photoelectron spectrometer....................................................24

Figure 2.4

Wien filter region of the photoelectron spectrometer. Ions are focused into the Wien
filter with 3-element quadrupole Q2 and mass selected inside the Wien filter. The mass
selected ions are refocused with 3-element quadrupole Q3 and pass through a shortfield-free region..............................................................................................................26

Figure 2.5

Construction of the Wien filter electric field plates. Five pairs of parallel stainless steel
shims on each side (+ and −) and two electrodes (dark grey) have voltages distributed togive a vertical electric field equivalent to infinite parallel plates. Teflon shims (lightgrey) are placed between the stainless steel shims to insulate them from each other. The
ion trajectory is into the plane of the page, and the magnetic field lines run from left toright. ...............................................................................................................................27

Figure 2.6

Vacuum chamber containing the final elements of the ion beam path and the electronenergy analysis and detection components.....................................................................29

Figure 2.7

Components of the laser system. The system is made up of an argon-ion laser and thevacuum chamber serves as an external build-up cavity. The back mirror of the laser andthe front cavity window are mounted on piezoelectric translators. The acousto-optic

xii
modulator (AOM), piezos, and photodiodes are part of a servo system which maintainefficient build-up within the vacuum chamber. ..............................................................30
Figure 2.8

Cutaway of the electron kinetic energy analyzer system. The deceleration lenses andanalyzer lenses (V1-V6, VHC) are made from molybdenum, and the concentrichemispheres of the analyzer are made from oxygen-free high conductivity copper. .....34

Figure 2.9

Gas-phase thermochemical cycles. The negative ion cycle combines electron affinity
measurements with gas phase acidity measurements to determine R—H bonddissociation energies.......................................................................................................39

Figure 3.1

Top panel: 488.0 nm photoelectron spectrum of C5H5− from reference 32. Bottom
panel: 351.1 nm magic angle photoelectron spectrum of C5H5− taken at 200 K. ..........45

Figure 3.2

351.1 nm photoelectron spectra of C5H5− taken at 200 K. The red curve was obtainedwith the laser at vertical polarization (θ = 0°), and the blue curve was obtained with thelaser at horizontal polarization (θ = 90°). .......................................................................46

Figure 3.3

Expanded view of the 351.1 nm photoelectron spectrum of C5H5− taken at 200 K withthe laser at vertical polarization (θ = 0°). .......................................................................47

Table 3.1

Relative experimental peak positions and anisotropy parameters for peaks appearing in
Figures 3.1 - 3.3..............................................................................................................48

Table 3.2

C5H5− fundamental vibrational frequencies obtained from B3LYP/6-311++G(d,p)
calculations.....................................................................................................................50

Table 3.3

C5H5− and C5H5• geometries obtained from B3LYP/6-311++G(d,p) calculations. ........51

Table 3.4

C5H5• fundamental vibrational frequencies obtained from B3LYP/6-311++G(d,p)
calculations.....................................................................................................................52

Figure 3.4

351.1 nm magic angle photoelectron spectrum of C5H5− taken at 200 K (black) andFranck-Condon simulations of detachment of 1A1' C5H5− to 2A2 (blue) and 2B1 (red)C5H5•. .............................................................................................................................53

Table 3.5

Assignments of C5H5• vibrational features observed in the photoelectron spectrum of
C5H5−. .............................................................................................................................58

Figure 4.1

363.8 nm magic angle photoelectron spectrum of pyrrolide at 298 K............................64

xiii
Figure 4.2

363.8 nm photoelectron spectra of pyrrolide taken at 298 K. The blue curve wasobtained with the laser at vertical polarization (θ = 0°), and the red curve was obtainedwith the laser at horizontal polarization (θ = 90°). .........................................................65

Table 4.1

Pyrrolide and pyrrolyl geometries obtained from the B3LYP/6-311++G(d,p)
calculations.....................................................................................................................67

Table 4.2

Pyrrolide and pyrrolyl fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations. .............................................................................68

Figure 4.3

Three highest occupied molecular orbitals of pyrrolide and the schematic representationof electron photodetachment from pyrrolide to form three electronic states of pyrrolyl
radical. ............................................................................................................................69

Figure 4.4

Franck-Condon simulations of detachment of 1A1 pyrrolide to 2A2 (blue) and 2B1 (red)pyrrolyl. The binding energy for the 2A2 origin peak was adjusted to match theexperimental spectrum (black) while the 2B1 origin peak position was adjusted to matchthe DFT term energy (~0.49 eV). ...................................................................................70

Figure 4.5

Relative atomic displacements in the normal modes for pyrrolyl radical. Three a1
modes for the 2A2 state of pyrrolyl which are active in the photoelectron spectrum, andone b2 mode for the 2B1 state of pyrrolyl which has an imaginary frequency. ...............71

Figure 4.6

Adiabatic potential energy surface of the two lowest electronic states ofcyclopentadienyl radical.................................................................................................77

Figure 4.7

Adiabatic potential energy surface of the two lowest electronic states of the pyrrolyl
radical. ............................................................................................................................77

Figure 4.8

Schematic illustration of the potential energy curves of the two lowest electronic statesof the pyrrolyl radical, corresponding to a slice of the potential energy surfaces shownin Figure 4.7 along the totally symmetric coordinate. The potential energy separations
were calculated at the B3LYP/6-311++G(d,p) level. .....................................................78

Table 5.1

Relative experimental peak positions for peaks labeled in Figure 5.1............................83

Figure 5.1

351.1 nm magic angle photoelectron spectrum of 1-imidazolide at 298 K. ...................84

Figure 5.2

351.1 nm photoelectron spectra of 1-imidazolide taken at 298 K. The blue curve wasobtained with the laser at vertical polarization (θ = 0°), and the red curve was obtainedwith the laser at horizontal polarization (θ = 90°). .........................................................85

xiv
Table 5.2

1-Imidazolide and 1-imidazolyl geometries obtained from B3LYP/6-311++G(d,p)
calculations.....................................................................................................................87

Table 5.3

1-Imidazolide and 1-imidazolyl fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations. .............................................................................88

Figure 5.3

Three highest occupied molecular orbitals of 1-imidazolide and the schematic
representation of electron photodetachment from 1-imidazolide to form three electronic
states of 1-imidazolyl. ....................................................................................................89

Figure 5.4

Franck-Condon simulation of detachment from 1A1 1-imidazolide to 2B1 1-imidazolyl(red) overlaid on the experimental spectrum (black)......................................................90

Figure 5.5

Portion of the 351.1 nm magic angle photoelectron spectrum of imidazolide anions
produced from the imidazole + HO− reaction at 298 K (black) and simulation of
detachment of 5-imidazolide (red). The peak at ~2.45 eV is the 1-imidazolide hot bandand the intense origin peak of 1-imidazolyl grows in at >2.5 eV. ..................................93

Figure 5.6

B3LYP/6-311+G(d,p) reaction enthalpies for proton transfer reactions at different
positions of imidazole.....................................................................................................94

Table 5.4

5-Imidazolide and 5-imidazolyl geometries obtained from B3LYP/6-311++G(d,p)
calculations.....................................................................................................................95

Table 5.5

5-Imidazolide and 5-imidazolyl fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations. .............................................................................96

Table 5.6

Geometry displacement parameters, K, in dimensionless normal coordinates for 2B1
1-imidazolyl – 1A1 1-imidazolide and 2B1 1-imidazolyl – 2A2 1-imidazolyl. .................99

Figure 6.1

351.1 nm magic angle photoelectron spectrum of pyrazolide at 298 K........................107

Figure 6.2

DFT reaction enthalpies evaluated for HO− deprotonation of pyrazole at different
positions. ......................................................................................................................108

Figure 6.3

351.1 nm magic angle photoelectron spectrum of 1-pyrazolide produced under liquid
nitrogen cooled flow tube conditions. ..........................................................................110

Table 6.1

Experimental peak positions and anisotropy parameters for peaks appearing in thephotoelectron spectrum of 1-pyrazolide. ......................................................................112

xv
Table 6.2

1-Pyrazolide and 1-pyrazolyl geometries obtained from B3LYP/6-311++G(d,p)
calculations...................................................................................................................113

Table 6.3

1-Pyrazolide and 1-pyrazolyl fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations. ...........................................................................114

Figure 6.4

Franck-Condon simulation of detachment of 1-pyrazolide. The red curve is thesimulation for detachment to 2A2 1-pyrazolyl, the blue curve is the simulation for
detachment to 2B1 1-pyrazolyl. .....................................................................................115

Figure 6.5

351.1 nm magic angle photoelectron spectrum of 5-pyrazolide produced in the reactionof HO− with pyrazole....................................................................................................118

Table 6.4

Experimental peak positions and assignments for peaks appearing in the photoelectron
spectrum of 5-pyrazolide (Figure 6.5). .........................................................................119

Table 6.5

5-Pyrazolide and 5-pyrazolyl geometries obtained from B3LYP/6-311++G(d,p)
calculations...................................................................................................................120

Table 6.6

5-Pyrazolide and 5-pyrazolyl fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations. ...........................................................................121

Figure 6.6

Franck-Condon simulation (red) of detachment of 5-pyrazolide based on the optimized
geometries and vibrational frequencies obtained from the B3LYP/6-311++G(d,p)
calculations overlaid on the experimental spectrum (black).........................................122

Figure 6.7

Relative atomic displacements and harmonic frequencies of normal modes of
5-pyrazolyl active in the photoelectron spectrum evaluated at theB3LYP/6-311++G(d,p) level of theory. .......................................................................123

Figure 6.8

Three highest occupied molecular orbitals of 1-pyrazolide and the schematic
representation of electron photodetachment from 1-pyrazolide to form three electronic
states of 1-pyrazolyl. ....................................................................................................125

Figure 6.9

Resonance structures of 1-pyrazolide showing migration of the negative charge aroundthe ring..........................................................................................................................129

Figure 7.1

351.1 nm magic angle photoelectron spectrum of vinyldiazomethyl anion at 298 K. See
Table 7.1 for assignments of labels. .............................................................................134

Table 7.1

Relative experimental peak positions and assignments for the peaks labeled in Figures
7.1 and 7.2. ...................................................................................................................135

xvi
Table 7.2

E- and Z-vinyldiazomethyl anion and radical geometries obtained fromB3LYP/6-311++G(d,p) calculations. ...........................................................................137

Table 7.3

E- and Z-vinyldiazomethyl anion and radical fundamental vibrational frequencies
obtained from B3LYP/6-311++G(d,p) calculations. ....................................................138

Figure 7.2

Expanded view of the 351.1 nm magic angle photoelectron spectrum ofvinyldiazomethyl anion at 298 K (black) and Franck-Condon simulation of detachmentof E-vinyldiazomethyl anion (red) and Z-vinyldiazomethyl anion (blue). ...................139

Figure 7.3

Relative atomic displacements and harmonic frequencies of normal modes ofE-vinyldiazomethyl radical evaluated at B3LYP/6-311++G(d,p) level. ......................141

Figure 7.4

Reaction profile for ring-closure of vinyldiazomethyl anion. Potential surface energies
are relative to E-vinyldiazomethyl anion and are computed at theB3LYP/6-311++G(d,p) level. Energies are in kcal mol-1. ...........................................145

Figure 7.5

Reaction profile for ring-closure of vinyldiazomethane to ultimately form 1H-pyrazole.
Potential surface energies are relative to E-vinyldiazomethane and are computed at theB3LYP/6-311++G(d,p) level. Energies are in kcal mol-1. ...........................................146

Figure 8.1

351.1 nm photoelectron spectrum of 2-oxepinoxide at 300 K taken at the magic angle(54.7°, black), parallel (0°, blue) and perpendicular (90°, red) to the photoelectron
collection axis...............................................................................................................150

Table 8.1

2-Oxepinoxide anion and 2-oxepinoxyl radical geometries in Cartesian coordinatesobtained from the B3LYP/6-31+G(d) calculations. .....................................................152

Table 8.2

2-Oxepinoxide anion and 2-oxepinoxyl radical fundamental vibrational frequencies
obtained from B3LYP/6-31+G(d) calculations. ...........................................................153

Figure 8.2

Franck-Condon simulation of 2-oxepinoxide 351.1 nm photodetachment at 0 K with acutoff level of 100% (black), 10% (blue), and 1% (red) of the origin intensity. The
position of the origin is indicated by the green line. The experimental spectrum isoverlaid in grey.............................................................................................................154

Figure 8.3

Franck-Condon simulation of 2-oxepinoxide 351.1 nm photodetachment at 300 K witha cutoff level of 100% (black), 10% (blue), and 1% (red) of the origin intensity. The
position of the origin is indicated by the green line.
The experimental spectrum isoverlaid in grey.............................................................................................................156

Figure 8.4

Potential energy curves illustrating the B3LYP computed vertical detachment energy(VDE) and electron affinity (EA). The top values were computed using the 6-31+G(d)xviibasis set, the middle values using the 6-311++G(d,p) basis set and the bottom values
using the aug-cc-pVTZ basis set. .................................................................................159
Table 8.3

Experimental and B3LYP computed electron affinities. ..............................................160

Figure 9.1

EAs of the 5-membered ring radicals with the corresponding number of nitrogen atoms
in the 5-membered ring. The black points are experimental values and the red points
are DFT calculated EAs. The solid line is a linear regression fit of the data to a line, andthe green point is the EA predicted for pentazolyl radical from the extrapolation. ......164

Figure 9.2

N—H bond dissociation energies of the 5-membered heterocyclic compounds as afunction of the number of nitrogen atoms in the 5-membered ring. The black points are
the experimental data, and the red points are DFT computed values. The solid line is alinear regression fit of the data to a line, and the green point is the N—H bonddissociation energy predicted for hydrogen pentazole from the extrapolation. ............166

Figure 9.3

298 K heats of formation of 5-membered ring radicals................................................168

________________________________________________
1
Introduction________________________________________________
1.1

Anion Photoelectron Spectroscopy
Photoelectron spectroscopy is a useful tool for studying the properties of atomic andmolecular anions, neutrals, and cations.1-3

Neutral photoelectron spectroscopy involves

measuring the kinetic energy of an electron removed from a neutral species with a photon,
resulting in a cation and a free electron. Similarly, anion photoelectron spectroscopy involves
measurement of the kinetic energy of an electron removed from an anion by a photon. Properchoice of the anionic species allows one to study unstable or short lived neutral species such
radicals and transition states.4
An atomic anion (M−) can absorb a photon of sufficient energy (hν) to detach an
electron (e−) with some kinetic energy, leaving a neutral, M− + hν Æ M + e−. In anion
photoelectron spectroscopy experiments, these electrons are collected and their kinetic energy
distribution is measured. This distribution reflects the internal energy of the resulting neutral
species. Conservation of energy requires that the energy of the photon absorbed in the
detachment (hν) be equal to the kinetic energy of the detached electron (eKE) plus thebinding energy of the electron (eBE), hν = eKE + eBE. If the energy of the incident photon
is large enough, more strongly bound electrons can be detached from the anion resulting inelectronically excited neutrals.
For atomic anions, the transitions from anion to neutral are purely electronic. With
photons of a single energy, there will be discrete electron kinetic energies, eachcorresponding to detachment of a particular anion state to an electronic state of the neutral.
As an example, Figure 1.1 shows the photoelectron spectrum of oxygen anion (O−), andreflects the two lowest spin-orbit states of O− (2P3/2,1/2) detaching to the three lowest states of2(3P2,1,0) For atomic transitions of O− (2P3/2,1/2) detaching to the three lowest states of oxygen
atom (3P2,1,0).5-7 for atomic transit

J' = 0J' = 1
J' = 2

J'' = 1/2
J'' = 3/2

Figure 1.1

363.8 nm (3.408 eV) photoelectron spectrum of atomic oxygen anion (O−).
The spectrum is composed of six transitions, from the O− (2P3/2,1/2) states to
the O (3P2,1,0) states.7

3oxygen atom (3P2,1,0).5-7 For the spectrometer described in Chapter 2, the linewidth of atomic
transitions is the instrumental linewidth, typically 6-12 meV fwhm Gaussian lineshape.
Molecular photoelectron spectra, on the other hand, can be considerably more
complex than atomic photoelectron spectra, due to the fact that many vibrational androtational levels may exist within a single electronic manifold.1, 2 This internal energy must
be taken into consideration when conservation of energy is applied to the system. Consider
the photoelectron spectroscopy of a stable diatomic anion, AB−.

This ion will have a

distribution of vibrational (Evib), rotational (Erot), and electronic (Eelec) energies. Likewise, theresulting neutral species will also have vibrational, rotational, and electronic energies,
yielding the following for conservation of energy:
AB−[E''vib(v'')+E''rot(J'')+E''elec]+hν Æ AB[E'vib(v')+E'rot(J') + E'elec] + e− (eKE)(1.1)

where a double prime indicates the initial state (the anion), and a single prime indicates the
final state (the neutral). Since our instrumental resolution is typically inadequate to observe
individual rotational levels, rotational broadening of an individual band is taken into accountby application of a rotational correction8 to the final energies obtained through:AB−[E''vib(v'') + E''elec] + hν Æ AB[E'vib(v') + E'elec] + e− (eKE)(1.2)

Figure 1.2 depicts the potential energy curves and vibrational energy levels for thishypothetical system.
The ion production method described in Chapter 2 produces ions which arethermalized to room temperature.9, 10 Unless there are low lying electronic states in the anion(such as in O−), there will typically be only one electronic state of the anion populated, with athermal distribution of vibrational levels. The transition from the anion ground electronic,
vibrational, and rotational state to the neutral ground electronic, vibrational, and rotational
state is the origin of the electronic absorption (photodetachment) band. The energy of this4transition is defined as the adiabatic electron affinity (EA) of the neutral species, EA (AB).
Adams

hν
Electron binding energy
Electron Kinetic Energy

~
A AB

~
X AB

T0

ElectronAffinity
EA(AB)

eBE = hν - eKE

AB−

Figure 1.2

Schematic representation of photodetachment from molecular anion AB− totwo possible electronic states of neutral molecule AB.

5transition is defined as the adiabatic electron affinity (EA) of the neutral species, EA(AB). A
similar transition between the ground state of the anion and an electronically excited state of
the neutral can yield an electronic energy splitting for the neutral, commonly referred to as aterm energy, T0. The term energy is simply the difference in binding energies between the
ground vibrational level of the ground electronic state and the ground vibrational level of theexcited electronic state: T0 = E 0ex - E 00 .
Since the vibrational energy levels of the anion and neutral are quantized, electrons
will be detached with discrete energies corresponding to the transition between a particularvibrational state in the anion, and a vibrational state of the neutral. Transitions from avibrationally excited anion, known as hot-bands, typically appear in the room temperature
spectra and contain vibrational information for the anion. Likewise, transitions from theground vibrational state of the anion yield vibrational information for the neutral species. By
precisely measuring the difference between the discrete electron energies, relative vibrationalenergies can be determined.
The identities of different electronic states appearing in a photoelectron spectrum aredetermined by the orbital from which the outgoing electron is detached. For example, in aclosed shell atomic anion, detachment of an electron from an s-orbital or p-orbital will
respectively result in doublet S and doublet P states of the neutral atom appearing in the
spectrum. Similarly, for a closed shell molecular anion, detachment of an electron in σ-typeor π-type orbitals will result in doublet σ and doublet π states appearing in the spectrum. Theenergy necessary for these detachments can be approximated by the absolute energy of theorbitals in which the detached electrons are found in the anion.

6
1.2

Photoelectron Intensities and Selection Rules
The transition dipole moment, M, provides valuable information about aspectroscopic transition. In particular, selection rules can be derived to determine whether or
not a transition is allowed and thus whether or not it is expected to be observed.
Additionally, the square of the transition dipole moment is proportional to the intensity of atransition, allowing for direct comparison of the strengths of different transitions.11, 12
Specifying the initial and final states of the system by wavefunctions Ψi (r,q) andΨf (r,q), respectively, and the transition moment operator by μ (r,q), the transition dipole
moment is given by integrating over all electronic (r) and nuclear (q) coordinates:

M = Ψ i (r , q) μ (r , q) Ψ f (r , q)

(1.3)

The Born-Oppenheimer approximation is invoked to separate the nuclear and atomic
motion from one another. This approximation is based on the assumption that the electrons
are much lighter and faster than the nuclei and can react quickly to changes in nuclear
positions. The wavefunction for the system can then be separated as:Ψ(r,q) = ψvib(q)ψelec(r,q)(1.4)

Substituting into the transition dipole moment gives:

M = ψ vib,i (q)ψ elec,i (r , q) μ (r , q) ψ vib, f (q)ψ elec, f (r , q)

(1.5)

If the transition moment operator is resolved into the sum of an electronic part and a nuclear
part the transition dipole moment becomes:

M = ψ vib,i (q)ψ elec,i (r , q) μelec (r ) + μnuc (q) ψ vib, f (q)ψ elec, f (r , q)
M = ψ vib ,i (q)ψ elec ,i (r , q) μelec (r ) ψ vib, f (q)ψ elec , f (r , q)
+ ψ vib,i (q)ψ elec ,i (r , q) μ nuc (q) ψ vib , f (q)ψ elec, f (r , q)

(1.6)

(1.7)

Recognizing that the nuclear transition moment and vibrational wavefunctions do not depend
upon the electronic coordinates, the second term in equation 1.7 can be rewritten, yielding:7

M = ψ vib ,i (q)ψ elec ,i (r , q) μelec (r ) ψ vib, f (q)ψ elec , f (r , q)

(1.8)

+ ψ vib ,i (q) μ nuc (q) ψ vib, f (q) ψ elec ,i (r , q) ψ elec , f (r , q)

Noticing that electronic wavefunctions of different electronic states are orthogonal, it is clearthat the second half of equation 1.8 is zero. Assuming the Condon approximation, theelectronic wavefunctions are nearly constant over the range of nuclear coordinates sampled
by the molecule and can be approximated as having an average internuclear separation of q0.
The remainder of the transition dipole moment can then be separated as:

M = ψ vib,i (q) ψ vib, f (q) ψ elec,i (r , q0 ) μelec (r ) ψ elec, f (r , q0 )(1.9)

The square of equation 1.9 is proportional to the relative intensity of a transition:
2

I ∝ M = ψ vib,i (q) ψ vib, f (q)2ψ elec,i (r , q0 ) μelec (r ) ψ elec , f (r , q0 )2

(1.10)

Where the electronic transition strength is |〈ψelec,i (r,q0)|μelec(r)|ψelec,f (r,q0)〉|2 and the FranckCondon factor is the square of the vibrational wavefunction overlap, |〈ψvib,i (q)|ψvib,f (q)〉|2. A
consequence of the Condon approximation is that for a given pair of electronic states, theelectronic transition strength will be the same for all vibrational transitions and the relative
intensities for the different vibrational transitions will be directly related to the FranckCondon factors.
In the case of electron detachment, the initial-state wavefunctions describe the anion
and the final-state wavefunctions describe the neutral species plus a free electron.13 This
leads to an important difference from typical electronic absorption spectroscopy: the total
spin of the system must still be conserved, but since the electron detaches with a spin of ± ½
ħ, the spin of the molecule must also change by the same amount. This allows for appearanceof both singlet and triplet states in a photoelectron spectrum when detaching from a doublet
anion, allowing for direct measurement of a singlet-triplet splitting. Since only one electron
is involved in the transition, a more general form of the spin selection rule for the molecule is8
ΔS = ± ½. When there are resonance states in the continuum, however, this spin selection
rule does not hold, such as in the case of N2− where there are shape and Feshbach resonances
and two electrons can be involved in a transition.13
Since the absorbed photon brings with it 1ħ of angular momentum, an angularmomentum selection rule exists for the system (molecule + unbound electron) of

Δ J = 0,±1, completely analogous to conventional bound-bound spectroscopy. The outgoing electron
may carry the angular momentum of the photon; if the electron is initially in an atomic s-type
orbital, conservation of angular momentum requires that the electron be promoted to a p-type
orbital, or detached as a p-wave. Similarly, an atomic p-type electron will be promoted tos-type or d-type orbitals, or detached as an s-wave or d-wave. For molecular orbitals, thedetachment process is more complicated. A picture analogous to atomic detachment suggeststhat detachment from a σ molecular orbital will result in an outgoing wave with a π-type
shape, and detachment of an electron from a π orbital will yield interfering σ-type and δ-type
waves.
Each electronic state appearing in the photoelectron spectrum of a polyatomic anion
is made up of one or more vibrational peaks. In atomic or molecular systems, there are 3N
degrees of freedom, where N is the number of atoms. Of these, 3 degrees of freedom
correspond to translation of the center of mass of the system, and for a nonlinear polyatomic
molecule, like those described later in this dissertation, 3 more degrees of freedom
correspond to the different rotations of the molecule as a whole. This leaves 3N-6 degrees of
freedom for molecular vibrations. Assuming the normal mode approximation, the 3N-6
vibrational modes are separable and the overall Franck-Condon factor becomes a product ofFranck-Condon factors for each vibrational mode.
Assuming the vibrations are all harmonic, vibrational selection rules can now be
determined. For a non-zero Franck-Condon factor, the product 〈ψvib,i (q)|ψvib,f (q)〉 must be9
totally symmetric; thus, totally symmetric vibrational modes can be active in a photoelectron
spectrum for any Δv. By symmetry, the Franck-Condon factors for non-totally symmetricvibrational modes are zero for Δv = ±1, ±3, ±5… but can be non-zero for Δv = 0, ±2, ±4….
In practice, non-totally symmetric vibrational modes have significant Franck-Condon factors
only for Δv = 0 unless there is a large change in the vibrational frequency between the anion
and the neutral.
Figure 1.2 shows potential curves in which there is only a small geometry change
upon photodetachment. This results in the band origin having the largest Franck-Condon
overlap which is thus the most intense feature in the spectrum, as seen in the curves on theleft side of Figure 1.2. In some cases, however, the geometry shift upon photodetachment
can be large. This will result in a small Franck-Condon factor for the band origin and muchlarger Franck-Condon factors for vibrationally excited levels of the neutral. The point in the
spectrum with the most intensity (and largest Franck-Condon factor) is often referred to as avertical detachment energy (VDE).

This energy corresponds to the energy required toremove an electron with no geometry change. Figure 1.3 shows hypothetical potential energy
curves for a harmonic oscillator where there is a large geometry change, and illustrates thedifferences between the band origin and VDE.

1.3

Nonadiabatic Effects
The above discussion of transition strengths holds well for many systems where theelectronic state density is low. When electronic states lie very close in energy, there can bestrong interactions between states. This usually indicates that non-adiabatic terms neglectedin the Born-Oppenheimer approximation become significant contributions to the potential
energy surface.

10
Mathematically, the potential energy surface can be represented by an expansion of
the Hamiltonian about a reference nuclear configuration in the basis of noncoupled electronic
states.14 Including up to second-order terms, the potential is:

Vi = V0 + ∑ Ψ i ∂∂QHα Ψ i Qα + 12 ∑ Ψ i ∂∂QH2 Ψ i Qα 2
2

α

−∑

α

α

Ψ i ∂∂QHα Ψ k Ψ k ∂∂QHα Ψ i

α , k ≠iEk − Ei

Qα 2(1.11)

Here, the index α covers all the normal modes, while i and k describe all the relevant
electronic states. The last term in Equation 1.11 is where interactions between different
electronic states can arise.

The matrix elements in this term are nonzero only when

considering the appropriate symmetry vibration which can couple the two electronic states.
For example, consider a molecule in the C2v point group with electronic states of A2 and B1
B

symmetry. The last term in Equation 1.11 is nonzero only when considering b2 vibrations
since a2 x b1 = b2. Also, notice that when the energy splitting between two electronic states issmall, this term can become very large.
This type of coupling can have large effects on the potential energy surfaces ofinteracting electronic states. For example, if the coupling term is large enough, the potential
surface can curve downward along a coupling coordinate, transforming a stationary point
which would otherwise be a minimum into a transition state with one or more imaginary
frequencies. When considering the interaction where Ek – Ei is negative, this term causes thepotential surface to become steeper along the coupling coordinate, and can shift vibrationalfrequencies to much larger and unexpected values.

Often, when electronic structure

calculations compute frequencies that are too large or too small for a system which isotherwise expected to have “normal” vibrational frequencies, it may be an indication thatelectronic states are interacting with one another.

11

VDE
Band
OriginFigure 1.3

Harmonic oscillator potential energy curves showing a large geometry
change upon photodetachment.

