PYROLYSIS AND SPECTROSCOPYOF CYCLIC AROMATIC COMBUSTION INTERMEDIATES
byGRANT THORNTON BUCKINGHAM
B.A. Carleton College 2010

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirement for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry2016This thesis entitled:
Pyrolysis and Spectroscopy of Cyclic Aromatic Combustion Intermediates
written by Grant Thornton Buckingham
has been approved for the Program of Chemical Physics_________________________________
G. Barney Ellison__________________________________
Veronica M. Bierbaum

Date_________________
The final copy of this thesis has been examined by the signatories, and wefind that both the content and the form meet acceptable presentation standardsof scholarly work in the above mentioned discipline.

ii

Buckingham, Grant Thornton (Ph.D., Chemical Physics)
Pyrolysis and Spectroscopy of Cyclic Aromatic Combustion Intermediates
Thesis Directed by Emeritus Professor G. Barney Ellison

We have studied the pyrolysis of aromatic combustion intermediates using
an array of detection techniques. The molecules investigated include cyclicaromatic molecules with hydrocarbon substituents (ethylbenzene, npropylbenzene, isopropylbenzene, and styrene), oxygen-containing substituents(anisole and phenol), triply substituted systems (vanillin), resonance stabilized
radicals (benzyl radical and tropyl radical) and phenyl radical. At the exit of a
resistively heated micro-reactor (1 mm inner diameter, 3 cm long), the pyrolysis
fragments are detected using photoionization mass spectrometry (PIMS), matrix
isolation vibrational spectroscopy, microwave spectroscopy, tunable VUVsynchrotron-based PIMS, and table-top VUV PIMS with photoelectron photoion
coincidence spectroscopy (PEPICO). This array of detection methods allows for theidentification of all possible fragments including metastables, radicals, and atoms.
The findings allow for detailed mechanistic information regarding which pathways
are active at different pyrolysis temperatures. The findings can also be used to help
identify products and individual isomers that are formed during the gas-phasethermal decomposition of aromatic systems. By providing direct experimental
pyrolysis data, models for fuel decomposition and soot formation can be improved
to help understand current combustion systems and eventually aid in the design ofsuperior fuel sources in the near future.

iii

ACKNOWLEDGMENTS
I think that I am on the verge of concluding one of the luckiest graduate school
experiences possible. The amount that I learned about chemistry, physics, and sciencein general is fantastic but the 6 years I had in Boulder may be some of the mostimportant years of my life. I arrived in Colorado as a young, ignorant, insecure, andpoorly defined person and I write this now as a confidant self-sufficient, and driven
man. For this I am deeply indebted to 1) my family, 2) to my two research advisors,
David Nesbitt and Barney Ellison, and 3) to the group of friends I found in Boulder. My
advisors taught me about chemical physics and life as a scientist but the other two
camps helped me find who I am. To all three groups I am eternally grateful.
For the first two-and-a-half years of my degree, I worked with David Nesbitt,
who may be the most careful and creative thinker and teacher I have ever met. The
completeness of his understanding is truly staggering and the vigor with which hepursues knowledge is something I will always strive to emulate. For the next 3.5 years I
worked with Barney Ellison learning a very different type of chemistry. Barney’s ability
to find and solve interesting problems, as well as amass a wonderful group ofcolleagues and friends to help him do it showed me how great science can really be.
Working with him and learning to navigate academia has been more fun than I thought
graduate school could be. The combination of these two mentors has allowed me togain such a wealth (and breadth) of knowledge that I can’t imagine a more rewarding
Ph.D. experience.
It has been a pleasure working with so many fun and intelligent people during
my time here. In the Nesbitt group, I benefitted from knowing Melanie Roberts, ChihHsuan Chang, Rob Roscioli, Kevin Early, Nick Dupuis, Mia Zutz, Andy Gisler, Mikeiv

Ziemkiewicz, Andrej Grubisic, Monika Gruetter-Kasumaj, Dan Nelson, Eric
Holmstrom, Tom Baker, and Julie Fiore and all of the JILA staff. In the Ellison group I
enjoyed working with Kimberly Urness, Jessie Porterfield, Josh Baraban, Tom Ormond,
Jong Kim, Fiona Deguillaume, John Stanton, John Daily, Qi Guan, Don David, the Ion
Super Group, Ronnie Bierbaum, Carl Lineberger, and Mathias Weber. I also had thepleasure or collaborating with Mark Nimlos and David Robichaud at NREL; Tim Zwier
and his very welcoming group at Purdue University; the ALS team at Berkeley: Musa
Ahmed, Tyler Troy, and Oleg Kostko; Mike McCarthy and Marie-Aline Martin at
Harvard CfA; Hans-Heinrich Carstensen from the University of Gent; Bill Peters andDavid Couch from the KM group; and Bryan Changala and Ben Spaun of Jun Ye’s
group.
I also need to acknowledge the tremendous experience I had at Carleton College,
where I learned about chemistry but also learned to love learning. The education I
received from the faculty there, especially Will Hollingsworth and Marion Cass, helped
me to succeed in graduate school but also showed me how to be passionate about
science.
The people I spend my free time with were hugely important to my time in
Boulder. My housemates at Armer: Jordan “Jo’j” Mirocha, Anthony Harness, Marek
Slipski, Emily Kraus, and Tristan Weber were always there for me to make me laugh.
Book Club with Eric Coughlin helped keep my life in balanced and coffee with Greg
Salveson helped keep my awake. Mia Zutz, Andy Gisler and Nick Dupuis were myconstant ski team and dinner crew. Galen Gorski, Kevin Early, Kevin Pollock, Matt
Nock, and Austin Hall were all there for me from a distance.
Finally, and most importantly, I want to acknowledge everything that my family
has done for me along the way. The unwavering support from my parents Kimv

Thornton and Kent Buckingham and sister Katherine Buckingham allowed me topersevere and thrive during this stage of my life, and all the stages that preceded it.
There is no doubt that all of my strengths as a person came from being lucky enough to
be born into the family that I was.

viCONTENTS

Chapter 1. Introduction
I. Motivation

1

II. Understanding Current Combustion Systems

2

III. Selecting Better Future Fuels

7

IV. Outline

11

References for Chapter 1

14

Chapter 2. Experimental Methods
I. Introductions

17

II. High Resolution Absorption Spectroscopy

18

III.

A. Difference Frequency Infrared Production

18

B. Slit Discharge Radical and Ion Production

19

Pyrolysis of Gas-Phase Biofuel Intermediates Using a Heated MicroReactor

22

A. Pyrolysis in a heated Micro-Reactor

23

B. Photoionization Mass Spectroscopy

27

i.

ii.
iii.

Photoionization Mass Spectrometry Using the 9th harmonicof an NdLYAG Laser and Reflectron Time-of-Flight
Detection

28

Photoionization Mass Spectrometry Using Synchrotron
Radiation and Time-of-Flight Reflectron Detection

29

Photoionization Mass Spectrometry Using Synchrotron
Radiation and Imaging Photoelectron Photoion
Coincidence Spectroscopy

30

vii

iv.

Photoionization Mass Spectrometry Using a Tabletop
Vacuum Ultraviolet Laser and Photoelectron Photoion
Coincidence Spectroscopy

32

C. Matrix Isolation Infrared Absorption Spectroscopy

35

D. Sample Preparation

36

References for Chapter 2

39

Chapter 3. High-Resolution Rovibrational Spectroscopy of Jet-Cooled Phenyl
Radical: The !19 Out-of-Phase Symmetric CH Stretch
Abstract

43

I. Introduction

44

II. Experimental

50

III. Results and Analysis

53

IV. Discussion
A. Nuclear Spin Weights

62

B. Planarity and In-Plane Ring Distortion

64

C. Intramolecular Vibrational Redistribution

69

V. Summary and Conclusions

75

References for Chapter 3

76

Chapter 4. Observation of the a1 CH Stretching Modes in Phenyl Radical using JetCooled Sub-Doppler Absorption Spectroscopy
Abstract

83

I. Introduction

83

II. Experimental

88

III. Results and Discussion
A. Spectral Observation and Analysis

89

B. Intensity Analysis

99
viii

IV. Summary and Conclusions

103

References for Chapter 4

104

Chapter 5. Hydropyrolysis of Ethylbenzene in a Heated Micro-Reactor
Abstract

107

I. Introduction

107

II. Experimental

112

III. Results and Discussion
A. Ethylbenzene Decomposition

113

B. Hydropyrolysis of Ethylbenzene

116

IV. Conclusions

125

References for Chapter 5

127

Chapter 6. The Thermal Decomposition of the Benzyl Radical in a Heated MicroReactor: I. Experimental Findings
Abstract

131

I. Introduction

132

II. Experimental

139

III. Results

144

A. Decomposition of Benzyl-d0 Radical

145

B. Decomposition of Benzyl-d2 Radical

155

C. Decomposition of Benzyl-d5 Radical

160

D. Decomposition of 13C-Benzyl Radical, C6H513CH2

161

IV. Discussion and Conclusions

167

References for Chapter 6

171

ix

Chapter 7. The Thermal Decomposition of the Benzyl Radical in a Heated MicroReactor: II. Pyrolysis of the Tropyl Radical
Abstract

179

I. Introduction

180

II. Experimental
A. Heated Micro-Reactor Pyrolysis Source

185

B. Photoionization Mass Spectrometryi.

Pulsed Vacuum Ultraviolet Radiation

186

ii.

Continuous Vacuum Ultraviolet Radiation

187

C. Matrix Isolation Fourier Transform Infrared Spectroscopy

187

D. Sample Preparation

188

III. Results and Discussion
A. Photoionization Mass Spectrometry

190

B. Matrix Isolation Spectroscopy

200

C. Cycloheptatrienyl or Norbornadiene as Tropyl Precursors?

205

IV. Conclusions

213

A. Pyrolysis of Benzyl Radical without Tropyl
References for Chapter 7

214
224

Chapter 8. Tabletop Vacuum Ultraviolet Laser-Based Photoionization Mass
Spectrometry with Photoelectron Photoion Coincidence Detection of Pyrolysis
Products
Abstract

233

I. Introduction

233

II. Experimental

237

III. Results and Discussionx

A. Isomer Identification

240

B. Disentangling Dissociative Ionization in Mass Spectra

242

IV. Conclusions

253

References for Chapter 8

254

Chapter 9. Full Bibliography

257

Appendix A

281

Appendix B

293

Appendix C

301

Appendix D

309

Appendix E

311

Appendix F

317

Appendix G

323

xi

TABLES
2.1

Summary of the experimental conditions for each experiment
performed with heated micro-reactor.

27

3.1

Ab initio theoretical and experimental rovibrational constants ofphenyl radical

66

4.1

The symmetry and band origin frequencies from theoreticalprediction, gas-phase, and Ar-matrix studies of phenyl radical andcation in the region of C−H stretching

92

4.2

Spectroscopic parameters of the phenyl radical in the ground state andexcited CH stretching modes

96

5.1

Vibrational band assignments of toluene

124

5.2

Vibrational band assignments of styrene

125

6.1

Experimental properties of benzyl and cyclohepatrienyl radicals

137

6.2

Summary of Experimental Conditions

143

7.1

Tropyl and Benzyl Radical Experimental Properties

183

xii

FIGURES
1.1

The formation of soot starting from simple acyclic combustion
intermediates up through 1.5 nm diameter soot particles

4

1.2

Propargyl radical in the ground electronic state5

1.3

Sample of lignin with the different linkages and their abundances

9

1.4

The three lignin monomers (monolignols)

10

1.5

Chemical structure of three isomers of C7H7, including their common
names and the symmetry of their electronic ground states

12

2.1

Cartoon diagram of pulsed slit jet discharge

21

2.2

Cut away view of the clover heater

25

2.3

Continuous flow heated micro-reactor assembly (inset) and larger
cutaway diagram

34

2.4

Sample probe for heating low-vapor pressure solids and liquids

37

3.1

Phenyl radical with its body fixed principle rotation axes labeled

45

3.2

Cartoon diagram of pulsed slit jet discharge

51

3.3

Overview rovibrational spectrum of the symmetric out-of-phase C-H
stretch, !19, in phenyl radical at Trot roughly equal 11(1)K

55

3.4

Q branch region of the phenyl radical out-of-phase CH symmetricstretch59

3.5

Scan of phenyl radical in the R branch region of the !19 CH stretch

61

3.6

Comparison of two simulated spectra, which highlight the effect of
nuclear spin statistics in the spectrum of phenyl radical in the !19
region63

3.7

Ab initio theoretical comparison (CCSD(T)/vtz-f12) between benzene
and phenyl radical, illustrating the subtle distortion of the carbon ringupon H atom abstraction and radical formation

69

3.8

Estimates of symmetry selected vibrational state density for a series ofaromatic and polycyclic aromatic hydrocarbon (PAH) molecules,
based on simple density functional predictions (B3LYP/6311++g(3df,3pd)) and an efficient backtracking algorithm

74

xiii

4.1

Structure of phenyl radical with carbon atom positions labeled

84

4.2

The displacement vectors for each vibrational mode in the region of
C−H stretching of phenyl radical

87

4.3

Rotationally resolved spectrum of the phenyl radical in the region of3068−3078 cm-1

90

4.4

Spectrum of the weak band observed at 3062 cm-1 with thecorresponding vibrational assignment

93

4.5

A portion of the rotationally resolved rovibrational spectrum in thevicinity of 3076 cm-1

94

4.6

Boltzmann plot of the single isolated lines in the !19 band leading to arotational temperature of about 11.2 K

100

4.7

The normalized integrated band intensity for the experimental (gasphase and Ar matrix) observation and the theoretical prediction

102

5.1

Schematic of the chemical pathways confirmed for the pyrolysis ofbenzaldehyde mixed with methyl nitrite

110

5.2

PIMS spectra of the thermal decomposition of dilute ethylbenzene(0.1% in He) at 900 K, 1200 K and 1300 K

114

5.3

PIMS spectra of the thermal decomposition of 0.5% methyl nitrite in apulsed He mico-reactor

116

5.4

A mixture of 0.1% ethylbenzene and 0.5% methyl nitrite waspyrolyzed at 900 K, 1200 K, and 1300 K in a micro-reactor and the
products were detected using 118.2 nm PIMS