12

1.4

Photoelectron Angular Distributions
Photoelectron intensities can also vary with the polarization of the incoming linearlypolarized photons.15, 16 By controlling the angle between the laser polarization and thephotoelectron collection axis it is possible to measure the relative angular distributions of aphotoelectron spectrum. Photoelectron intensities have angular distributions according to:I∝

σ total[1 + β P2 (cosθ )]4π(1.12)

where P2(cos θ ) is the second Legendre polynomial [½(3cos2θ −1)], θ is the angle between
the laser polarization and the photoelectron collection axis, σtotal is the total photodetachment
cross section, and β is the anisotropy parameter which can range from −1 to 2.15, 16
Detachment from different symmetry orbitals will yield different photoelectron
angular distributions. For example, detachment of an electron in an atomic s-orbital, such as
from H−, will result in an outgoing p-wave, which will be oriented along the laser polarizationand have a cos2θ behavior (β = 2). On the other hand, detachment of an electron in an
atomic p-orbital, such as from O−, will result in both s- and d-waves. These may interfere
and, depending on the photon energy, yield values for β typically between -1 and 0.
Detachment from molecular orbitals is more complicated, but previous measurements haveshown that β < 0 for detachment from π-type molecular orbitals and β > 0 for detachmentfrom σ-type molecular orbitals.17, 18
For the results described later in this dissertation, the anisotropy parameter gives
clues to the type and symmetry of the molecular orbital from which electrons are detached.
Additionally, it is also used to distinguish features arising from different electronic states inthe same spectrum. Most of the spectra presented have the laser polarization at the magic
angle (θ = 54.7°) to the photoelectron collection axis. The magic angle is the angle whereP2(cosθ ) = 0, and the photoelectron intensities are simply proportional to the total

13
photodetachment cross section and are free from angular effects. Values of the anisotropy
parameter are estimated by relative intensity measurements with the laser polarization parallel(θ = 0°) and perpendicular (θ = 90°) to the photoelectron collection axis:

β≈

1.4

1

I 0º − I 90º
2 I 0º + I 90º(1.13)

Dissertation Outline
This dissertation is split among several chapters. Chapter 2 describes the anion

photoelectron spectrometer in detail, including components of the optical system. It also
describes the general computational methods used for simulation and analysis of the spectra.
Chapters 3-8 each describe the experimental results obtained for a single parent molecule.
Chapter 3 describes the results obtained from the cyclopentadiene molecule (C5H6).
Cyclopentadiene is a 5-membered ring molecule consisting only of carbon and hydrogen.
Spectra of the corresponding anion, cyclopentadienide (C5H5−), have been previously
obtained;19 however, the spectra presented here have much higher resolution and allow for
determination of more of the vibrational properties of the analogous neutral, cyclopentadienyl
radical (C5H5•), a well-known Jahn-Teller molecule.20, 21
The photoelectron spectra obtained from the pyrrole system (C4H5N) are the subjectof Chapter 4. Similar to cyclopentadiene, pyrrole is a five-membered ring molecule; unlike
cyclopentadiene, pyrrole contains carbon, hydrogen, and nitrogen. By substituting a nitrogen
atom for a C—H group, cyclopentadiene and pyrrole are isoelectronic. This substitution
eliminates Jahn-Teller effects in the pyrrolyl radical system, though other non-adiabatic
effects influence the appearance of the spectra.
Substitution of a nitrogen atom for another C—H group results in other isoelectronicsystems (C3H4N2). If the substitution is such that the nitrogen atoms are separated in the14
5-membered ring, the result is the imidazole molecule, the subject of Chapter 5. If the twonitrogen atoms are adjacent in the 5-membered ring, pyrazole is the result, and is the subjectof Chapter 6. Although isomers, the photoelectron spectra of the corresponding anions,imidazolide and pyrazolide, appear very different. Non-adiabatic effects still influence both
spectra, but the effects are small in the imidazolyl radical system and much larger in thepyrazolyl radical system. Additionally, photodetachment signals are observed for isomericanions of imidazolide and pyrazolide.

These anions retain the ring structure, but are

deprotonated at a carbon-site instead of the nitrogen-site.
Chapter 7 discusses the results from another isoelectronic system, vinyldiazomethane(C3H4N2). Vinyldiazomethane is a ring-opened isomer of imidazole and pyrazole. The
backbone of the molecule has the structure of pyrazole where a C—N bond is broken and thering is opened. The corresponding anion (the vinyldiazomethyl anion) is not produced fromdeprotonation as the anions described in other chapters, but is actually synthesized in the flow
tube in a molecule/anion reaction between N2O and allyl anions (C3H5−). Two conformers (Eand Z) of the vinyldiazomethyl anion are expected to be formed in the flow tube, but analysisof the detachment signals indicates only one is formed in detectable amounts.
Chapter 8 focuses on the photoelectron spectra of the 2-oxepinoxide anion, whose
corresponding radical (2-oxepinoxyl) is a suspected reactive intermediate formed during the
combustion of benzene and has been the focus of several theoretical investigations. The
spectra presented here represent some of the first experimental results obtained on this radical
species and are performed with the ultimate goal of obtaining the heat of formation of the
2-oxepinoxyl radical.
Chapter 9 provides a summary of the experimental results presented in thisdissertation and includes a discussion of the trends observed in the photoelectron spectrum of
the 5-membered ring anions. These trends are used in an extrapolation to approximate the15properties of 5-membered ring systems which contain more nitrogen atoms. Future work
which builds on the results presented here is also discussed.

16

Chapter 1 References
1

J.H.D. Eland, Molecular Photoelectron-Spectroscopy. Journal of Physics E-Scientific
Instruments, 1978. 11(10): p. 969-977.

2

K.M. Ervin and W.C. Lineberger, Photoelectron Spectroscopy of Negative Ions, inAdvances in Gas Phase Ion Chemistry, N.G. Adams and L.M. Babcock, Editors.
1992, JAI Press: Greenwich. p. 121-166.

3

P. Chen, Photoelectron Spectroscopy of Reactive Intermediates, in Unimolecular andBimolecular Reaction Dynamics, C.Y. Ng, T. Baer, and I. Powis, Editors. 1994,
Wiley & Sons: New York. p. 371-425.

4

P.G. Wenthold and W.C. Lineberger, Negative ion photoelectron spectroscopy
studies of organic reactive intermediates. Accounts of Chemical Research, 1999.
32(7): p. 597-604.

5

C.E. Moore, Atomic Energy Levels. NSRDS-NBS. 1952, Washington: US GPO
Circular No. 467.

6

D.M. Neumark, K.R. Lykke, T. Andersen, and W.C. Lineberger, Laser
Photodetachment Measurement of the Electron-Affinity of Atomic Oxygen. PhysicalReview A, 1985. 32(3): p. 1890-1892.

7

K.M. Ervin, W. Anusiewicz, P. Skurski, J. Simons, and W.C. Lineberger, 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(41): p.
8521-8529.

8

P.C. Engelking, Approximate Rotational Band Shifts. Journal of Physical Chemistry,
1986. 90(19): p. 4544-4545.

9D.G. Leopold, K.K. Murray, and W.C. Lineberger, Laser PhotoelectronSpectroscopy of Vibrationally Relaxed CH2− - a Reinvestigation of the Singlet Triplet
Splitting in Methylene. Journal of Chemical Physics, 1984. 81(2): p. 1048-1050.

10D.G. Leopold, K.K. Murray, A.E.S. Miller, and W.C. Lineberger, Methylene - a
Study of the X 3B1 and 1A1 States by Photoelectron-Spectroscopy of CH2− and CD2−.
Journal of Chemical Physics, 1985. 83(10): p. 4849-4865.

11

P.F. Bernath, Spectra of Atoms and Molecules. 1995, New York: Oxford University
Press.

12

D.C. Harris and M.D. Bertolucci, Symmetry and Spectroscopy An Introduction toVibrational and Electronic Spectroscopy. 1978, New York, NY: Oxford University
Press.

17
13

J. Simons, Detachment Processes for Molecular Anions, in Photoionization andPhotodetachment, C.Y. Ng, Editor. 1999, World Scientific: River Edge, NJ. p. 9581010.

14

R.G. Pearson, A symmetry rule for predicting molecular structure. Journal of theAmerican Chemical Society, 1969. 91: p. 4947-4955.

15

J. Cooper and R.N. Zare, Angular distribution of photoelectrons. Journal of Chemical
Physics, 1968. 48: p. 942-943.

16

J.L. Hall and M.W. Siegel, Angular dependence of the laser photodetachment of thenegative ions of carbon, oxygen, and hydrogen. Journal of Chemical Physics, 1968.
48: p. 943-945.

17

R.F. Gunion, M.K. Gilles, M.L. Polak, and W.C. Lineberger, Ultraviolet
Photoelectron-Spectroscopy of the Phenide, Benzyl and Phenoxide Anions, with
Abinitio Calculations. International Journal of Mass Spectrometry and Ion Processes,
1992. 117(1-3): p. 601-620.

18

P.G. Wenthold, M.L. Polak, and W.C. Lineberger, Photoelectron spectroscopy of theallyl and 2-methylallyl anions. Journal of Physical Chemistry, 1996. 100(17): p.
6920-6926.

19

P.C. Engelking and W.C. Lineberger, Laser Photoelectron Spectrometry of C5H5− Determination of Electron-Affinity and Jahn-Teller Coupling in Cyclopentadienyl.
Journal of Chemical Physics, 1977. 67(4): p. 1412-1417.

20

B.E. Applegate, A.J. Bezant, and T.A. Miller, The Jahn-Teller and related effects inthe cyclopentadienyl radical. II. Vibrational analysis of the A 2A2'' - X 2E1'' electronictransition. Journal of Chemical Physics, 2001. 114(11): p. 4869-4882.

21

B.E. Applegate, T.A. Miller, and T.A. Barckholtz, The Jahn-Teller and related
effects in the cyclopentadienyl radical. I. The ab initio calculation ofspectroscopically observable parameters. Journal of Chemical Physics, 2001.
114(11): p. 4855-4868.

________________________________________________
2
Methodology________________________________________________
2.1

Overview
The anion photoelectron spectrometer used to perform the experiments described inthis dissertation consists of four major parts: the ion source, the ion optics and mass selection
elements, the laser system, and the electron energy analyzer. A flowing afterglow ion source
is used to create a beam of negative ions. The ion beam is accelerated, shaped and focused,and passed through a Wien velocity filter for mass selection. The mass selected ion beam isthen focused and decelerated for interaction with a photon beam.

Photoelectrons areproduced at the interaction region and a portion of them are energy analyzed using ahemispherical energy analyzer. The electrons are detected using a position sensitive detector
which digitizes the position of the electrons for analysis by a computer. This chapter focuseson the components of the anion photoelectron spectrometer in detail, and is accompanied by adiscussion of the computational methods used to simulate and analyze the spectra.

2.2

Ion Source
The experiments described later in this thesis were performed using a common ion

source, known as a microwave discharge flowing afterglow ion source. The microwave
discharge ion source is made of two components, a brass microwave cavity, and a quartz
discharge cylinder.1, 2 Figure 2.1 illustrates the construction of the microwave dischargesource. Helium buffer gas, purified by passing through a liquid nitrogen cooled molecularsieve trap, is passed through the quartz discharge cylinder at a rate of 6-8 standard liters per
minute (controlled by an adjustable Tylan flow controller), along with a trace amount of O2(7-10 standard cm3 per minute). The discharge is ignited by means of a Tesla coil, which
produces a plasma containing unbound electrons.

The electrons absorb the 2.45 GHz19
radiation which maintains the plasma. The microwave radiation is provided by means of an
Opthos MPG 4 Microwave Power Generator. Typical discharge ignition is achieved by

BrassMicrowave
QuartzCavityCylinder
He/O2

Reagent
InletsFigure 2.1

Microwave Discharge Ion Source. A brass microwave cavity encloses aquartz discharge cylinder. The microwave discharge is connected to the flow
tube where ions react with neutral reagents introduced through movable ring
inlets.

20
microwave radiation which maintains the plasma.3 The microwave radiation is provided
from an Opthos MPG 4 Microwave Power Generator. Typical discharge ignition is achievedby providing ~70 W of forward power to the brass cavity and energizing the Tesla coil on thequartz cylinder. Once a stable discharge is produced, the forward power is reduced and the
majority of the radiation is absorbed by the plasma; correspondingly, the reflected powershould be near zero, protecting the magnetron in the microwave generator. The forward
power may be adjusted to achieve maximum ion signal. For production of atomic oxygenanion (O−), for example, the smallest forward power necessary to maintain the discharge
produces the maximum ion signal.
Atomic oxygen anions are used as the main anion precursor and are produced in the
discharge by dissociative electron attachment to O2:

O2 + e − → O2−* → O − + O(2.1)

Two movable ring inlets are located just downstream from the microwave discharge and used
to introduce precursor molecules into the flow tube for subsequent reaction. For example,
introduction of methane (CH4) into the flow tube results in production of hydroxide (HO−),another major ion precursor, via a hydrogen transfer reaction with O−:

O − + CH 4 → HO − + CH 3

(2.2)

These inlets may be used to titrate a reaction by adjusting their relative position in the flow
tube for the appropriate reaction distance (reaction time).
Once created, the anions continue down the flow tube where they are thermalized toroom temperature by 104 – 105 collisions with the He buffer gas.1, 2 The source region is
pumped by a 330 l/s Stokes Roots Blower. A typical flow tube pressure is 0.4 Torr (~50 Pa)
at 7.3 standard liters per minute of He. Surrounding the flow tube is a jacket which may befilled with liquid nitrogen to aid in further cooling of the ions to roughly 200 K.

21
2.3

Ion Optics and Mass Selection
The ions are gently extracted from the flow tube through a small aperture (~1 mm) ina nosecone at 0-3 volts, where they encounter the first of a series of adjustable ion optics anddeflectors (called the afterglow lenses); here the ions are focused and accelerated to 736 eV
kinetic energy. A 2000 l/s 6-inch diffusion pump maintains the vacuum chamber pressure in
the afterglow lens region at about 10-4 Torr (~10-2 Pa). Figure 2.2 illustrates construction of
the flow tube and afterglow lenses. The nosecone, first three lens elements (L1-L3) and firstdeflector (D1H and D1V) are mounted in the vacuum chamber as a single assembly. The
potentials on L1, L2, and L3 can be adjusted separately and deflector D1 is referenced to theL3 potential and has independent horizontal and vertical deflections. The next series oflenses and second deflectors are also mounted as a single assembly with an aperture plate on
the front. The first assembly fits over the second assembly to ensure the ion trajectories are
not affected by the ground potential of the vacuum chamber walls. Lens element L4 is also
adjustable and deflector D2 is referenced to the L4 potential and has independent horizontal
and vertical deflections. Lens elements L5 and L6 are given constant potentials of 400 and
736 volts, respectively. Table 2.1 lists typical potentials for all adjustable elements.
Beyond lens element L6 is another pair of deflectors (D3) which are used to steer theanions, now traveling at 736 eV kinetic energy, through a 10° bend in the ion flight path.
These deflectors allow the anions to be steered thought the 10° bend, while neutrals andphotons collide with the vacuum chamber walls. Following the 10° bend is a 3-element
quadrupole einzel lens (Q1) which is used to focus and deflect the ion beam4 through anaperture plate, A2. The ion current is measurable on this aperture by means of a Keithley
600A electrometer. Typical total ion beam currents are 10-30 nA. This region is pumped by
a 3000 l/s 8-inch diffusion pump to a pressure of 10-6 Torr (~10-4 Pa). Figure 2.3 depicts thisregion of the instrument.

A pair of deflectors (DL4) follow, and another 3-element

22
quadrupole lens (Q2)sits behind a gate valve. This ion optics is used to focus and deflect theion beam into the mass selection region.

Roots Pump
330 liter/sec
10−4 Torr
L1 L2 L3

Cooling Jacket
Flow Tube 0.4 Torr

N

L3

L4 L5 L6

A1

D1V D1HD2V D2H

6-inchDiffusion Pump

Figure 2.2

Flow tube and afterglow ion optics. The nosecone is labeled N, accelerationlenses are labeled L1-L6, deflectors are labeled D1 and D2, and the apertureis labeled A1.

23

Gate Valve10−6 Torr

Q1

A2

A1
D3

D4

6-inchDiffusion Pump

Figure 2.3

10 degree deflectors (D3), first 3-element quadrupole einzel lens (Q1),second aperture (A2) and fourth deflectors (D4).

24

Element
Nosecone
L1
L2
L3
D1 Horizontal
D1 Vertical
L4
D2 Horizontal
D2 Vertical
L5
L6
D3 Vertical
D3 Horizontal
Q1 Inner Top
Q1 Inner Bottom
Q1 Inner Right
Q1 Inner Left
Q1 Outer Horizontal
Q1 Outer Vertical
D4 H
D4 V
Q2 Inner Top
Q2 Inner Bottom
Q2 Inner Right
Q2 Inner Left
Q2 Outer Horizontal
Q2 Outer Vertical
E-Field Voltage
B-Field Voltage
Q3 Inner Top
Q3 Inner Bottom
Q3 Inner Right
Q3 Inner Left
Q3 Outer Horizontal
Q3 Outer Vertical

Table 2.1

Typical Potential
1V
25 V
160 V
175 V
0V
0V
181.7 V
0V
0V
400 V
736 V
0V
±75 V
19 V
20 V
-20 V
-20 V
26 V
-27 V
13 V
8V
17 V
15 V
-18 V
-17 V
21 V
-22 V
60 V
3.2 V for O−
18 V
7V
-14 V
-14 V
14 V
-16 V

Element
Q4 Inner Right
Q4 Inner Left
Q4 Inner Top
Q4 Inner Bottom
Q4 Outer Vertical
Q4 Outer Horizontal
D5 Horizontal
D5 Vertical
DL1
DL2
DL3
DL4
Interaction Region
Computer eKE
Computer V3
Manual Te
V1
V2
V3
V4
VHC
VIN
VOUT
Contact Potential
V5
V6
1st MCP
Last MCP

Typical potentials for the photoelectron spectrometer.

Typical Potential
25 V
40 V
-33 V
-37 V
38 V
-39 V
-47 V
24 V
736 V
134 V
360 V
35 V
35 V
2.06
0.1
3.97 V
17.29 V
16.04 V
0.572 V
15.28 V1.958 V3.38 V
1.352 V
-0.428 V
28.62 V
245.4 V
250.7 V3573 V25
quadrupole lens (Q2) sits behind a gate valve. These ion optics are used to focus and deflect
the ion beam into the mass selection region.
The mass selection region used in the photoelectron spectrometer is a velocity filter
consisting of perpendicular electric and magnetic fields, a Wien filter. The Wien filter sits in
a common assembly with the second and third 3-element quadrupole lenses (Q2 and Q3) as
illustrated in Figure 2.4. This region is pumped to a pressure of 10-7 Torr (~10-5 Pa) by a 250
l/s Varian Turbomolecular Pump.
The velocity filter operates according to the following principles. The force on an
ion from an electric field is the product of the charge of the ion, q, and the electric field,

K K
K
E : F = qE . The force on an ion from a magnetic field depends not only upon the charge ofK

the ion and direction and magnitude of the field, B , but on the direction and magnitude of the

K

K

K

K

ion’s velocity, v : F = qv × B . By setting these two forces equal and opposite, it is possible
for an ion to pass through the Wien filter undeflected. All ions have the net charge of a single
electron, and thus all feel the same force from the electric field. Since all ions have the samekinetic energy (½ mv2 = 736 eV), there will be a velocity distribution of the ions, with thelighter ions traveling faster and the heavier atoms traveling slower, resulting in different
forces for different mass ions. Ions which are too heavy or too light are deflected andneutralized by the walls of the Wien filter. To select which mass ion is passed through the
Wien filter undeflected, the magnetic field is adjusted while the electric field is kept constant.
The magnetic field is created via a water cooled electromagnet powered by a 0-10A / 0-60V
Kepco regulated DC voltage supply. The electric field sector is constructed as illustrated in
Figure 2.5; voltages are provided by a voltage divider circuit powered by a 0-40mA / 0-500V
power supply. The Wien filter has a mass resolution of m/Δm ≈ 40, which is sufficient for
separation of O−, HO−, and the deprotonated species described later with masses of 60-90
amu.

26

Q2

Wien Filter

Q3

10−7 Torr

250 liter/second
Turbo Pump

Figure 2.4

Wien filter region of the photoelectron spectrometer. Ions are focused into
the Wien filter with 3-element quadrupole Q2 and mass selected inside the
Wien filter. The mass selected ions are refocused with 3-element quadrupole
Q3 and pass through a short field-free region.

27

0

+1

+2

+3

+4

+5

+

K
E
K
B

−

−1

Figure 2.5

−2

−3

−4

−5

Construction of the Wien filter electric field plates. Five pairs of parallel
stainless steel shims on each side (+ and −) and two electrodes (dark grey)
have voltages distributed to give a vertical electric field equivalent to infinite
parallel plates. Teflon shims (light grey) are placed between the stainlesssteel shims to insulate them from each other. The ion trajectory is into theplane of the page, and the magnetic field lines run from left to right.

28
There are several more ion optics following the mass selection region: a 3-element
quadrupole (Q3), a short field free region, an aperture (A3), another 3-element quadrupole(Q4), a pair of deflectors (D5), and a series of deceleration lenses (DL1-DL4).

The

deceleration lenses slow the ions to 35V as well as focus them for interaction with a laser
beam. Ions are collected in a Faraday cup and their current is measured using a Keithley 602
Electrometer.

Ion currents of 1-2000 pA are achievable.

Figure 2.6 illustrates thecomponents in the final chamber of the photoelectron spectrometer which is pumped to 10-8
Torr (~10-6 Pa) by a 280 l/s Varian turbomolecular pump.

2.4

Ultraviolet Laser System
The 35 eV ion beam is intersected perpendicularly in the interaction region with a363.8 or 351.1 nm photon beam from a continuous wave Spectra Physics 2045 Argon-Ion
Laser. Before reaching the interaction region, the laser photons encounter several opticalcomponents which alter the laser light in order for the vacuum cavity to double as a powerbuild-up cavity.5, 6 A diagram of the laser system is given in Figure 2.7. The laser cavity
itself consists of a highly reflective back mirror, an etalon, a plasma tube, and an output
coupler. Operation at 363.8 or 351.1 nm is selected with the appropriate output coupler. An
etalon is placed in the laser cavity to allow for operation on a single cavity mode within the363.8 or 351.1 nm Doppler profile. The argon ion laser output power is typically ~1 W for
single mode operation at 363.8 or 351.1 nm.
Several mirrors direct the laser light through a telescope where the light is focused
into the interaction region. An acousto-optic modulator (AOM), piezoelectric translators(piezo) on the back mirror of the laser and front cavity window, and two photodiodes are used
in a servo system which maintains the power build-up in the vacuum chamber. Power
increases approximately 100 fold in the build-up cavity.

A half wave plate is placed

29
immediately before injection into the power build-up cavity to allow for rotation of the laser
polarization within the cavity.

10−8 Torr
A3

Faraday Cup

Position
Sensitive
Detector

DL1 DL2 DL3 DL4
Q4

D5
V6
Interaction
Region

V1

V4

V2
V3
Hemispherical
EnergyAnalyzer

V5

VHC
VIN
VOUT

280 liter/second
Turbo Pump

Figure 2.6

Vacuum chamber containing the final elements of the ion beam path and theelectron energy analysis and detection components.

30

Ion Beam
Mirror Piezo

Photodiode

AOM
Telescope

Photodiode

Mirror
Piezo

Figure 2.7

EtalonPlasma Output
Tube Coupler

λ/2
plate
MirrorCavityWindows

Components of the laser system. The system is made up of an argon-ion
laser and the vacuum chamber serves as an external build-up cavity. The
back mirror of the laser and the front cavity window are mounted on
piezoelectric translators. The acousto-optic modulator (AOM), piezos, andphotodiodes are part of a servo system which maintain efficient build-up
within the vacuum chamber.

31
immediately before injection into the power build-up cavity to allow for rotation of the laser
polarization within the cavity.
The build-up cavity operates similar to the laser cavity but has no gain medium. For
intensity to buildup constructively within the cavity, the cavity length must be a half-integer
multiple of the laser wavelength. The build-up cavity length, however, is sensitive to external
factors such as thermal fluctuations, mechanical vibrations from the pumps, or acoustic noisedue to the pumps or other room noise. To maintain the build-up, these fluctuations are
responded to by the servo system. A direct response can be achieved by adjusting the piezos
on the front cavity window which directly changes the cavity length. The piezo on the backmirror of the laser can also be used to restore the system to resonance. The piezo is used toadjust the length of the laser cavity, thereby changing the laser wavelength.
Similarly, the AOM is used to slightly change the laser wavelength/frequency.
Within the AOM is a quartz crystal which is connected to an acoustic transducer. An
acoustic frequency of 55-85 MHz is applied to the quartz crystal, producing a standing wave
in the crystal, which acts as a Bragg diffraction grating.7 The incoming laser beam isdiffracted and the frequency of the diffracted laser light is offset by the frequency of thestanding wave in the crystal. Additionally, a 1 MHz dither is added on top of the 55-85 MHz
offset. The 55-85 MHz offset allows for small frequency adjustments and optically isolates
the laser and build-up cavities from one another, preventing any light which reflects off thebuild-up cavity from returning to the laser cavity and causing interference. The 1 MHz dither
serves to provide an error signal for the servo electronics, which adjust the build-up cavity
piezo, laser cavity piezo, or AOM frequency offset to maintain the resonance condition of thebuild-up cavity. Each of the piezos and the AOM are used to respond to different frequencies
of noise in the system. The slowest frequencies (0 – 100 Hz) are responded to by the piezos
on the build-up cavity. The piezo on the back mirror of the laser is used to react to higher

32frequency noise (100 Hz – 10 kHz). The highest frequency noise (10 kHz – 1 MHz) is dealt
with by the AOM.
Depending on the state in which the system is last left, build-up may not happen on
its own by simply turning on the electronics. The electronics should be left with the active
stabilization turned off. Assuming the optics are aligned close enough to allow for the lightto resonate within the vacuum cavity, the alignment is optimized by adjusting the telescope,
the AOM tilt angle, the two mirrors immediately in front of the cavity, and the front cavity
window. Next, the servo electronics are switched to active stabilization on. At this point thephase delay between the AOM and piezos can be adjusted. While monitoring the signal on
the photodiode located at the back side of the vacuum chamber, the phase delay should be set
to the point where the cavity is close to or even starting to have build-up occur. This is when
there are the most resonances occurring. At this point, the gain for the laser piezo can be
adjusted which should allow build-up to begin. The build-up can be optimized by small
adjustments to the phase delay, the laser piezo gain as well as the gain on the AOM servo.
Occasionally, it may be necessary to adjust the bias positions of the laser and buildup cavity
piezos.

2.5

Photoelectron Energy Analysis and Detection
At the position where the ion and laser beams intersect (the interaction region)

photoelectrons are ejected in all directions. A portion of the electrons are ejected in a
downward direction and pass through a 5° half angle opening in the interaction region where
they enter the energy analysis region of the instrument.8 The components of the energy
analyzer are pictured in Figure 2.8, which depicts a cutaway showing the path thephotoelectrons take through the analyzer.

33
The hemispherical energy analyzer operates at a constant transmission energy,
presently ~4.0 eV. Since electrons may be photodetached with kinetic energies ranging from
0 eV to the photon energy (~3.5 eV), the electrons must be accelerated before they enter thehemispherical analyzer if they are to be transmitted and detected. A series of lenses is used toaccelerate and focus the incoming electrons to the proper energy and position. The electrons
are first accelerated to 5-10 times their initial energy with lens elements 1 and 2. Lens
elements 2, 3, and the input stack Herzog Corrector are also constructed to form a zoom lens
which focus the electrons to the entrance of the hemispheres while giving them theappropriate transmission energy. Passing through the hemispherical analyzer, the electrons
are spread in space, with slower electrons near the center hemisphere and faster electrons near
the outer hemisphere. Electrons which are too slow or too fast collide with the hemispheres
and are neutralized.

Only electrons within ~2% of the transmission energy are passed

through the entire analyzer. These electrons are accelerated and imaged onto a position
sensitive detector using lens elements 4, 5, and 6. The position sensitive detector consists ofa stack of 5 microchannel plates coupled to a resistive anode detector (Quantar Technology
3318A). The minimum detectable electron energy is approximately 0.1 eV, though thetransmission of electrons slower than about 0.3 eV is significantly lower than unity due topoor transmission through the energy analyzer. Normally, electron energies smaller than 0.3
eV are not measured.