117

5.5

Matrix isolation IR spectra of gas mixtures (benzene, ethylbenzene andmethyl nitrite, ethylbenzene, and methyl nitrite) flowed through aheated micro-reactor

121

5.6

Matrix isolation IR spectra of gas mixtures (ethylene, ethylbenzene
and methyl nitrite, ethylbenzene, and methyl nitrite) flowed through aheated micro-reactor

123

6.1

Proposed benzyl radical decomposition pathways

134

6.2

Dilute ethylbenzene heated in a pulsed helium tubular micro-reactor
studied using PIMS at four different reactor temperatures, 350 K, 1300
K, 1400 K, and 1500 K

146

xiv

6.3

Matrix isolation IR absorbance spectra of dilute ethylbenzene in apulsed helium reactor at 300 K and 1500 K

148

6.4

The acetylenic stretch region of the vibrational spectrum is shown forthe decomposition of ethylbenzene in pulsed neon at 300 K and 1500 K

150

6.5

Photoionization efficiency curves for m/z 76 and m/z 65 measured
during the 1500 K decomposition of 0.2% ethylbenzene in a CW
helium reactor

151

6.6

PIMS spectra recorded during the decomposition of benzyl bromide at
1200 K and 1300 K in a CW reactor

153

6.7

Thermal decomposition of benzyl bromide in a CW argon reactor
measured at the tunable synchrotron lights source at the SLS

154

6.8

Dilute ethylbenzene-d5 (C6H5CD2CD3) heated in a pulsed helium
tubular micro-reactor studied using PIMS at three different reactor
temperatures, 300 K, 1300 K, and 1500 K

156

6.9

PIMS spectra of C6H5OCH3 decomposition at 1300 K and the outcomeof mixing 0.02% C6H5OCH3 with 0.25% CD3ONO, a known D atom
source, in pulsed helium

158

6.10

Dilute ethylbenzene-d5 (C6D5CH2CH3) heated in a pulsed helium
tubular micro-reactor studied using PIMS at three different reactor
temperatures, 350 K, 1300 K, and 1400 K

161

6.11

Dilute ethylbenzene-13C (C6H513CH2CH3) heated in a pulsed helium
tubular micro-reactor studied using PIMS at three different reactor
temperatures, 300 K, 1300 K, and 1500 K

163

6.12

Photoionization efficiency curves confirming the identity of m/z 91 as
C5H4=C=CH2 and m/z 90 as C5H4-C≡CH

164

6.13

Photoionization efficiency curves confirming the identity of m/z 65
and 66 as cyclopentadienyl radical

165

6.14

PIMS for the decomposition of C6H5CD2Br at 1250 K in a CW Ar
reactor at the SLS

167

7.1

The Benson Pathway for the pyrolysis of benzyl radical

181

7.2

The 9.5 eV PIMS spectra of bitropyl (C7H7-C7H7) pyrolyzed in acontinuous flow micro-reactor heated to 600 K

191

xv

7.3

PIMS spectra of bitropyl (C7H7-C7H7) recorded in a continuous flow
micro-reactor at temperatures of: 400 K, 1100 K, 1200 K, 1300 K, 1400
K, and 1500 K

193

7.4

Photoionization efficiency curves for m/z 65 recorded during thethermal decomposition of bitropyl (C7H7-C7H7) in a continuous flow
micro-reactor in He at 900 K , 1200 K, and 1500 K

195

7.5

Neon matrix absorbance IR spectrum of the 1200 K pyrolysis of
bitropyl (C7H7-C7H7) in a pulsed micro-reactor

197

7.6

PIMS spectra comparing the thermal decomposition of the benzyl
radical and the tropyl radical

199

7.7

Neon matrix absorbance IR spectra of the pyrolysis of bitropyl(C7H7-C7H7) in a pulsed micro-reactor at 400 K, 1000 K, 1100 K, and1200 K in the region of 3030 cm-1

202

7.8

Neon matrix absorbance IR spectra of the pyrolysis of bitropyl(C7H7-C7H7) at 400 K, 1000 K, 1100 K, and 1200 K in the region of1450 cm-1

204

7.9

Isomerization pathways of norbornadiene, norcaradiene,
cycloheptatriene, and toluene

206

7.10

The neon matrix absorbance IR spectrum of cycloheptatriene (C7H8)
pyrolysis in a pulsed micro-reactor at 1300 K

208

7.11

PIMS spectra from the pyrolysis of cycloheptatriene in pulsed heliumat three reactor temperatures, 300 K, 1400 K, and 1600 K

210

7.12

PIMS spectra from the pyrolysis of norbornadiene in pulsed helium atthree reactor temperatures, 400 K, 1300 K, and 1600 K

212

7.13

The “extended” Benson Fragmentation pathways of the benzyl radical

216

7.14

Fragmentation pathways of the benzyl radical including thenorbornadienyl pathway and "-methyl phenyl pathway

219

8.1

Continuous flow heated micro-reactor assembly and larger cutaway
diagram239

8.2

Photoelectron spectrum of 1-butyne flowed through an unheated CW
micro-reactor

241

8.3

Photoelectron spectrum of 1,3-butadiene, 1-butyne, and a mixture of
the two flowed through an unheated CW micro-reactor

242

xvi

8.4

Room temperature PIMS spectra recorded using 118.2nm light ofdilute (0.07%) 2-butanol in a pulsed He micro-reactor

244

8.5

Room temperature PIMS spectra of 2% 2-butanol in a CW He microrecorded using an XUV laser and a PEPICO detector

245

8.6

PIMS of 0.07% 2-butanol in a pulsed He micro-reactor at 1200 K,
recorded with a 118.2 nm ionization source

247

8.7

PIMS of 2% 2-butanol pyrolyzed in CW helium flowed through a
micro-reactor heated with 3 Amps operating at 13 Volts, recorded
using an XUV laser and a PEPICO detector

248

8.8

Photoelectron spectrum of the species with m/z 15 observed during 2butanol pyrolysis, assigned to be methyl radical

250

8.9

Photoelectron spectrum of the species with m/z 28 observed during 2butanol pyrolysis, assigned to be ethylene

251

8.10

Photoelectron spectrum of the species with m/z 56 observed during 2butanol pyrolysis, cautiously assigned to a mixture of 1-butene and2-butene

252

8.11

Photoelectron spectrum of the species coincident with m/z 44
observed during 2-butanol pyrolysis, potentially assigned to a mixtureof acetaldehyde and vinyl alcohol

253

xvii

Chapter 1
Introduction

I. Motivation
Global climate change is one of the largest and most complicated issues
humanity has faced thus far. What makes this problem so challenging is that it will
require the efforts of so many different fields of study. Among the more obvious ways
to attack the problem is by means of scientific and engineering advancements tounderstand the processes involved and better plan future efforts. There exist many
more subtle challenges, however, that will require the efforts of political scientists,sociologists, and even philosophers as we attempt to prioritize the health the planet andthe prosperity of the people on that planet simultaneously. In the same way that manydifferent fields all have a part to play in this challenge, there are also myriad avenues
within science and engineering that contribute to an efficient and effective solution.
There are currently material scientists working to push the limits of solar energy
conversion, engineers trying to model the characteristics of flames and engines, andthere are significant contributions yet to be made on the fundamental chemical level as
we try to understand combustion on the molecular level. The latter pursuit can often
feel far removed from the dramatic immediacy of climate change, but in some ways, itis the most important struggle because only from a deep understanding can a truly
revolutionary solution emerge.

1II. Understanding Current Combustion Systems
As the power and efficiency of computational methods continues to increase,modelers are able to quantitatively characterize complex kinetic systems. Chemical andmechanical engineers are currently constructing systems of reactions to simulate theprocesses of combustion and pyrolysis (decomposition in the absence of oxygen) given
a set of starting conditions and reactant fuel molecules. Unfortunately, even for a
relatively simple system such as one with only a single fuel with no oxygen present, thenumber of products and intermediates detected can be quite large. In an experiment
where toluene (C6H5-CH3) was heated to 1800 K in the absence of oxygen, at least 64
different species were detected.[1] In order to model the kinetics of this system one must
initially allow for the reactions of all 64 molecules with each other, which leads to 2080
different potential bimolecular reactions, each with its own rate constants.

Sincereaction rates depend on concentration, temperature, and rate constant for each, thissystem is radically undetermined with only the few experimentally available
parameters.

A system like this can be dramatically simplified by building up

complexity slowly, with dilute samples (i.e. only unimolecular reactions) and smaller
precursors. This is the aim of our research program. By starting with simple systems
and short reaction times, we can hope to identify all the nascent products of a
unimolecular thermal decomposition. If the exact mechanism of the decomposition
products can be identified, then the reaction modelers can dramatically simplify theirschemes and make progress toward more complete kinetic schemes. This “bottom-up”
technique requires starting with fundamental molecules but promises to provide a more
complete picture of a system, however small for now.

2

A large problem for efficient combustion is the issue of incomplete combustion.
Ideal combustion results in complete conversion of hydrocarbons and oxygen (O2) into
carbon dioxide and water (see Reaction 1.1 for the combustion of octane).
C8H18 + 12½ O2 → 8 CO2 + 9 H2O(1.1)

If, instead, some of the carbon and hydrogen atoms in the reactant (octane) react
differently to produce other products (e.g. soot) then less of the exothermicity (heat of
combustion) can be utilized to do mechanical work. This incomplete combustion canalso result in formation of CO (see Reaction 1.2), which is poisonous.
C8H18 + 8½ O2 → 8 CO + 9 H2O(1.2)

The processes by which fuel is incompletely combusted and converted to sootremain poorly understood. Key progress was made in 1984 to predict the exact methodby which small acyclic non-aromatic molecules can react to form first monocyclicaromatic molecules (e.g. benzene) and subsequently polycyclic aromatic hydrocarbons(PAH).[2]The latter half of this mechanism, hydrogen-abstraction-C2H2-addition(HACA) is included in Fig. 1.1, which is a summary figure reproduced from a 2005
review of combustion-derived PAH involvement in environmental chemistry.[3] PAHs
are important in modeling combustion because they act as a sink for hydrogen andcarbon atoms. These resonance stabilized PAH molecules resist oxidation and thermal
decomposition meaning that once formed, they can persist and prevent the eventual
conversion to CO2 and H2O, effectively sequestering potentially useful energy fromfurther reactions.

3Fig. 1.1 The formation of soot starting from simple acyclic combustion
intermediates up through 1.5 nm diameter soot particles. The design of the figure isbased on a figure from a 2000 review paper[4] with the inclusion of the HACA reaction
mechanisms to convert phenyl radical (lower left) to bicyclic naphthyl radical (C10H7)
first described by Frenklach.[2]4The stability of PAH brings to bear an important factor in combustion modeling,
quantum mechanical stability via electron delocalization. One of the main reasons
PAHs are significant in combustion is the marked stability that accompanies
aromaticity. When organic structures allow electrons to delocalize from a single atomsuch that they have non-trivial probability density spread among multiple atoms, thesestructures are more stable than comparable molecules that lack electron delocalization.
Benzene is a special example of electron delocalization that satisfies Hückel’s Rule,
which defines aromatic as a cyclic system with 4n + 2 " electrons (where n is any nonnegative integer).

A more general example of electron delocalization is found in

propargyl radical, which can be drawn in two resonance structures (see Fig. 1.2).

Fig. 1.2. Propargyl radical in the ground electronic state. The top two resonances
structures are drawn using the generalized valence bond (GVB) description.[5] The
lower two drawings are in the more standard form but represent the same electronic
state structures.
Resonance stabilized radicals play an important role in combustion as they
contribute to the radical pool that is evoked to describe reactivity of complex
combustion systems.[6] The radical pool is a means of simplifying reaction schemes

5when there are hundreds or thousands of species involved. The notion is that oncesome homolytic bond cleavages begin to occur, a set of radicals is formed that mediate
reactionsthroughResonance-stabilized

a

set

of

radicals

propagation
likepropargyl

reactionsradicalwithneutral(CH2CCH),

molecules.

allyl

radical(CH2CHCH2), cyclopentadienyl radical (C5H5), benzyl radical (C6H5CH2), and tropyl
radical (C7H7) are all predicted to play important roles. Due to their relative stability,
reactions between these radicals is especially important and many pathways to form
PAH rely on this small set of molecules.[7-9] In these systems it is often possible tosample products after pyrolysis of fuel molecules by use of shock tubes. A limitation tothese experiments is that a shock tube can only run a few times per day, meaning signal
averaging on dilute molecules is limited. In most cases, reactive intermediates likeradicals cannot be detected and their presence can only be inferred from the stableproducts detected.

Given the immense number of potential mechanisms at play,inferring intermediates by the final products detected can be difficult and often leads toambiguous conclusions even with the aid of clever isotope labeling. For this reason,
short timescale pyrolysis experiments are very valuable as they allow for detection of
nascent products and can often permit the direct detection of unstable intermediates
like radicals.
Current gasoline contains large amounts of substituted aromatic rings. As muchas ¼ of current liquid gasoline consists of aromatic ring structures, which include
benzene (C6H6), toluene (C6H5CH3), ethylbenzene (C6H5CH2CH3), xylenes (o, m, p - CH3C6H4-CH3) and some longer side-chain molecules.

Since this class of molecules isalready present in everyday systems, it is necessary to understand the decomposition
mechanisms, what intermediates may exist, and what final products are formed.
Although these molecules have been studied extensively in shock tubes and even6

idealized flame systems, in most cases only the stable, long lived produces can bedetected and little is known about the radical intermediates formed. Additionally,
many experiments are able to detect some subset of the products but are often incapableof detecting all products formed, which leaves the experimentalist vulnerable tooverlooking unexpected products. In the cases where unforeseen products are formed,
techniques that detect every product are very important. By gaining information about
the intermediates and nascent products, much more can be learned about the chemicalmechanisms responsible for the decomposition.

III. Selecting Better Future Fuels
Climate change, to put it simply, is the result of anthropogenic deposition of CO2
into the atmosphere that can absorb infrared (IR) light emitted from the earth’s surface
that would otherwise escape to space. As the concentration of CO2 in the atmosphere
increases, the opacity of the atmosphere increases causing heating of the planet. The
long-term solution will likely be to replace all combustion-based industries with thosethat do not lead to release of CO2, e.g. solar, wind, or nuclear energy. Unfortunately,
these methods are still being developed and will take some time before they canefficiently replace combustion.