2.6

Data Collection
The position sensitive detector is wired to a position computer (Quantar 2401B)

which analyzes the outputs of the resistive anode detector to determine the spatial position ofa single detached electron. The position computer is attached to an oscilloscope (for real time
viewing of the electron positions on the detector) as well as a PC for data collection. Since34
only a very small energy region (2% of the 4 eV transmission energy) is detected at a time, a
LabVIEW program is used to accumulate a complete photoelectron spectrum (from 0.3 toPosition
Sensitive
Detector
Deceleration Lens Stack

Faraday
Cup

VIR

Ions

V5

V1

5 – 10x
acceleration

V2

Zoom
Lens

V6

4 – 4.5x
magnification

V4

V3

6x
acceleration
VHC

VHC

VIN

VOUT

Figure 2.8

Cutaway of the electron kinetic energy analyzer system. The decelerationlenses and analyzer lenses (V1-V6, VHC) are made from molybdenum, and theconcentric hemispheres of the analyzer are made from oxygen-free highconductivity copper.

35
only a very small energy region (2% of the 4 eV transmission energy) is detected at a time, a
LabVIEW program is used to accumulate a complete photoelectron spectrum (from 0.3 to~3.5 eV). This program handles the process of adjusting the electron analyzer voltages tocollect photoelectrons of different energies by sending out digital signals to a digital to analog
encoder chip located within a voltage programmer (JILA LC036). A photoelectron spectrum
is collected in LabVIEW by specifying a starting kinetic energy, total number of spectral
segments, and the spacing between spectral segments.
For proper determination of the electron kinetic energy, the photoelectron spectrumof a reference ion is obtained to fix the absolute energy scale. Since it is atomic and theneutral’s electron affinity9 is precisely known (1.46111 eV), O− is typically used for thispurpose. Additionally it is usually the major precursor ion and is formed in the flow tube
before and while most experiments are performed.

A second calibration is performed

periodically to determine any nonlinearity in the energy scale. This is done by measuring theenergies of all the peaks in the tungsten anion (W−) photoelectron spectrum and comparing
with the absolute positions tabulated by Charlotte Moore.10

A small (< 0.5%) energy

compression is typically observed. One final correction to the photoelectron energies must be
applied since the anions are traveling at 35 eV of kinetic energy when the electrons aredetached. This correction converts the electron kinetic energies from the laboratory frame tothe center of mass frame of the anions. The final electron kinetic energy can be determinedby:eKE = hν − ⎡ EAref + γ ( eKEref − eKElab ) + Wme
⎣

(

1
M ref

)

− M1 ⎤⎦(2.3)

where hν is the photon energy, EAref is the electron affinity of the reference ion, γ is theenergy compression factor, (eKEref – eKElab) is the lab frame difference in the kinetic energiesof the reference ion and the molecule of interest, W is the kinetic energy of the ion beam, me36
is the electron mass, Mref is the mass of the reference ion, and M is the mass of the moleculeof interest.
2.7

Spectral Simulation and Assignment
In order to assign and interpret the photoelectron spectra, electronic structure

calculations are performed using the Gaussian suite of software.11 Optimized moleculargeometries and harmonic vibrational frequencies are obtained for each anion/neutral pair.
The B3LYP12, 13 method of Density Functional Theory14 (DFT) is typically used with amodest basis set [6-31+G(d) or 6-311++G(d,p)].

Using these geometry and frequency

results, Franck-Condon factors are obtained for the transitions between the anion and theneutral using the FCFGAUS and PESCAL programs.15-17 The DFT calculations also allow
for computation of the EA by taking the DFT energy difference between anion and neutral(including zero point energies).
The FCFGAUS program extracts the geometries, normal mode vectors, andvibrational frequencies for the anion and neutral from the Gaussian output and outputs
Duschinsky rotation matrices, normal mode displacement vectors, molecular geometries, andnormal mode frequencies in a form used by PESCAL. The PESCAL program computes
Franck-Condon factors, treating molecular vibrations as independent harmonic oscillators andtransforming the anion normal modes into linear combinations of the neutral normal
modes.15, 18-21 The raw Franck-Condon factors are convoluted with an experimental linewidth(Gaussian or Lorentzian lineshape with full width at half maximum of 8-15 meV), resulting
in simulated spectrum based solely on the electronic structure calculations.
Generally, for systems free from non-adiabatic effects, the Franck-Condon simulation
matches the experimental spectra quite well. This allows for precise determination of theband origin and electron affinity, and assignment of vibrational bands to the appropriate
normal vibrations. These vibrational assignments typically confirm that the normal modes

37
whose vibrational motions occur along the geometry displacements from anion to neutral are
the modes active in the photoelectron spectra.
When non-adiabatic effects are present, there will be large discrepancies between theexperimental spectrum and the Franck-Condon simulation. In general, the Frank-Condon
simulation will overestimate the intensities of vibrational bands in systems where nonadiabatic effects are important.

Also, there may be extra vibrational bands in theexperimental spectrum which are missing from the Franck-Condon simulation. This is often
due to non-totally-symmetric modes appearing in the experimental spectrum which havecalculated Franck-Condon factors of zero (due to symmetry). These two effects tend to offset
one another in an intensity borrowing manner, where the intensity of some bands are lowered
while other not normally allowed bands gain intensity. If reliable assignments can be made,photoelectron spectra showing non-adiabatic effects can yield information which is notnormally anticipated, such as the frequencies of non-totally symmetric vibrations whichappear in the spectrum.

2.8

Thermodynamics
For many of the systems discussed in this dissertation, thermodynamic parameters

can be evaluated using thermochemical cycles.

Figure 2.9 illustrates three gas-phasethermochemical cycles. Using the negative ion cycle, the electron affinity (EA) of the system
of interest is combined with the enthalpy of deprotonation (ΔacidH0) of the parent system andthe ionization energy (IE) of the hydrogen atom (13.5984 eV) to determine a bonddissociation enthalpy for breaking an R—H bond (D0):
D0(R—H) = ΔacidH0(RH) – IE(H) + EA(R)(2.3)

Often, values for the 298 K gas phase acidity (ΔacidG298) or deprotonation enthalpy(ΔacidH298) are available in the literature.22 Conversion of the 298 K deprotonation enthalpy to

38
0 K is achieved by considering the contributions from integrated heat capacities, which canbe derived from the Gaussian output files, but are explicitly written as:298

Δ acid H 0 (RH) = Δ acid H 298 (RH) − ∫ dT[C P (R − ) + C P (H + ) − C P (RH)]

(2.4)

0

Error bars for the thermal corrections derived from the Gaussian output files are assumed tobe small, on the order of 0.1 kcal mol-1.
Using these bond dissociation energies, an absolute heat of formation can be obtained
for the R system if the heat of formation of the parent system is available in the literature:ΔfHT(R) = ΔfHT(RH) + DT(RH) – ΔfHT(H)where T represents the temperature in K.

(2.5)

39

R + H+ + e−
R+ + H + e−

EA(R)

PA(R)
RH+ + e−R− + H+

IE(R)
IE(H)
AE(R+,RH)

IE(RH)

ΔacidH0(RH)

R+H
D0(RH)

ProtonAffinity
Cycle

Figure 2.9

Negative
Ion
Cycle

RH

Appearance
EnergyCycle

Gas-phase thermochemical cycles. The negative ion cycle combines electron
affinity measurements with gas phase acidity measurements to determine
R—H bond dissociation energies.

40

Chapter 2 References
1

D.G. Leopold, K.K. Murray, and W.C. Lineberger, Laser PhotoelectronSpectroscopy of Vibrationally Relaxed CH2− - a Reinvestigation of the Singlet Triplet
Splitting in Methylene. Journal of Chemical Physics, 1984. 81(2): p. 1048-1050.

2

D.G. Leopold, K.K. Murray, A.E.S. Miller, and W.C. Lineberger, Methylene - a
Study of the X 3B1 and 1A1 States by Photoelectron-Spectroscopy of CH2− and CD2−.
Journal of Chemical Physics, 1985. 83(10): p. 4849-4865.

3

F.C. Fehsenfeld, K.M. Evenson, and H.P. Broida, Microwave Discharge Cavities
Operating at 2450 MHz. Review of Scientific Instruments, 1965. 36(3): p. 294-298.

4

M. Marynowski, W. Franzen, and M. Elbatanouny, Analysis of the Properties of an
Electrostatic Triplet Quadrupole Lens Used as an Electron-Beam Transport Device.
Review of Scientific Instruments, 1994. 65(12): p. 3718-3723.

5

K.M. Ervin, J. Ho, and W.C. Lineberger, A Study of the Singlet and Triplet-States ofVinylidene by Photoelectron-Spectroscopy of H2C=C−, D2C=C−, and HDC=C− Vinylidene Acetylene Isomerization. Journal of Chemical Physics, 1989. 91(10): p.
5974-5992.

6

K.M. Ervin and W.C. Lineberger, Photoelectron Spectroscopy of Negative Ions, inAdvances in Gas Phase Ion Chemistry, N.G. Adams and L.M. Babcock, Editors.
1992, JAI Press: Greenwich. p. 121-166.

7

I.C. Chang, Acousto-Optic Devices and Applications, in Handbook of Optics, M.
Bass, Editor. 1995, McGraw-Hill: New York. p. 12.1-12.54.

8

C.S. Feigerle, Ph.D. Thesis. 1983, University of Colorado: Boulder, CO.

9

C. Valli, C. Blondel, and C. Delsart, Measuring electron affinities with thephotodetachment microscope. Physical Review A, 1999. 59(5): p. 3809-3815.

10C.E. Moore, Atomic Energy Levels. NSRDS-NBS. 1952, Washington: US GPO
Circular No. 467.

11

M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, J.A. Montgomery, Jr. , T. Vreven, K.N. Kudin, J.C. Burant, J.M.
Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegama, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.

41
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, and
J.A. Pople, Gaussian 03, Revision B.05. 2003, Gaussian, Inc.: Pittsburgh, PA.
12

C.T. Lee, W.T. Yang, and R.G. Parr, Development of the Colle-Salvetti CorrelationEnergy Formula into a Functional of the Electron-Density. Physical Review B, 1988.
37(2): p. 785-789.

13

A.D. Becke, Density-Functional Thermochemistry .3. The Role of Exact Exchange.
Journal of Chemical Physics, 1993. 98(7): p. 5648-5652.

14

R. Krishnan, J.S. Binkley, R. Seeger, and J.A. Pople, Self-Consistent MolecularOrbital Methods .20. Basis Set for Correlated Wave-Functions. Journal of Chemical
Physics, 1980. 72(1): p. 650-654.

15

K.M. Ervin, T.M. Ramond, G.E. Davico, R.L. Schwartz, S.M. Casey, and W.C.
Lineberger, Naphthyl radical: Negative ion photoelectron spectroscopy, FranckCondon simulation, and thermochemistry. Journal of Physical Chemistry A, 2001.
105(48): p. 10822-10831.

16

K.M. Ervin, FCFGAUS, Fortran program. 2003, Reno:
http://www.chem.unr.edu/~ervin/pes; University of Nevada, Reno.

17

K.M. Ervin, PESCAL, Fortran program. 2004, Reno:
http://www.chem.unr.edu/~ervin/pes; University of Nevada, Reno.

18

P. Chen, Photoelectron Spectroscopy of Reactive Intermediates, in Unimolecular andBimolecular Reaction Dynamics, C.Y. Ng, T. Baer, and I. Powis, Editors. 1994,
Wiley & Sons: New York. p. 371-425.

19

I. Ozkan, Franck-Condon Principle for Polyatomic-Molecules - Axis-Switching
Effects and Transformation of Normal Coordinates. Journal of Molecular
Spectroscopy, 1990. 139(1): p. 147-162.

20

R. Botter, V.H. Dibeler, J.A. Walker, and H.M. Rosenstock, Experimental andTheoretical Studies of Photoionization-Efficiency Curves for C2H2 and C2D2. J.
Chem. Phys., 1966. 44: p. 1271-1278.

21

T.E. Sharp and H.M. Rosenstock, Franck-Condon factors for Polyatomic Molecules.
J. Chem. Phys., 1964. 41: p. 3453-3463.

22

J.E. Bartmess, "Negative Ion Energetics Data", in NIST Chemistry Webbook, NIST
Standard Reference Database Number 69, J.P. Linstrom and W.G. Mallard, Editors.
June 2005, National Institue of Standards and Technology: Gaithersburg MD, 20899.

________________________________________________
3
Cyclopentadienide, C5H5−________________________________________________
3.1

Introduction
The cyclopentadienyl radical, C5H5•, has a long history. Well known as a Jahn-Teller

system, cyclopentadienyl radical has been the subject of numerous theoreticalinvestigations.1-11 First experimentally investigated in 1956,12 it is now recognized as anintermediate in hydrocarbon combustion and may possibly be involved in soot formation.13-21
There have since been many experimental investigations of the cyclopentadienyl radical,
including absorption,22 electron impact ionization,23 electron paramagnetic resonance,24
fluorescence,25-30 kinetics,31 and anion photodetachment.32 Many of these experimental andtheoretical investigations have sought to extract information about the molecular structure,Jahn-Teller coupling constants, and Jahn-Teller stabilization energy, and only a few of thetheoretical studies have tried to predict or interpret the experimental spectra. For this thesis,
the anion photodetachment experiments were repeated with a higher energy photon (3.53 eV
versus 2.54 eV) and much better photoelectron energy resolution, in order to obtain a more
precise EA for C5H5• and to determine vibrational frequencies of C5H5•.
H

H H
C

H

C
C
H

C

H H

C

−

C

C

C

H

Cyclopentadiene

H

H

H

C

C

H

C

C

C
H

Cyclopentadienide

H

H

C
C
H

Cyclopentadienyl Radical

The cyclopentadienyl radical readily binds an electron in the gas phase to form
cyclopentadienide (C5H5−). Being a closed shell molecule in the D5h point group, the groundstate of C5H5− is 1A1'. The highest occupied molecular orbital of C5H5− is doubly degenerate
and has e1'' symmetry. Logically, it follows that the ground state of C5H5• should be 2E1'' and43
be doubly degenerate.

The Jahn-Teller theorem states there is at least one non-totallysymmetric vibration along which the orbital energy degeneracy can be lifted.33

Thesymmetric direct product of the electronic state symmetry gives the symmetry of the linear
Jahn-Teller active vibrational modes which eliminate the degeneracy:
E1'' x E1'' = A1' + E2'(3.1)

Here, A1' represents the electronic coupling symmetry and E2' represents the Jahn-Teller
mixing; the symmetry of the linearly active Jahn-Teller vibrational modes is accordingly e2'.
There are 4 doubly degenerate e2' vibrational modes in C5H5•.
The photoelectron spectrum of C5H5− will thus represent a transition between the 1A1'
ground state of C5H5− and the Jahn-Teller 2E1'' ground state of C5H5•. Since there are only
two vibrational modes of a1' symmetry (i.e., totally symmetric modes) in C5H5, it is expectedthat the photoelectron spectrum will be composed of the band origin and vibrational bands
corresponding to the Jahn-Teller active modes and the two totally symmetric vibrationalmodes.

3.2

Experiment
The cyclopentadiene dimer (C10H12, Aldrich 98%) was cracked to produce

cyclopentadiene (C5H6) according to the literature procedure.34 Once produced, C5H6 wasstored under liquid nitrogen to prevent redimerization unless the sample was being used. Dueto its high vapor pressure and to maintain a steady flow rate into flow tube, C5H6 was kept at
0 °C when in use by submersing the sample in an ice water bath. Hydroxide ions (HO−),created using the microwave discharge ion source as described in Chapter 2, were used todeprotonate the C5H6 introduced downstream to create C5H5−. Typical ion beam currents of
C5H5− were ~300 pA. In some experiments, liquid nitrogen was used to cool the flow tube
and produce vibrationally cooler ions; in these experiments, slightly smaller ion beams were44
produced (~200 pA). The laser wavelength used for photodetachment of C5H5− was 351.1 nm
and the measurement of the oxygen EA was used in calibration of the absolute energy scale.

3.3

Results
Figure 3.1 (bottom panel) shows the 351.1 nm magic angle (θ = 54.7°) photoelectron

spectrum of C5H5− taken under liquid nitrogen cooled flow tube conditions (~200 K). The
room temperature spectrum shows broadening of the vibrational bands due to contributions
from hot band transitions. The spectra are dominated by an intense band origin appearing at
1.812 ± 0.005 eV (labeled A), four moderately intense peaks (labeled E, F, H and I), andmany small features all appearing to higher binding energy of peak A. Figure 3.1 (top panel)also shows the previously obtained photoelectron spectrum32 of C5H5−, where the smallfeatures are obscured by the strong band origin and poorer energy resolution.
Figure 3.2 shows the 351.1 nm photoelectron spectra of C5H5− taken at laser
polarizations parallel (θ = 0°, vertical polarization) and perpendicular (θ = 90°, horizontal
polarization) to the experimental photoelectron collection axis. These spectra were also
obtained under liquid nitrogen cooled flow tube conditions.

Values for the anisotropy

parameter (β) for features in the spectrum were determined, with most features having

β ≈ −0.9 except several small features between peaks A and E where β lies between −0.3 and−0.7. Figure 3.3 shows an expanded view of the θ = 0° spectrum and reveals several smallfeatures which appear with only minimal relative intensity in the θ = 90° and θ = 54.7°
spectra. Table 3.1 summarizes the peak positions relative to the origin.
In order to better assign the vibrational features of the photoelectron spectra, C5H5−and C5H5• geometry optimizations and frequency calculations were performed at theB3LYP/6-311++G(d,p) level of density functional theory (DFT) using the methods describedin Chapter 2. Table 3.2 lists the C5H5− vibrational frequencies obtained from this45

Electron Binding Energy (eV)
2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.7

1.6

1.5

1.4

200

150

100

Photoelectron Counts

50

0
5000

A

4000

3000

2000

1000

H

F
E

2.0

1.9

I

0
2.6

2.5

2.4

2.3

2.2

2.1

1.8

Electron Binding Energy (eV)Figure 3.1

Top panel: 488.0 nm photoelectron spectrum of C5H5− from reference 32.
Bottom panel: 351.1 nm magic angle photoelectron spectrum of C5H5− taken
at 200 K.

46

7000
6000

Photoelectron CountsAθ = 90°θ = 0°

5000
4000
3000
2000

I

1000

F

H E

0
2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

1.8

1.7

Electron Binding Energy (eV)Figure 3.2

351.1 nm photoelectron spectra of C5H5− taken at 200 K. The red curve wasobtained with the laser at vertical polarization (θ = 0°), and the blue curve
was obtained with the laser at horizontal polarization (θ = 90°).

47

180

A

160

Photoelectron Counts

140

B

120
100

DC

80

I

60

H

40

E
G

F

20
0
2.2

2.1

2.0

1.9

1.8

Electron Binding Energy (eV)Figure 3.3

Expanded view of the 351.1 nm photoelectron spectrum of C5H5− taken at
200 K with the laser at vertical polarization (θ = 0°).

48

Peak Label
A
B
C
D
E
F
G
H
I

Table 3.1

Peak Position (cm-1)
0500 ± 15 cm-1
650 ± 30 cm-1
790 ± 10 cm-1
890 ± 15 cm-1
1080 ± 10 cm-1
1325 ± 35 cm-1
1520 ± 15 cm-1
1720 ± 15 cm-1

β–0.95 ± 0.05
–0.35 ± 0.08
–0.37 ± 0.08
–0.68 ± 0.15
–0.88 ± 0.05
–0.94 ± 0.05
–0.89 ± 0.05
–0.91 ± 0.05
–0.92 ± 0.05

Relative experimental peak positions and anisotropy parameters for peaks
appearing in Figures 3.1 - 3.3.

49
in Chapter 2. Table 3.2 lists the C5H5− vibrational frequencies obtained from this calculation
and Table 3.3 lists the C5H5− molecular geometry.
DFT geometry optimization of C5H5• fails to converge when constrained to D5hsymmetry, presumably because of the Jahn-Teller effect. The symmetry of the neutral system
is lowered from D5h to C2v by displacement along the e2' Jahn-Teller active normal modes.
Lowering of the symmetry results in the degenerate 2E1'' state transforming into 2A2 and 2B1
Bstates in the C2v point group. It has been shown that for C5H5• there is no quadratic JahnTeller mixing.6 This lack of quadratic mixing causes the resulting lower symmetry states toenergetically relax by the same amount from the symmetric D5h point, resulting in degenerate,
but displaced, 2A2 and 2B1 states.
B

Geometry optimizations and frequency calculations on these C2v states were
performed at the B3LYP/6-311++G(d,p) level and are summarized in Tables 3.3 and 3.4.
Stationary points were located for both A2 and B1 electronic symmetries. These states were
not found to be exactly degenerate at this level of theory; the 2A2 state was found to be lower
by ~0.00095 eV.
In order to make qualitative assignments of the photoelectron spectra, FranckCondon simulations were carried out, as outlined in Chapter 2, using the results from the D5hanion and C2v neutrals. The results are presented in Figure 3.4. The origin positions andintensities of the Franck-Condon simulations were adjusted to match the experimental EA
and origin intensity. Additionally, the C2v stationary points each had a single vibrationalmode of b2 symmetry with a low frequency (~65 cm-1) which was omitted from thesimulation. These modes should correspond to the low barrier and nearly flat pseudorotation
coordinate which transforms C5H5• between 2A2 and 2B1.
B

From the simulations in Figure 3.4, it is clear that the intensities of most vibrational
bands are overestimated, while the intensities of a few others are underestimated. However,50
this is not totally unexpected, since the Franck-Condon simulation was performed assuming
the 2A2 and 2B1 states are stationary points, that the vibrational modes are harmonic
B

Mode
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14

Table 3.2

Nuclear Motions
C—H stretch
C—C stretch
In plane C—H bend
Out of plane C—H bend
In plane C—H stretch
In plane ring distortion
In plane C—H bend
Out of plane C—H bend
In plane C—H stretch
In plane ring distortion
In plane C—H bend
In plane ring bend
Out of plane C—H bend
Out of plane ring bendFrequency (cm-1)
3174
1143
1271
636
3152
1455
1013
602
3128
1370
1058
846
744
613

C5H5− fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations.

Symmetry
a1'
a1'
a2'
a2''
e1'
e1'
e1'
e1''
e2'
e2'
e2'
e2'
e2''
e2''

51

6
10
5
4
9

1

7

−

2
3
8

Point GroupElectronic StateC—C Bonds (Å) 1-2
2-3
3-4
C—H Bonds (Å) 1-6
2-7
3-8∠CCC (°)
1-2-3
2-3-4
5-1-2
∠HCC (°)
6-1-2
7-2-3
8-3-4

Table 3.3

D5hA1'
1.41531.41531.41531.08551.08551.0855108.00108.00108.00126.00126.00126.001

C2v
A2
1.3981
1.4692
1.35851.07961.08241.0805108.77107.66107.15126.43125.01
127.272

C2v
2B1
1.4364
1.3692
1.48211.08281.07981.0815107.28108.23108.99125.50127.08124.85B

C5H5− and C5H5• geometries obtained from B3LYP/6-311++G(d,p)
calculations.

52

Mode

Nuclear Motions

Symmetry

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18
ν19
ν20
ν21
ν22
ν23
ν24

C—H stretch
C—H stretch
C—H stretch
C—C stretch + C—H bend
C—C stretch + C—H bend
Ring breathing
C—C stretch
C—C stretch
In plane C-C-C bend
Out of plane C—H bend
Out of plane C—H bend
Out of plane C—H bend
Out of plane C—H bend
Out of plane C—H bend
Out of plane C—H bend
Out of plane C—H bend
C—H stretch
C—H stretch
In plane C—H bend
In plane C—H bend
In plane C—H bend
In plane C—H bend
In plane C—H bend
In plane ring distortion

a1
a1
a1
a1
a1
a1
a1
a1
a1
a2
a2
a2
b1
b1
b1
b1
b2
b2
b2
b2
b2
b2
b2
b2

2

A2 Frequency(cm-1)
3240
3232
3201
1565
1443
1142
1081
1033
847
908
819
524
886
723
678
489
3216
3202
1395
1294
1215
968
920
69

Table 3.4
C5H5• fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations.

2

B1 Frequency(cm-1)
3239
3216
3200
1485
1385
1142
1058
944
846
901
721
514
894
819
679
499
3232
3204
1528
1295
1217
1071
928
60
B

53

7000
Experiment
2B Franck-Condon Simulation
1
2A Franck-Condon Simulation
2

Photoelectron Counts

6000
5000
4000
3000
2000
1000
0
2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

1.8

1.7

Electron Binding Energy (eV)Figure 3.4

351.1 nm magic angle photoelectron spectrum of C5H5− taken at 200 K(black) and Franck-Condon simulations of detachment of 1A1' C5H5− to 2A2(blue) and 2B1 (red) C5H5•.
B54
this is not totally unexpected, since the Franck-Condon simulation was performed assuming
the 2A2 and 2B1 states are stationary points, that the vibrational modes are harmonicB

oscillators, and that the system is free from non-adiabatic effects. These assumptions do notnecessarily describe the C5H5• system well.
Despite these inconsistencies, several conclusions can be drawn from the simulations.
It is clear which peak corresponds to the band origin and that the strong features in theexperimental spectrum most likely correspond to totally symmetric (a1) vibrations within theC2v point group. In D5h, these modes have a1', e1', or e2' symmetry. Additionally, it can beconcluded that the actual geometry displacements in the detachment must be smaller than the
DFT calculations predict. This would result in more of the intensity appearing in the origin
peak, as experimentally observed. Careful examination of the simulated Franck-Condon
factors and the DFT vibrational frequencies of C5H5•, coupled with a comparison between the
DFT computed vibrational motions of C5H5− and C5H5•, reveal that the vibrational modes
appearing in the photoelectron spectrum are primarily due the Jahn-Teller active modes (e2').
Although they are allowed, the two totally symmetric (a1') modes do not appear in theFranck-Condon simulations with significant intensity.

3.4

Discussion
The peak labeled A in Figures 3.1-3.3 is identified as the C5H5• X 2E1'' ← C5H5−

X 1A1' band origin. The binding energy of this peak corresponds to the electron affinity of
C5H5•, 1.812 ± 0.005 eV. Considering the discrepancies between the experimental spectra
and the Franck-Condon simulations, unambiguous assignment of the vibrational features
observed here requires further analysis.
Previously,32 the EA of C5H5• was assigned as 1.786 ± 0.020 eV and an 800 ± 40
cm-1 vibration was observed and assigned to the totally symmetric mode (a1') of D5h C5H5•.
This assignment was based on the 983 cm-1 totally symmetric mode observed in K+(C5H5−).

55
Considering the smallest C5H5− a1' DFT vibrational frequency is 1143 cm-1 and thecorresponding a1 frequency in both 2A2 and 2B1 C5H5• is 1142 cm-1, this peak must be
Breassigned. This feature must correspond to the vibration of 890 ± 15 cm-1 observed here,
which is attributed to a Jahn-Teller active mode. A vibrational frequency of 1170 ± 40 cm-1
was also observed previously. This most likely corresponds to the vibration of 1080 ± 10
cm-1 observed here. Finally, a vibrational frequency of 1690 ± 40 cm-1 was previously
observed in the photoelectron spectrum of C5H5−, which must correspond to the 1520 ± 15
and 1720 ± 15 cm-1 vibrations observed here.
Additional simulations which include non-adiabatic effects are currently underway;35
without these simulations, definitive assignment of the majority of the smaller features
observed in the photoelectron spectra presented here is difficult. However, based on theFranck-Condon simulation results the following tentative assignments are made: the peaklabeled E in Figures 3.1-3.3 corresponds to one quantum of ν9 in 2A2 or 2B1 C5H5•; the peakBlabeled F corresponds to one quantum of ν8 in 2A2 or one quantum of ν7 in 2B1; the peakBlabeled H corresponds to one quantum of ν5 in 2A2 or one quantum of ν4 in 2B1.
B

Alternatively, the frequencies observed here can be compared with those observed inother experiments to allow for more reliable assignments. In their recent laser induced
fluorescence measurements,6, 30 Miller et al. observed many of the fundamental vibrationalfrequencies of C5H5•. In particular, they observed a single a1' mode of 1071 cm-1 and three ofthe four e2' modes of 1320, 1041, and 872 cm-1. They also observed a2', and e1' modes of1244 and 1001 cm-1, respectively. Overtones of the a2'', e1'', and two e2'' modes were also
observed and their frequencies were halved to obtain fundamental frequencies of 681, 766,
861, and 576 cm-1, respectively. These frequencies all compare reasonably well with thoseobtained for C5H5− from the DFT calculations (Table 3.2).