While these superior technologies are still being

developed, a short-term solution is worth considering that is replacing fossil fuels with
biofuels. The advantage to biofuels is that although CO2 is still released during biofuel
combustion, the process of making biofuels (growing plants) actively removes CO2 fromthe atmosphere. In this way, one full cycle of growing, converting to liquid fuel, then
combusting does not add new carbon to the atmospheric system. This is in stark
contrast to fossil fuel burning, which converts solid-phase carbon that is sequestered in7the earth’s crust into gas-phase CO2.

This unidirectional deposition of CO2 isproblematic as the earth’s energy demands continue to increase. Biofuel combustion is
likely inferior to many potential replacements, but biofuel has the potential to be ashort-term alternative while superior methods are being developed.
The challenges faced by biofuel designers are 1) to identify what would be agood biofuel feed stock and 2) to learn to manufacture it cheaply enough to competewith current fuels. The answer to the first question requires knowledge of what plantmatter is available (or can be made available) and if it thermally decomposes in a waythat can be efficient and safe. A source of plant matter that is currently not being used
for biofuels but is available in large amounts is lignin, shown in Fig. 1.3.

8

β"O"4%%%%%%%%(50"60%),
α"O"4%%%%%%%%%%%%%%%(8%),
5"5%%%%%%%%%%%%%(10"20%),
β"5%%%%%%%%%%%%%%%(5"10%),
β"1%%%%%%%%%%%%%%%%%%%%(7%),β"β%%%%%%%%%%%%%%%%%%%%(2%),
5"O"4%%%%%%%%%%%%%%%%(5%),,

Fig. 1.3. Sample of lignin with the different linkages and their abundances
shown in different colors. The percent abundance values are taken from book “Lignin
and lignins: advances in chemistry”.[10] The above image is taken from the book
“Reaction Pathways and Mechanisms in Thermocatalytic Biomass Conversion II”.[11]Lignin is a polymer that is found in plant cell walls and is the second mostabundant biopolymer on earth after cellulose. The reason that lignin is not currently
used more in industry is that, although it is a polymer, it lacks the predictable andrepeating linkages found in simpler polymers like cellulose. Lignin has three subunits
and each has multiple potential linkage sites. The three subunits (monolignols) are
shown in Fig. 1.4 along with their common names, coniferyl alcohol, coumaryl alcoholand synapyl alcohol.

9coniferyl*alcohol*-coumaryl*
alcohol*-

synapyl*alcohol*-

Fig. 1.4. The three lignin monomers (monolignols)

The three monomers each have an aromatic ring with vinyl alcohol groups andeither one or two methyl ether or OH groups.

In order to render lignocellulosic

biomass viable for efficient processing, large lignin polymers must be fragmented anddeoxygenated, a process that remains too costly to allow lignocellulosic fuel sources toreplace fossil fuels. Recent progress in this field has used a process called integratedhydropyrolysis and hydroconversion (IH2),[12] whereby raw biomass is converted toliquid-phase hydrocarbons at relatively low pyrolysis temperatures, between 350°C and
480°C. Recent work at the National Renewable Energy Laboratory[13] has begun toelucidate potential chemical mechanisms active during the IH2 process. This work has
shown that in the gas-phase thermal decomposition of benzaldehyde, hydrogen atoms
can mediate the selective removal of the aldehyde group from the ring. Thishydropyrolysis process is likely to yield similar results for other substituted aromatic
systems that can function as analogs for lignin monomers. Further exploration into
these mechanisms will help to expand the understanding of and improve the future
efficiency of the biomass conversion.

10

If and when lignin based liquid fuels become competitive with fossil fuel derived
gasoline, the crucial questions to answer will be how do these fuels combust, what
products are formed, and how does the combustion differ from standard gasoline
combustion?

These questions are a core motivation for studying cyclic aromatic

molecules in pyrolysis environments, which constitutes the bulk of the present work.

IV. Outline
This thesis is organized essentially in chronological order with respect to when
the research was performed. Chapter 2 provides an overview of the experimental
techniques used. Chapters 3 and 4 summarize work that was done in the group of Dr.
David Nesbitt. Rovibrational spectra were analyzed for the simplest aromatic radical,
phenyl radical (C6H5). A well-assigned IR spectrum of this radical will be crucial in
identifying phenyl radical in combustion systems and the nearly 200 individuallyassigned rovibrational features will hopefully be used to confirm mechanisms like
HACA and others in real combustion environments.
Chapters 5 – 8 summarize the work done in the group of Dr. Barney Ellison,
where we worked closely with the combustion modeling community to study pyrolysis
processes directly. By selecting important combustion precursors and intermediates tothermally decompose, we can establish pyrolysis pathways and help simplify radicallyunderdetermined systems of reactions that are currently challenging for combustion
modelers. Chapter 5 details the efforts made to isolate the role hydrogen atoms canplay in removing substituents from aromatic molecules in the gas-phase. This work
quickly motivated a detailed study of one of the most common intermediates in alkylsubstituted aromatic species decomposition, benzyl radical, summarized in Chapter 6.

11

Benzyl radicals are formed during the pyrolysis of any species with the formula
C6R5CR3, where R can be carbon or hydrogen atoms. Thermal decomposition of suchmolecules will form C6R5CR2, which exhibits large resonance stabilization due to theradical electron delocalizing into the ring structure. The resonance stabilization of thesimplest benzyl radical (C6H5CH2) has been calculated by comparing experimental heatsof formation and is found to be 11.4 ± 1 kcal/mol, as summarized in Chapter 7.
The study of benzyl radical proved to be much more complicated than had been
anticipated and eventually spurred the study of two other C7H7 isomers, tropyl radical
and 2,5-norbornadienyl radical. The three C7H7 radicals are shown in Fig. 1.5.

HC

H
C
C

HC

CH2•

CH

H
C
HC

H

CH

•

HC

HC
HC

CH
CH

C
H

C
H

benzyl radical~
X 2B1

tropyl radical~
X 2E2"

HC
HC

HH

CH 2
C•
•
CH
C

C

C

CH

CH

CH
H

norbornadienyl+radicalnorboradienyl radical~
X 2B1

Fig. 1.5. Chemical structure of three isomers of C7H7, including their common
names and the symmetry of their electronic ground states.
Chapter 7 provides a detailed explanation of the decomposition of tropyl radical andpresents more findings related to the decomposition of benzyl radical that werediscovered after the publication of our work on benzyl radical.[14]Chapter 8 details the recent work done in collaboration with Dr. William Petersand David Couch from the research group of Dr. Margaret Murnane and Dr. Henry

12

Kapteyn. We have coupled our pyrolysis source to a tabletop extreme ultraviolet (XUV)
laser and advanced ion imaging detector and have demonstrated a powerful new
method to record PIMS spectra. Chapter 8 includes a description of this experimentalong with preliminary data that demonstrate the utility of this combined experimental
approach.

13

References for Chapter 1[1] T. C. Zhang, L. D. Zhang, X. Hong, K. W. Zhang, F. Qi, C. K. Law, T. H. Ye, P. H.
Zhao and Y. L. Chen. An experimental and theoretical study of toluene pyrolysis
with tunable synchrotron VUV photoionization and molecular-beam mass
spectrometry. Combustion and Flame 2009, 156, 2071-2083.
[2] M. Frenklach, D. W. Clary, W. C. Gardiner and S. E. Stein. Twentieth Symposium(International) on CombustionDetailed kinetic modeling of soot formation inshock-tube pyrolysis of acetylene. Symposium (International) on Combustion 1984,
20, 887-901.
[3] A. L. C. Lima, J. W. Farrington and C. M. Reddy. Combustion-Derived Polycyclic
Aromatic Hydrocarbons in the Environment—A Review. Environmental Forensics
2005, 6, 109-131.
[4] H. Richter and J. B. Howard. Formation of polycyclic aromatic hydrocarbons andtheir growth to soot - a review of chemical reaction pathways. Progress in Energy
and Combustion Science 2000, 26, 565-608.
[5] W. A. Goddard, T. H. Dunning, W. J. Hunt and P. J. Hay. Generalized valence bonddescription of bonding in low-lying states of molecules. Accounts of Chemical
Research 1973, 6, 368-376.
[6] K. Brezinsky. The high-temperature oxidation of aromatic hydrocarbons. Progress in
Energy and Combustion Science 1986, 12, 1-24.
[7] J. D. Savee, T. M. Selby, O. Welz, C. A. Taatjes and D. L. Osborn. Time- and IsomerResolved Measurements of Sequential Addition of Acetylene to the Propargyl
Radical. The Journal of Physical Chemistry Letters 2015, 4153-4158.
[8] S. A. Skeen, H. A. Michelsen, K. R. Wilson, D. M. Popolan, A. Violi and N. Hansen.
Near-threshold photoionization mass spectra of combustion-generated highmolecular-weight soot precursors. Journal of Aerosol Science 2013, 58, 86-102.
[9] J. A. Miller, M. J. Pilling and J. Troe. Unravelling combustion mechanisms through aquantitative understanding of elementary reactions. Proceedings of the Combustion
Institute 2005, 30, 43-88.

14[10] C. Heitner, D. Dimmel and J. Schmidt, Lignin and lignans: advances in chemistry, CRC
press, 2010, p. 1-10.
[11] D. Robichaud, M. Nimlos and G. B. Ellison in Pyrolysis Mechanisms of Lignin Model
Compounds Using a Heated Micro-Reactor, Eds.: M. Schlaf and Z. C. Zhang,
Springer Singapore, 2016, pp. 145-171.
[12] T. L. Marker, L. G. Felix, M. B. Linck and M. J. Roberts. Integrated Hydropyrolysis
and Hydroconversion (IH2) for the Direct Production of Gasoline and Diesel
Fuels or Blending Components from Biomass, Part 1: Proof of Principle Testing.
Environmental Progress & Sustainable Energy 2012, 31, 191-199.
[13] A. K. Vasiliou, J. H. Kim, T. K. Ormond, K. M. Piech, K. N. Urness, A. M. Scheer, D.
J. Robichaud, C. Mukarakate, M. R. Nimlos, J. W. Daily, Q. Guan, H.-H.
Carstensen and G. B. Ellison. Biomass Pyrolysis: Thermal Decomposition
Mechanisms of Furfural and Benzaldehyde Journal of Chemical Physics 2013, 139,
104310.
[14] G. T. Buckingham, T. K. Ormond, J. P. Porterfield, P. Hemberger, O. Kostko, M.
Ahmed, D. J. Robichaud, M. R. Nimlos, J. W. Daily and G. B. Ellison. The
thermal decomposition of the benzyl radical in a heated micro-reactor. I.
Experimental findings. The Journal of Chemical Physics 2015, 142, 044307.

15

16

Chapter 2
Experimental Methods

I. Introduction
The work of my thesis was done in two stages, first in the group of Dr. David J.
Nesbitt, where I performed high-resolution absorption spectroscopy on unstableradicals and ion. The second phase of my degree took place in the labs of Dr. G. Barney
Ellison, where I studied the unimolecular decomposition of combustion relevantintermediates. The work performed with Dr. Nesbitt required a specialized molecularsource to produce high densities of non-interacting gas-phase radicals and ions coupled
to a stable, tunable infrared (IR) light source that allowed for careful scanning of the
C-H, N-H, and O-H stretching regions of the vibrational spectrum. The instrument
resolution of roughly 60 MHz allowed for assignment of individual rovibrational lines
and in a molecule like phenyl radical, for example, greater than 100 individualtransitions can be assigned.

In the Ellison lab, we were able to heat gas-phaseprecursors and directly detect fragments of the nascent decomposition pathways that
occur within the first roughly 100 microseconds of pyrolysis. By combining multiple
different precursors, unique isotopomers, and multiple complementary detection
techniques, we were able to make important conclusions about how fundamental
molecules and radicals decompose when heated to a range of temperatures.
Given the broad array of techniques used, it will be necessary to provide a moredetailed experimental description for each project in the appropriate chapters later on.

17

This section will serve to describe general technical features that span many projects
and familiarize the reader with the types of experiments performed.

II. High Resolution Absorption Spectroscopy
The ability to obtain well resolved rovibrational spectra of phenyl radical(C6H5 X̃ 2A1) in the gas phase relies largely on several experimental advantages: a) highradical densities per quantum state accessible with low temperature discharge
expansions, b) sub-Doppler compression of velocity broadening in the slit jet, c) longabsorption path geometry and d) a precisely tunable frequency stabilized IR lightsource. The experimental apparatus has been described in previous papers[1-2] thus onlya brief summary of details relevant to the present work will be presented here.

A. Difference Frequency Infrared Production
Infrared light is produced by spatially overlapping two visible laser beams in atemperature controlled, periodically poled lithium niobate crystal, which results indifference frequency generation of tunable light in the 2.5-4 !m region. The two visible
light sources are i) a fixed frequency argon ion (Ar+) laser operating at 514 nm (19429.88
cm-1) and ii) a tunable ring dye laser (R6G dye) between 17500 to 16000 cm-1. The Ar+
laser is locked via servo-loop to the transmission fringes of an external Fabry-Perotcavity (FSR = 250 MHz), which in turn is locked to a polarization-stabilized heliumneon (HeNe) laser to achieve long-term frequency stability over multiple days.
Scanning progress of the dye laser is measured with respect to fringes on the same
ultrastable

Fabry-Perotcavity,yielding

comparable

precisionforfrequency

measurement on the difference frequency infrared (IR) light. Absolute frequencies are18

determined by a reference absorption line of jet cooled methane (R(4), 3067.3000 cm-1)
observed under identical slit jet conditions,[3] providing a calibrated IR frequency axis
with typical rms reproducibility of 8 MHz (< 0.0003 cm-1) for the transitions reported.
The IR light is divided into two beams by a 50:50 beam splitter, with one portion
measured directly on a liquid nitrogen cooled InSb reference detector. The remainder of
the IR light is then directed through the slit jet discharge/Herriott cell vacuum
chamber, extracted after 16 passes and re-collimated onto a matched liquid nitrogencooled InSb signal detector. Noise between signal and reference detector is actively
balanced using high bandwidth (1 MHz) home-built active subtraction electronics, withthe weak absorption signals at the 50 kHz discharge modulation frequency obtained via
lock-in detection to provide additional signal-to-noise enhancement.