56
Although the photoelectron spectrum is expected to be composed of the origin bandand vibrational features corresponding to only totally symmetric (a1') and Jahn-Teller active(e2') modes, it is possible for other vibrational modes to appear in the photoelectron spectrum
since overtones of non-totally symmetric modes are explicitly allowed. Fundamentals arealso vibronically allowed and can often be seen if off-diagonal coupling is strong.
Considering that Miller et al. observed fundamentals of forbidden modes, it is likely that offdiagonal coupling in C5H5• is strong enough to induce appearance of these modes in thephotoelectron spectrum. Considering this and the above observations, assignments of the
bands labeled in Figures 3.1-3.3 are made in Table 3.5. Peaks H and I are not assigned here
but are most likely overtones of peaks E and F with strong anharmonicity.
The EA determined for C5H5• can be used to obtain the C—H bond dissociationenergy (D0) of C5H6 using the methods outlined in Chapter 2. Literature values36 for the
298 K deprotonation enthalpy and heat of formation of C5H6 are 353.9 ± 2.2 and33.2 kcal mol-1. Using thermodynamic information from Gaussian output files and Equation
2.3, D0(C5H5—H) = 80.6 ± 2.3 kcal mol-1. Assuming an error bar of ± 1.0 kcal mol-1 for the
C5H6 heat of formation, the heat of formation of C5H5• can be determined from Equations 2.4
and 2.5, ΔfH298(C5H5•) = 64.6 ± 2.7 kcal mol-1.

3.5

Conclusion
The C5H5− photoelectron spectra presented here suggest only the slightest change inthe molecular structure upon photodetachment, resulting in a spectrum dominated by anintense origin peak. Considering that the Jahn-Teller modes will be significantly anharmonicand possibly have extensive mixing, it is difficult to make assignments of the vibrationalfeatures without performing additional simulations which include non-adiabatic effects.
These simulations are currently underway.35 Regardless of these difficulties, the EA of C5H5•

57
is unequivocally assigned as 1.812 ± 0.005 eV and many of the vibrational features observedin the photoelectron spectrum have been assigned to modes of 2E1'' C5H5•.

58

Mode

Nuclear Motions

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14

C—H stretch
C—C stretch
In plane C—H bend
Out of plane C—H bend
In plane C—H stretch
In plane ring distortion
In plane C—H bend
Out of plane C—H bend
In plane C—H stretch
In plane ring distortion
In plane C—H bend
In plane ring bend
Out of plane C—H bend
Out of plane ring bend

Label inFigures
3.1-3.3
—
—
—
C
—
—
—
D
—
G
F
E
—
B

Observed Frequency(cm-1)

Symmetry***
650 ± 30***790 ± 10*1325 ± 351080 ± 10
890 ± 15*500 ± 15

a1'
a1'
a2'
a2''
e1'
e1'
e1'
e1''
e2'
e2'
e2'
e2'
e2''
e2''

* These vibrational modes were not observed in the photoelectron spectrum.

Table 3.5

Assignments of C5H5• vibrational features observed in the photoelectron
spectrum of C5H5−.

59

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1

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24

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61
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26

L. Yu, S.C. Foster, J.M. Williamson, M.C. Heaven, and T.A. Miller, Rotationally
Resolved Electronic-Spectrum of Jet-Cooled Cyclopentadienyl Radical. Journal of
Physical Chemistry, 1988. 92(15): p. 4263-4266.

27

L. Yu, J.M. Williamson, and T.A. Miller, Rotationally Resolved Electronic-Spectrumof Jet-Cooled Deuterated Cyclopentadienyl Radical. Chemical Physics Letters, 1989.
162(6): p. 431-436.

28

L.A. Yu, D.W. Cullin, J.M. Williamson, and T.A. Miller, High-Resolution Laser
Spectroscopy of Asymmetrically Deuterated Cyclopentadienyl Radicals - a Study ofVibronic Degeneracy Resolution and Jahn-Teller Distortion. Journal of Chemical
Physics, 1993. 98(4): p. 2682-2698.

29

S. Sun and E.R. Bernstein, Vibronic Structure of the Cyclopentadienyl Radical and
Its Nonrigid Van-Der-Waals Cluster with Nitrogen. Journal of Chemical Physics,
1995. 103(11): p. 4447-4454.

30

B.E. Applegate, A.J. Bezant, and T.A. Miller, The Jahn-Teller and related effects inthe cyclopentadienyl radical. II. Vibrational analysis of the A 2A2'' - X 2E1'' electronictransition. Journal of Chemical Physics, 2001. 114(11): p. 4869-4882.

31

K. Roy, M. Braun-Unkhoff, P. Frank, and T. Just, Kinetics of the cyclopentadienedecay and the recombination of cyclopentadienyl radicals with H-atoms: Enthalpy offormation of the cyclopentadienyl radical. International Journal of Chemical
Kinetics, 2002. 34(3): p. 209-222.

32

P.C. Engelking and W.C. Lineberger, Laser Photoelectron Spectrometry of C5H5− Determination of Electron-Affinity and Jahn-Teller Coupling in Cyclopentadienyl.
Journal of Chemical Physics, 1977. 67(4): p. 1412-1417.

33

H.A. Jahn and E. Teller, Stability of polyatomic molecules in degenerate electronic
states. I. Orbital degeneracy. Proc. R. Soc. (London), 1937. A161: p. 220-235.

34

R.B. Moffett, Cyclopentadiene and 3-chlorocyclopentene. Organic Syntheses, 1952.
32: p. 41-44.

35

T. Ichino, A.J. Gianola, W.C. Lineberger, and J.F. Stanton, Manuscript in
preparation.

36

J.E. Bartmess, "Negative Ion Energetics Data", in NIST Chemistry Webbook, NIST
Standard Reference Database Number 69, J.P. Linstrom and W.G. Mallard, Editors.
June 2005, National Institue of Standards and Technology: Gaithersburg MD, 20899.

________________________________________________
4
Pyrrolide, C4H4N−________________________________________________
4.1

Introduction
Substitution of an N—H group for the CH2 group in cyclopentadiene results in the

pyrrole molecule (C4H5N) which is isoelectronic with cyclopentadiene.

The analogous

deprotonated anions, pyrrolide (C4H4N−) and cyclopentadienide, are isoelectronic with eachother. The corresponding neutrals, the pyrrolyl radical and the cyclopentadienyl radical, arealso isoelectronic with one another. In addition, the substitution lowers the symmetry from
D5h to C2v. Considering this, it is surprising that there have been few studies on pyrrolyl as aperturbation to the cyclopentadienyl system.1
Photodetachment of pyrrolide has been reported,2 as has photoionization of pyrrole.3
A short time ago, photofragment velocity map imaging was applied to the pyrrole system todetermine the N—H bond dissociation energy (D0) of pyrrole.4 More recently, H Rydberg
atom photofragment translational spectroscopy was employed to very precisely measure theN—H bond dissociation energy of pyrrole.5

Here, the electron affinity (EA) and thevibrational and electronic structure of the pyrrolyl radical are determined. Combining the
pyrrolyl EA with the pyrrole N—H bond dissociation energy, a precise gas-phase acidity forthe pyrrole molecule is obtained. The work presented here is also available as a publication6(J. Phys. Chem. A, 2004, 108, 10326-10335).
H
N

H

C
C

N

H H

C

C

C

H

H

Pyrrole

−

C

N

H H

C

C

C

H

C
H

Pyrrolide

H

H

C
C
H

Pyrrolyl Radical

63
4.2

ExperimentHydroxide ions (HO−), created using the microwave discharge ion source as

described in Chapter 2, were used to deprotonate pyrrole (Sigma, 98%), introduced
downstream, to create pyrrolide. Typical pyrrolide ion beam currents were ~150 pA. In
some experiments, liquid nitrogen was used to cool the flow tube and produce vibrationallycooler ions; in these experiments, slightly smaller ion beams were produced (~75 pA). The
laser wavelength used for photodetachment of pyrrolide was 363.8 nm and measurement of
the EA of oxygen was used to calibrate the absolute energy scale.

4.3

Results
The 363.8 nm (3.408 eV) magic-angle photoelectron spectrum of pyrrolide at ~298 K

is shown in Figure 4.1. The spectrum shows a Franck-Condon envelope with at least three
active vibrations. The peak at 2.145 ± 0.010 eV is assigned as the electronic band origin,corresponding to the EA of pyrrolyl. This value is close to that of 2.39 ± 0.13 eV obtained byBrauman in a very early, low-resolution threshold photodetachment measurement.2 Threepyrrolyl vibrational frequencies are resolved in the spectrum at 925 ± 65, 1012 ± 25, and1464 ± 20 cm-1 from the band origin. A weak peak at lower binding energy is identified as apyrrolide vibrational hot-band with a frequency of 874 ± 95 cm-1.
Figure 4.2 shows the 298 K photoelectron spectrum with the laser polarization
parallel (θ = 0°) and perpendicular (θ = 90°) to the photoelectron collection axis. A value of–0.5 ± 0.1 was obtained for the anisotropy parameter, β, for the strong features in the
spectrum. The β value, however, is smaller across the weak continuous portion of thespectrum (eBE > 2.5 eV), approaching zero. This variation of β suggests that the spectrum
may represent electron detachment from more than one molecular orbital of pyrrolide.

64

140

Photoelectron Counts

120
100
80
60
40
20
0
3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

Electron Binding Energy (eV)Figure 4.1

363.8 nm magic angle photoelectron spectrum of pyrrolide at 298 K.

1.9

65

300

θ = 90°θ = 0°

Photoelectron Counts

250

200

150

100

50

0
3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

Electron Binding Energy (eV)Figure 4.2

363.8 nm photoelectron spectra of pyrrolide taken at 298 K. The blue curve
was obtained with the laser at vertical polarization (θ = 0°), and the red curve
was obtained with the laser at horizontal polarization (θ = 90°).

66
To aid in the assignment and interpretation of the photoelectron spectra, electronic
structure calculations were carried out on the pyrrole/pyrrolide/pyrrolyl system as outlined in
Chapter 2.

Geometry optimizations and frequency calculations were performed at theB3LYP/6-311++G(d,p) level of Density Functional Theory (DFT) for ground state pyrrolide(1A1), as well as the ground (2A2) and first excited (2B1) states of pyrrolyl. The optimizedBgeometries for these three stationary points are listed in Table 4.1 and vibrational frequencies
are listed in Table 4.2; these results compare well with a recent DFT study.7
The DFT calculations show that these states all have C2v symmetry. Figure 4.3
depicts the highest occupied molecular orbitals of pyrrolide. Detachment of an electron fromthe highest occupied molecular orbital forms the ground state of pyrrolyl, which has A2electronic symmetry. Likewise, detachment from the second highest occupied molecularorbital forms excited state pyrrolyl, with B2 electronic symmetry. Detachment from the a1(σ)
orbital is inaccessible with the photon energy used in this experiment. The DFT calculations
find a minimum for 2A2 pyrrolyl and a transition state for 2B1 pyrrolyl, as indicated by theBimaginary frequency listed in Table 4.2. These two stationary points are connected via apseudorotation path on the ground adiabatic potential energy surface.6
Examination of the geometries provides insight into which vibrational modes will be
activated upon photodetachment. There are significant differences of the bond lengths in thering between the anion ground state and the two states of the radical. It is therefore expectedthat those vibrational modes which contain significant motion along the ring coordinates will
be active in the Franck-Condon profile.
Using the geometry and frequency results from the DFT calculations, Franck-Condon
simulations were performed for the transitions between 1A1 pyrrolide and 2A2 or 2B1 pyrrolyl,Bas outlined in Chapter 2. The simulation, depicted in Figure 4.4, assumes identical total
photodetachment cross sections for the two radical states and neglects the imaginary
frequency in the 2B1 state. Three totally symmetric vibrational modes in each state ofB

67
pyrrolyl are determined to be active in the Franck-Condon profile; the corresponding normal
mode displacements for the 2A2 state are depicted in Figure 4.5. Fundamentals, overtones,
and combination bands of the three active modes make up the vibrational features.

N1

H9

C5
C4

C2

−

H7

Electronic StateN—C Bonds (Å)
C—C Bonds (Å)
C—H Bonds (Å)

Table 4.1

N

N

N

C3

H8

∠CNC (°)∠NCC (°)∠CCC (°)∠HCN (°)∠HCC (°)

H6

1-2
2-3
3-4
2-6
3-7
5-1-2
1-2-3
2-3-4
6-2-1
7-3-2

Pyrrolide1
A1
1.36251.40351.41951.08651.0847105.13112.06105.38120.54127.202

A2
1.34411.45981.36101.08341.0798104.74112.38105.25120.99126.26

Pyrrolyl2B11.38561.36391.49471.07941.0806106.84111.04105.54120.17128.29B

Pyrrolide and pyrrolyl geometries obtained from the B3LYP/6-311++G(d,p)
calculations.

68

Pyrrolide
Mode
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18
ν19
ν20
ν21

Table 4.2

Symmetry
A1
A1
A1
A1
A1
A1
A1
A1
A2
A2
A2
B1
B1
B1
B2
B2
B2
B2
B2
B2
B2
B

B

B

B

B

B

B

B

B

B

1

A1
Frequency (cm-1)
3174
3131
1462
1375
1168
1104
1018
879
764
683
608
753
679
652
3152
3126
1444
1300
1229
1031
893

Pyrrolyl
2A2Frequency (cm-1)
3245
3201
1556
1433
1203
1090
1043
882
904
817
488
836
710
540
3226
3197
1355
1293
1079
935
817

2

B1
Frequency (cm-1)
3248
3225
1477
1368
1137
1071
936
870
887
753
505
878
702
326
3240
3217
1456
1285
1079
931
2167i
B

Pyrrolide and pyrrolyl fundamental vibrational frequencies obtained fromB3LYP/6-311++G(d,p) calculations.

69

H

a2(π)

H
C

H

a2

C

C

C

H

N

H

b1(π)
H

a1

H
C

C

C

H

H

b1

2

B1

a1

b1

2

A1

a1

C

C

C

-e

a2

H
C

H

A2

a2

C

N

a1(σ)

b1 2

a2

H

b1

N

1

A1

a1

Figure 4.3

Three highest occupied molecular orbitals of pyrrolide and the schematic
representation of electron photodetachment from pyrrolide to form threeelectronic states of pyrrolyl radical.

70

140
Experiment
2B Franck-Condon Simulation
1
2A Franck-Condon Simulation
2

Photoelectron Counts

120
100
80
60
40
20
0
3.3

3.2

3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

Electron Binding Energy (eV)Figure 4.4

Franck-Condon simulations of detachment of 1A1 pyrrolide to 2A2 (blue) and2
B1 (red) pyrrolyl. The binding energy for the 2A2 origin peak was adjusted
to match the experimental spectrum (black) while the 2B1 origin peak
position was adjusted to match the DFT term energy (~0.49 eV).
B

B

71

N

H
C

C
C

N

H H
C

C

H

H
C

C
H

C

H

H

a1 mode, 1433 cm-1

a1 mode, 882 cm-1

ring distortion

CNC bend

N

H
C

H

C
C

C

C

H

H

a1 mode, 1043 cm

H

C

C
C

N

H H

-1

H

b2 mode, 2167i cm-1

CCH bend + CC stretch
Figure 4.5

Relative atomic displacements in the normal modes for pyrrolyl radical.
Three a1 modes for the 2A2 state of pyrrolyl which are active in thephotoelectron spectrum, and one b2 mode for the 2B1 state of pyrrolyl which
has an imaginary frequency.
B

72
pyrrolyl are determined to be active in the Franck-Condon profile; the corresponding normal
mode displacements for the 2A2 state are depicted in Figure 4.5. Fundamentals, overtones,
and combination bands of the three active modes make up the vibrational features observed in
the spectra.
Comparison between the experimental photoelectron spectrum and the FranckCondon simulations (Figure 4.4) makes it clear that the main features in the photoelectron
spectrum are due to detachment to the 2A2 ground state of pyrrolyl. The DFT calculations
predict a pyrrolyl EA of 2.116 eV, in excellent agreement with the experimental value of
2.145 ± 0.010 eV. The Franck-Condon analysis also predicts the 2B1 state to appear in theBspectrum with a term energy of 0.49 eV according to the DFT calculations.

No suchprominent features appear in the spectrum which can be ascribed to the 2B1 state. The
B

absence of such features must originate from the lifetime broadening along the vibrationalmode with an imaginary frequency.
4.4

Discussion
The pyrrolyl radical is isoelectronic with the cyclopentadienyl radical, C5H5•. The

highest occupied molecular orbitals in C5H5• are doubly degenerate, have e1'' symmetry, andare analogous to the a2 and b1 orbitals illustrated in Figure 4.3. Replacement of one C—H
group with a nitrogen atom converts C5H5• into pyrrolyl and reduces the maximum symmetry
from D5h to C2v. As a consequence, the highest occupied molecular orbitals in pyrrolyl are nolonger required by symmetry to be degenerate. Since the b1 orbital of pyrrolyl has a large
contribution from the nitrogen p orbital it will be more stable than the a2 orbital which has anode at the nitrogen position. This will result in pyrrolyl having a ground state of 2A2, asconfirmed by the Franck-Condon analysis. ESR measurements of the pyrrolyl radical inaqueous solution also find a 2A2 ground state.8

73
It is straightforward to rationalize the geometry change between the anion groundstate and the radical ground state by recognizing the bonding character of the anion orbitalfrom which electron detachment takes place. The a2 orbital has bonding character between
C2 and C3 and between C4 and C5, while antibonding character is found between C3 and C4(Figure 4.3; see the figure in Table 4.1 for atom numbering). Detachment of an electron fromthis orbital reduces these bonding and antibonding effects. Thus, the C2—C3 and C4—C5
bonds lengthen, while the C3—C4 bond shortens upon electron detachment to form the 2A2
state of the radical, as found in the DFT calculations (Table 4.1)
The anisotropy parameter (β) for the intense features assigned to the 2A2 state isabout –0.5.

As outlined in Chapter 1, this anisotropy is consistent with electron

photodetachment from a π type molecular orbital, lending additional support to theassignment made here. The observed change of the β value from –0.5 to near zero hints at
contribution to the spectrum from another electronic state of pyrrolyl. Figure 4.4 shows that
the Franck-Condon simulation of detachment to the 2B1 state of pyrrolyl predicts a prominent,Bwell-resolved photoelectron spectrum beginning about 0.5 eV above the ground-state origin.
No such spectrum is observed, but there appears to be a weak continuum in this region.
This state is “missing” because of a large imaginary frequency at the 2B1 stationaryBpoint. DFT predicts a first order transition state for the 2B1 state with a single b2 mode with aBlarge imaginary frequency of 2167i cm-1. All other calculated frequencies are real, with thesmallest frequency being 326 cm-1. The displacements for the imaginary mode are depicted
in Figure 4.5. This vibration corresponds to the pseudorotation coordinate between the 2B1
B

and 2A2 stationary points. Following this coordinate from the 2B1 stationary point, it isB

downhill in energy all the way to the 2A2 potential minimum.
The magnitude of the imaginary frequency shows the steepness of the potential
energy surface along this normal coordinate. The shape of the potential surface is directly

74
related to lifetime broadening in transition-state spectra.9 Such a large magnitude imaginary
frequency for the pyrrolyl 2B1 state suggests that the spectrum would be broadenedBsubstantially. The Franck-Condon simulation in Figure 4.4 was performed with a full widthat half maximum of 15 meV. This linewidth represents the instrument resolution and isappropriate for the 2A2 ground state but not for the 2B1 transition state. If the 2B1 state has
BBboth substantial lifetime broadening and a photodetachment cross section comparable to theground state, then the spectrum would appear as a weak, broad, featureless continuum, as
observed. The Franck-Condon simulation for the 2A2 ground state shows little intensity
above 2.7 eV electron binding energy, and yet the experimental spectrum exhibits a weak tailup to about 2.1 eV, with a significantly different β value. All of these observations lead tothe conclusion that the broad tail of the photoelectron spectrum above an electron binding
energy of 2.6 eV reflects transitions to the 2B1 state of pyrrolyl, with an intrinsic width ofB

perhaps 0.1 eV.
The imaginary frequency of the 2B1 state arises from non-adiabatic (vibronicBcoupling) effects with the nearby 2A2 ground state. Comparing pyrrolyl with C5H5• may help
illustrate the problem at hand.

Figure 4.6 shows a qualitative picture of the adiabatic

potential energy surfaces of C5H5• along two first-order Jahn-Teller active normal
coordinates, as discussed recently by Miller et al.10, 11 The degenerate ground states, 2E1'' inD5h symmetry, are subject to Jahn-Teller effects, resulting in lower symmetry (C2v) stationary
points which are no longer degenerate. The C2v minima are connected by a pseudorotation
path (the moat in Figure 4.6), and there is a small barrier along this coordinate. For pyrrolyl,
the corresponding adiabatic potential energy surfaces can be drawn along a1 and b2 normal
coordinates, as shown in Figure 4.7. The picture reflects the fact that the symmetry is
reduced from D5h to C2v, lifting the degeneracy, and giving rise to two stationary points, aminimum (a2 electronic symmetry) and a transition state (b1 electronic symmetry).

75
Looking at a pseudorotation path similar to C5H5•, it is clear that it is indeed downhill
in energy from the 2B1 saddle point on the potential surface to the 2A2 minimum. The
B

imaginary frequency of 2167i cm-1 is consistent with a time of ca. 10 fs to move away fromthe saddle point, which can be taken as an estimate of the lifetime of the transition state.
With such a small lifetime, any population which arrives at the 2B1 transition state point on
B

the potential surface will quickly relax toward the 2A2 minimum.
Figure 4.8 shows a one-dimensional slice of the potential energy surfaces of pyrrolyl
along the C2v (a1) axis. DFT calculations were employed to evaluate the energetic relation
between the 2A2 and 2B1 electronic symmetries at the stationary points. At the 2A2 minimumBgeometry, the upper potential (2B1) is 1.3 eV higher. In contrast, at the 2B1 saddle point
BB

geometry, the upper potential energy (2A2) is located only 0.17 eV above the 2B1 saddle point.
B

In terms of the potential energy surface expansion in Equation 1.11, b2 vibrations canvibronically couple the 2A2 and 2B1 states. At the 2B1 stationary point, the energy separation
B

B

is small, and the coupling term in Equation 1.11 becomes large and negative. If themagnitude of the coupling term exceeds the other quadratic term, the stationary point will
become a saddle point. The imaginary frequency of 2167i cm-1 for a b2 mode is consistent
with this.

At the 2A2 stationary point, however, there is a relatively large energy separation,

which suggests that vibronic coupling effects will be minimal. This region is, of course, theportion of the configuration space explored in the photoelectron spectrum of the pyrrolyl 2A2
ground state. As discussed before, the band origin of the 2A2 state (corresponding to thepyrrolyl radical EA) is 2.145 ± 0.010 eV. Without resolvable features for the 2B1 excitedB

state, assignment of the term energy is difficult. According to the DFT calculations this value
is 0.49 eV.
With the above measurement of the pyrrolyl EA, the bond dissociation energy can
now be determined for breaking the pyrrole N—H bond, as outlined in Chapter 2. However,
the recently determined pyrrole N—H bond dissociation energy5 (93.92 ± 0.11 kcal mol-1)

76
has an error bar much smaller than usually obtained through the methods of Chapter 2.
Consequently, the EA determined here can be combined with this bond dissociation energy todetermine a precise value for the 0 K pyrrole deprotonation enthalpy, ΔacidH0(C4H4N—H) =358.0 ± 0.4 kcal mol-1. Using thermodynamic information from the Gaussian output files,
conversion to the 298 K deprotonation enthalpy and gas phase acidity yields
ΔacidH298(C4H4N—H) = 359.4 ± 0.4 kcal mol-1 and ΔacidG298(C4H4N—H) = 351.9 ± 0.4 kcal
mol-1, respectively. This compares well with but improves upon the literature values12 ofΔacidH298(C4H4N—H) = 358.6 ± 2.2 kcal mol-1 and ΔacidG298(C4H4N—H) = 350.9 ± 2.0
kcal mol-1.

4.5

Conclusion
The photoelectron spectrum of pyrrolide reveals the EA of pyrrolyl radical to be

2.145 ± 0.010 eV. This EA value and the pyrrole N—H bond dissociation energy werecombined in a thermochemical cycle to yield the gas-phase acidity of pyrrole,
ΔacidG298(C4H4N—H) = 351.9 ± 0.4 kcal mol-1. The Franck-Condon simulation of thephotoelectron spectrum confirms that the ground state of pyrrolyl is 2A2 and identifies threeharmonic vibrational frequencies active in the spectrum (925 ± 65, 1012 ± 25, and 1464 ± 20cm-1). These totally symmetric modes of 2A2 pyrrolyl have large displacements along the ring
coordinates.

The first excited state of pyrrolyl, 2B1, appears broad and featureless,Boverlapping the tail of the 2A2 portion of the spectrum. DFT calculations predict a saddle
point for the 2B1 stationary point, with a b2 imaginary frequency of 2167i cm-1 indicating thatB

the potential energy surface is rather steep along this coordinate.

Substantial lifetime

broadening is responsible for the appearance of the 2B1 state in the photoelectron spectrum.
B

The term energy for the 2B1 state is 0.49 eV according to the DFT calculations, but thisBenergy cannot be determined from the spectrum.

77

e2′
Figure 4.6

Adiabatic potential energy surface of the two lowest electronic states ofcyclopentadienyl radical.

a1
Figure 4.7

e2′

b2

Adiabatic potential energy surface of the two lowest electronic states of thepyrrolyl radical.

78

Energy2B1(TS)0.169 eV

1.294
eV
2A
2

0.575 eV

Totally symmetric coordinate

Figure 4.8

Schematic illustration of the potential energy curves of the two lowest
electronic states of the pyrrolyl radical, corresponding to a slice of thepotential energy surfaces shown in Figure 4.7 along the totally symmetriccoordinate. The potential energy separations were calculated at theB3LYP/6-311++G(d,p) level.

79

Chapter 4 References
1

G.B. Bacskay, M. Martoprawiro, and J.C. Mackie, An ab initio quantum chemical
study of the electronic structure and stability of the pyrrolyl radical: Comparison
with the isoelectronic cyclopentadienyl radical. Chemical Physics Letters, 1998.
290(4-6): p. 391-398.

2

J.H. Richardson, L.M. Stephenson, and J.I. Brauman, Photodetachment of Electrons
from Large Molecular Systems - Pyrrolate Ion - Electron Affinity of C4H4N. Journalof the American Chemical Society, 1975. 97(5): p. 1160-1162.

3

P.J. Derrick, L. Asbrink, O. Edqvist, and E. Lindholm, PhotoelectronSpectroscopical Study of Vibrations of Furan, Thiophene, Pyrrole andCyclopentadiene. Spectrochimica Acta, Part A: Molecular Spectroscopy, 1971. A
27(12): p. 2525-2537.

4

J. Wei, A. Kuczmann, J. Riedel, F. Renth, and F. Temps, Photofragment velocity map
imaging of H atom elimination in the first excited state of pyrrole. Physical
Chemistry Chemical Physics, 2003. 5(2): p. 315-320.

5

B. Cronin, M.G.D. Nix, R.H. Qadiri, and M.N.R. Ashfold, High resolution
photofragment translational spectroscopy studies of the near ultraviolet photolysis ofpyrrole. Physical Chemistry Chemical Physics, 2004. 6(21): p. 5031-5041.

6

A.J. Gianola, T. Ichino, R.L. Hoenigman, S. Kato, V.M. Bierbaum, and W.C.
Lineberger, Thermochemistry and electronic structure of the pyrrolyl radical. Journalof Physical Chemistry A, 2004. 108(46): p. 10326-10335.

7

J.C. Rienstra-Kiracofe, D.E. Graham, and H.F. Schaefer, Medium ring compounds
and their anions: a systematic density functional theory study. Molecular Physics,
1998. 94(5): p. 767-787.

8

A. Samuni and P. Neta, Electron-Spin Resonance Study of Reaction of Hydroxyl
Radicals with Pyrrole, Imidazole, and Related Compounds. Journal of Physical
Chemistry, 1973. 77(13): p. 1629-1635.

9D.M. Neumark, Transition-State Spectroscopy Via Negative-Ion Photodetachment.
Accounts of Chemical Research, 1993. 26(2): p. 33-39.

10

B.E. Applegate, T.A. Miller, and T.A. Barckholtz, The Jahn-Teller and related
effects in the cyclopentadienyl radical. I. The ab initio calculation ofspectroscopically observable parameters. Journal of Chemical Physics, 2001.
114(11): p. 4855-4868.

11

B.E. Applegate, A.J. Bezant, and T.A. Miller, The Jahn-Teller and related effects inthe cyclopentadienyl radical. II. Vibrational analysis of the A 2A2'' - X 2E1'' electronictransition. Journal of Chemical Physics, 2001. 114(11): p. 4869-4882.

80
12J.E. Bartmess, "Negative Ion Energetics Data", in NIST Chemistry Webbook, NIST
Standard Reference Database Number 69, J.P. Linstrom and W.G. Mallard, Editors.
June 2005, National Institue of Standards and Technology: Gaithersburg MD, 20899.

________________________________________________
5
Imidazolide, C3H3N2−________________________________________________
5.1

Introduction
Substitution of a N atom for a C—H group in cyclopentadiene results in the pyrrole

molecule; substitution of a second N atom for another C—H group can result in two isomers
depending upon the arrangement of the two N atoms in the ring. If the two N atoms are
adjacent in the ring, the result is the pyrazole molecule; if the two N atoms are spaced in thering, the result is the imidazole molecule.

As in the pyrrole system, deprotonation of

imidazole takes place at the N site to form 1-imidazolide anion. In imidazole, however, acarbon site is also quite acidic1, 2 and is deprotonated by HO− to form an imidazolide isomer,
5-imidazolide. The gas phase acidities (ΔacidG298) of the imidazole N and C5 protons are
342.6 ± 0.4 and 380 ± 4 kcal mol-1, respectively. Although these ions have the same mass,
they should have different photoelectron signatures and, if formed in the flow tube, bothshould appear in the photoelectron spectrum. The photoelectron spectra obtained reveal
vibrational and electronic structure of the corresponding neutral radicals.

The resultspresented here are also available in a recent publication3 (J. Phys. Chem. A, 2005, 109,
11504-11514).
H
N

H

C

C
C

N

H H

N

H

−

C
C

C

C

1-Imidazolide

C
C

N

H

Imidazole

N

H H

N

H

1-Imidazolyl Radical

H

82
H

H
N

−C

C
C

N

H

C
C

N

5.2

H

N

H

H

5-ImidazolideC

5-Imidazolyl Radical

ExperimentHydroxide ions are created using the microwave discharge ion source as described in

Chapter 2. Imidazole (Sigma, 99%) is introduced downstream by flowing a stream of helium
through a crystalline imidazole sample heated to ~50 °C. The imidazole is deprotonated by
HO− to form imidazolide ions. Typical imidazolide ion beam currents were ~150 pA. The
laser wavelength used for photodetachment of imidazolide was 351.1 nm.

In someexperiments O− was used as the deprotonation agent. Measurement of the EA of oxygen wasused in calibration of the absolute energy scale.

5.3

Results
The 351.1 nm (3.531 eV) magic angle photoelectron spectrum of 1-imidazolide isshown in Figure 5.1. The most intense peak, labeled A, is assigned as the ground state band
origin, and the EA of 1-imidazolyl is determined to be 2.613 ± 0.006 eV. Several peaks areobserved to higher binding energy, representing vibrational levels of 1-imidazolyl. Table 5.1
lists the energies of the observed peaks relative to the origin. Figure 5.2 shows the 298 K
photoelectron spectrum with the laser polarization parallel (θ = 0°) and perpendicular (θ =90°) to the photoelectron collection axis. A value of –0.30 ± 0.08 was obtained for the
anisotropy parameter (β) of the major features in the spectrum.
DFT calculations were performed to make assignments of the photoelectron spectra,
as outlined in Chapter 2. Geometry optimizations and frequency calculations were performed

83

Peak Label
A
B
C
D
E
F

Table 5.1

Peak Position (cm-1)
0955 ± 151365 ± 201575 ± 30
1925 ± 252325 ± 30

Relative experimental peak positions for peaks labeled in Figure 5.1.

84

1000

A

Photoelectron Counts

800

600

C B
400

F
200

E

D

0
3.4

3.3

3.2

3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

Electron Binding Energy (eV)Figure 5.1

351.1 nm magic angle photoelectron spectrum of 1-imidazolide at 298 K.

85

1400

θ = 90°θ = 0°

Photoelectron Counts

1200
1000
800
600
400
200
0
3.4

3.3

3.2

3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

Electron Binding Energy (eV)Figure 5.2

351.1 nm photoelectron spectra of 1-imidazolide taken at 298 K. The blue
curve was obtained with the laser at vertical polarization (θ = 0°), and the red
curve was obtained with the laser at horizontal polarization (θ = 90°).

86
at the B3LYP/6-311++G(d,p) level of DFT for ground state 1-imidazolide (1A1), and the 2B1,
B

2

B2, and 2A1 states of 1-imidazolyl. Table 5.2 lists the optimized geometries and Table 5.3
Blists the harmonic vibrational frequencies obtained at these four stationary points. As withthe pyrrole system, these stationary points all have C2v symmetry. Figure 5.3 depicts themolecular orbitals of 1-imidazolide, illustrating how the orbitals have been reordered for thissystem. Detachment from the highest occupied molecular orbital of 1-imidazolide will resultin the ground state of 1-imidazolyl, 2B1. Detachment from lower energy molecular orbitals ofB

1-imidazolide results in excited states of 1-imidazolyl, 2B2 and 2A2. The DFT calculations
Bfind a minimum for 2B1 1-imidazolyl, but transition states for both 2B2 and 2A2 1-imidazolyl
BB

as indicated by the b1 vibrations with imaginary frequencies.
A Franck-Condon simulation was performed for detachment of 1A1 1-imidazolide to2

B1 1-imidazolyl and is depicted in Figure 5.4. The simulation reproduces the observedBexperimental spectrum reasonably well. The DFT calculations predict an EA of 2.592 eV, ingood agreement with the experimental value. Thus, the observed spectrum is assigned to the2

B1 ground state of 1-imidazolyl. A 2B1 ground state for 1-imidazolyl was also observed inBBaqueous solution.4 With the Franck-Condon simulation, assignments of the vibrational peaks
can now be made. Peak A corresponds to the vibrationless origin. A small 1-imidazolide
hot-band peak appears to lower binding energy of peak A with a frequency of 1165 ± 45 cm-1.
The first vibrational peak, B, is a totally symmetric C—C stretching mode (ν6). The nextpeak, C, is another totally symmetric mode with C—N stretching motions (ν3). The overtoneof ν6 and the ν3 + ν6 combination band appear as peaks E and F, respectively.
Peak D, however, does not appear in the Franck-Condon simulation. Comparing theposition of this peak with the DFT computed vibrational frequencies for 2B1 1-imidazolyl,Bthis mode could correspond to an overtone of a b1 mode (2ν11) or a b2 mode (2ν18). Even87
quanta of nontotally symmetric modes are symmetry-allowed and can be observed in the
spectra

N1

H8

C5
C4

C2

H6

N3

H7

Electronic StateN—C Bonds (Å)
1-2 / 3-2
1-5 / 3-4
C—C Bonds (Å)
4-5
C—H Bonds (Å)
2-9
4-7 / 5-8
1-2-3
∠NCN (°)
2-1-5 / 2-3-4∠CNC (°)
1-5-4 / 3-4-5∠NCC (°)
6-2-1 / 6-2-3∠HCN (°)
7-4-3 / 8-5-1

Table 5.2

1-Imidazolide
1A1
1.34891.37081.39171.08601.0847116.30102.69109.16121.85
121.65

1-Imidazolyl
2B11.37171.3172
1.47441.08161.0825116.03103.10108.88121.98123.00B

2

B2
1.33201.37941.3634
1.07511.0765105.61111.23105.96127.20121.05B

1-Imidazolide and 1-imidazolyl geometries obtained fromB3LYP/6-311++G(d,p) calculations.

2A2
1.33981.44281.33831.0804
1.0779115.31103.64108.70122.35119.98

88

1-Imidazolide
Mode

Symmetry

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18

a1
a1
a1
a1
a1
a1
a1
a2
a2
b1
b1
b1
b2
b2
b2
b2
b2
b2

Table 5.3

1A1Frequency(cm-1)
3172
3147
1443
1254
1184
1085
935
777
666
809
734
696
3145
1466
1304
1231
1100
931

1-Imidazolyl
2

B1
Frequency(cm-1)
3231
3218
1436
1313
1166
982
919
974
560
867
756
511
3205
1523
1289
1188
1016
780
B

2

B2
Frequency(cm-1)
3298
3280
1530
1371
1159
1116
906
833
647
787
758
403i
3258
1396
1269
1227
1045
733
B

2

A2
Frequency(cm-1)
3274
3236
1594
1302
1131
1037
922
891
454
848
746
924i
8436
3248
1345
1275
956
904

1-Imidazolide and 1-imidazolyl fundamental vibrational frequencies obtained
from B3LYP/6-311++G(d,p) calculations.

89

b1(π)

b1

H
N
H

C

C

C

H

b2 2

N

a2
b2(σ)

b1
B1

b2

b1
2

B2

a2

b2

2

A2

a2

H
N
H

C

C

C

H

-e

N

b1
H

a2(π)

N
H

C

C
N

Figure 5.3

b2

C

1

A1

H

a2

Three highest occupied molecular orbitals of 1-imidazolide and theschematic representation of electron photodetachment from 1-imidazolide toform three electronic states of 1-imidazolyl.

90

1200
Experiment2
B1 Franck-Condon Simulation

Photoelectron Counts

1000

800

600

400

200

0
3.4

3.3

3.2

3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

Electron Binding Energy (eV)Figure 5.4

Franck-Condon simulation of detachment from 1A1 1-imidazolide to 2B1
1-imidazolyl (red) overlaid on the experimental spectrum (black).
B

91
quanta of nontotally symmetric modes are symmetry-allowed and can be observed in the
spectra. A nonzero Franck-Condon factor for these overtones originates from a large change
in the corresponding vibrational frequencies between the initial and final states of thetransition. In the 1-imidazolide system, however, there is no significant Franck-Condon
factor for either 2ν11 or 2ν18, as seen by the lack of a vibrational peak in the region of peak D.
Alternatively, overtone peaks can gain intensity from Fermi resonance.5 The Fermi
resonance mixes two vibrational levels of the same vibrational symmetry, and the extent of
the mixing is larger when the energy separation of the two unperturbed vibrational levels is
smaller. Peak D is located very close to peak C whose Franck-Condon factor is very large
according to the simulation. The two peaks are separated by only ~200 cm-1. The mixing of
the two levels takes place through anharmonic coupling.
Numerical differentiation of the analytic second derivatives is implemented in theGaussian 03 program package to obtain cubic force constants. The cubic force constants forthe 2B1 state of 1-imidazolyl were calculated at the stationary point and are transformed intoBthe basis of the dimensionless normal coordinates. The calculated force constants for (ν3, ν11,ν11) and (ν3, ν18, ν18) are 25 and 209 cm-1, respectively. A simple 2 by 2 matrix is set up foreach anharmonic coupling to evaluate the corresponding Fermi resonance mixing, using the
DFT harmonic frequencies. The mixing between ν3 and 2ν11 is less than 1%, with an energy
shift of less than 1 cm-1. The mixing between ν3 and 2ν18, however, is about 10%, with anenergy shift of 19 cm-1. These calculations suggest that the Fermi resonance effects are muchlarger for ν3 and 2ν18 despite the larger separation of the energy levels. Therefore, peak D is
assigned as 2ν18, borrowing intensity from the Franck-Condon factor of peak C (ν3) throughthe Fermi resonance.
Figure 5.5 shows a portion of the photoelectron spectrum observed at lower binding
energy which displays several small features. When O− was used to deprotonate imidazole,

92
these features are absent from the spectra and only the spectrum of 1-imidazolide is observed,
virtually identical to Figure 5.1. Since HO− is a stronger base than O−, deprotonation of a lessacidic site of imidazole may be possible with HO− but not with O−. DFT calculations were
performed to explore the different acidities of imidazole, with the results depicted in Figure5.6. The DFT calculations predict ΔacidH298 to be 349.6 kcal mol-1 for the N1 position, in
excellent agreement with an experimental value3 of 349.7 ± 0.5 kcal mol-1. Combined with areported deprotonation enthalpy6 of H2O (ΔacidH298 = 390.27 ± 0.03), the enthalpy of HO−
reaction with imidazole to yield 1-imidazolide and H2O is –40.6 kcal mol-1 (experimental
value is –40.6 ± 0.5 kcal mol-1). The DFT calculations find the other protons in imidazole tobe much less acidic. The calculated deprotonation enthalpies, ΔacidH298, are 389.8, 404.4, and387.9 kcal mol-1 for the C2, C4, and C5 positions, respectively. As shown in Figure 5.6, HO−deprotonation of imidazole is slightly exothermic at C2 and C5, but endothermic for C4
deprotonation. The deprotonation enthalpy of the hydroxyl radical7 is 382.70 ± 0.10 kcal
mol-1, and the DFT calculation results suggest that deprotonation at all of the carbon sites ofimidazole by O− is endothermic. These DFT results suggest that 5-imidazolide detachment ismost likely responsible for the small features observed in the spectrum.
To assign these features, DFT geometry optimizations and frequency calculations
were performed on 5-imidazolide and 5-imidazolyl. Table 5.4 lists the geometries obtained
from these calculations and Table 5.5 lists the frequencies. Using these results, a FranckCondon simulation was performed and is overlaid on the experimental spectrum in Figure5.5. DFT predicts an EA of 5-imidazolyl of 1.986 eV, in excellent agreement with the peaklocated at 1.992 ± 0.010 eV. A progression with a frequency of 900 ± 70 cm-1 is observed in
the spectrum and the simulation predicts two in-plane ring bending modes to be active in thephotoelectron spectrum. These modes have harmonic frequencies of 859 and 931 cm-1 and93
are ν13 and ν12, respectively. This progression reflects the differences in the angles of the
five-membered ring between the anion and the neutral, particularly the N1-C5-C4 angle.

16
Experiment5-Imidazolyl Franck-Condon Simulation

14

Photoelectron Counts

12
10
8
6
4
2
0
2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

Electron Binding Energy (eV)Figure 5.5

Portion of the 351.1 nm magic angle photoelectron spectrum of imidazolide
anions produced from the imidazole + HO− reaction at 298 K (black) andsimulation of detachment of 5-imidazolide (red). The peak at ~2.45 eV is the1-imidazolide hot band and the intense origin peak of 1-imidazolyl grows in
at >2.5 eV.

94

H
H

N1
C5
C4

C2

H
+ HO

ΔrxnH298

N1

H

C5

N3

C2

C4

H

+ H2O

- 40.6 kcal mol-1(exp. - 40.6 ± 0.5)

+ H2O

- 0.5 kcal mol-1

H +H O+ 14.2 kcal mol-1+ H2O- 2.4 kcal mol-1

H

N3

H
H
N1

H

C5

C2

C4
H

N3
H

N1

H

C5

C2

C4

2

N3

H
N1
C5
C4

C2

H

N3

H

Figure 5.6

B3LYP/6-311+G(d,p) reaction enthalpies for proton transfer reactions at
different positions of imidazole.

95

H8
N1
C5
C4

C2

H6

5-Imidazolide

H7Electronic StateN—C Bonds (Å)

C—C Bonds (Å)
N—H Bonds (Å)
C—H Bonds (Å)∠NCN (°)∠CNC (°)∠NCC (°)∠HNC (°)∠HCN (°)∠HCN (°)

Table 5.4

5-Imidazolyl

N3

1

1-2
2-3
3-4
5-1
4-5
1-8
2-6
4-7
1-2-3
2-1-5
2-3-4
1-5-4
3-4-5
8-1-2
6-2-1
7-4-5

A'
1.377
1.317
1.396
1.406
1.400
1.007
1.084
1.084
110.4
111.8
103.9
99.0
115.0
124.5
123.8
126.3

2

A'
1.368
1.315
1.384
1.370
1.363
1.007
1.079
1.076
111.1
105.7
107.2
108.5
107.5
127.8
123.2
130.0

5-Imidazolide and 5-imidazolyl geometries obtained fromB3LYP/6-311++G(d,p) calculations.

96

1-Imidazolide

Table 5.5

Mode

Symmetry

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18

a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a''
a''
a''
a''
a''

1

A'
Frequency(cm-1)
3638
3171
3164
1509
1387
1328
1271
1190
1100
1070
1022
941
892
784
687
674
655
507

1-Imidazolyl
2A'
Frequency(cm-1)
3656
3277
3246
1518
1434
1408
1315
1249
1149
1106
1056
931
859
818
784
673
556
439

5-Imidazolide and 5-imidazolyl fundamental vibrational frequencies obtained
from B3LYP/6-311++G(d,p) calculations.

97
are ν13 and ν12, respectively. This progression reflects the differences in the angles of the
five-membered ring between the anion and the neutral, particularly the N1-C5-C4 angle.

5.4

Discussion

With two N atoms in the five-membered ring, the relative energies of the highest
occupied molecular orbitals of 1-imidazolide are different compared to the pyrrolide system.
Since the a2 orbital of 1-imidazolide is mainly composed of nitrogen p orbitals it will bestabilized relative to the b1 orbital which has only minor contributions from the nitrogen p
orbitals. This stabilization is in fact much larger than in the pyrrolide system, such that the a2
orbital is lower in energy than a b2 orbital. The energetic ordering of the 1-imidazolide
molecular orbitals is illustrated in Figure 5.3. Not including zero point energies, the DFT
calculations predict that the term energy for the 2B2 state is 0.84 eV, while that of the 2A2
Bstate is 0.95 eV. Either the laser photon energy is insufficient for detachment to these excitedstates or the kinetic energy of the photoelectrons may be too low (< 0.15 eV) for efficient
detection.
With these electronic states lying so closely together, non-adiabatic effects canbecome important. For the 1-imidazolyl radical ground state, there is reasonable agreementof the Franck-Condon simulation with the experimental spectrum, indicating that nonadiabatic effects are minimal in the vicinity of the 2B1 ground state. Near the 2B2 and 2A2
B

B

stationary points, the non-adiabatic effects become more significant. These two excited states
couple strongly with each other through b1 vibrational modes and are transformed intotransition states. There is also significant vibronic coupling between the 2B1 ground state andB

the 2A2 state, as evidenced by the unexpected 8436 cm-1 b2 vibration calculated for the 2A2
state.
Coupling between the 2B1 ground state and 2A2 excited state may also be responsibleB

for the appearance of the Fermi resonance peak in the spectrum of the 2B1 state. Looking atB

98
the potential energy surface expansion of Equation 1.11, vibronic coupling between the 2B1
B

and 2A2 states occurs through b2 vibrational modes. Vibronic coupling effects on the 2B1
B

surface near its equilibrium geometry are marginal because of the relatively large energy
separation with the 2A2 state. The DFT calculations show that the 2A2 surface lies 1.9 eVhigher in energy than the 2B1 surface at its equilibrium geometry.
B

The cubic force constant for Φ3,18,18 is explicitly written as:Φ 3,18,18 =∂ 3V∂ ⎛ ∂ 2V ⎞=⎜⎟∂Q3∂Q18∂Q18 ∂Q3 ⎝ ∂Q18 2 ⎠(5.1)

This cubic force constant is a gradient of the quadric force constant for the ν18 b2 mode along
the totally symmetric ν3 mode. Looking again to Equation 1.11, the ν18 quadratic force
constant depends on the vibronic coupling between the 2B1 and 2A2 states. Consequently, theB

cubic force constant Φ3,18,18 can exhibit the dependence of the vibronic coupling on the ν3
motion.
In the Franck-Condon simulation shown in Figure 5.4, the parameters of thefollowing equation are calculated.8(5.2)

Q = JQ' + K

Here, Q' and Q are matrices for the normal coordinates of the imidazolide and imidazolyl
ground states, respectively; J is a matrix for Duschinsky rotation of the two sets of normal
coordinates; K is a column vector representing the geometry displacement between the twostates. The components of the K vector for all of the a1 modes of the 2B1 state of 1-imidazolylB

are shown in Table 5.6. The largest displacement takes place along ν3, and the magnitude ofthe displacement is 1.249 in dimensionless units. The second largest value is ν6, with adisplacement of 0.919. These large displacements manifest themselves in the photoelectron
spectrum as vibrational progressions for these modes.

99

Table 5.6

a1 modeK(2B1 – 1A1)

K(2B1 – 2A2)

1
2
3
4
5
6
7

0.036
0.031
1.249
0.376
0.322
0.919
0.534

0.075
0.081
2.768
1.094
0.026
1.488
1.322

B

B

Geometry displacement parameters, K, in dimensionless normal coordinates
for 2B1 1-imidazolyl – 1A1 1-imidazolide and 2B1 1-imidazolyl – 2A2
1-imidazolyl.
B

B

100
In the same way, the K vector can be calculated for the geometry shift from the 2B1
B

ground state to the 2A2 excited state of imidazolyl; its components are also shown in Table
5.6. The geometry shift along ν3 is by far the largest between the two imidazolyl states,
having a displacement of 2.768. The large K vector component for the N—C stretching mode
is not surprising as Table 5.2 shows that the N1—C5 (N3—C4) is very different between the2

B1 and 2A2 states. Assuming for a moment that the 2B1 and 2A2 states have identical normalBB

modes (i.e. parallel modes) and that the potential energy surfaces are harmonic, the K vector
represents the slopes of the 2A2 potential energy surface along the a1 modes at the equilibriumgeometry of the 2B2 state. Under this assumption, the energy separation between the 2B1 andB

B

2A2 states changes by ~0.6 eV as the geometry shifts along the ν3 mode by one dimensionlessunit.

The nuclear motions for this mode is asymmetric NCN stretching, and thecorresponding mode in pyrrolyl was found to be responsible for strong vibronic coupling
between the 2A2 and 2B1 states of pyrrolyl.9 Taking the magnitude of the vibronic couplingBmatrix as 0.15 eV, which represents moderately strong vibronic coupling,10-14 the quadratic
coupling constant for the ν18 mode of 1-imidazolyl changes by 90 cm-1 as the energy
separation decreases from 1.9 to 1.3 eV. Thus it is reasonable that vibronic coupling between
the 2B1 and the 2A2 states of imidazolyl contributes to the relatively large anharmonicity,B

which causes the Fermi resonance effects.

Thermodynamics

Using the experimentally determined EA for 1-imidazolyl, it is now possible toderive the imidazole N—H bond dissociation energy using the methods outlined in Chapter 2.
Experimental values3 for the imidazole gas phase acidity and deprotonation enthalpies are
ΔacidG298 = 342.6 ± 0.4 kcal mol-1, ΔacidH298 = 349.7 ± 0.5 kcal mol-1, and ΔacidH0 = 348.4 ± 0.5

kcal mol-1. Combining the EA with the 0 K deprotonation enthalpy yields D0(imidazole N-H)

101
= 95.1 ± 0.5 kcal mol-1. In turn, the bond dissociation energy can be combined with theimidazole heat of formation7 (ΔfH298 = 31.8 ± 0.1 kcal mol-1) to determine the heat offormation of 1-imidazolyl radical, ΔfH298 = 76.2 ± 0.6 kcal mol-1.
With an experimental value for the imidazole C5 gas phase acidity, it would bepossible to determine the imidazole C5—H bond dissociation in a similar way. Experimental
considerations make it difficult to directly measure the acidity of a proton in the presence ofmore acidic protons in the same molecule.

The photoelectron measurements on

5-imidazolide, however, definitively show that it is energetically possible to deprotonate
imidazole at the C5 position using HO−. Since these measurements also indicate that it is notenergetically possible to deprotonate imidazole at the C5 position using O−, the gas phase
acidity of the C5 proton of imidazole is bracketed between the acidities of H2O and HO,
yielding ΔacidG298(imidazole C5—H) = 380 ± 4 kcal mol-1. Combining this value with the
5-imidazolyl EA of 1.992 ± 0.010 eV and thermal corrections from the DFT results,
D0(imidazole C5—H) = 119 ± 4 kcal mol-1. From D0, the heat of formation of 5-imidazolyl

radical can also be determined, ΔfH298 = 100 ± 4 kcal mol-1.

5.5

Conclusion

The photoelectron spectrum of imidazolide, produced by HO− deprotonation of
imidazole in helium buffer gas, has been measured. The EA of 1-imidazolyl radical isdetermined to be 2.613 ± 0.006 eV. Franck-Condon simulations based on DFT calculations
for the ground states of 1-imidazolide and 1-imidazolyl reproduce the observed spectrum
reasonably well, indicating that two totally symmetric vibrations are active in the spectrumwith C—C stretching and C—N stretching motions. The experimental frequencies of thesevibrations are 955 ± 15 and 1365 ± 20 cm-1, respectively. Additionally, a small overtone
feature is observed in the spectrum with a frequency of 1575 ± 30 cm-1, gaining intensity due
to a Fermi resonance. Combining the EA of 1-imidazolyl with a value for the N—H bond102
deprotonation enthalpy of imidazole in a thermodynamic cycle, the imidazole N—H bonddissociation energy is determined to be D0 = 95.1 ± 0.5 kcal mol-1.
The imidazolide photoelectron spectrum also included several peaks originating from
5-imidazolide. These features reveal the EA of 5-imidazolyl radical to be 1.992 ± 0.010 eV.
When O− was used as the deprotonation agent, these signals were absent from the spectrum,suggesting that the acidity of the proton at the C5 position of imidazole is bracketed by thoseof H2O and OH radical and is ΔacidG298 = 380 ± 4 kcal mol-1. The acidity is corrected forthermal effects and combined with the EA of 5-imidazolyl in a thermodynamic cycle to yield
the C5—H bond dissociation energy of imidazole, D0 = 119 ± 4 kcal mol-1. The relatively
large error bar arises from the uncertainty of the acidity, and experiments on
1-methylimidazole are underway to allow for more accurate determination of the C—H bonddissociation energy.15

103

Chapter 5 References
1

J. Catalan, R.M. Claramunt, J. Elguero, J. Laynez, M. Menendez, F. Anvia, J.H.
Quian, M. Taagepera, and R.W. Taft, Basicity and Acidity of Azoles - the Annelation
Effect in Azoles. Journal of the American Chemical Society, 1988. 110(13): p. 41054111.

2

R.W. Taft, F. Anvia, M. Taagepera, J. Catalan, and J. Elguero, Electrostatic
Proximity Effects in the Relative Basicities and Acidities of Pyrazole, Imidazole,
Pyridazine, and Pyrimidine. Journal of the American Chemical Society, 1986.
108(12): p. 3237-3239.

3

A.J. Gianola, T. Ichino, R.L. Hoenigman, S. Kato, V.M. Bierbaum, and W.C.
Lineberger, Photoelectron spectra and ion chemistry of imidazolide. Journal of
Physical Chemistry A, 2005. 109(50): p. 11504-11514.

4

A. Samuni and P. Neta, Electron-Spin Resonance Study of Reaction of Hydroxyl
Radicals with Pyrrole, Imidazole, and Related Compounds. Journal of Physical
Chemistry, 1973. 77(13): p. 1629-1635.

5

G. Herzberg, Molecular Specta and Molecular Structure. Vol. II. Infrared andRaman Spectra of Polyatomic Molecules. 1950, New York: Van Nostrand.

6

K.M. Ervin and V.F. DeTuro, Anchoring the gas-phase acidity scale. Journal of
Physical Chemistry A, 2002. 106(42): p. 9947-9956.

7J.E. Bartmess, "Negative Ion Energetics Data", in NIST Chemistry Webbook, NIST
Standard Reference Database Number 69, J.P. Linstrom and W.G. Mallard, Editors.
June 2005, National Institue of Standards and Technology: Gaithersburg MD, 20899.

8

P. Chen, Photoelectron Spectroscopy of Reactive Intermediates, in Unimolecular andBimolecular Reaction Dynamics, C.Y. Ng, T. Baer, and I. Powis, Editors. 1994,
Wiley & Sons: New York. p. 371-425.