These lock-in

detection signals are then further analyzed in the time domain by gated integration anddifferential baseline correction before, during and after the slit jet pulse, with all
resulting data and frequency diagnostics stored in a computer for later analysis.

B. Slit Discharge Radical and Ion Production
The spectroscopic target species are produced via C-X bond fragmentation of
halogenated precursors in a slit supersonic jet expansion, yielding typical signal-tonoise on the strongest absorption peaks of roughly 50:1. The mechanism for C-X bondfission is presumed to be electron dissociative attachment to form the radical-

center + X , which, favors use of halogenated precursor species with weakest C-X bondstrengths. Indeed, phenyl radical, for example, can be efficiently synthesized in the
discharge

withchlorobenzene(C6H5Cl),

bromobenzene(C6H5Br),and

iodobenzene (C6H5I), with signal intensities greatly limited by reduced room19

temperature vapor pressures of the latter. As a compromise between vapor pressure,control, and formation efficiency, the carrier gas Ne70 (70 % neon, 30 % helium) is
bubbled through liquid-phase chlorobenzene cooled at 0° C, with the gas mixture
further diluted with Ne70 to produce 0.5 % C6H5Cl gas samples in the stagnation region
at 280 Torr total pressure. The gas is supersonically expanded through a JILA-built
solenoid-driven slit valve capable of generating intense pulses with tunable duration(1000 !s, < 50 !s rise time) that exit through a 4 cm long, 300 !m wide slit into a
vacuum chamber. This supersonic expansion is triggered at a repetition rate of 19 Hz,
with pressure in the vacuum chamber maintained less than 50 mTorr by a 560
liter/second Roots blower (Leybold WS200).

During each gas pulse, a 50 kHz

modulated square wave voltage (around 500 V) is applied across an insulated 1 mmregion located immediately prior to the expansion orifice, as illustrated in Fig. 2.1.[1]This voltage is sufficient to produce a glow discharge with 280 mA peak current
through the sample and carrier gas mixture, which causes dissociative electron
attachment and homolytic cleavage of the C-X bonds to produce the desired radicals.

20

Fig. 2.1. Cartoon diagram of pulsed slit jet discharge. During the 1000 !s gas
pulse, the carrier gas and precursor flow into the discharge region where a square wave
modulated voltage (−500 V) is applied with a 50 kHz frequency. The electric field
induced glow discharge results in efficient electron dissociative attachment of the weakcarbon−halogen bond in the precursor. The products in the expansion are rapidly
cooled to 11(1) K in the slit supersonic expansion.

The slit jet expansion rapidly cools the phenyl radical down to roughly 11(1) K,
where infrared light 5 mm downstream from the nozzle orifice allows direct absorption
by the newly formed radical molecules. Near shot noise limited absorption sensitivities
on the order of 0.002 % (A = 2 x 10-5) in a 10 kHz bandwidth are made possible by fast
servo loop subtraction of signal and reference detector technical common mode noiseon the IR laser. The signal is further augmented by increasing the absorption path
length with a 16-pass Herriott cell, which is aligned directly below the slit orifice. This21

alignment minimizes molecular velocity parallel to the beam path, lowers the Doppler
width of the measured absorption peaks by an additional 5-10 fold (in Ne70), andthereby increases the peak absorption cross section by an equivalent 5-10 fold. In
combination with the much slower (1/r vs. 1/r2) density drop off in a slit vs. pinhole jet,this yields a net sensitivity enhancement for direct IR absorption on high reactive
radicals by some 3-4 orders of magnitude. Combined with typical[4] integrated
absorption strengths (5-20 km/mol) for hydrocarbon species such as phenyl radical,
these absorption sensitivities translates into minimum detectable number densities of2 x 108 cm-3 quantum state-1 in a detection region 5 mm downstream of the slit jet
orifice.

III. Pyrolysis of Gas-Phase Biofuel Intermediates Using a Heated Micro-Reactor
Gas-phase samples of hydrocarbons and oxygenated hydrocarbons have been
pyrolyzed in a heated micro-reactor and a set of detection techniques has been
employed to detect the products. By combining multiple techniques with very different
detection mechanisms, we are able to identify all products that exit the nozzle,
including radicals, metastables, and atoms. The reactor itself will be described first,
including recent efforts to computationally model the flow of gasses through the reactor
at elevated temperatures and with large pressure gradients. After the exit of the reactor,
the detection schemes used include: i) photoionization mass spectrometry using the 9th
harmonic of an Nd:YAG laser and time-of-flight (TOF) detection, ii) photoionization
mass spectrometry using synchrotron radiation and TOF detection, iii) photoionization
mass spectrometry using tunable synchrotron radiation and imaging photoelectron
photoion coincidence spectroscopy (iPEPICO), iv) photoionization mass spectrometry

22

using table-top high harmonic generation and photoelectron photoion coincidence(PEPICO) spectroscopy, v) matrix-isolation IR absorption spectroscopy, and vi) laserinduced fluorescence (LIF).

A. Pyrolysis in a Heated Micro-Reactor
To study pyrolysis, shock tubes have long been an attractive method as they
allow for controlled conditions and myriad detection schemes.

Due to their low

repetition rates, however, it is challenging to study the dilute initial products and only
stable products can be detected. An excellent complement to this technique is a heatedmicro-reactor often called a Chen Nozzle, which was developed in the lab of Dr. Peter
Chen.[5-7]Initially made of silicon carbide (SiC), these reactors have 1 mm innerdiameter (I.D.), 2 mm outer diameter (O.D.) and can be cut to any length but are often
between 1 and 3 cm. SiC is used because its electrical resistance is such that it can beconveniently and controllably heated to temperatures of up to 2000 K with simple
variable transformers and/or low cost proportional-integral-derivative (PID) servo
controllers. One of the main advantages of micro-reactors is that for a given set of flow
conditions, it is possible to thermally decompose samples over short times scales so that
only the initial decomposition is observed.
In recent years, the micro-reactor has been used to study the thermal
decomposition of fuels[8] by coupling to a matrix isolation spectroscopy scheme.[9]Initially used primarily as a source of organic radicals[10-13] for IR spectroscopic analysis,
the value of the reactor as a means to study pyrolysis quickly emerged and since has
been used more as a means to study thermal decomposition.[14-16] A shortcoming of the
micro-reactor set-up eventually emerged that the temperature and pressure inside the23reactor are poorly characterized. Although the wall temperature can be measured
directly with Type-C thermocouples and the upstream/downstream pressure can bemonitored, these two variables cannot be easily solved for inside the reactor. In orderto provide useful information to reaction modelers, the temperature and pressure must
be known within the reactor so that the products detected can be entered into
complicated kinetic schemes that are the basis of combustion researchers looking tobetter characterize complex chemical environments.
Collaboration with Dr. John W. Daily of the Mechanical Engineering Department
has provided significant progress towards being able to directly predict the temperatureand pressure at all points within the reactor. Using computational fluid dynamics(CFD) simulations of inert carrier gasses like He, Ar, or Ne, it was possible to carry out
simulations of these two important variables (gas temperature and pressure) as afunction of wall temperature, pressure differential, and mass flow rate. The findings
are summarized in a 2014 review paper,[17] which establishes a comprehensive
investigation of fluid flow through a heated micro-reactor. In order to validate thepredictions of these simulations, a recent effort has been made to design newexperiments that will make accessible the gas inside a heated reactor. To monitor thetemperature of the gas inside the reactor, a novel heater was designed that canradiatively heat a fused silica tube (1 mm I.D., 2 mm O.D.) without obscuring opticalaccess to the interior.[18]24Fig. 2.2. Cut away view of the clover heater. Gas flows through the fused silica
tube (1) in the center, while radiation from the four heater elements (2), of which three
are shown here, is concentrated onto the micro-reactor by the elliptical mirrors (3). To
the left, one parabolic mirror (4) from the custom IR thermometer illustrates how
emission from the micro-reactor can be collected via slits (5) in between the ellipticalmirrors. The inset schematic (upper right) depicts the simultaneous radiative heating of(red) and optical access to (purple) the micro-reactor (blue).

The heater, referred to as a “clover heater” due to its shape, is shown[18] in Fig. 2.2.
Fused silica allows for ultraviolet (UV) spectroscopy to be done on the gas inside thereactor and as a proof of concept, the laser induced fluorescence (LIF) spectrum of
dilute (5 %) nitric oxide (NO) in argon can be recorded on gas flowing through theheated reactor.

These first experiments on NO provide a rotational temperature

measurement that is in good agreement with the computational models generated by
the CFD simulations.

25Each experiment performed has slightly different requirements in terms ofpumping speed, pumping mechanisms, and detection mechanisms. For this reason, it
has been necessary to use many different configurations of carrier gas, flow condition(pulsed and continuous CW), pressures, and reactor geometries. These conditions are
summarized in Table 2.1.

26

Experiment

CarrierGas

Flow

Lights Source

Backing
Pressure

OutletPressure

ReactorGeometry

1500 Torr

2 x 10-6
Torr

I.D. 1 mm
HeatedLength: 15 mm

Photoionization
Mass Spectrometryin Colorado

HeliumPulsed118.2 nm(10.487 eV)
Nd:YAG
Laser

Photoionization
Mass Spectrometryat the ALSa

HeliumContinuous

TunableSynchrotron

50 Torr2 x 10-5
Torr

I.D. 0.6 mm
HeatedLength: 10 mm

Photoionization
Mass Spectrometryat the SLSb

Argon

Continuous

TunableSynchrotron

50 Torr

5 x 10-6
Torr

I.D. 1 mm
HeatedLength: 15 mm

800 – 1500
Torr

1 x 10-4
Torr

I.D. 1mm
HeatedLength: 15 mm

1000 Torr

2 x 10-6
Torr

I.D. 1 mm
HeatedLength: 10 mm

Photoionization
Mass Spectrometry
with Tabletop
VUVc + PEPICOd

HeliumContinuous

Ti:sapphire
High
Harmonic
Generation

Matrix Isolation IR
Spectroscopy inColorado

Neon

Pulsednone

Table 2.1. Summary of the conditions for each experiment performed withheated micro-reactor.
aAdvanced Light Sourceb
Swiss Light SourcecVacuum ultravioletdPhotoelectron photoion coincidence

B. Photoionization Mass Spectrometry
One of the most useful experimental techniques for identifying the pyrolysis
fragments that exit a heated reactor is mass spectrometry. Within a relatively shortamount of time, the mass-to-charge ratio (m/z) of nearly all species can be identified ata wide range of temperatures. The only limit one regarding molecule detection is theenergy of the ionization source employed.

In Colorado, at both the University ofColorado Boulder and the National Renewable Energy Laboratory (NREL), we utilize #= 118.2 nm (Ephoton = 10.487 eV) as the ionization source. This frequency of light is ideal27

for ionizing pyrolysis products because it is sufficiently energetic to ionize nearly allorganic species except for a short list of exceptions. The molecules with ionizationenergy (IE) greater than 10.487 eV cannot be observed and this set of molecules includes
H2O, CO2, CO, HCC, CH4, HC≡CH, and CH2=CH2. Most other organic molecules willlose an electron upon absorbing a # = 118.2 nm photon and can then be detected using
electrostatic ion optic techniques. The disadvantages of the 9th harmonic of an Nd:YAG
in our labs in Colorado are: i) the repetition rate is low, either 10 Hz or 30 Hz; ii)
fragments made that have IEs greater than 10.487 eV cannot be detected and thus
mechanisms forming such fragments are challenging to confirm; iii) some moleculesthat have IEs lower than 10.487 eV will undergo dissociative ionization; in this process acation is formed with enough excess energy that it can fragment to a different cationand one (or more) neutral species; and iv) many isomers may exist at a given m/z so afeature in a PIMS spectrum does not specifically identify what species is actuallypresent. To cope with these shortcomings, our group has multiple complementary
PIMS techniques using different ionization sources and different detections schemes
such that all four shortcomings can be overcome. Each PIMS experimental techniquewill be described separately to highlight the differences between them.

i. Photoionization Mass Spectrometry Using the 9th Harmonic of an Nd:YAG Laser
and Reflectron Time-of-Flight Detection
PIMS with a pulsed micro-reactor[9] combines the 9th harmonic of a Nd:YAG laser(118.2 nm or 10.487 eV). The laser is synchronized to a Parker general pulsed valve
backed by a helium-hydrocarbon mixture. The pulsed valve operates at 10 Hz or 30 Hzwith an opening time of 1 msec. A 1 mm skimmer is placed about 1 cm from the exit of28

the SiC reactor to select only the component of the expansion traveling in thelongitudinal direction with respect to the reactor tube. The pressure in the chamber atthe exit of the reactor is 10-6 Torr.
The internally frequency-tripled output (# = 355 nm) of a Nd:YAG laser (SpectraPhysics Quanta-Ray Lab 130) is directed into a cell that is pressurized to 150 Torr with a
10:1 argon:xenon mixture. This mixture is optimized to promote sum-frequency
generation that creates # = 118.2 nm VUV photons. Generation of 118.2 nm photons by
Xe/Ar tripling cells has been studied previously.[19] Use of 30 mJ pulse-1 power at 355
nm from the YAG laser and the established tripling efficiency of 1 x 10-5 in pure xenon[2021]

as well as the transmission efficiency of the MgF2 lens at the exit of the cell indicatesthat we should produce roughly 30 nJ pulse-1 at 118.2 nm light. This VUV laser light isdirected into a vacuum chamber maintained at 10-7 Torr by a 1200 L/s Pfieffer TPU 1201
P turbomolecular pump and is then intersected with the skimmed output of thesupersonic expansion. After ionization the molecules in the beam are accelerated into a
Jordan reflectron time of flight spectrometer with an MCP detector producing PIMS
spectra at 10.487 eV. The pressure in the flight tube is maintained at around 10-7 Torr bya smaller turbomolecular pump.

ii. Photoionization Mass Spectrometry Using Synchrotron Radiation and Reflectron
Time-of-Flight Detection
A similar experimental setup and detection scheme is used at the Advanced
Light Source (ALS) at Lawrence Berkeley National Labs (LBNL), Chemical Dynamics
Beamline endstation 9.0.2. Using the synchrotron light for PIMS adds an experimental

29degree of freedom: tunable VUV radiation.[22] The energy of the synchrotron photons at
Beamline 9.0.2 can be tuned between 7.4 eV and 30 eV, sufficiently high to ionize all
species produced in the heated reactor. The synchrotron has a repetition rate of 500
MHz and so data acquisition is much more rapid than in the laser based techniques
performed in Colorado. By measuring ion current at a single mass-to-charge ratio(m/z) while varying the photon energy it is possible to record photoionization spectra,
which can be used to identify individual isomers at a given m/z by observing theionization threshold. Signal averaging is used to increase the signal-to-noise ratio andtypical spectra are the composite of between 5 x 104 and 2 x 105 scans. To control theflow of gas through the reactor, an MKS mass flow controller is used along with aslightly smaller (I.D. = 0.6 mm) SiC reactor. Typical flow conditions include setting theflow controller to 200 standard cm3 min-1 (SCCM) backed by 5 atm of He. This yields apressure of 100 Torr between the mass flow controller and entrance of the reactor androughly 10-4 Torr at the exit of the nozzle. With a 1 mm skimmer 1 to 2 cm downstream,
the pressure in the ionization chamber can be maintained at roughly 5 x 10-6 Torr by alarge turbomolecular pump.

iii. Photoionization Mass Spectrometry Using Synchrotron Radiation and Imaging
Photoelectron Photoion Coincidence Spectroscopy
The Swiss Light Source (SLS) at the Paul Scherrer Institute in Villigen
Switzerland is able to provide photons at even lower energy than the ALS.