9

A.J. Gianola, T. Ichino, R.L. Hoenigman, S. Kato, V.M. Bierbaum, and W.C.
Lineberger, Thermochemistry and electronic structure of the pyrrolyl radical. Journalof Physical Chemistry A, 2004. 108(46): p. 10326-10335.

10

J. Eiding, R. Schneider, W. Domcke, H. Koppel, and W. Vonniessen, Abinitio
Investigation of the Multimode Dynamic Jahn-Teller Effect in the X 2E1g State of theBenzene Cation. Chemical Physics Letters, 1991. 177(3): p. 345-351.

11

H. Muller, H. Koppel, and L.S. Cederbaum, Topology and Dynamics of Vibronically
Coupled Potential-Energy Surfaces - General-Aspects and Application to the Ozone
Cation. New Journal of Chemistry, 1993. 17(1-2): p. 7-29.

12M. Mayer, L.S. Cederbaum, and H. Koppel, Ground-State Dynamics of NO3 Multimode Vibronic Borrowing Including Thermal Effects. Journal of Chemical
Physics, 1994. 100(2): p. 899-911.

104
13

S. Mahapatra, H. Koppel, and L.S. Cederbaum, Impact of nonadiabatic coupling
between the conically intersecting X 2A1 and A 2B2 states of NO2 on the negative ion
photoelectron spectra of NO2-. Journal of Chemical Physics, 1999. 110(12): p. 56915701.

14

B.E. Applegate, A.J. Bezant, and T.A. Miller, The Jahn-Teller and related effects inthe cyclopentadienyl radical. II. Vibrational analysis of the A 2A2'' - X 2E1'' electronictransition. Journal of Chemical Physics, 2001. 114(11): p. 4869-4882.

15

S.M. Villano, A.J. Gianola, S. Kato, V.M. Bierbaum, and W.C. Lineberger,
Manuscript in preparation.

________________________________________________
6
Pyrazolide, C3H3N2−________________________________________________
6.1

Introduction

As previously discussed, pyrazole is an isomer of imidazole. In pyrazole the nitrogenatoms in the ring are adjacent whereas in imidazole they are separated. Deprotonation ofpyrazole takes place primarily at the nitrogen site to form 1-pyrazolide, but as with imidazole,
it is possible to deprotonate a pyrazole carbon site with HO− to form 5-pyrazolide. The gas
phase acidities (ΔacidG298) of the pyrazole N and C5 protons are 346.4 ± 0.3 and 380 ± 4 kcal
mol-1, respectively. In contrast with imidazole, the removal of the proton at the pyrazole C5
position was facile, and quite large photoelectron signals from 5-pyrazolide were observed
simultaneous with the 1-pyrazolide photoelectons. Here, the EA and vibrational structure of1- and 5- pyrazolyl are measured. These results are newly available as a publication1 (J.
Phys. Chem. A, 2006, DOI: 10.1021/jp057499+).
H
N

H

C

N
C

N

H

C

C

H

−

C

H

H

H

C

C

N
C

H

5-Pyrazolide

HC
H

1-Pyrazolyl Radical

N
N

C

H

H

N

−C

N
C

1-Pyrazolide

Pyrazole

C

C

H

H

N

H

N

C
H

5-Pyrazolyl Radical

106
6.2

ExperimentHydroxide ions are created using the microwave discharge ion source as described in
Chapter 2. Pyrazole is introduced into the flow tube by flowing a stream of helium through acrystalline pyrazole sample at room temperature.

Deprotonation of pyrazole forms

pyrazolide ions, and typical ion beam currents were ~250 pA. In some experiments liquid
nitrogen was used to cool the flow tube to aid in cooling of the ions. In these experiments,
slightly smaller ion beam currents were observed, ~150 pA. Additionally, pyrazolide ionswere also formed by reaction of pyrazole with oxygen anion (O−) or fluoride (F−) with similar
ion beam currents. F− is produced by adding a trace amount of NF3 downstream of the
microwave discharge. The laser wavelength used for photodetachment of pyrazolide was351.1 nm. Measurements of the EAs of oxygen and iodine2 (3.059 eV) were used to calibrate
the absolute energy scale.

6.3

Results

Figure 6.1 shows the 351.1 nm magic angle photoelectron spectrum of pyrazolideions produced from the reaction of pyrazole with HO−. Intense peaks are observed at eBE >2.9 eV, along with several weaker peaks at eBE < 2.6 eV. Spectra were also measured withpyrazolide ions produced from the reaction of pyrazole with O− and F−. The spectra obtainedwith these reactant ions are virtually identical with that in Figure 6.1 at eBE > 2.9 eV, butthere are no photoelectron signals observed at eBE < 2.6 eV.
DFT calculations were performed to explore the acidities of pyrazole. Reaction
enthalpies

fordeprotonationofpyrazole

by

HO−

were

computedatthe

B3LYP/6-311++G(d,p) level. The DFT results were combined with an experimental value ofthe H2O deprotonation enthalpy3 (ΔacidH298 = 390.27 ± 0.03 kcal mol-1) to derive the reaction
enthalpies listed in Figure 6.2. Deprotonation at the N1 site is the most exothermic, with a107reaction enthalpy of –35.4 kcal mol-1. For deprotonation at the carbon sites, only formationof 5-pyrazolide is predicted to be exothermic, with a reaction enthalpy of –5.8 kcal mol-1.

400Photoelectron Counts

300

200

100

0
3.3

3.2

3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

Electron Binding Energy (eV)Figure 6.1

351.1 nm magic angle photoelectron spectrum of pyrazolide at 298 K.

1.9

108

H
H

ΔrxnH298

N1
C5

N2

+ HO

H

C4 C3
H

N1
C5

N2

+ H2O

- 35.4 kcal mol-1(exp. - 36.7 ± 0.4)

+ H2O

+ 9.9 kcal mol-1+ H2O+ 6.6 kcal mol-1+ H2O- 5.8 kcal mol-1

C4 C3
H

H

pyrazoleH

1-pyrazolide
H
H

N1
C5

N2

C4

C3

H

3-pyrazolide
H
H

N1
C5

N2

C4 C3
H

4-pyrazolide
H
N1
C5

N2

C4 C3
H

H

5-pyrazolide

Figure 6.2

DFT reaction enthalpies evaluated for HO− deprotonation of pyrazole atdifferent positions.

109
reaction enthalpy of –35.4 kcal mol-1. For deprotonation at the carbon sites, only formationof 5-pyrazolide is predicted to be exothermic, with a reaction enthalpy of –5.8 kcal mol-1.
When O− [ΔacidH298(OH) = 382.6 ± 0.07 kcal mol-1] 4 or F− [ΔacidH298(HF) = 371.331 ± 0.003
kcal mol-1] 3 is used, deprotonation at all carbon sites is endothermic. These results suggest
assignment of the higher binding energy features to 1-pyrazolide detachment and the lower
binding energy features to 5-pyrazolide detachment.
Figure 6.3 shows an expanded region of the higher binding energy portion of thespectrum, corresponding to detachment of 1-pyrazolide. This spectrum was taken with thesame experimental conditions as used for the spectrum in Figure 6.1 except that the flow tube
was cooled with liquid nitrogen to vibrationally cool the ions. Angular measurements were
performed and anisotropy parameters (β) were measured for the different peaks in Figure 6.3,
with β values ranging from -0.2 to -0.65. Intuitively, among all of the peaks observed, theone at the lowest binding energy (peak A) is expected to be the origin of the groundelectronic state of 1-pyrazolyl. Table 6.1 lists the relative experimental peak positions and β
values for features labeled in Figure 6.3. The peak labeled B has β = –0.65 ± 0.09, which issignificantly different from that of the origin peak, β = –0.25 ± 0.09. Observation of distinct
anisotropy parameters indicates detachment from different molecular orbitals of the anion,that is, access to different electronic states of the neutral.
To aid in the assignment of these features, DFT calculations were performed on
1-pyrazolide and 1-pyrazolyl.

Geometry optimizations and frequency calculations were

performed at the B3LYP/6-311++G(d,p) level and C2v stationary points were located for 1A1
1-pyrazolide, and 2A2, 2B1, and 2B2 1-pyrazolyl. The DFT calculations predict a minimum for
B

B

1

A1 1-pyrazolide, 2A2 and 2B1 1-pyrazolyl, but a transition state for 2B2 1-pyrazolyl. Tables
BB

6.2 and 6.3 list the geometries and frequencies obtained from these calculations. These
results predict a 2A2 ground state for 1-pyrazolyl, with an EA of 2.945 eV. This predicted EA

110
is close to the peak at 2.935 ± 0.005 eV. Considering this, the ground state of 1-pyrazolyl is
assigned as 2A2. The DFT results also predict term energies of 0.031 and ~0.1 eV for 2B1
B

500

B

Photoelectron Counts

400

300

A
E

200

F
D

100

C
0
3.2

3.1

3.0

2.9

Electron Binding Energy (eV)Figure 6.3

351.1 nm magic angle photoelectron spectrum of 1-pyrazolide produced
under liquid nitrogen cooled flow tube conditions.

111
is close to the peak at 2.935 ± 0.005 eV. Considering this, the ground state of 1-pyrazolyl is
assigned as 2A2. The DFT results also predict term energies of 0.031 and ~0.1 eV for 2B1 andB

2

B2 1-pyrazolyl, respectively. The term energy of the 2B1 state is surprisingly close to theBBpeak spacing between peaks A and B in Figure 6.3. More important than the good agreementwith the experimental peak spacing is the observation of a near-degeneracy of the 2A2 and 2B1
Bstates.
Franck-Condon simulations were performed for detachment of 1-pyrazolide to the2A2 and 2B1 states of 1-pyrazolyl, as outlined in Chapter 2. Fixing the 2A2 origin position andBintensity to those of peak A and the 2B1 origin position and intensity to those of peak B yields
B

the simulations found in Figure 6.4, where the agreement between the Franck-Condon
simulation and the experimental spectrum is generally poor. The peaks labeled C and D arecompletely missed by the simulations and the intensity of the bands at higher eBE areseverely overestimated. Switching of the assignments (i.e. 2B1 as the ground state and 2A2 as
B

the excited state) does not alleviate the discrepancy.

This disagreement is surprising,considering that Franck-Condon simulations of pyrrolide and imidazolide detachment
reproduce the experimental spectra rather well. This simulation, however, is reminiscent of
the Franck-Condon simulation of cyclopentadienide detachment, where several bands were
missing from the spectrum while the intensities of other bands were overestimated.
These observations lead to the conclusion that the geometry displacements from 1A1
1-pyrazolide to 2A2 and 2B1 1-pyrazolyl may be overestimated by the DFT calculations.
B

Additionally, vibronic coupling (a.k.a. Pseudo-Jahn-Teller) effects contribute to thedisagreement between the Franck-Condon simulation and the experimental spectrum.
Simulations of the 1-pyrazolide photoelectron spectrum are currently being carried out by
establishing a diabatic model Hamiltonian5-7 for the neutral states around the equilibriumgeometry of the anion ground state. This model Hamiltonian takes into account nonadiabatic
effects which are neglected in the Franck-Condon simulation in Figure 6.4. Parameterization

112
of the model Hamiltonian is accomplished with ab initio calculations using the equations-ofmotion

Peak Label

β

A
B
C
D
E
F

– 0.25 ± 0.09
– 0.65 ± 0.09
– 0.45 ± 0.15
– 0.20 ± 0.09
– 0.45 ± 0.13
– 0.30 ± 0.12

Table 6.1

Peak Position
Relative to peak A—
0.032 ± 0.001 eV
554 ± 15 cm-1
697 ± 15 cm-1
1016 ± 20 cm-1
1323 ± 20 cm-1

Peak Position
Relative to peak B–0.032 ± 0.001 eV—
289 ± 15 cm-1
435 ± 15 cm-1
754 ± 20 cm-1
1060 ± 20 cm-1

Experimental peak positions and anisotropy parameters for peaks appearing
in the photoelectron spectrum of 1-pyrazolide.

113

H8

N1
C5
C4
H7

N2

1-Pyrazolide

C3
H6Electronic State
N—N Bonds (Å)
1-2
N—C Bonds (Å)
1-5 / 2-3
C—C Bonds (Å)
3-4 / 4-5
C—H Bonds (Å)
4-7
3-6 / 5-8
1-2-3 / 2-1-5∠NNC (°)
2-3-4 / 1-5-4∠NCC (°)
3-4-5∠CCC (°)
6-3-2 / 8-5-1∠HCN (°)
7-4-3 / 7-4-5∠HCC (°)

Table 6.2

1A1
1.3621
1.34911.40441.08531.0824107.89110.85102.52120.36
128.74

1-Pyrazolyl
2A21.27501.43001.38611.0805
1.0773108.34109.98103.36
119.63128.322B11.4676
1.29911.43741.08161.0813107.53110.96103.02
121.39128.491-Pyrazolide and 1-pyrazolyl geometries obtained fromB3LYP/6-311++G(d,p) calculations.

B

2

B2
1.2396
1.35771.40511.07591.0789111.54105.72105.48122.04127.26B

114

1-Pyrazolide
Mode

Symmetry

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18

a1
a1
a1
a1
a1
a1
a1
a2
a2
b1
b1
b1
b2
b2
b2
b2
b2
b2

Table 6.3

1A1
Frequency(cm-1)
3194
3148
1471
1215
1127
1026
930
756
693
785
696
631
3151
1404
1363
1165
1031
914

1-Pyrazolyl
2A2Frequency(cm-1)
3269
3235
1510
1390
1121
1007
912
835
564
860
700
408
3239
1336
1287
990
950
599

2

B1
Frequency(cm-1)
3234
3212
1521
1300
1054
924
786
859
188
882
705
480
3216
1477
1283
1180
946
539
B

2

B2
Frequency(cm-1)
3285
3243
1609
1391
1199
1075
913
794
286i
863
765
101
3279
1422
1283
1161
1039
627
B

1-Pyrazolide and 1-pyrazolyl fundamental vibrational frequencies obtained
from B3LYP/6-311++G(d,p) calculations.

115

Photoelectron Counts

500

Experiment
2A Franck-Condon Simulation
2
2B Franck-Condon Simulation
1

400

300

200

100

0
3.25

3.20

3.15

3.10

3.05

3.00

2.95

2.90

2.85

Electron Binding Energy (eV)Figure 6.4

Franck-Condon simulation of detachment of 1-pyrazolide. The red curve isthe simulation for detachment to 2A2 1-pyrazolyl, the blue curve is thesimulation for detachment to 2B1 1-pyrazolyl.
B116
of the model Hamiltonian is accomplished with ab initio calculations using the equations-ofmotion coupled-cluster theory (EOMIP-CCSD).8, 9 The simulation reproduces the observed
spectrum very well and confirms the assignment of the origin peak is correct; that is, theground state of 1-pyrazolyl is 2A2 and 2B1 is the first excited state. Details of this simulation
B

procedure as well as the results of the simulation will be presented in a forthcoming paper.10
As explained before, the lower eBE portion of the spectrum in Figure 6.1 is attributed
to detachment from 5-pyrazolide. Figure 6.5 shows an expanded view of the lower energy
portion of the photoelectron spectrum of pyrazolide produced from the HO− reaction withpyrazole. The experimental conditions for this spectrum are virtually identical with thosefrom Figure 6.1, but with a longer data integration time. Table 6.4 lists the observed peak
positions of peaks labeled in Figure 6.5.

Angular measurements yield an anisotropy

parameter of 0.35 ± 0.05 for the features in Figure 6.5. DFT calculations were carried out toobtain the optimized geometries and harmonic vibrational frequencies of the 5-pyrazolide
anion and 5-pyrazolyl ground states. The calculations find minima in Cs symmetry for theelectronic ground states of both 5-pyrazolide (1A') and 5-pyrazolyl (2A'). The optimized
geometries are listed in Table 6.4 and the frequencies are listed in Table 6.5. The DFT
calculations predict an EA of 5-pyrazolyl of 2.109 eV, very close to the experimental peak at
2.104 ± 0.005 eV.
Using these results, a Franck-Condon simulation was performed for the transition
from 5-pyrazolide to 5-pyrazolyl and is depicted in Figure 6.6, where the agreement with theexperimental spectrum is nearly perfect. This simulation utilizes a vibrational temperature of
400 K for the anion, and shows a small hot-band transition near a binding energy of 2.0 eV,corresponding to one quanta of an in-plane C4-C5-N1 bending mode (ν13) of 5-pyrazolide;the experimental frequency of this mode is 880 ± 20 cm-1. The Franck-Condon simulation
identifies the peak labeled a in Figure 6.5 as the band origin and also predicts three vibrations

117
to be strongly active in the photoelectron spectrum, ν6, ν9, and ν12. There is also some
activation of the ν13 vibrational mode which does not become significant until higher
quantum numbers. This mode is very close in frequency to ν12 and not resolvable in theexperimental spectrum. The experimental fundamental frequencies of 890 ± 15, 1110 ± 35,
and 1345 ± 30 cm-1 compare well with the ν12, ν9, and ν6 DFT frequencies of 914, 1162, and1372, respectively.

These modes correspond to ring breathing, asymmetric C3-C4-C5

stretching, and C4-C5-N1 bending, respectively. The DFT computed atomic displacements
for these modes are depicted in Figure 6.7. Table 6.4 summarizes the assignments of the
5-pyrazolide photoelectron spectrum.

6.4

Discussion

As mentioned in the results section, the photoelectron spectrum of 1-pyrazolide
demonstrates noadiabatic effects in the low-lying electronic states of 1-pyrazolyl.

Thissubject will be discussed in detail elsewhere,10 so only a brief, qualitative account will begiven here.
Comparing 1-pyrazolide with 1-imidazolide, the change in arrangement of the Natoms in the 5-membered ring affects the energetic relationship between the highest occupied
molecular orbitals.

In 1-imidazolide, the highest occupied π molecular orbital has b1

symmetry (within the C2v point group), and is mainly composed of C atom p orbitals. The
next highest occupied π molecular orbital of 1-imidazolide has a2 symmetry, and has large
contributions from N atom p orbitals which stabilize this molecular orbital by a large amount.
This results in a 2B1 ground state and a 2A2 excited state for 1-imidazolyl radical with a large
Benergy separation between these two states.

This large spacing leads to very small

nonadiabatic effects in the ground state around its equilibrium geometry, with little effect on
the photoelectron spectrum.

118

500

e

Photoelectron Counts

b

h

400

300

k

200

n m
p

100

i

l

o

a

j
g
f

d

c

a*

0
2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

Electron Binding Energy (eV)Figure 6.5

351.1 nm magic angle photoelectron spectrum of 5-pyrazolide produced in
the reaction of HO− with pyrazole.

119

*a *

Peak Position
Relative to peak a (cm-1)-880 ± 20

a

0

000

b

0890 ± 151210

c1110 ± 35910

d1345 ± 30

610e1780 ± 20

12 02

f

1990 ± 35

9101210g2225 ± 35

6101210h2665 ± 25

12 30

i

2990 ± 50

91012 02j

3115 ± 25

61012 02k3555 ± 35

12 04

l

3790 ± 35

9101230m

4000 ± 45

6101230n

4440 ± 35

12 50

o

4670 ± 35

91012 04p

4880 ± 35

61012 04

Peak Label

Table 6.4

assignment

1310

Experimental peak positions and assignments for peaks appearing in thephotoelectron spectrum of 5-pyrazolide (Figure 6.5).

120

H6
N1
C5
C4
H8

N2

C—H Bonds (Å)∠NNC (°)∠NCC (°)∠CCC (°)∠CCN (°)∠CNN (°)∠NNH (°)∠NCH (°)∠CCH (°)

Table 6.5

5-Pyrazolyl

1

2

C3
H7

Electronic State
N—N Bonds (Å)
N—C Bonds (Å)
C—C Bonds (Å)

5-Pyrazolide

1-2
1-5
2-3
3-4
4-5
1-6
3-7
4-8
1-2-3
2-3-4
3-4-5
4-5-1
5-1-2
2-1-6
2-3-7
5-4-8

A'
1.380
1.371
1.335
1.414
1.412
1.006
1.086
1.084
101.7
111.7
108.5
100.1
118.1
116.6
118.9
125.6

5-Pyrazolide and 5-pyrazolyl geometries obtained fromB3LYP/6-311++G(d,p) calculations.

A'
1.363
1.340
1.333
1.417
1.374
1.006
1.080
1.076
104.4
112.8
102.2
109.8
110.8
119.9
119.1
128.5

121

Table 6.6

Mode

Symmetry

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18

a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a''
a''
a''
a''
a''

5-Pyrazolide

5-Pyrazolyl

1

2

A'
Frequency (cm-1)
3654
3175
3149
1492
1391
1348
1247
1178
1104
1056
1030
922
901
840
753
696
650
527

A'
Frequency (cm-1)
3676
3275
3232
1526
1442
1372
1317
1261
1162
1048
1038
914
886
884
770
670
572
440

5-Pyrazolide and 5-pyrazolyl fundamental vibrational frequencies obtained
from B3LYP/6-311++G(d,p) calculations.

122

600
Experiment
5-Pyrazolyl Franck-Condon Simulation

Photoelectron Counts

500

400

300

200

100

0
2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

Electron Binding Energy (eV)Figure 6.6

Franck-Condon simulation (red) of detachment of 5-pyrazolide based on the
optimized geometries and vibrational frequencies obtained from theB3LYP/6-311++G(d,p) calculations overlaid on the experimental spectrum(black).

123

H

H

C
C

C
N

C
C

N

H

C
N

H

C
C

N

H

a' mode, 914 cm-1
ring distortion

Figure 6.7

HH

a' mode, 1162 cm-1
ring breathing

C
N

H

N

H
a' mode, 1372 cm-1
C-C stretch

Relative atomic displacements and harmonic frequencies of normal modes of
5-pyrazolyl active in the photoelectron spectrum evaluated at theB3LYP/6-311++G(d,p) level of theory.

124
With the N atoms adjacent in the 1-pyrazolide ring, the energetic relationshipbetween the two highest occupied π orbitals is changed.

Figure 6.8 illustrates thephotodetachment process for 1-pyrazolide. Here, both the a2 and b1 orbitals have comparable
contributions from the p orbitals of the two N atoms, resulting in π orbitals which are very
close in energy. Thus the 2A2 and 2B1 states of 1-pyrazolyl, formed by electron detachment
Bfrom the corresponding orbitals of 1-pyrazolide, are nearly degenerate. Because they are so
close in energy, vibronic interactions of the pseudo-Jahn-Teller type11, 12 (a.k.a. vibronic
coupling) between the two π states becomes significant around the equilibrium geometries ofboth states.

Therefore, Franck-Condon simulation based on the Born-Oppenheimer

approximation fails, as seen in Figure 6.4.

The near-degeneracy might have prevented

observation of a clear electron spin resonance spectrum of 1-pyrazolyl radical in alkalineaqueous solution.13 Further vibronic interactions arise from the participation of anothernearby electronic state, 2B2, as shown in Figure 6.8. This three-state14 interaction will be fullyBinvestigated in a forthcoming paper.10
The change in the arrangement of the N atoms and, thus, the highest occupied
molecular orbitals also becomes apparent in the EA of 1-pyrazolyl radical. Since the highest
occupied molecular orbital has large contributions from N atom p orbitals, it is stabilized,
resulting in a larger EA for 1-pyrazolyl when compared with 1-imidazolyl (2.938 ± 0.005 eV
vs. 2.613 ± 0.006 eV).
Using Equation 2.3, the EA of 1-pyrazolyl can be combined in a thermodynamic
cycle with the pyrazole deprotonation enthalpy1 (ΔacidH0 = 106.4 ± 0.4 kcal mol-1) to obtain
the pyrazole N—H bond dissociation energy: D0 = 106.4 ± 0.4 kcal mol-1. With the 298 K
heat of formation of pyrazole15 [ΔfH298(pyrazole) = 42.88 ± 0.20 kcal mol-1], and thermal
corrections from the DFT calculations, the heat of formation of 1-pyrazolyl radical can now
be determined using Equation 2.5: ΔfH298(1-pyrazolyl) = 98.7 ± 0.5 kcal mol-1.

125

C

H

2

H

H

a2(π)

C

N

a2
b2

C

b1

a2

C

N

b2

-e

H
C

C

C

N

a2

N

b2

Figure 6.8

b1

C

H

H

a2

b1

b2

N

b2(σ)

B2

H

H

H

2

B1

C

N

b1(π)

2

A2

b1

1

A1

Three highest occupied molecular orbitals of 1-pyrazolide and the schematic
representation of electron photodetachment from 1-pyrazolide to form threeelectronic states of 1-pyrazolyl.

126
In a similar way, the pyrazole C5—H bond dissociation energy can be obtained if thepyrazole C5—H deprotonation enthalpy is available. The results described above indicatethat 5-pyrazolide was formed in the flow tube by HO− deprotonation of pyrazole but it was
not formed in the flow tube when O− or F− was used as the deprotonation agent. Thisobservation allows bracketing of the pyrazole C5—H gas phase acidity between that of H2O3
(383.68 ± 0.02 kcal mol-1) and HO4 (376.7 ± 0.2 kcal mol-1) or HF3 (365.577 ± 0.003
kcal mol-1). This yields a pyrazole C5—H gas phase acidity of ΔacidG298 = 380 ± 4 kcal mol-1.
With thermal corrections from the DFT calculations, Equation 2.3 yields the pyrazole C5—H
bond dissociation energy: D0 = 121 ± 4 kcal mol-1. Similar to above, the 5-pyrazolyl radical
heat of formation can be determined using Equation 2.5: ΔfH298(5-pyrazolyl) = 113 ± 4kcal mol-1.
Reaction 6.1 below shows proton transfer between 5-imidazolide and pyrazole toform imidazole and 5-pyrazolide. The C5 proton acidity of imidazole was bracketed between
the acidities of H2O and HO, identical to that of the C5 proton of pyrazole described above,
resulting in identical gas phase acidities for the C5 protons of imidazole and pyrazole. Taftand coworkers developed an electrostatic model16, 17 to predict the difference in acidity
between different nitrogen heterocycles. This model predicts a difference in acidity between
the C5 protons of imidazole and pyrazole of zero. The DFT predicted free energy difference
for Reaction 6.1 is –3.5 kcal mol-1. The observation of larger photoelectron intensities from
5-pyrazolide versus 5-imidazolide may also support the idea that the C5 position of pyrazoleis slightly more acidic than that of imidazole.

C5
N1
H

H

H

H

H

C4

C4

C4

C4

N3
C2

H

+

C5
N1

H

H

C3

H

H

C5
N1

N2
H

N3

+

C5

C2

N1
H

H

C3

H

N2

ΔrxnG298 = 0 ± 6 kcal mol-1

(6.1)

127

C4
C5
N1

H

H

H
H

− C3
N2

H H

+

C4
C5
N1

C3

H

H

C5
N1

N2

H

H

C4

C4

C3

H

+

C5

N2

N1

H

H

C3

H

N2

ΔrxnG298 = 33.6 ± 4.0 kcal mol-1(6.2)

Reaction 6.2 represents proton transfer between pyrazolide and pyrazole to form
pyrazole and 5-pyrazolide. The large difference in acidity between the N1 and C5 positionsof pyrazole1 can easily be recognized as arising from extensive conjugation in 1-pyrazolide
which is not present in 5-pyrazolide. In 1-pyrazolide, the negative charge is free to movearound the ring (see Figure 6.9); in 5-pyrazolide, however, the negative charge is effectively
localized on C5.

6.5

Conclusion

The photoelectron spectrum of 1-pyrazolide has been measured, and the EA of the
1-pyrazolyl radical is determined to be 2.938 ± 0.005 eV, with a 2A2 ground state.
Combining the EA in a thermodynamic cycle with the pyrazole N—H deprotonation
enthalpy, the N—H bond dissociation energy of pyrazole is determined to be 106.4 ± 0.4 kcal
mol-1.

The photoelectron spectrum of 1-pyrazolide indicates non-adiabatic effects are

significant between low-lying states of 1-pyrazolyl, based on the poor agreement of theFranck-Condon simulations with the experimental spectrum.