TheCoincidence Endstation at the SLS is readily able to provide photons at energies as low
as 6 V, which is important when identifying isomers with small ionization energies likebenzyl radical[23] (IE = 7.2487 ± 0.0006 eV) and cycloheptatrienyl radical[24] (IE = 6.221 ±30

0.006 eV). In addition, the experiment at the SLS has the added advantage of animaging photoelectron photoion coincidence (iPEPICO) detector, which can select only
the ion counts that are detected in coincidence with electrons that are ejected with close
to zero kinetic energy.
Because the pumping scheme makes use of cryogenic pumps, it is necessary forthe carrier gas in these experiments to be argon. A temperature-controlled sample
bubbler was used to house the liquid samples and was either heated or cooleddepending on the sample vapor pressure and desired concentration in the gas flow. A
backing pressure of approximately 50 Torr of argon was passed through the bubbler.
The ratio of the vapor pressure at the sample temperature to the argon backing pressure
was used to approximate the concentration of the target species in the argon beam. Thisgas mixture was then expanded through a 150 !m pinhole directly into the SiC reactor(1 mm I.D.). A 2 mm skimmer was placed about 2-3 cm from the exit of the SiC reactor
to select only the component of the beam traveling in the longitudinal direction withrespect to the reactor tube. The flow rate is calculated to be less than 20 SCCM based on
the backing pressure of the argon through the bubbler. This flow is slow enough thatthe expansion at the exit of the reactor is not choked enough to be supersonic and isinstead an effusive flow.
The continuous flow enters the source chamber, which is evacuated by a 5000 L s1

Leybold COOLVAC 5000 CL cryogenic pump and a 1250 L s-1 Pfeiffer TPH 1201 UP

turbomolecular pump. The ionization chamber is pumped by a 1500 L s-1 Leybold
COOLVAC 1500 CL cryopump and a 500 L s-1 Pfeiffer TMH 521 YP turbomolecularpump. The total high pumping capacity of 8250 L s-1 is backed by an oil free foreline
system to minimize background and contamination in the system.

31

At the photon energies used here, a magnesium fluoride (MgF2) lens is included
to attenuate higher harmonics that would yield spurious ion counts on the detector.
Unfortunately the MgF2 lens has decreasing transmission at lower photon energies so
the photon flux below 7 eV is small compared to the higher energy portion of thespectrum. For this reason, PIMS spectra with photon energies of 7 eV yielded smaller
ion counts and thus required longer collection times, which made PIEs at these energies
too costly to perform.

iv. Photoionization Mass Spectrometry using a Tabletop Vacuum Ultraviolet Laserand Photoelectron Photoion Coincidence Spectroscopy
In order to combat issues of dissociative ionization, isomer ambiguity, andinability to ionize molecules with large ionization energies, we have coupled the microreactor to a laser system that generates harmonics of 800 nm Ti:sapphire laser light. The
laser system was built in the group of Dr. Margaret Murnane and Dr. Henry Kapteyn
and is modeled after the KMLabs Inc. DragonTM system. The laser system used here isable to perform with 10 kHz repetition rate, which is important due to the low countrates required for coincidence detection.

After amplifying the 30 fs output of a

Ti:sapphire mode locked oscillator, the power of the IR light is on the order of 20 W.
This is focused into a waveguide made of a fused silica capillary (150 !m I.D. x 2.5 cmlong) in which Xe is flowed, generating odd harmonics in the range of the 3rd harmonic(roughly 4.8 eV) up through the 11th harmonic (roughly 17.6 eV); higher harmonics up
to at the 17th harmonic are possible. The molecules wit the highest IE relevant to ourexperiments[25] is H2 (IE = 15.4 eV) so only the 9th harmonic and lower are necessary
here. By filtering of higher harmonics with different rare gas filter schemes, harmonics

32

can be selected by making only minor alterations to the system. The laser system was
not modified for these experiments and a more detailed description can be found in aprevious publication and citations therein.[26]The micro-reactor assembly in this experiment can be used without a pulse valve
since the source chamber is pumped by a large turbomolecular pump (1650 L s-1
Leybold MAG W 2010 C). Roughly 1 cm downstream of the reactor exit is a 300 !mskimmer that separates the source chamber from the ionization chamber and insuresthat a narrow molecular beam reaches the laser interaction region. In order to keep thepressure low enough for the detector, a 50 !m orifice was brazed into the stainless steel
tubing just upstream of the reactor. This allows for a backing pressure of 1 to 2 atm,
downstream pressures in the source chamber of 10-4 Torr, and ionization chamberpressures of 10-7 Torr during continuous flow of He. The reactor assembly, shown in
Fig. 2.3, has a few new design features. A small stainless steel disc is included that has
electrical feedthroughs installed to help stabilize the delicate (0.005”) wires leading to
the Type-C thermocouple. To limit the flow and maintain sufficiently low pressure for
the PEPICO detector, a 50 !m orifice was brazed onto the metal just upstream of thereactor.

33

Fig. 2.3. Continuous flow heated micro-reactor assembly (inset) and larger
cutaway diagram. The 50 !m orifice limits the gas flow so that a pulsed valve is notneeded to maintain appropriate upstream and downstream pressures. New design
features are highlighted that help stabilize the assembly to prevent misalignment andincrease thermocouple lifetime.
The detection mechanism in this experiment also adds important experimental
information. Rather than a standard TOF configuration, the design of the ion optics in
the interaction chamber allows us to detect both the cations and electrons formed froma single laser pulse. The count rate is intentionally attenuated so that only 1 in 10 laser
pulses leads to a photoionization event, which would be challenging to maintainwithout the high repetition rate of the laser system (10 kHz). For a given ionization
event, it is possible to correlate individual cations and their corresponding
photoelectrons.

The detectors for both the cations and the electrons are position

sensitive so given a sufficiently small ionization volume, the distance traveled in the
directions transverse to the flight axis can be used to calculate the kinetic energy of thecharged particles. By plotting only the electrons detected in coincidence with a given

34

cation m/z, a photoelectron spectrum (PE) can be recorded. Although the instrument
resolution is limited to about 0.75 eV, features in these photoelectron spectra can be
identified and used as a diagnostic of given isomers. As long as two isomers have IEs
that differ by more than roughly 1 eV, this detection scheme can be used to identify
which isomers are present in a given m/z. Additionally, if a molecule in the molecularbeam dissociatively ionizes, this detector can be used to identify the fragment mass as a
product of dissociative ionization. The photoelectrons detected in coincidence with thedissociative ionization fragment will correspond to the PE spectrum of the parent
molecule and not the fragment. As long as the PE spectra (or simply the IE) of theparent and the fragment are both known, peaks resulting from dissociative ionization
can be removed from the recorded PIMS spectra during post-processing. These aremajor advantages over the Nd:YAG based TOF PIMS that has been the workhorse ofthis experimental technique in the past.

C. Matrix Isolation Infrared Absorption Spectroscopy
Since all polyatomic molecules possess diagnostic vibrational spectra, IR
spectroscopy is an excellent technique for identifying molecular species and serves as
valuable complement to PIMS experiments. The matrix IR spectrometer has been
described before.[27-28] Briefly, a 1 mm I.D. SiC reactor is coupled to a pulsed valve
assembly very similar to the PIMS system described above. The upstream pressure is
between 800 and 1000 Torr and with the pulsed valve operating with 400 µs opening
time at 15 Hz, the pressure at the reactor exit is typically 10-6 Torr. The pressure in thischamber is maintained with a small turbomolecular pump. The output of the reactor isaimed at a cryogenically cooled CsI window 4 cm downstream that is maintained at 5 K

35

by a helium compressor. The carrier gas for this experiment is neon, which condenses
into a solid matrix upon colliding with the 5 K window. A standard matrix is formed byflowing 200 Torr of the sample mixture from a 3 L gas manifold with about one hourdeposition time. After dosing is complete, the reactor assembly is rotated 90º out of theway and the CsI window is lowered into the beampath of a commercial Fouriertransform IR (FTIR) spectrometer (Nicolet 6700). The MCT-A detector has a spectral
range of 4000 to 600 cm-1 and is operated with 0.25 cm-1 resolution with typical spectra
constructed by averaging 500 scans. Although the neon matrix is chemically inert, it
does affect the vibrational spectra of entrained molecules. Average matrix shifts are on
the order of 1 to 10 cm-1 from the unperturbed gas-phase values.

D. Sample Preparation
In all experiments using the micro-reactor, sample preparation is done in one of
two ways.

If the sample is volatile enough, a tank of pre-mixed gasses is made andfilled to between 1 and 5 atm. If the sample does not have sufficient vapor pressure atroom temperature, the solid or liquid sample is held immediately upstream of thereactor in a temperature-controlled vial and pure carrier gas is flowed over it. Thismethod assumes the vapor pressure (at the elevated temperature) is continuously
reached while the gas is flowing. Whether the target molecule is a stable precursor oran unstable intermediate, a dilute mixture of gas is entrained in an inert carrier gas,either He, Ne, or Ar. Bimolecular chemistry can usually be avoided if the precursor
pressure is less than or equal to 0.1 % of the total pressure. Prepared mixtures range
from 800 Torr to 2000 Torr so sample pressures of 0.8 Torr to 2 Torr are often
sufficiently dilute. If a more dilute sample is required (e.g. 10 times more dilute), a pre-36

made sample tank (pressure = P0) can be evacuated to pressure = 0.1*P0 and thenrefilled to P0 with carrier gas. This method can be repeated as long as sufficient mixing
time is allotted. By introducing carrier gas in quick bursts, this diffusion time can beshortened.
When the desired precursor does not have sufficient vapor pressure for a premade tank, the sample must be heated to raise the vapor pressure into the range of
0.1 %. To prevent condensation of the sample on gas-manifold surfaces, a customdesigned sample probe was built that allows the solid/liquid to be placed directly
behind the pulse valve and/or reactor. The sample probe, shown in Fig. 2.4, is smallenough to be inserted into the 3/8” or 1/4” I.D. stainless steel tubing that leads to thepulse valve.

Gas Inlet!

Gas Outlet!

Thermocouple !
Cartridge
Heater !

O-ring!
Sample Vial
Chamber!

Fig. 2.4. Sample probe for heating low-vapor pressure solids and liquids. Graycolored material is stainless steel and the bronze color is copper, which provides goodheat transfer. The O-ring near the end makes a seal with the tubing so that gas mustflow into the inlet, around the sample vial, and then through the gas outlet, which
ensures good sample pickup into the carrier gas flow.
The sample probe has a type-K thermocouple and heating element installed in
the metal that jackets the sample vial, which allows for careful temperature control of37

the sample using PID temperature controllers. To prevent condensation in the pulse
valve and neighboring metal surfaces, temperature controlled water (or other solvents)
can be circulated through custom-designed water jackets that are inside the vacuum
chamber. For difficult samples with small vapor pressures (e.g. vanillin), the entire
assembly inside the vacuum chamber including the pulse valve, reactor, and gas lines
was maintained at roughly 98° C, while the solid sample of vanillin was held at
between 90° and 120° C. A more robust heating system would make use of a circulating
fluid with a higher boiling point than water to allow even higher metal wall
temperatures.

38

References for Chapter 2[1] S. Davis, M. Farnik, D. Uy and D. J. Nesbitt. Concentration modulation spectroscopy
with a pulsed slit supersonic discharge expansion source. Chemical Physics Letters
2001, 344, 23-30.
[2] F. Dong, S. Davis and D. J. Nesbitt. Slit discharge IR spectroscopy of a jet-cooled
cyclopropyl radical: Structure and intramolecular tunneling dynamics. Journal of
Physical Chemistry A 2006, 110, 3059-3070.
[3] A. S. Pine. High-resolution methane nu-3-band spectra using a stabilized tunabledifference-frequency laser system. Journal of the Optical Society of America 1976, 66,
97-108.
[4] M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, V.
Zakrzewski, J. Montgomery Jr, R. E. Stratmann and J. Burant in Gaussian 98,
revision a. 7; gaussian, Vol. 12 Inc., Pittsburgh, PA, 1998.
[5] P. Chen, S. D. Colson, W. A. Chupka and J. A. Berson. Flash Pyrolytic Production of
Rotationally Cold Free-Radicals in a Supersonic Jet - Resonant Multiphoton
Spectrum of the 3P 2A2'' Origin Band of CH3. Journal of Physical Chemistry 1986, 90,
2319-2321.
[6] P. Chen, J. B. Pallix, W. A. Chupka and S. D. Colson. Resonant multiphoton
ionization spectrum and electronic-structure of CH radical - new states andassignments above 50,000 cm-1. Journal of Chemical Physics 1987, 86, 516-521.
[7] D. W. Kohn, H. Clauberg and P. Chen. Flash Pyrolysis Nozzle for Generation of
Radicals in a Supersonic Jet Expansion. Review of Scientific Instruments 1992, 63,
4003-4005.
[8] A. L. Brown, D. C. Dayton, M. R. Nimlos and J. W. Daily. Design andcharacterization of an entrained flow reactor for the study of biomass pyrolysis
chemistry at high heating rates. Energy and Fuels 2001, 15, 1276-1285.
[9] X. Zhang, A. V. Friderichsen, S. Nandi, G. B. Ellison, D. E. David, J. T. McKinnon, T.
G. Lindeman, D. C. Dayton and M. R. Nimlos. Intense, hyperthermal source oforganic radicals for matrix-isolation spectroscopy. Review of Scientific Instruments
2003, 74, 3077-3086.

39[10] A. V. Friderichsen, J. G. Radziszewski, M. R. Nimlos, P. R. Winter, D. C. Dayton, D.
E. David and G. B. Ellison. The Infrared Spectrum of the Matrix-Isolated Phenyl
Radical. Journal of the American Chemical Society 2001, 123, 1977-1988.
[11] S. Nandi, P. A. Arnold, B. K. Carpenter, M. R. Nimlos, D. C. Dayton and G. B.
Ellison. Polarized infrared absorption spectra of matrix-isolated allyl radicals.
Journal of Physical Chemistry A 2001, 105, 7514-7524.
[12] S. Nandi, S. J. Blanksby, X. Zhang, M. R. Nimlos, D. C. Dayton and G. B. Ellison.
Polarized infrared absorption spectrum of matrix-isolated methylperoxyl
radicals, CH3OO X 2A ''. Journal of Physical Chemistry A 2002, 106, 7547-7556.
[13] E. B. Jochnowitz, X. Zhang, M. R. Nimlos, M. E. Varner, J. F. Stanton and G. B.
Ellison. Propargyl Radical: ab initio Anharmonic Modes and the Polarized
Infrared Absorption Spectra of Matrix-Isolated HCCCH2. Journal of Physical
Chemistry A 2005, 109, 3812-3821.
[14] X. Zhang, A. T. Maccarone, M. R. Nimlos, S. Kato, V. M. Bierbaum, G. B. Ellison, B.
Ruscic, A. C. Simmonett, W. D. Allen and H. F. Schaefer. Unimolecular thermalfragmentation of ortho-benzyne. Journal of Chemical Physics 2007, 126, -.
[15] A. M. Scheer, C. Mukarakate, D. J. Robichaud, G. B. Ellison and M. R. Nimlos.
Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated
Anisoles: An Investigation of Aromatic Growth. Journal of Physical Chemistry A
2010, 114, 9043-9056.
[16] A. Vasiliou, K. M. Piech, X. Zhang, M. R. Nimlos, M. Ahmed, O. Kostko, A. Golan,
D. L. Osborn, J. W. Daily, J. F. Stanton and G. B. Ellison. The products of thethermal decomposition of CH3CHO. Journal of Chemical Physics 2011, 135, 1430614311.
[17] Q. Guan, K. N. Urness, T. K. Ormond, D. E. David, G. B. Ellison and J. W. Daily.
The Properties of a Micro-Reactor for the Study of the Unimolecular
Decomposition of Large Molecules. International Reviews in Physical Chemistry
2014, 33, 447-487.
[18] J. H. Baraban, D. E. David, G. B. Ellison and J. W. Daily. An optically accessible
pyrolysis microreactor. Review of Scientific Instruments 2016, 87, 014101.

40

[19] N. P. Lockyer and J. C. Vickerman. Single photon ionisation mass spectrometry
using laser-generated vacuum ultraviolet photons. Laser Chemistry 1997, 17, 139159.
[20] J. Boyle and L. Pfefferle. Study of higher hydrocarbon production during
ethylacetylene
pyrolysis
usinglaser-generated
vacuum-ultraviolet
photoionization detection. Journal of Physical Chemistry 1990, 94, 3336-3340.
[21] R. E. Bandy, C. Lakshminarayan, R. K. Frost and T. S. Zwier. The Ultraviolet
Photochemistry of Diacetylene - Direct Detection of Primary Products of theMetastable C4H2* + C4H2 Reaction. Journal of Chemical Physics 1993, 98, 5362-5374.
[22] P. A. Heimann, M. Koike, C. W. Hsu, D. Blank, X. M. Yang, A. G. Suits, Y. T. Lee,
M. Evans, C. Y. Ng, C. Flaim and H. A. Padmore. Performance of the vacuum
ultraviolet high-resolution and high-flux beamline for chemical dynamics studies
at the Advanced Light Source. Review of Scientific Instruments 1997, 68, 1945-1951.
[23] G. C. Eiden, F. Weinhold and J. C. Weisshaar. Photoelectron-spectroscopy of freeradicals with cm-1 resolution - the benzyl cation. Journal of Chemical Physics 1991,
95, 8665-8668.
[24] K. H. Fischer, P. Hemberger, A. Bodi and I. Fischer. Photoionisation of the tropyl
radical. Beilstein Journal of Organic Chemistry 2013, 9, 681-688.
[25] D. Shiner, J. M. Gilligan, B. M. Cook and W. Lichten. H2, D2, and HD ionizationpotentials by accurate calibration of several iodine lines. Physical Review A 1993,
47, 4042-4045.
[26] E. Gagnon, A. S. Sandhu, A. Paul, K. Hagen, A. Czasch, T. Jahnke, P. Ranitovic, C.
Lewis Cocke, B. Walker, M. M. Murnane and H. C. Kapteyn. Time-resolved
momentum imaging system for molecular dynamics studies using a tabletop
ultrafast extreme-ultraviolet light source. Review of Scientific Instruments 2008, 79,063102.
[27] A. Vasiliou, K. M. Piech, X. Zhang, M. R. Nimlos, M. Ahmed, A. Golan, O. Kostko,
D. L. Osborn, J. W. Daily, J. F. Stanton and G. B. Ellison. The products of thethermal decomposition of CH3CHO. Journal of Chemical Physics 2011, 135, 1430614311.
[28] T. K. Ormond, A. M. Scheer, M. R. Nimlos, D. J. Robichaud, J. W. Daily, J. F. Stantonand G. B. Ellison. Polarized Matrix Infrared Spectra of Cyclopentadienone:41

Observations, Calculations, and Assignment for an Important Intermediate in
Combustion and Biomass Pyrolysis. Journal of Physical Chemistry A 2014, 118, 708718.

42!

Chapter 3
High-Resolution Rovibrational Spectroscopy of Jet-Cooled Phenyl
Radical: The !19 Out-of-Phase Symmetric CH Stretch

Abstract
Phenyl radical has been studied via sub-Doppler infrared spectroscopy in a slit
supersonic discharge expansion source, with assignments for the highest frequency b2
out-of-phase C-H symmetric stretch vibration (!19) unambiguously confirmed by≤ 6 MHz (0.0002 cm-1) agreement with microwave ground state combination differencesof McMahon et al. [Astrophys. J. 590, L61-64 (2003)]. Least squares analysis of over 100
resolved rovibrational peaks in the sub-Doppler spectrum to a Watson Hamiltonian
yields precision excited-state rotational constants and a vibrational band origin(!0 = 3071.8915(4) cm-1) consistent with a surprisingly small red-shift (0.9 cm-1) withrespect to Ar matrix isolation studies by Friderichsen et al. [J. Am. Chem. Soc. 123, 1977
(2001)]. Nuclear spin weights and inertial defects confirm the vibrationally averaged
planarity and 2A1 rovibronic symmetry of phenyl radical, with analysis of the rotational
constants consistent with a modest C2v distortion of the carbon backbone frame due topartial sp rehybridization of the s C radical-center. Most importantly, despite thenumber of atoms (N = 11) and vibrational modes (3N-6 = 27), phenyl radical exhibits aremarkably clean jet cooled high-resolution IR spectrum that shows no evidence of
intramolecular vibrational relaxation (IVR) phenomena such as local or non-local
perturbations due to strongly coupled nearby dark states. This provides strong support

!

43

!

for the feasibility of high-resolution infrared spectroscopy in other cyclic aromatichydrocarbon radical systems.

I. Introduction
Phenyl radical (C6H5, Fig. 3.1) is the product of homolytic CH bond cleavage inbenzene and is arguably one of the most fundamental aromatic hydrocarbon radical
species in all of chemistry. By virtue of its aromatic character and unpaired electron,
phenyl constitutes a highly reactive radical species known to play an important role inchemistry ranging from geologic formation of hydrocarbon fuels to industrial
synthesis.[1-2] Of particular interest, this radical is also strongly implicated as a crucial
chemical intermediate in soot formation, combustion and polycyclic aromatichydrocarbon (PAH) chemistry in the interstellar medium.[3-9]In multiple areas ofchemistry, reaction schemes detailing the production and depletion of ring hydrocarbon
species often invoke phenyl radical as a proposed intermediate,[6, 10-12] which continues
to provide strong motivation for detailed first principles understanding of its
fundamental properties.

!

44

!

Fig. 3.1. Phenyl radical with its body fixed principal rotation axes labeled. For
C2v point group symmetry analysis, the x,y,z axes are taken as a right hand coordinate
system pointing along the C, A, B principal axes, respectively. The arrows indicate thedisplacement vectors for the b2 symmetry out-of-phase symmetric CH stretch (!19)
reported herein.

In contrast to its widespread chemical impact and importance, there is aremarkable lack of detailed spectroscopic information on phenyl radical, in particular
high-resolution infrared spectroscopy in the gas phase. Indeed, a deeper understandingof the structural properties, reaction kinetics and dynamics of radicals in general can beobtained via high-resolution infrared spectroscopy of highly reactive intermediates,
both in ground and vibrationally excited states. In particular, such experiments offer
benchmark tests under ideal laboratory conditions with which to confirm high level
theoretical predictions, as well as provide insight into complicated reactions pathways
under more challenging and exotic chemical conditions. One of the reasons for this isclearly high chemical reactivity, which makes it challenging to prepare phenyl radical insufficiently high densities per quantum state to allow such detailed characterization to
be feasible. This is particularly true for relatively large polyatomic radical species with

!

45

!

small rotational constants such as phenyl radical, for which rotational partition
functions under conventional temperature conditions can be quite large and thus yieldcorrespondingly low populations per quantum state.
In combustion chemistry, phenyl radical is a proposed intermediate in bothsynthetic and destructive pathways of PAH systems.[3]The importance of phenyl

radical is especially relevant for heavy oil and tar sand deposits, which greatly exceed
the existing reserves of conventional oil, but require additional chemical processing due
to their significant PAH content[13]. In order to convert such reserves into cleaner andmore efficient fuel sources, the hydrogen-to-carbon ratio can be increased by catalytichydrocracking, which uses homolytic bond cleavage of large aromatic molecules toyield lower molecular weight combustible reagents.[14]Indeed, for conjugatedhydrocarbon systems such as those present in heavy fuel sources, phenyl radical is
directly produced through breaking the biaryl C-C bonds, a process known ashydrogenolysis.[15] In addition, mechanisms for combustion of even simple aliphatic
fuels also highlight phenyl radical as a critical species in the net production of aromatics
and therefore a key intermediate on the path to formation of macroscopic soot.[5]Although much less determined at present, phenyl radical is thought to play anequally central role in astrochemistry. In particular, it has been definitively establishedthat PAHs are indeed present in the interstellar medium (ISM)[16-17] with phenyl radical
a likely key player in the underlying chemistry. For example, reaction schemes such as
those presented by Wang and Frenklach[3] highlight phenyl radical as a critical reactive
intermediate in the conversion from the simplest aromatics (e.g. benzene) to larger PAH
species.

This so called hydrogen-abstraction-C2H2-addition (HACA) is initiated by

activation of benzene via removal of a hydrogen atom to yield phenyl radical, which

!

46

!

then reacts with acetylene to form H + phenylacetylene.

Subsequent hydrogen

abstraction from the #-carbon followed by a second C2H2 addition produces adiacetylenic radical. This species can rapidly undergo electron rearrangement toproduce a bicyclic aromatic radical, which in turn may either proceed with further C2H2
additions or terminate via hydrogen atom addition to yield naphthalene. Indeed, such
HACA pathways have been adapted from combustion chemistry and applied to PAH
formation in carbon rich stars[3, 5, 17-18] where the high temperatures (roughly 1000 K) can
provide sufficient energy to overcome the significant activation barriers for such areaction scheme.
Interestingly, such combustion motivated HACA mechanisms are challenging to
invoke in explaining the formation of PAHs in the ISM, where the much lower
temperatures (roughly 10 K) would strongly limit such activated processes. Of
particular relevance, therefore, crossed molecular beam experiments by Tielens et al.[19]have recently discovered a barrierless reaction between phenyl radical andvinylacetylene, whereby naphthalene can be produced in a single collision event, evenat low temperatures similar to those found in the ISM. Clearly, definitive confirmationof such mechanisms for PAH synthesis in astrochemical environments will requireconsiderable additional work, for which observational spectroscopy of relevant reaction
intermediates such as phenyl radical may offer critical support. Indeed, phenyl radical
is indicated as an intermediate for both low and high temperature reaction pathways,
for which advanced knowledge about phenyl radical is needed to facilitate accurate
predictions of its concentration and chemical activity. By way of example, the precursor
molecule benzene has recently been observed spectroscopically in the proto-planetarynebula CRL 618.[20] Infrared, mm and/or microwave detection of phenyl radical spectra

!