A term energy for the

1-pyrazolyl first excited state (2B1) of 0.032 ± 0.001 eV is also observed. The detailed
B

analysis of the vibronic structure of the 1-pyrazolide photoelectron spectrum will be given in
a separate paper.10
The HO− deprotonation of pyrazole yields a minor product ion, 5-pyrazolide, as
identified by Franck-Condon fitting of the photoelectron spectrum based on the results of
B3LYP/6-311++G(d,p) DFT calculations. The EA of 5-pyrazolyl radical is 2.104 ± 0.005
eV. Fundamental vibrational frequencies of 890 ± 15, 1110 ± 35, and 1345 ± 30 cm-1 havebeen assigned to an in-plane CCN bending mode and two in-plane ring stretching modes,

128
respectively of 2A' 5-pyrazolyl.

An extensive vibrational progression appears in the

spectrum, suggesting a substantial change in the CCN angle from the anion to the neutral.
This change in angle is corroborated by the DFT calculations. Formation of 5-pyrazolide is
not observed in the O− or F− reactions with pyrazole, which allows the gas phase acidity ofthe pyrazole C5—H to be bracketed between the acidities of H2O and HO, yielding
ΔacidG298(pyrazole, C5—H) = 380 ± 4 kcal mol-1. The EA of 5-pyrazolyl and the pyrazole

C5—H gas phase acidity are combined in a thermodynamic cycle to yield the C5—H bonddissociation energy of pyrazole D0 = 121 ± 4 kcal mol-1. To more precisely determine a C5-H
bond dissociation energy, experiments on 1-methylpyrazole are currently underway.18

129

H
H

C4
C5

C3

N1

C5

C3

N1

H
H

C4

H

H

N2

H

H

N2

H

C4
C5
N1

− C3

N1

N1

Figure 6.9

C3

H

N2

N2
H

C4
C5

C5

H

H
H

C4

H

C3
N2

H

H

C4
C5
N1

C3

H

N2

Resonance structures of 1-pyrazolide showing migration of the negative
charge around the ring.

130

Chapter 6 References
1

A.J. Gianola, T. Ichino, S. Kato, V.M. Bierbaum, and W.C. Lineberger,
Thermochemical Studies of Pyrazolide. Journal of Physical Chemistry A, 2006.
10.1021/jp057499+.

2

D. Hanstorp and M. Gustafsson, Determination of the Electron-Affinity of Iodine.
Journal of Physics B-Atomic Molecular and Optical Physics, 1992. 25(8): p. 17731783.

3

K.M. Ervin and V.F. DeTuro, Anchoring the gas-phase acidity scale. Journal of
Physical Chemistry A, 2002. 106(42): p. 9947-9956.

4J.E. Bartmess, "Negative Ion Energetics Data", in NIST Chemistry Webbook, NIST
Standard Reference Database Number 69, J.P. Linstrom and W.G. Mallard, Editors.
June 2005, National Institue of Standards and Technology: Gaithersburg MD, 20899.

5

H. Koppel, W. Domcke, and L.S. Cederbaum, Multimode Molecular-Dynamics
Beyond the Born-Oppenheimer Approximation. Advances in Chemical Physics, 1984.
57: p. 59-246.

6

Y.J. Bomble, K.W. Sattelmeyer, J.F. Stanton, and J. Gauss, On the vertical excitationenergy of cyclopentadiene. Journal of Chemical Physics, 2004. 121(11): p. 52365240.

7

A. Hazra and M. Nooijen, Comparison of various Franck-Condon and vibronic
coupling approaches for simulating electronic spectra: The case of the lowest
photoelectron band of ethylene. Physical Chemistry Chemical Physics, 2005. 7(8): p.
1759-1771.

8J.F. Stanton and J. Gauss, Analytic Energy Derivatives for Ionized States Describedby the Equation-of-Motion Coupled-Cluster Method. Journal of Chemical Physics,
1994. 101(10): p. 8938-8944.

9J.F. Stanton, Coupled-cluster theory, pseudo-Jahn-Teller effects and conicalintersections. Journal of Chemical Physics, 2001. 115(22): p. 10382-10393.

10

T. Ichino, A.J. Gianola, W.C. Lineberger, and J.F. Stanton, Nonadiabatic effects inthe photoelectron spectrum of the pyrazolide anion: Three-state interactions of thepyrazolyl radical. Submitted.

11

R.G. Pearson, A symmetry rule for predicting molecular structure. Journal of theAmerican Chemical Society, 1969. 91: p. 4947-4955.

12

G. Fischer, Vibronic Coupling. 1984, London: Academic Press.

13

A. Samuni and P. Neta, Electron-Spin Resonance Study of Reaction of Hydroxyl
Radicals with Pyrrole, Imidazole, and Related Compounds. Journal of Physical
Chemistry, 1973. 77(13): p. 1629-1635.

131
14

S. Matsika and D.R. Yarkony, Conical intersections of three electronic states affect
the ground state of radical species with little or no symmetry: Pyrazolyl. Journal of
the American Chemical Society, 2003. 125(41): p. 12428-12429.

15

P. Jimenez, M.V. Roux, C. Turrion, and F. Gomis, Thermochemical Properties of NHeterocyclic Compounds .1. Enthalpies of Combustion, Vapor-Pressures andEnthalpies of Sublimation, and Enthalpies of Formation of Pyrazole, Imidazole,
Indazole, and Benzimidazole. Journal of Chemical Thermodynamics, 1987. 19(9): p.
985-992.

16

R.W. Taft, F. Anvia, M. Taagepera, J. Catalan, and J. Elguero, Electrostatic
Proximity Effects in the Relative Basicities and Acidities of Pyrazole, Imidazole,
Pyridazine, and Pyrimidine. Journal of the American Chemical Society, 1986.
108(12): p. 3237-3239.

17

J. Catalan, R.M. Claramunt, J. Elguero, J. Laynez, M. Menendez, F. Anvia, J.H.
Quian, M. Taagepera, and R.W. Taft, Basicity and Acidity of Azoles - the Annelation
Effect in Azoles. Journal of the American Chemical Society, 1988. 110(13): p. 41054111.

18

S.M. Villano, A.J. Gianola, S. Kato, V.M. Bierbaum, and W.C. Lineberger,
Manuscript in preparation.

________________________________________________
7
Vinyldiazomethyl Anion, C3H3N2−________________________________________________
7.1

Introduction

Formation of substituted and unsubstituted pyrazoles can be accomplished byreactions of alkynes with diazomethanes.1-3 Both 1H- and 3H- pyrazoles are formed fromthese reactions. Once formed, 1H-pyrazoles have very stable ring structures, due to theiraromatic character, but collision induced dissociation (CID) studies of 1-pyrazolide haveshown ring opening followed by rapid fragmentation.4 The ring opening typically resultsfrom rupture of the N—N bond. Thermal ring opening of the non-aromatic 3H-pyrazole
compounds at a C—N bond to form vinyldiazo compounds has been known for quite sometime.3 Ring opening is most likely when steric effects favor it, as well as when the resulting
vinyldiazo compound is stabilized by conjugation. These vinyldiazo compounds are often anintermediate to loss of N2 and formation of a vinylcarbene, followed by subsequent
cyclopropene formation.
H

H

C

C

H

H

H
H

H

C
C

C

C
C

E-Vinyldiazomethyl
Anion

C
C

N
N

E-Vinyldiazomethyl
Radical

H

H

C
N

N

C

H

H

C
N

H

N

N

N

Z-Vinyldiazomethyl
Anion

Z-Vinyldiazomethyl
RadicalBierbaum and coworkers succeeded in synthesizing deprotonated vinyldiazomethane(the vinyldiazomethyl anion) in the gas phase in 1977.5 The vinyldiazomethyl anion, a ring
opened isomer of pyrazolide and imidazolide, was synthesized in the flow tube here and its
photoelectron spectrum has been measured. The method used for its synthesis is briefly
reviewed, and is followed by a description of the experimental spectra obtained. DFT

133
calculations suggest that two isomers of the vinyldiazomethyl anion should be formed in theflow tube, but Franck-Condon simulations show that only one is formed in detectableamounts.

The thermodynamics of ring closure of vinyldiazomethane and thevinyldiazomethyl anion are discussed.

7.2

Experiment

Vinyldiazomethyl anions are produced in a three step process by first reacting O−
with an excess amount of propene (C3H6) introduced downstream of the microwave discharge
to produce HO− in a hydrogen transfer reaction, similar to the reaction of O− with methane:O − + C3 H 6 → HO − + C3 H 5(7.1)

The propene which remains unreacted is deprotonated by HO− to produce allyl anions(C3H5−):HO − + C3 H 6 → H 2O + C3 H 5−

(7.2)

A trace amount of nitrous oxide (N2O) is added further downstream, which reacts with theallyl anions to produce vinyldiazomethyl anions (C3H3N2−) and water:5-8

N 2 O + C3 H 5− → C3 H 3 N 2− + H 2O

(7.3)

This method produces vinyldiazomethyl anion beams of ~200 pA. The laser wavelength
used for photodetachment of vinyldiazomethyl anion was 351.1 nm, and the absolute energy
scale was calibrated via the measurement of the EA of oxygen. In some experiments liquid
nitrogen was used to cool the flow tube to aid in cooling the ions.

7.3

Results

The 351.1 nm magic angle spectrum of the vinyldiazomethyl anion at 298 K is shownin Figure 7.1. The spectrum shows two regions of detachment, one between binding energies
of 1.7 and 2.4 eV and one between binding energies of 2.4 and 3.4 eV. Angular distribution

134
measurements reveal different anisotropies for the different portions of the spectra. For the
lower binding energy region, β = 0.0 ± 0.1; for the higher binding energy region, β = 0.52 ±

1600
1
1400

2
3

Photoelectron Counts

1200
1000

4

800
600

5

9 8
10

400

6
7

11
12

200

A

B

0
3.5

3.3

3.1

2.9

2.7

2.5

2.3

2.1

1.9

1.7

Electron Binding Energy (eV)Figure 7.1

351.1 nm magic angle photoelectron spectrum of vinyldiazomethyl anion at
298 K. See Table 7.1 for assignments of labels.

135

Table 7.1

Peak Label

Relative Peak Position (cm-1)

E-Vinyldiazomethyl
Assignment

B-414 ± 20

1210

A-179 ± 15

1310

1

0

000

2

189 ± 101310

3410 ± 151210

4607 ± 1512101310

5818 ± 2012101302

61030 ± 25

12021310

71250 ± 25810

81451 ± 25

810131091687 ± 30

8101210101868 ± 30410

112051 ± 35

4101310122273 ± 35

4101210

Relative experimental peak positions and assignments for the peaks labeledin Figures 7.1 and 7.2.

136
measurements reveal different anisotropies for the different portions of the spectra. For the
lower binding energy region, β = 0.0 ± 0.1; for the higher binding energy region, β = 0.52 ±0.06. This suggests that the two different regions correspond to detachment to two different
electronic states of vinyldiazomethyl radical.
The lower binding energy region shows resolvable vibrational structure, and thetallest peak is assigned as the band origin, yielding an EA of 1.867 ± 0.007 eV. Table 7.1
summarizes the relative peak positions for peaks labeled in Figures 7.1 and 7.2. The higher
binding energy region has a vertical detachment energy of 2.809 ± 0.010 eV. It is very broad
and without clearly resolvable structure, indicating that there is a large geometry change upon
photodetachment to this state. It is not possible to precisely determine the band origin of thisstate, but the term energy is estimated as 0.6 eV – near the region where the photoelectron
signals of this band begin.
To further aid in the assignments of the photoelectron spectrum, DFT calculations
were performed on the vinyldiazomethyl anion and vinyldiazomethyl radical at theB3LYP/6-311++G(d,p) level.

Ground state geometry optimizations and frequency

calculations were performed which located E,Z isomers of the vinyldiazomethyl anion andradical. Minima of Cs symmetry were located for both isomers of the anion and the radical.
The optimized geometries are listed in Table 7.2 and the frequencies are listed in Table 7.3.
These radical states have a' (σ) electronic symmetry, which is consistent with the anisotropy
observed for this portion of the spectrum. The E- and Z- labels reflect the positions of atoms
relative to the C3—C4 bond. The DFT calculations predict an EA for E-vinyldiazomethyl
radical of 1.869 eV and an EA for Z-vinyldiazomethyl radical of 1.872 eV. If formed in theflow tube, both isomers are expected to contribute to the photoelectron spectrum.
Using the geometry and frequency results, Franck-Condon simulations were
performed for transitions from the E- or Z-vinyldiazomethyl anion to the E- or137
Z-vinyldiazomethyl radical. Figure 7.2 shows the Franck-Condon simulation of detachmentof E-vinyldiazomethyl anion (red) and Z-vinyldiazomethyl anion (blue), as well as an

H7

H7

H7

C5

C5
H8

H8

H6
C4

H6

C5

C4

H8

C3

C3

N2

N2
N1

Electronic State
N1—N2 (Å)
N2—C3 (Å)
C3—C4 (Å)
C4—C5 (Å)
C4—H6 (Å)
C5—H7 (Å)
C5—H8 (Å)∠N1-N2-C3 (°)∠N2-C3-C4 (°)∠C3-C4-C5 (°)∠C3-C4-H6 (°)∠C4-C5-H7 (°)∠C4-C5-H8 (°)

Table 7.2

N2

N1

C3

H6
C4

N1

E-Vinyldiazomethyl

Z-Vinyldiazomethyl

Anion

anion

1

A'
1.16901.26541.42431.36291.10091.08531.0858186.77121.14
127.42116.86121.01121.22radical2A'
1.15071.2815
1.41851.35861.09071.08341.0836189.98122.34121.16119.94121.27120.581A'
1.1671
1.26701.42981.36681.09251.08641.0861
173.15238.05130.16114.10120.86121.91

Radical2A'
1.15031.28031.42051.36091.08531.08391.0848169.88236.16126.01115.32121.01121.78E- and Z-vinyldiazomethyl anion and radical geometries obtained fromB3LYP/6-311++G(d,p) calculations.

138

E-Vinyldiazomethyl
Mode

Symmetry

ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18

a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a''
a''
a''
a''
a''

Table 7.3

Anion
1
A'
Frequency(cm-1)
3190
3111
2959
2034
1599
1465
1353
1290
1109
897
622
445
183
962
718
626
566
138

Radical2A'
Frequency(cm-1)
3244
3146
3095
1955
1540
1425
1366
1288
1100
890
584
439
186
984
945
635
477
137

Z-Vinyldiazomethyl
Anion
1
A'
Frequency(cm-1)
3181
3103
3057
2026
1565
1460
1395
1287
1074
904
663
475
146
983
714
620
580
205

Radical2A'
Frequency(cm-1)
3230
3161
3136
1951
1542
1421
1376
1293
1073
888
615
469
158
1003
928
566
484
198

E- and Z-vinyldiazomethyl anion and radical fundamental vibrationalfrequencies obtained from B3LYP/6-311++G(d,p) calculations.

139

Electron Binding Energy (eV)
2.3

2.2

2.1

2.0

1.9

2

1400

1.8

1.7

1

3

1200
1000

4
800

5

600

Photoelectron Counts

400

12

200

11 10

9

8

7

A

6

B

0
1400
1200
1000
800
600
400
200
0

2.3

2.2

2.1

2.0

1.9

1.8

1.7

Electron Binding Energy (eV)Figure 7.2

Expanded view of the 351.1 nm magic angle photoelectron spectrum ofvinyldiazomethyl anion at 298 K (black) and Franck-Condon simulation of
detachment of E-vinyldiazomethyl anion (red) and Z-vinyldiazomethyl
anion (blue).

140

Z-vinyldiazomethyl radical. Figure 7.2 shows the Franck-Condon simulation of detachmentof E-vinyldiazomethyl anion (red) and Z-vinyldiazomethyl anion (blue), as well as an
expanded view of the experimental spectrum (black). To best match the appearance of hot
bands in the experimental spectrum, the simulation used a vibrational temperature for theanions of 250 K. The simulations origin positions are set to match the DFT predicted EAs.
From Figure 7.2, it is clear that the spectrum matches the E-vinyldiazomethyl anion
detachment simulation nearly perfectly, with little indication of contributions from
Z-vinyldiazomethyl anion detachment. The most intense peak is confirmed to be the groundstate band origin, verifying the EA of E-vinyldiazomethyl radical as 1.867 ± 0.007 eV.
Detachment of E-vinyldiazomethyl anion results in activation of four vibrational modes ofE-vinyldiazomethyl radical. The experimental fundamental frequencies extracted from thespectrum of 189 ± 10, 410 ± 15, 1250 ± 25, and 1868 ± 30 cm-1 compare well with thecorresponding DFT frequencies of 186, 439, 1288, and 1955 cm-1, respectively.

Peakassignments are given in Table 7.1 and Figure 7.3 shows the relative atomic displacements
for the vibrations active in the photoelectron spectrum.

7.4

Discussion

The Franck-Condon simulation of vinyldiazomethyl anion detachment shows that thephotoelectron signals primarily originate from detachment of E-vinyldiazomethyl anion withlittle contribution from the Z-vinyldiazomethyl anion.

The relative intensities of theexperimental peaks labeled 1 and 2, however, do not match the relative intensities of thesimulation of E-vinyldiazomethyl anion detachment perfectly.

Assuming that the total

photodetachment cross sections for E- and Z-vinyldiazomethyl anion are identical, if there is
a 1:1 ratio of E- and Z-vinyldiazomethyl anion present in the ion beam, the origin intensity ofthe Z- isomer would be approximately twice that of the E- isomer. With ~7% of the Z-

141
isomer present in the ion beam, the relative intensities of peaks 1 and 2 are better matched,with only marginal effects on the other features of the spectrum.

H

H
C
H

H

C

C
H

H

C
C

C

N

N

N

N
a' mode, 186 cm-1
C-C-N bend

a' mode, 439 cm
C-C-C bend

H

H

C
H

H

C
C

C
H

H

C
C

N

Figure 7.3

-1

N
N

N

a' mode, 1288 cm-1
C-C stretch

a' mode, 1955 cm-1
N-N stretch

Relative atomic displacements and harmonic frequencies of normal modes ofE-vinyldiazomethyl radical evaluated at B3LYP/6-311++G(d,p) level.

142
isomer present in the ion beam, the relative intensities of peaks 1 and 2 are better matched,with only marginal effects on the other features of the spectrum.
Franck-Condon simulations to the excited states of the vinyldiazomethyl radical were
not performed here, due to the lower symmetry structures expected for these states. When Cs
symmetry was imposed on π-type excited states of the E- and Z-vinyldiazomethyl radicals,
only transition states were found. Since the ground states of the E- and Z- vinyldiazomethyl
radical have Cs symmetry, any non-planar geometries input into the electronic structure
calculations relax to the planar ground states regardless of the molecular orbital population
chosen.

Time dependent DFT methods were attempted to calculate structures andfrequencies for the π-type excited states of the E- and Z-vinyldiazomethyl radicals; however,the processing time required to complete these calculations was prohibitive.
The DFT calculations on the anions suggest that the E-vinyldiazomethyl anion ismore stable than Z-vinyldiazomethyl anion by ~1.8 kcal mol-1.

For mostly

E-vinyldiazomethyl anion to be present in the ion beam suggests a significant barrier toprevent the conversion between the two isomers.
explore the isomerization energetics.

DFT calculations were performed to

A structure of C1 symmetry was located as thetransition state between the E- and Z- vinyldiazomethyl anion isomers. This transition state
was located approximately 8 kcal mol-1 above E-vinyldiazomethyl anion. This barrier mustbe large enough for preferential formation or stabilization of mostly the E- isomer in the flowtube.
As mentioned in the introduction, 3H-pyrazoles are known to ring open to form
vinyldiazomethanes when heated.3 Ring closure of deprotonated vinyldiazomethane, then, is
expected to form a deprotonated 3H-pyrazole. DFT was again used to explore the energetics
of such ring formation. C5 deprotonated 3H-pyrazole is the expected product of ring closure,and a Cs minimum was located for this ion, with an energy approximately 22 kcal mol-1 above

143
E-vinyldiazomethyl anion. A C1 transition state was located between Z-vinyldiazomethyl
anion and C5 deprotonated 3H-pyrazole.

This barrier was located approximately 46

kcal mol-1 above E-vinyldiazomethyl anion. Such a large endothermicity and barrier suggests
it is nearly impossible to form a 5-membered ring anion from vinyldiazomethyl anion under
the current flow tube conditions.

Figure 7.4 summarizes the potential surface points

computed for this proposed ring closure.
Vinyldiazomethane, though, is known to slowly isomerize to 1-H pyrazole underroom temperature conditions.9 Ledwith and Parry10 and Hart and Brewbaker11, 12 studied therate of cyclization and proposed a mechanism which involves 3H-pyrazole as anintermediate. DFT calculations were again used to study the energetics of the ring closing
mechanism at various points along the reaction path, beginning with vinyldiazomethane andending with 1H-pyrazole. According to these calculations, E-vinyldiazomethane is slightly
more stable than Z-vinyldiazomethane (~0.6 kcal mol-1), with a barrier similar in height tothe deprotonated form (~8 kcal mol-1).

The barrier to ring closure and formation of

3H-pyrazole is larger, about 27 kcal mol-1.

Overall, production of 3H-pyrazole fromvinyldiazomethane is about 6 kcal mol-1 exothermic. The barrier to shifting a hydrogen atomfrom the pyrazole C3 to N1 position is similar to the ring closure barrier, about 27 kcal mol-1
higher than 3H-pyrazole.

It is ~29 kcal mol-1 exothermic to form 1H-pyrazole from

3H-pyrazole.

This large stabilization is presumably due to the aromatic character of

1H-pyrazole.

These results seem to corroborate the experimental observation of slow

formation of 1H-pyrazole from pure vinyldiazomethane.
computed potential surface points for this ring closure.

Figure 7.5 summarizes the

144
7.5

Conclusion

The photoelectron spectrum of vinyldiazomethyl anion has been obtained and the EAof vinyldiazomethyl radical is 1.867 ± 0.007 eV. Franck-Condon simulations indicate thattheE-vinyldiazomethyl

anion

isomerisprimarily

formed

in

the

flow

tube;

Z-vinyldiazomethyl anion is not formed in significant amounts to contribute to the spectrum.
Vibrational frequencies of E-vinyldiazomethyl radical are observed in the photoelectron
spectrum of 189 ± 10, 410 ± 15, 1250 ± 25, and 1868 ± 30 cm-1. These modes respectively
correspond to C-C-N bending, C-C-C bending, C—C stretching and N—N stretching modesof E-vinyldiazomethyl radical.

Comparing the vinyldiazomethyl anion photoelectron

spectrum with that of pyrazolide, there are no photoelectrons corresponding to detachment ofvinyldiazomethyl anion in the pyrazolide spectrum, nor are there photoelectrons
corresponding to detachment of pyrazolide in the vinyldiazomethyl anion spectrum. This
indicates that of ring closure in the vinyldiazomethyl anion and ring opening of pyrazolide
does not occur in the flow tube. Theoretical calculations suggest formation of pyrazolide in
the flow tube is virtually impossible under the experimental conditions.

145

46.2

21.8
H

8.1
0

1.8

H

H

C
H

H

C
N

N

C
N

C
C

C
N

C

H

C

H
H

N

H

C

N

Figure 7.4

Reaction profile for ring-closure of vinyldiazomethyl anion. Potential
surface energies are relative to E-vinyldiazomethyl anion and are computedat the B3LYP/6-311++G(d,p) level. Energies are in kcal mol-1.

146

26.6
20.7
8.3
0

0.6

H

H

C
H
H

N

N

Figure 7.5

C

H
H

N

C
H

C
C

C
N

C

C

H

C
N

H

H

H

C
N

-5.8

C

H

C

H

H

H

N

H

N

-34.6

Reaction profile for ring-closure of vinyldiazomethane to ultimately form
1H-pyrazole. Potential surface energies are relative to E-vinyldiazomethane
and are computed at the B3LYP/6-311++G(d,p) level. Energies are in kcal
mol-1.

147

Chapter 7 References
1

A.N. Kost and I.I. Grandberg, Progress in pyrazole chemistry. Advances in
Heterocyclic Chemistry, 1966. 6: p. 347-429.

2

R.H. Wiley, ed., Chemistry of Heterocyclic Compounds, Vol. XXII: Pyrazoles,
Pyrazolines, Pyrazolidines, Indazoles, and Condensed Rings. 1967.

3

M.P. Sammes and A.R. Katritzky, The 3H-Pyrazoles. Advances in Heterocyclic
Chemistry, 1983. 34: p. 1-52.

4

G.W. Adams, J.H. Bowie, and R.N. Hayes, Negative-Ion Fragmentations ofDeprotonated Heterocycles - the Pyrazole and Imidazole Ring-Systems. Rapid
Communications in Mass Spectrometry, 1992. 6(1): p. 54-57.

5

V.M. Bierbaum, C.H. DePuy, and R.H. Shapiro, Gas-Phase Reactions of Anions with
Nitrous-Oxide and Carbon-Dioxide. Journal of the American Chemical Society,1977. 99(17): p. 5800-5802.

6

S.R. Kass, J. Filley, J.M. VanDoren, and C.H. DePuy, Nitrous-Oxide in Gas-Phase
Ion Molecule Chemistry - a Versatile Reagent for the Determination of Carbanion
Structure. Journal of the American Chemical Society, 1986. 108(11): p. 2849-2852.

7

C.H. DePuy, J.M. Van Doren, S. Gronert, S.R. Kass, E.L. Motell, G.B. Ellison, and
V.M. Bierbaum, Gas-phase negative-ion chemistry of diazomethane. Journal of
Organic Chemistry, 1989. 54(8): p. 1846-50.

8

R.A.J. OHair, S. Gronert, and C.H. DePuy, New insights into the gas-phase anion
chemistry of nitrous oxide. European Mass Spectrometry, 1995. 1(5): p. 429-436.

9

C.D. Hurd and S.C. Lui, Vinyldiazomethane. Journal of the American Chemical
Society, 1935. 57: p. 2656-2657.

10

A. Ledwith and D. Parry, 1,3-Dipolar cycloaddition reactions of diazo alkanes. II.
Kinetics of the thermal and photochemical formation of pyrazole from diazopropene.
Journal of the Chemical Society B: Physical Organic, 1967(1): p. 41-42.

11

H. Hart and J.L. Brewbaker, 1,3-Bisdiazopropane. Preparation and cyclization topyrazole. Journal of the American Chemical Society, 1969. 91(3): p. 706-711.

12

J.L. Brewbaker and H. Hart, Cyclization of 3-diazoalkenes to pyrazoles. Journal of
the American Chemical Society, 1969. 91(3): p. 711-715.

________________________________________________
82-Oxepinoxide, C6H5O2−________________________________________________
8.1

Introduction

This chapter describes experiments performed measuring the photoelectron spectrumof 2-oxepinoxide, C6H5O2− in order to establish the EA of the corresponding neutral, the2-oxepinoxy radical. The 2-oxepinoxy radical is believed to be an intermediate formed
during the combustion of benzene, as originally proposed by Carpenter.1

Theoreticalinvestigations on the oxidation of the phenyl radical2 and unimolecular decomposition of thephenylperoxy radical3 followed, both which include the 2-oxepinoxy radical as anintermediate to the formation of cyclopentadienyl radical and CO2. These studies and anothertheoretical report on the unimolecular decomposition of the 2-oxepinoxy radical4 suggest thatthe 2-oxepinoxy radical should be a stable intermediate formed during benzene combustion atintermediate temperatures.
O

O

O

H

H

H

H

H

2(3H)-Oxepinone

O

−

H H

H

H

O

O

H

H H

H

2-Oxepinoxide

H

H

H

2-Oxepinoxyl Radical

Recently, the gas phase acidity of 2(3H)-oxepinone was determined5 as the first stepin a series of three experiments designed to establish the heat of formation of 2-oxepinoxy
radical. Determination of the EA of the 2-oxepinoxyl radical, reported here, is the second
step. Combination of the 2(3H)-oxepinone gas phase acidity with the 2-oxepinoxyl radical
EA in a thermodynamic cycle (as described in Chapter 2) will yield information about a C-H

149
bond dissociation energy of 2(3H)-oxepinone. The third experiment in the series is thecalorimetric measurement of the heat of formation of 2(3H)-oxepinone. Combination of thisheat of formation with the bond dissociation energy of 2(3H)-oxepinone will yield the heat offormation of the 2-oxepinoxy radical.

8.2

Experiment2(3H)-Oxepinone was obtained from Carpenter and coworkers, prepared as described
previously,5 and was introduced into the flow tube by flowing a stream of helium over theliquid sample.