47

!

in collocated regions of space could provide critical supporting evidence for suchreaction mechanisms as well as information on kinetics of PAH formation.[21] This, inturn, could significantly advance future prospects for successful search for these species
in the interstellar medium with new far-IR/sub-mm telescope capabilities coming online.[22-24] Such detection and confirmation efforts would greatly profit from accurate
spectroscopic characterization by laboratory studies of gas phase phenyl radical, toward
which the present work is directed.
The infrared C-H stretch region of phenyl radical has been studied previously inthe condensed phase via matrix isolation spectroscopy,[25] but of the 5 normal mode
stretches predicted in the infrared CH stretch energy region, there has been remarkably
little success at obtaining further information by high-resolution gas phase efforts.[26]Indeed, there are only two sources of high-resolution spectroscopic data on phenyl
radical to date, i) pioneering microwave studies by McMahon et al.[27] and ii) a recent
communication from our group[26] on high-resolution infrared studies of phenyl in a slit
jet discharge. We have since then considerably improved our experimental conditions
for phenyl radical production, and recently exploited this experimental capability forhigh-resolution spectral search throughout the 3000-3200 cm-1 near-infrared region.
Such efforts have yielded detailed gas phase spectra for 3 of the 5 fundamental CH
stretch spectral bands in this near-IR region. The focus of this work is a more complete
description and analysis of the b2 out-of-phase symmetric CH stretch mode, !19, with
spectral results for two weaker a1 symmetry CH stretching modes (!1 and !2) currently
under analysis and to be presented elsewhere.
What is particularly noteworthy about this observation is that, despite therelatively large number of atoms in phenyl radical, the CH stretch spectra can be clearly

!

48

!

rovibrationally resolved, unambiguously assigned, and successfully analyzed. This isfar from an obvious result, since large polyatomic hydrocarbon molecules withcorrespondingly many vibrational modes can exhibit extensive coupling between
excited zero order vibrational “bright states” and the dense bath of near resonant “darkstates” that are energetically accessible at C-H stretch frequencies.[28-31] The lack ofstrong rovibrational state coupling evident in the high-resolution phenyl radical spectra
indicates

surprisinglysmallcomplications

due

to

intramolecularvibrational

redistribution (IVR) of energy, which bodes promisingly for high-resolution
spectroscopy of other cyclic aromatic radical species, as well as potentially even more
complicated PAH radical systems.
The outline of this chapter is as follows. The experimental setup is describedbriefly in Sec. II, including a description of the pulsed slit jet discharge and present
methods of producing frequency stabilized tunable IR light. Sec. III presents results andanalysis of the spectroscopic data, with discussion of the relevant experimental findings
in Sec. IV. In particular, the rotational constants and nuclear spin statistics offer insight
into the structural planarity, in-plane distortion and rovibronic symmetry of themolecule, while the simplicity of the gas phase rovibrational spectrum at thefundamental vibrational level indicates a remarkable lack of unimolecular dynamics
due to intramolecular vibrational coupling effects. Sec. V summarizes the main results
and conclusions.

!

49

!

II. Experimental
The ability to obtain well resolved rovibrational spectra of phenyl radical in thegas phase relies largely on several experimental advantages: i) high radical densities perquantum state accessible with low temperature discharge expansions, ii) sub-Doppler
compression of velocity broadening in the slit jet, iii) long absorption path geometry
and iv) precisely tunable frequency stabilized IR light source. The experimental setup
has been described in previous papers[29, 32] thus only a brief summary of details relevantto the present work will be presented here.
The spectroscopic target phenyl radical species is produced via CX bondfragmentation of monohalobenzenes (C6H5X) in a slit supersonic jet expansion, yielding
typical signal-to-noise on the strongest absorption peaks of roughly 50:1.

The

mechanism for CX bond fission is presumably electron dissociative attachment to form
the radical center + X-, which would clearly favor use of halogenated precursor species
with weakest CX bond strengths. Indeed, phenyl radical can be efficiently synthesizedin the discharge with bromobenzene and iodobenzene, but with signal intensities
greatly limited by reduced vapor pressures at room temperature. As a compromise
between vapor pressure, control and formation efficiency, the carrier gas Ne70 (70 %neon, 30 % helium) is therefore bubbled through liquid-phase chlorobenzene cooled at
0° C, with the gas mixture further diluted with Ne70 to produce 0.5 % C6H5Cl gas
samples in the stagnation region at 280 Torr total pressure. The gas is supersonically
expanded through a JILA-built solenoid-driven slit valve capable of generating intense
pulses with tunable duration (1000 $s, < 50 $s rise time) that exit through a 4 cm long,
300 $m wide slit into a vacuum chamber. This supersonic expansion is triggered at
repetition rate of 19 Hz, with pressure in the vacuum chamber maintained < 50 mTorr

!

50

!

by a 560 liter/second Roots blower (Leybold WS200). During each gas pulse, a 50 kHz
modulated square wave voltage (around 500 V) is applied across an insulated 1 mmregion located immediately prior to the expansion orifice, as illustrated in Fig. 3.2. This
voltage is sufficient to produce a glow discharge with 280 mA peak current through thesample and carrier gas mixture, which causes dissociative electron attachment andhomolytic cleavage of the CCl bond to produce the desired phenyl radical.

Figure 3.2. Diagram of pulsed slit jet discharge. During the 1000 ms gas pulse,
the carrier gas and precursor flow into the discharge region where a square wave
modulated voltage (-500 V) is applied with a 50 kHz frequency. The electric field
induced glow discharge results in efficient electron dissociative attachment of the weakcarbon-halogen bond. This high concentration of phenyl radicals is then rapidly cooled
to roughly 11(1) K in the slit supersonic expansion.

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The slit jet expansion rapidly cools the phenyl radical down to roughly 11(1) K,
where infrared light 5 mm downstream from the nozzle orifice permits directabsorption by the newly formed radical molecules. Near shot-noise-limited absorption
sensitivities on the order of 0.002 % (A = 2 x 10-5) in a 10 kHz bandwidth are madepossible by fast servo loop subtraction of signal and reference detector technicalcommon mode noise on the IR laser. The signal is further augmented by increasing theabsorption path length using a 16-pass Herriott cell, which is aligned directly below theslit orifice. This alignment minimizes molecular velocity parallel to the beam path,lowers the Doppler width of the measured absorption peaks by an additional 5-10 fold(in Ne70), and thereby increases the peak absorption cross section by an equivalent 5-10
fold. In combination with the much slower (1/r vs. 1/r2) density drop off in a slit vs.
pinhole jet, this yields a net sensitivity enhancement for direct IR absorption on highly
reactive radicals by some 3-4 orders of magnitude. Combined with typical integrated
absorption strengths[33] (5-20 km/mol) for hydrocarbon species such as phenyl radical,
these absorption sensitivities translates into minimum detectable number densities of
2 to 8 x 108 cm-3 quantum state-1 in a detection region 5 mm downstream of the slit jet
orifice.
Infrared light is produced by spatially overlapping two visible laser beams in atemperature controlled, periodically poled lithium niobate crystal, which results indifference frequency generation of tunable light in the 2.5-4 $m region. The two visible
light sources are i) a fixed frequency argon ion (Ar+) laser operating at 514 nm(19429.88 cm-1) and ii) a tunable ring dye laser (R6G dye) between 17500 to 16000 cm-1.
The Ar+ laser is locked via servo-loop to the transmission fringes of an external
Fabry-Perot cavity (FSR = 250 MHz), which in turn is locked to a polarization-stabilized

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helium-neon (HeNe) laser to achieve long-term frequency stability over multiple days.
Scanning progress of the dye laser is measured with respect to fringes on the same
ultrastable

Fabry-Perotcavity,yielding

comparable

precisionforfrequency

measurement on the difference frequency IR light. Absolute frequencies are determinedby a reference absorption line of jet cooled methane (R(4), 3067.3000 cm-1) observed
under identical slit jet conditions,[34] providing a calibrated IR frequency axis withtypical rms reproducibility of 8 MHz (< 0.0003 cm-1) for the transitions reported.
The IR light is divided into two beams by a 50:50 beam splitter, with one portion
measured directly on a liquid nitrogen cooled InSb reference detector. The remainder of
the IR light is then directed through the slit jet discharge/Herriott cell vacuum
chamber, extracted after 16 passes and recollimated onto a matched liquid nitrogencooled InSb signal detector. Noise between signal and reference detector is actively
balanced using high bandwidth (1 MHz) home-built active subtraction electronics, withthe weak absorption signals at the 50 kHz discharge modulation frequency obtained via
lock-in detection to provide additional signal-to-noise enhancement.

These lock-in

detection signals are then further analyzed in the time domain by gated integration anddifferential baseline correction before, during and after the slit jet pulse. All resulting
data and frequency diagnostics are stored in a computer for later analysis.

III. Results and Analysis
Phenyl radical is predicted to be a planar, near-oblate asymmetric top with arigid structure described by C2v point group symmetry with x, y, z axes pointing along
the C, A and B principal axis directions (see Fig. 3.1). Normal mode analysis predicts

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five infrared active fundamental modes in the 3000-3200 cm-1 region consisting ofpredominantly CH stretching character. There are 3 CH stretch vibrations of a1
symmetry, which in order of decreasing frequency can be predominantly characterized
as: i) in-phase, symmetric stretch of all 5 CH bonds (a1, !1, 3192 cm-1), ii) out-of-phase,
symmetric stretch between the ortho (i.e., carbon atoms 2,6) and para (i.e., carbon atom 4)
CH bond positions (a1, !2, 3180 cm-1), and iii) in–phase, antisymmetric stretch of thecorresponding ortho/meta CH bonds (a1, !3, 3160 cm-1). In addition, there are 2 CH
stretch vibrations of b2 symmetry: i) out-of-phase, symmetric stretch (b2, !19, 3182 cm-1)
and ii) out-of-phase, antisymmetric stretch (b2, !20, 3166 cm-1) of the ortho/meta CH bonds.
The predicted frequencies for these 5 vibrations are based on anharmonic scaling(0.9637(31)) B3LYP/6-31++g(3pd,3df) predictions of Dong et al.,[32-33] which in turn havebeen benchmarked against a comprehensive set of high-resolution CH stretch spectra
for hydrocarbon radicals. Of the five IR active CH stretching modes, the b2 symmetric
out-of-phase CH stretch (!19) is predicted from simple density functional calculations[B3LYP/6-311++g(3df,3pd)]

to

have

the

largestintegrated

infraredintensity(S0 = 17.3 km/mol).[33] A sample overview spectral region for the phenyl radical in theslit jet discharge absorption spectrometer is shown in Fig. 3.3, revealing extensive
rotationally resolved P, R branch progressions and a clear Q branch feature near3071.9 cm-1. Typical isolated line absorbances are roughly 0.04 %, which for the above
integrated absorption cross section translates into a typical number density perquantum state of about 3 x 109 radicals cm-3 in the slit jet probe region androughly 5 x 1010 radicals cm-3 extrapolated up toward the discharge slit orifice. Although
this represents greater than 10-fold lower formation efficiency than observed for otherhydrocarbon radicals in the slit jet discharge geometry, this nevertheless translates into a

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respectable S/N of roughly 20 for typical absorbance sensitivities of 0.002 % in our

detection bandwidth.

Fig. 3.3. Overview rovibrational spectrum of the symmetric out-of-phase C-H
stretch, !19, in phenyl radical at Trot roughly equal 11(1)K. The band origin is at3071.8915 cm-1, with extensive P, Q, and R branch structure clearly evident. In spite ofspectral overlap due to the near oblate top nature of phenyl radical, the combination ofsupersonic jet cooling and sub-Doppler transverse velocity compression allows for
assignment and analysis of over 100 fully resolved rovibrational transitions.

As discussed in detail below, this rotational structure is indeed predominantly
due to the symmetric out-of-phase CH stretch (!19) band, with a dipole derivative
moment along the A principal axis and thus an A-type band dominated by DKa = 0 and
DKc = ±1 transitions. Closer inspection also reveals additional B-type band transitions
from the much weaker a1 totally symmetric n1 CH stretch vibration fortuitously
occurring in the same spectral region, which is not considered herein. The !19 Q-branchfeature near 3071.9 cm-1 is in good agreement with the Ar matrix isolation studies(!19 = 3072 cm-1) performed by Friderichsen et al..[25] The high-resolution capabilities of

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the slit jet expansion are often sufficient for resolving spin-rotation fine structure andsometimes even nuclear spin-electron spin hyperfine structure in these open shell
systems. In the case of phenyl radical, however, these fine structure contributions are
known from microwave studies to be quite small (%cc = 4.8 MHz) and in fact are seen
only as unresolved additional broadening beyond the sub-Doppler limit.[27] To beconsistent with our other studies, however, we will nevertheless refer to total angularmomentum simply by the end-over-end rotational quantum number N, although ateven higher-resolution these levels would be split into doublets by coupling withelectron spin (S = ½).
Due to H atom loss and ring distortion of the C radical center, the degeneracy of
the in-plane principal rotation axes is broken (A ≥ B > C), which makes phenyl radical anear-oblate asymmetric top.

Although the total angular momentum N remains a

conserved quantum number, the unsigned body fixed projections of N along the A andC principal axes, Ka and Kc, offer a further useful designation of energy levels by NKaKc.
Although these projection quantum numbers are only approximate for an asymmetric
top, the even (e) or odd (o) nature of Ka and Kc is rigorous, since these correspond to ± 1
eigenvalues for body fixed C2(y) and C2(x) rotation operators that commute with thetotal Hamiltonian. As a result, the rotational energy levels of phenyl radical can bereadily solved by expanding the total asymmetric top Hamiltonian in a Wang basis set
comprised of symmetric/antisymmetric linear combinations of oblate symmetric top
functions (e.g.