Hydroxide ions, created using the microwave discharge ion source as

described in Chapter 2, were used to deprotonate 2(3H)-oxepinone yielding the desired
2-oxepinoxide anion as well as an unknown ion of m/z = 65. Stable ion beams of ~100 pA of
2-oxepinoxide were produced using this method. In some experiments, liquid nitrogen wasused to cool the flow tube, aiding in the cooling of the ions. In these experiments, slightly
less intense ion beams were formed (~50 pA). In all experiments, the laser wavelength used
for photodetachment was 351.1 nm and calibration of the absolute energy scale wasaccomplished via measurement of the EA of oxygen.
8.3

Results

Figure 8.1 shows the room temperature 351 nm photoelectron spectra of
2-oxepinoxide anion with the laser polarization at the magic angle (black), 0° (blue), and 90°(red) to the photoelectron collection axis. The spectrum consists of a broad photoelectronband with several sharper features which show no indication of a pattern. The angularmeasurements result in an anisotropy parameter, β, of –0.18 ± 0.07 at the peak of the broad
detachment signal at 2.254 ± 0.010 eV.
DFT calculations were performed on the 2-oxepinoxide anion and 2-oxepinoxyl
radical to assist in the assignment of the photoelectron spectra, as previously outlined in

150
Chapter 2.

The B3LYP method was used with the 6-31+G(d) basis set.

Optimized

geometries and harmonic vibrational frequencies were obtained for the ground states of the140θ = 0°
θ = 90°θ = 54.7°

Photoelectron Counts

120
100
80

EA = 1.85 ± 0.10 eV

60
40
20
0
3.3

3.1

2.9

2.7

2.5

2.3

2.1

1.9

1.7

1.5

Electron Binding Energy (eV)Figure 8.1

351.1 nm photoelectron spectrum of 2-oxepinoxide at 300 K taken at themagic angle (54.7°, black), parallel (0°, blue) and perpendicular (90°, red) tothe photoelectron collection axis.

151
Chapter 2.

The B3LYP method was used with the 6-31+G(d) basis set.

Optimized

geometries and harmonic vibrational frequencies were obtained for the ground states of theanion and neutral and are listed in Tables 8.2 and 8.3. For the 2-oxepinoxide anion, aminimum with non-planar structure (C1 symmetry) was obtained. For 2-oxepinoxyl radical, aminimum with planar structure (Cs symmetry) was obtained. These calculations predict an
EA of ~1.76 eV, close to the region of the experimental spectrum where the photodetachment
signals begin.
With the anion having C1 symmetry and the neutral having Cs symmetry, all 33
vibrational modes are allowed in the photoelectron spectrum.

Large normal mode

displacements are expected for the out-of-plane bending vibrations of the neutral, whichshould result in an extended vibrational progression for these modes.

Franck-Condon

simulations using the optimized geometries and frequencies were attempted, but, due to thelarge number of allowed and active vibrational modes (all 33 modes were found to be active),meaningful results have not been obtained.
For the Franck-Condon simulations described in previous chapters, a “cutoff” valueof 0.001% of the origin intensity was used. This means that transitions whose intensities
were less than 0.001% of the origin were omitted from the simulation. At this cutoff level,
the 0 K Franck-Condon simulation of 2-oxepinoxide photodetachment is outside of ourprocessing capabilities. Larger cutoff values were used successfully, however. Figure 8.2
shows the 0 K Franck-Condon simulation of 2-oxepinoxide detachment with cutoff values of
100%, 10%, and 1% of the origin intensity. In all of these simulations, the origin was given
an intensity of 100 counts and a position matching the DFT computed EA (1.7567 eV). Fromthe simulations, a clear trend is visible, with the rightmost portion of a particular simulation
being a good approximation to a simulation with a smaller cutoff value. Simulations witheven smaller cutoff values (0.1% and 0.01%) also show that this trend continues.

152

2-Oxepinoxide Anion
Coordinates

C1
C2
C3
C4
C5
C6
O7
O8
H9
H10
H11
H12
H13

X
-0.47811.2816
0.4982-0.8864-1.6714-1.8983
2.49440.6015-0.3080
1.0827-1.2043-2.4856-2.92542-Oxepinoxyl Radical

C1
C2
C3
C4
C5
C6
O7
O8
H9
H10
H11
H12
H13

Table 8.1

Y
-1.48790.12901.2974
1.4688-0.85400.5253-0.0069-1.0107-2.4750
2.18512.5097-1.41440.8641Z
0.1423-0.00170.1631-0.09200.2329-0.11910.1861-0.58140.57720.3978-0.21420.7016-0.2534

Coordinates
X-0.7059
1.3026
0.6116-0.7354-1.7956-1.8434
2.50860.6223-0.8346
1.3234-0.9649-2.7454-2.8284

Y-1.5087
0.00001.28471.5911-0.6875
0.7292-0.0924-1.2322-2.5859
2.10372.6554-1.2168
1.1871Z
0.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00002-Oxepinoxide anion and 2-oxepinoxyl radical geometries in Cartesian
coordinates obtained from the B3LYP/6-31+G(d) calculations.

153

2-Oxepinoxide
Mode

2-Oxepinoxyl
Mode

Symmetry

Frequency (cm-1)
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18
ν19
ν20
ν21
ν22
ν23
ν24
ν25
ν26
ν27
ν28
ν29
ν30
ν31
ν32
ν33
Table 8.2

3160
3142
3126
3091
3074
1692
1648
1571
1506
1415
1401
1267
1239
1199
1053
1031
954
930
907
865
825
746
724
702
682
653
577
508
420
478
346
191
115

Frequency (cm-1)
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18
ν19
ν20
ν21
ν22
ν23
ν24
ν25
ν26
ν27
ν28
ν29
ν30
ν31
ν32
ν33

a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a''
a''
a''
a''
a''
a''
a''
a''
a''
a''

3228
3220
3205
3185
3170
1786
1617
1558
1480
1453
1418
1345
1268
1215
1094
1049
951
892
801
696
509
421
359
992
941
851
795
739
640
536
376
133
73

2-Oxepinoxide anion and 2-oxepinoxyl radical fundamental vibrationalfrequencies obtained from B3LYP/6-31+G(d) calculations.

154

DFT EA = 1.7567
120
1%
10%
100%

Simulated Photoelectron Counts

1.0e+9100

8.0e+880

6.0e+860

4.0e+840

2.0e+820

0.0

Experimental Photoelectron Counts

1.2e+9

0
3.3

3.1

2.9

2.7

2.5

2.3

2.1

1.9

1.7

1.5

Electron Binding Energy (eV)Figure 8.2

Franck-Condon simulation of 2-oxepinoxide 351.1 nm photodetachment at 0
K with a cutoff level of 100% (black), 10% (blue), and 1% (red) of the origin
intensity. The position of the origin is indicated by the green line. The
experimental spectrum is overlaid in grey.

155
Figure 8.3 shows Franck-Condon simulations performed at 300 K to explore theeffects of hot band transitions. Again, cutoff values of 100%, 10%, and 1% of the origin
intensity were used, with an origin intensity and position identical to that used for the 0 K
simulations. With a cutoff value of 100%, the simulation is identical to that at 0 K, indicatingthat all hot-band transitions have smaller Franck-Condon factors than that of the origin. The
simulations with cutoff values of 10% and 1% clearly show how significantly the hot-band
transitions affect the spectrum.

Inclusion of these hot-band transitions increases thecomputation time dramatically, however.

8.4

Discussion

Unfortunately, as can be seen in Figures 8.2 and 8.3, there is no indication of
convergence of the simulation with the cutoff level as small as 1%.

Franck-Condon

simulations at 0 K with cutoff levels of 0.1% and 0.01% also show no indication of
convergence. Without a reasonable Franck-Condon simulation, another method is needed toallow for the determination of the EA of the 2-oxepinoxyl radical.

By estimating thedifference between the DFT computed vertical detachment energy (VDE) and EA andapplying that difference to the experimental VDE, an approximate experimental EA may be
determined.
At the B3LYP/6-31+G(d) level, the computed EA is 1.7567 eV and the VDE is
2.0792 eV; the difference between these two values is 0.3225 eV.

Figure 8.4 shows

hypothetical potential energy curves for the 2-oxepinoxide system illustrating these energy
differences. Applying this difference to the experimentally observed VDE of 2.254 ± 0.010
eV results in an EA of approximately 1.931 eV, near the region of the spectrum where thedetachment signals start. This estimation is somewhat consistent with the 300 K FranckCondon simulations where the EA is slightly higher in energy than the start of thephotoelectron signals.

To better estimate the difference between the VDE and EA,

156
calculations with the larger 6-311++G(d,p) basis set were performed, yielding an EA of
1.8166 eV and VDE of 2.1353 eV, for a difference of 0.3187 eV. Applying this difference to

DFT EA = 1.7567
120
1%
10%
100%

Simulated Photoelectron Counts

5e+9

100

4e+9

80

3e+9

60

2e+9

40

1e+9

20

0

Experimental Photoelectron Counts

6e+9

0
3.3

3.1

2.9

2.7

2.5

2.3

2.1

1.9

1.7

1.5

Electron Binding Energy (eV)Figure 8.3

Franck-Condon simulation of 2-oxepinoxide 351.1 nm photodetachment at
300 K with a cutoff level of 100% (black), 10% (blue), and 1% (red) of theorigin intensity. The position of the origin is indicated by the green line.
The experimental spectrum is overlaid in grey.

157
calculations with the larger 6-311++G(d,p) basis set were performed, yielding an EA of
1.8166 eV and VDE of 2.1353 eV, for a difference of 0.3187 eV. Applying this difference tothe experimental VDE yields an EA of 1.935 eV. With the aug-cc-pVTZ, the EA computedin this way is 1.918 eV.
As shown in the earlier chapters in this dissertation, DFT often predicts radical EAs
extremely well. Table 8.3 summarizes the experimental and DFT electron affinities. With anaverage absolute error of 0.017 eV, B3LYP with the 6-311++G(d,p) basis set is expected to
predict the EA of 2-oxypinoxy quite accurately. Considering this and the above discussion of
the VDE and EA differences, we assign the EA of the 2-oxepinoxyl radical as 1.85 ± 0.10 eV.
With the EA of the 2-oxepinoxyl radical, it is possible to obtain the bond dissociationenergy of 2(3H)-oxepinone using the methods outlined in Chapter 2. From the literature,5 the298 K deprotonation enthalpy of 2(3H)-oxepinone is 352 ± 2 kcal mol-1. Using thermal
corrections from the DFT calculations, ΔacidH0 = 350 ± 2 kcal mol-1. Combining this with the
EA from above yields a bond dissociation energy of D0 = 80 ± 3 kcal mol-1. This value isclose to the B3LYP/6-311++G(d,p) computed D0 of 74 kcal mol-1.
Several sharp features extend beyond the broad congested photodetachment signal
which have a slightly different anisotropy.

Since the deprotonation enthalpy5 of2(3H)-oxepinone is 352 ± 2 kcal mol-1 and that6 of H2O is 390.27 ± 0.03 kcal mol-1, thereaction of 2(3H)-oxepinone with hydroxide is exothermic by ~38 kcal mol-1. Such a large
exothermicity could fragment the ring structure, and such an isomer of 2-oxepinoxide may be
the signal carrier for these sharp features. Experiments using F− as the deprotonation agent of2(3H)-oxepinone are currently underway to help determine if these features originate fromsuch an ion. Observation5 of an ion with m/z 65 formed in the reaction of 2(3H)-oxepinone
with hydroxide indicates that the ring structure may not stay intact.

Unimoleculardecomposition of 2-oxepinoxide could form CO2 and a mass 65 ion with molecular formula158
C5H5−.

Photoelectron experiments on this ion indicate the structure is not that of

cyclopentadienide but most likely an open chain isomer.

8.5

Conclusion

The photoelectron spectrum of 2-oxepinoxide anion reveals that there is a large
geometry change upon photodetachment, indicated by a broad congested spectrum with nodiscernable structure with a VDE of 2.254 ± 0.010. Franck-Condon simulations based on
DFT computed geometries and frequencies also show that a broad spectrum is expected. The
Franck-Condon simulations fail to reproduce the experimental spectrum within a reasonableamount of time to allow for precise determination of the EA of 2-oxepinoxyl. In spite of thisdifficulty, the EA of 2-oxepinoxyl radical is assigned as 1.85 ± 0.10 eV from the estimationof the difference between the EA and VDE. This combines with a previously reported gas
phase acidity of 2(3H)-oxepinone to establish the bond dissociation energy of2(3H)-oxepinone to be 80 ± 3 kcal mol-1.

159

VDE
2.0792 eV
2.1353 eV
2.1144 eV
EA
1.7567 eV
1.8166 eV
1.7788 eV

Figure 8.4

Potential energy curves illustrating the B3LYP computed vertical detachment
energy (VDE) and electron affinity (EA). The top values were computed
using the 6-31+G(d) basis set, the middle values using the 6-311++G(d,p)
basis set and the bottom values using the aug-cc-pVTZ basis set.

160

RadicalCyclopentadienyl
Pyrrolyl
1-Imidazolyl
5-Imidazolyl
1-Pyrazolyl
5-Pyrazolyl
E-Vinyldiazomethyl
2-Oxepinoxyl
2-Oxepinoxyl
2-Oxepinoxyl

Table 8.3

Experimental
EA(eV)
1.812 ± 0.005
2.145 ± 0.010
2.613 ± 0.006
1.992 ± 0.010
2.935 ± 0.005
2.104 ± 0.005
1.867 ± 0.007———

Basis Set
6-311++G(d,p)
6-311++G(d,p)
6-311++G(d,p)
6-311++G(d,p)
6-311++G(d,p)
6-311++G(d,p)
6-311++G(d,p)
6-31+G(d)6-311++G(d,p)
aug-cc-pVTZ

B3LYP
EA
(eV)
1.763
2.116
2.592
1.986
2.945
2.109
1.869
1.757
1.817
1.779

Experimental and B3LYP computed electron affinities.

EAexp –
EAB3LYP
(eV)
0.049
0.029
0.021
0.006
-0.010-0.005-0.002———

161

Chapter 8 References
1

B.K. Carpenter, Computational Prediction of New Mechanisms for the Reactions ofVinyl and Phenyl Radicals with Molecular-Oxygen. Journal of the American
Chemical Society, 1993. 115(21): p. 9806-9807.

2

C. Barckholtz, M.J. Fadden, and C.M. Hadad, Computational study of themechanisms for the reaction of O2(3Σg) with aromatic radicals. Journal of Physical
Chemistry A, 1999. 103(40): p. 8108-8117.

3

M.J. Fadden, C. Barckholtz, and C.M. Hadad, Computational study of theunimolecular decomposition pathways of phenylperoxy radical. Journal of Physical
Chemistry A, 2000. 104(13): p. 3004-3011.

4

M.J. Fadden and C.M. Hadad, Unimolecular decomposition of the 2-oxepinoxy
radical: A key seven-membered ring intermediate in the thermal oxidation ofbenzene. Journal of Physical Chemistry A, 2000. 104(34): p. 8121-8130.

5

S.M. Kroner, M.P. DeMatteo, C.M. Hadad, and B.K. Carpenter, The gas-phase
acidity of 2(3H)-oxepinone: A step toward an experimental heat of formation for the
2-oxepinoxy radical. Journal of the American Chemical Society, 2005. 127(20): p.
7466-7473.

6

K.M. Ervin and V.F. DeTuro, Anchoring the gas-phase acidity scale. Journal of
Physical Chemistry A, 2002. 106(42): p. 9947-9956.

________________________________________________
9
Summary and Conclusions________________________________________________
9.1

Introduction

There has recently been interest in homonuclear polynitrogen compounds as highenergy density materials.1-3 One polynitrogen species, N5−, was experimentally detected
through collision induced dissociation electrospray ionization mass spectrometry4 and laser
desorption ionization time-of-flight mass spectrometry5 of substituted p-phenylpentazoles.
Theoretical studies suggest that N5− is a stable 5-membered ring compound, pentazolide.6-9
Its decomposition into azide anion (N3−) and N2 is predicted to be exothermic by 14-16
kcal mol-1, and the heat of formation of pentazolide is predicted9 to be ΔfH298 = 62.1 ± 3.6
kcal mol-1.

Determination of these thermodynamic values through experiments ischallenging, considering the difficulty in synthesis of pentazolide. Unequivocal synthesis orobservation of pentazole (N5H) has not yet been reported.10
The pentazolide anion is isoelectronic with the other 5-membered ring anions
described earlier in this dissertation (cyclopentadienide, pyrrolide, imidazolide, andpyrazolide). By examining the systematic substitution of N atoms into the 5-membered ring,it may be possible to predict some of the properties of pentazole/pentazolide/pentazolyl
radical through extrapolation of the experimental values described in previous chapters.
This chapter summarizes the previous chapters and makes the extrapolations
discussed above based on similarities and trends observed in the experimental data. Since
most of the compounds discussed in this dissertation are isoelectronic (with the exception
C6H5O2−, discussed in Chapter 8), it is expected that these trends may provide usefulpredictions for the properties of other isoelectronic species yet unobserved (such as thetriazole and tetrazole systems) in addition to N5−. Insights into potential experimental or163
theoretical difficulties which may arise in the collection and interpretation of these data arealso discussed.
9.2

Electron Affinities

Electron affinities of the 5-membered ring radicals cyclopentadienyl, pyrrolyl,
1-imidazolyl and 1-pyrazolyl were determined using negative ion photoelectron
spectroscopy. Their values are 1.812 ± 0.005, 2.145 ± 0.010, 2.613 ± 0.006, and 2.938 ±0.005 eV, respectively. Figure 9.1 displays the electron affinity of these radicals as a functionof the number of nitrogen atoms in the ring. Also displayed are B3LYP/6-311++G(d,p)
computed electron affinities for 1,2,3-triazolyl, 1,2,4-triazolyl, and tetrazolyl. As a first
approximation, these data are used in a linear regression to predict an EA for the pentazolyl
radical of ~4.6 eV. Vertical detachment energies of 5.7 and 5.6 eV have been previously
predicted for pentazolide.9, 11
DFT predicts the EAs of the 1,2,3- and 1,2,4- triazolyl radicals to be 3.39 and 3.48
eV, respectively. The EA of tetrazolyl is predicted to be 4.16 eV. Notice that these energies
are close to or greater than the photon energy used in these experiments of 3.408 or 3.531 eV.
It is expected that experiments on these systems will require a higher energy photon to allow
for determination of the EA of these radicals.
Electron affinities of other species were also determined, including isomers of
1-imidazolyl and 1-pyrazolyl. The EAs of these carbon centered radicals, 5-imidazolyl and5-pyrazolyl are 1.992 ± 0.010 and 2.104 ± 0.005 eV, respectively.

The EA ofvinyldiazomethyl radical, a ring opened isomer of these isoelectronic species, was also foundto be 1.867 ± 0.007 eV.
The EA of the 2-oxepinoxy radical was not precisely established in this dissertation.
A large geometry change upon photodetachment of the 2-oxepinoxide anion strongly
activates all vibrational modes of the radical in the photoelectron spectrum, resulting in a

164highly congested spectrum with little indication of the position of the band origin. Despitedifficulties in simulating the spectrum, an EA of 1.85 ± 0.10 eV was assigned. DFT

5.0
DFT Value
Experimental Value
Extrapolated Value

4.5

N

N
N

Electron Affinity (eV)

N

N

N

N
N

4.0

N

N
N

N

3.5
N

N

N

3.0
N

N

N

2.5
N

N

2.0
1.5
0

1

2

3

4

5

# N atoms

Figure 9.1

EAs of the 5-membered ring radicals with the corresponding number ofnitrogen atoms in the 5-membered ring. The black points are experimental
values and the red points are DFT calculated EAs. The solid line is a linear
regression fit of the data to a line, and the green point is the EA predicted for
pentazolyl radical from the extrapolation.

165

highly congested spectrum with little indication of the position of the band origin. Despitedifficulties in simulating the spectrum, an EA of 1.85 ± 0.10 eV was assigned.

DFT

calculations predict an EA for the 2-oxepinoxyl radical of 1.82 eV at theB3LYP/6-311++G(d,p) level.
9.2

Bond Dissociation Energies and Heats of Formation

Bond dissociation energies were established for several of the 5-membered ringparent compounds for breaking a C—H or N—H bond using a thermodynamic cycle
combining the gas phase acidity of the parent compound with the EA of the radical. Figure
9.2 displays the N—H bond dissociation energy as a function of the number of nitrogenatoms in the ring. The N—H bond dissociation energies of pyrrole, imidazole, and pyrazole
are 93.92 ± 0.11,12 95.1 ± 0.5 and 106.4 ± 0.4 kcal mol-1. N—H bond dissociation energies
for 1,2,3-triazole, 1,2,4-triazole, and tetrazole were computed at the B3LYP/6-311++G(d,p)
level and are also shown in Figure 9.2. A linear regression of the data suggests the N—Hbond dissociation energy of hydrogen pentazole (N5H) is ~114 kcal mol-1.
For two of these heterocyclic compounds, imidazole and pyrazole, C—H bonddissociation energies were determined by combining the EAs of the corresponding radicals
with the C—H gas phase acidity determined via bracketing observed in the photoelectron
spectrometer. The carbon-5 proton on both imidazole and pyrazole is acidic enough to be
deprotonated by HO− but not acidic enough for deprotonation by O−, resulting in identical gas
phase acidities of ΔacidG298 = 380 ± 4 kcal mol-1. The EAs and gas phase acidities combine togive C5—H bond dissociation energies of 119 ± 4 and 121 ± 4 kcal mol-1 for imidazole andpyrazole, respectively.

The C—H bond dissociation energies of cyclopentadiene and2(3H)-oxepinone, also determined using a thermodynamic cycle, are 80.6 ± 2.3 and 81 ± 3kcal mol-1, respectively.

166

Bond Dissociation Energy (kcal mol-1)

120
DFT Value
Experimental Value
Extrapolated Value

115

N
N

N
N

110

N

N
N

N
N

N

105

N

N
N

N

N

100

N

95

N

N
N

N

90
1

2

3

4

5

# N atoms

Figure 9.2

N—H bond dissociation energies of the 5-membered heterocyclic
compounds as a function of the number of nitrogen atoms in the 5-memberedring. The black points are the experimental data, and the red points are DFT
computed values. The solid line is a linear regression fit of the data to a line,
and the green point is the N—H bond dissociation energy predicted forhydrogen pentazole from the extrapolation.

167
Comparison of the bond dissociation energies determined here can reveal the relative
stability of the different radical species. Cyclopentadiene, with the smallest C—H or N—H
bond dissociation energy, is expected to have the most stable radical formed upon breaking
this bond. This radical will be followed in stability by the pyrrolyl radical, the 1-imidazolyl
radical and the 1-pyrazolyl radical. The carbon radicals 5-imidazolyl and 5-pyrazolyl are
likely to follow this.
Comparing the heats of formation of these radicals can verify this simple argument.
From the bond dissociation energies and the heats of formation of the parent compounds,13
the radical heats of formation can be determined. The 298 K heat of formation of thecyclopentadienyl radical is 64.6 ± 2.7 kcal mol-1. The 298 K heat of formation of the pyrrolyl
radical12 is 69.15 ± 0.35 kcal mol-1. The 298 K heats of formation of 1-imidazolyl and1-pyrazolyl are 76.2 ± 0.6 and 98.7 ± 0.5 kcal mol-1, respectively.

The 298 K heats offormation of the carbon radicals 5-imidazolyl and 5-pyrazolyl are 100 ± 4 and 113 ± 4 kcal
mol-1, respectively. Figure 9.3 summarizes these values.

9.3

Electronic and Vibrational Structure

Aside from the thermodynamic parameters discussed above, electronic andvibrational structure of the neutral species can be extracted from the anion photoelectron
spectra. The photoelectron spectrum of the high-symmetry 1A1' cyclopentadienide anion
reflects the presence of Jahn-Teller interactions in the doubly degenerate
cyclopentadienyl radical.

2

E1''

The spectrum is dominated by an intense origin band, but

vibrational bands corresponding to the Jahn-Teller active vibrations are clearly observed.
Additionally, unexpected vibrational bands appear in the spectrum, and are assigned to nontotally symmetric modes which gain intensity due to strong off-diagonal coupling. Since thepentazolide and cyclopentadienide systems are isoelectronic and of the same symmetry (D5h),
it is expected that, if bound, pentazolyl will exhibit similar electronic and vibrational structure

168
as the cyclopentadienyl radical, with a spectrum dominated by the band origin and JahnTeller active vibrations.
120

HN

N

N

N

110

HN

N

-1

ΔfH298 (kcal mol )

100

90

80
N

N

N

70

60

Figure 9.3

298 K heats of formation of 5-membered ring radicals.

169
as the cyclopentadienyl radical, with a spectrum dominated by the band origin and JahnTeller active vibrations.
With the maximum symmetry lowered from D5h to C2v, Jahn-Teller interactions are
not present in the pyrrolyl, 1-imidazolyl, and 1-pyrazolyl systems.

Franck-Condon

simulations do, however, show that these systems are not free from non-adiabatic effects, andthe spectra show interesting effects attributed to vibronic coupling between low lying
electronic states. In pyrrolyl, the ground state features are well resolved but the vibronic
coupling effects cause the first excited state to appear broad and featureless. This is due totransformation of the expected potential energy minimum into a transition state.14 In the
1-imidazolyl system, however, the vibronic coupling effects are minimal and the spectrum of
the electronic ground state appears very Franck-Condon like. A small unexpected featureappears in the spectrum and is assigned to an overtone of an in-plane non-totally symmetricvibration, gaining intensity from a Fermi-Resonance which is affected by vibronic coupling.15
The photoelectron spectrum of 1-pyrazolide shows the ground and first excited statesof 1-pyrazolyl to be very close in energy. Franck-Condon simulations on these states fail toreproduce the experimental spectra, with two peaks missing which are experimentallyobserved and the intensities of most other vibrational bands severely overestimated. The
second electronic excited state further complicates the spectra and is transformed into atransition state by the vibronic coupling effects with the ground and first excited states.
Simulations which take the nonadiabatic effects into consideration predict the spectrum very
well, allowing for confident assignments of the vibrational features observed.16, 17 It is
expected that the photoelectron spectrum of the 1,2,3-triazolide, 1,2,4-triazolide, andtetrazolide anions may also have analogous vibronic coupling effects, which may cause
difficulties in assignment of the spectra.
The photoelectron spectra of the isoelectronic 5-imidazolide and 5-pyrazolide are
reproduced nearly perfectly by the Franck-Condon simulations. Several vibrational features

170
are observed in the spectra which correspond primarily to in-plane ring distortion or ringbreathing vibrations of the corresponding neutral radicals. Franck-Condon simulations of thevinyldiazomethyl anion detachment show that the spectrum originates from theE-vinyldiazomethyl anion isomer, with little or no indication of Z-vinyldiazomethyl anion
formation in the flow tube.
9.4

Future Work

Photoelectron spectra of one more 5-membered heterocyclic ion, 1,2,3-triazolide, arebeing obtained in another of Professor Lineberger’s laboratories. Once finalized, the EA of
the corresponding radical and the parent N—H bond dissociation energy can be added to theextrapolations performed here to lend additional support to the predicted properties of thepentazole system.
Experiments on N-methyl substituted imidazolide and pyrazolide are being
performed on the photoelectron spectrometer used here to obtain the EA of the C5-methylimidazolyl and C5-methyl-pyrazolyl radicals.

These measurements, coupled withcorresponding measurements of the C5—H gas phase acidities will allow for a precise
determination of the C5—H bond dissociation energies of these compounds.18 Assuming themethyl substitution is only a small perturbation, this will provide a useful comparison to theC5—H bond dissociation energies obtained for imidazole and pyrazole; preliminary results
indicate this to be the case.
Continuing the investigation of heterocyclic compounds, experiments are planned on
the 6-membered rings which are isoelectronic with benzene (pyridine, pyrimidine, etc).
These experiments will provide EAs, bond dissociation energies, and heats of formation forthese systems, which at present are not well known. Additionally, all of the experiments
which are planned or currently in progress will reveal vibrational structure of the unstableradical species formed from the photodetachment of the closed shell anions.

171

Chapter 9 References
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4

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7

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10

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11

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172
12

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13

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14

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15

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16

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17

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18

S.M. Villano, A.J. Gianola, S. Kato, V.M. Bierbaum, and W.C. Lineberger,
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