12{| NK > ± | N − K >} for K = even/odd), where K is the signed

projection along the C axis. Rotational constants for phenyl radical are then obtained by
iterative assignment of absorption peak frequencies to rovibrational transitions

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(N’Ka’Kc’ ← N”Ka”Kc”) in a least squares fitting program based on a Watson asymmetric top
Hamiltonian. Due to the low rotational temperatures (roughly 11(1) K) in the jet
expansion, these fits are sufficient to reproduce the transition frequencies to within
experimental precision without need for higher order centrifugal distortion terms, andthus the choice of reduction and representation is arbitrary.
For a near oblate asymmetric top, the eigenenergies for a given N decrease withincreasing Kc, which implies most favorable Boltzmann factors for low Ka values with
Kc roughly = N. Furthermore, for a planar near-oblate top (i. e., A ( B ( 2C), the structureof each Kc subband is shifted by ( 2(C − B ) K c ( - B K c , which for alternate Kc values
approximately matches the ( 2B spacing in the P and R branch progressions. The net
result is a closely spaced progression of transitions with a natural propensity forspectral overlap. We therefore begin the assignment using only spectrally fully resolved
and isolated features in the P and R branch region. The advantage of this approach isthat it permits immediate confirmation from 2-line combination differences based upon
microwave spectroscopy of Thaddeus et al.,[27] to which the present assignments matchwithin 6 MHz (0.0002 cm-1). Therefore, the ground state constants are held fixed at the
microwave values, with the excited vibrational state rotational constants (A’, B’, and C’)
and vibrational band origin !0 subject to refinement in this first round of least squares
fit.
This initial P/R branch analysis provides a sufficiently good fit to the Watson
Hamiltonian to enable confident spectral prediction of fully resolved albeit weaker
spectral structure within the central Q branch. With the inclusion of these additional Q
branch lines in the fit, the spectral predictions are now sufficiently accurate to

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confidently identify the weaker individual P and R branch lines (N ≠ Kc and/or*N ≠ *Kc) that do not satisfy requirements for both high Boltzmann population and
Hönl-London factor line strength. This iterative process results in more than 100
spectrally isolated transitions that can be unambiguously assigned and least squares fit
to extract high-resolution rotational constants (see Table 3.1) and a band origin. Even
without centrifugal distortion terms, the random errors in such a least squares analysis
yield a residual standard deviation of 0.00037 cm-1 (11 MHz), i.e., roughly comparable torms experimental error in our frequency measurements.

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Fig. 3.4. Panel (a) shows the Q branch region of the phenyl radical out-of-phase
CH symmetric stretch, band origin (3071.8915 cm-1) labeled with a black arrow. The
simulated spectrum using experimental fits is shown below in red. Panels (b) and (c)show rovibrational energy level diagrams highlighting the N = 2 ← 2 and 6 ← 6Q-branch manifold transitions. Color coded arrows in panel (a) correspond totransitions in panels (b) and (c).

At closer inspection, a small Q branch region for !19 phenyl radical is expanded
in Fig. 3.4, with the band origin designated with a black arrow. The predicted spectrum(downward, red line) is generated using experimental least squares fit constants andrecapitulates the high-resolution experimental spectrum (upward, black line) of phenyl
radical quite well. As seen in Fig. 3.4, due to compensating Hönl-London factors, thestrongest peaks in the Q-branch occur near the band origin and correspond totransitions that originate from higher energy states where Ka” is roughly equal N” andKc” is small, with corresponding Q-branch transitions out of the lower energy states
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(i.e., where K”a is small and Kc” is roughly equal N”) shifted significantly to higher (∆Kc
= -1) and lower (∆Kc = +1) transition frequencies from !0. If we focus on the Q branchtransitions with small frequency shifts, these form clear blue (∆Kc = +1) and red(∆Kc = -1) shaded progressions out of Kc” = 0 and K”c = 1, respectively, converging upon
one another near the band origin. This behavior can be rationalized by the right handpanels in Fig. 3.4, which illustrate the approximate energy level dependence on N, Ka,
and Kc for pairs of Q-branch transitions. For low N values, the energy difference
between Kc = 0 and the mean of the split Kc =1 levels is approximately independent of N(i.e., ∆E ( (C - B) Kc2), while the Kc =1 asymmetry splitting for low N(∆E ( ½ (A - B)N(N + 1)) grows quadratically with N. With increasing N, therefore, theN’N’1 ← N”N”0 and N’N’0 ← N”N”1 progressions necessarily shift to the blue and red,respectively. For sufficiently high N, however, the asymmetric top energy level patternapproaches a more prolate top behavior, with the NN0 and NN1 states converging to form
pairs of Kc asymmetry split levels for Ka = N. The net result is that the N’N’1 ← N”N”0 and
N’N’0 ← N”N”1 Q-branch progressions converge to the same transition frequency. Indeed,
for negligible changes in upper and lower state rotational constants, this would predicta perfect convergence of transitions at the band origin, as is the qualitative trend
evident in Fig. 3.4. With a reliable spectral assignment in hand, we can similarly look ata sample data region in the P and R branches (see Fig. 3.5). As expected for a near oblate
planar asymmetric top (A ( B ( 2C), the spectral structure is dominated by nearlyoverlapping progressions in ∆N = ∆Kc = ± 1, where the congestion arises from the nearcommensurate match between P/R branch spacing and the shift in alternate Kc subband origins for an A-type band (∆Kc = ± 1). Nevertheless, agreement between

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experimental data and the partially overlapping asymmetric top predictions is clearlyextremely good.

Fig. 3.5. Scan of phenyl radical in the R branch region of the !19 CH stretch. The
simulated spectrum based on experimentally fit rotational and vibrational constants isshown below in red. The spectral congestion observed in these rotational structuresarises naturally for a perpendicular band in a planar, near oblate asymmetric top(A ( B ( 2C), for which the P/R branch spacings are near commensurate with sub-band
spacings as a function of Kc.

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IV. Discussion
A. Nuclear spin weights
As a first point of discussion, relative intensities in the rotationally resolved
rovibrational spectrum can be used to provide additional confirmation on theunderlying nuclear spin statistics and thus rovibronic symmetry of the phenyl radical
ground state.

Although the most complete treatment of nuclear spin statistics in“floppy” molecules is provided by molecular permutation group theory, this is notessential in phenyl radical, for which results are readily obtained simply by assuming arigid structure with C2v point group symmetry.[35] From a point group perspective,
phenyl radical has two sets of equivalent hydrogen nuclei, each of which undergo
feasible exchange by low barrier rotation of the molecule around the C2 axis. The fourhydrogen atoms (I = ½) have either spin up or spin down, which translates into 24 = 16
total spin states for the molecule. This forms a reducible representation of the C2v point
group, which can be reduced with the C2v character table to yield ,nuc spin = 10A1⊕6B2.
To obtain nuclear spin weights, we note that Fermi-Dirac statistics requires the total
wavefunction (-tot) to be antisymmetric with respect to exchange of equivalenthydrogen atoms and thus two equivalent pairs of fermions must transform
symmetrically with respect to C2 rotation. Furthermore, the eigenvalues for C2 rotation
around the B axis are +1 and -1 for asymmetric top eigenfunctions with Ka + Kc = evenand odd, respectively. In conjunction with a totally symmetric A1 ground vibronic state,
this corresponds to a 10:6 ratio of nuclear spin statistical weights for Ka + Kc = even/odd
states, respectively.

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Fig. 3.6. Comparison of two simulated spectra, which highlight the effect of
nuclear spin statistics in the spectrum of phenyl radical in the !19 region. The middle
panel shows experimental absorption spectra, whereas the lower and upper panels
present intensity predictions for correct (10:6) vs. inverted (6:10) nuclear spin statisticalweights anticipated for Ka+Kc = even:odd levels in the ground state. These nuclear spinweights confirm the 2A1 electronic symmetry of the ground state and thus the . natureof the C radical center.

Spectroscopic confirmation of the correct nuclear spin weighting can be found in
Fig. 3.6, which shows a small segment of the Q branch for the !19 stretch of phenyl
radical. The top panel shows stick plot predictions calculated with the correct nuclear

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spin ratio of 10:6 for Ka + Kc = even:odd. Conversely, the stick plot in the bottom panel isthe same prediction but with nuclear spin statistics reversed, i.e., 6:10 for Ka + Kc =even:odd. Comparison of the two simulated stick plots with the high-resolution results
clearly confirms a 10:6 ratio for Ka + Kc = even:odd to correctly reproduce the spectral
data. It is worth noting that overall wavefunction symmetry with respect to rotation
around the B axis is directly influenced by and therefore reports on the electronic
symmetry of the ground state. Specifically, for a putative p radical ground state with B1electronic symmetry, C2 rotation around the B principal axis would invoke an
additional sign change, and thus the nuclear spin weights would shift from 10:6 to 6:10
for Ka + Kc = even:odd states. In conjunction with the above discussion of electronic
symmetry, these 10:6 nuclear spin weights can be taken as independent spectral
confirmation of a ground state A1 rather than B1 electronic symmetry, which in turnrequires a . vs / electronic structure for the radical center on the C atom. This is also
consistent with aromatic stabilization of phenyl radical, which significantly lowers theenergy of the / electron cloud and makes homolytic bond cleavage to form the . radical
center the energetically preferred pathway.

B. Planarity and In-plane Ring Distortion
The rovibrationally analyzed spectra also provide additional information on thevibrationally averaged planarity of phenyl radical, as well as evidence for a static inplane distortion of the 6-membered ring. Specifically we consider the inertial defect (*),
i.e., * = Ic – Ia – Ib , where Ii is obtained from a sum over all masses times the distancesquared from the corresponding principal; rotation axis. For a perfectly rigid, classicalmolecule with all mass constrained to a plane, the inertial defect should vanish

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identically, although systematic shifts in D arise due to higher order quantum effects
such as finite electron mass, zero point motion of the nuclei, as well as in-plane vs. outof-plane vibrational motion.[36-38] Table 3.1 summarizes the experimental inertial defects
for phenyl radical in ground and vibrationally excited states. The ground state value
for phenyl is small (∆ = + 0.046 amu Å2) and in fact remarkably close to benzene[39-41] inits ground vibrational state (∆ = + 0.044 amu Å2). This is consistent with a planar
aromatic molecular structure for phenyl radical, with the slight positive contribution toD arising from in-plane vibrational zero point effects. It is worth noting that this inertial
defect increases by nearly 2-fold in the !19 excited state (∆ = +0.089 amu Å2), which isalso consistent with additional CH stretch motion in one of the in-plane vibrational
degrees of freedom.

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Table. 3.1. Ab initio theoretical and experimental rovibrational constants. High level ab initio predictions[42] (leftthree columns), obtained at a fixed CCSD(T)/vtz-f12 level to facilitate quantitative comparison, reflect i) benzene,ii)“sp2 phenyl” (i.e., equilibrium benzene geometry with one hydrogen removed), and iii) phenyl radical. The threerightmost columns represent experimentally determined values for i) benzene, [41] ii) ground state phenyl radical[27],and iii) ν19 phenyl radical excited in the out-of-phase symmetric CH stretching mode. To aid quantitative comparison,
fractional changes in each quantity with respect to theoretical and experimental values for benzene are indicated in %.

!

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Although benzene and phenyl radical have the same number of carbon atoms
and /-electrons, homolytic cleavage of a CH bond results in a less saturated radical
carbon that is no longer purely sp2 hybridized, now mixing in partial sp character inorder to generate a . radical. To investigate distortion of the phenyl backbone awayfrom 6-fold symmetry, we compare both theoretical and experimental rotational
constants for a progression of C6 ring molecules in the absence or presence of such
rehybridization effects due to radical formation. As summarized in Table 3.1, theleftmost three columns reflect theoretical rotational constant values calculated at a highbut uniform ab initio level[42] (MOLPRO, CCSD(T)/vtz-f12) for i) benzene, ii) a fictitious
fully “sp2 phenyl” species clamped at the D6h benzene geometry but one hydrogen atom
removed along the B axis, and iii) phenyl radical itself with complete relaxation of the
carbon backbone allowed. Percent changes in each of these theoretical values withrespect to benzene are indicated in parentheses. Similarly, the next three columns
represent the experimental rotational constant values for i) ground state benzene, andphenyl radical in the ii) ground state and iii) with one quanta of excitation in the !19
stretch, again with fractional changes in the quantities shown in parentheses.
In the theoretically predicted values, there is clearly a substantial increase(10.2 %) in the A rotational constant between benzene and phenyl radical. However, the“sp2 phenyl” limit (i.e., H atom removal with no change in nuclear coordinates of the C6
frame) accounts for only roughly 75 % of the observed difference. This indicates that H
atom loss cannot be the only source of increase in A and that indeed the phenyl radicalring is more compressed with respect to the A-axis. The shifts in B provide further
evidence for such compression of the carbon ring. Specifically, since the H atom
removed originates on the B-axis, there must be identically zero change in B between

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benzene and “sp2 phenyl” resulting purely from H atom removal. Experimentally,
however, the B constant for phenyl radical decreases by an additional 1.6 % as a result of
rehybridization, providing confirmation that the two pairs of meta/ortho C atoms must
be on average stretched further away from the B axis than in benzene.
In the absence of a full isotope substitution study, such changes in rotational
constants are insufficient to determine the nuclear coordinates of phenyl radical.
However, a simple rehybridization picture consistent with these changes is illustrated
in Fig. 3.7, with coordinates taken from theoretical calculations. The unique carbon
atom in phenyl radical has two filled bonding molecular orbitals with a third orbital
only half occupied, thus resulting in an admixture of sp and sp2 character. Partial sp
character yields a greater propensity toward linear vs.trigonal planar geometry thatcauses the C-C-C bond angle to be substantially larger (roughly + 6°) than theconventional 120° value for sp2 bonding. Simply summarized, rehybridization of the C
radical center results in i) decreased distance of the unique carbon atom to the A axis andii) increased distance of the two adjacent (para) carbon atoms from the B axis. Such inplane distortion is consistent with experimental results in Table 3.1, which reveal ananomalously large fractional increase in A and decrease in B from phenyl radical tobenzene. Alternately stated, despite having the same number of carbon atoms and /electrons as benzene, the one half filled bonding orbital in phenyl radical clearly has
measurable effects on the ring backbone geometry of the molecule. Finally, the phenyl
radical data in Table 3.1 indicate very good agreement between experiment and theory(< 1 %). Moreover, since these predictions ignore zero point quantum averaging overthe vibrational wavefunction, the remaining discrepancies are largely systematic, for
which

!

one

can

empiricallycompensate

68

byscaling

to

the

ratio

of

!

experimental/theoretical values for a related molecule such as benzene. The result ofsuch scaling improves the rotational constant agreement between theory andexperiment by an additional order of magnitude (roughly 0.1 %).

2.4092!

2.4431!
1.3703

1.3910!

!
!
2.7819!

!

!
120.0°

2.7098!

126.1°!

116.4°

Fig. 3.7. Ab initio theoretical comparison (CCSD(T)/vtz-f12) between benzene(left) and phenyl radical (right), illustrating the subtle distortion of the carbon ring upon
H atom abstraction and radical formation.[42] The greater sp hybridization character ofthe unique radical carbon atom center both i) increases the C-C-C bond angle as well asii) decreases the adjacent C-C bond lengths.

C. Intramolecular Vibrational Dynamics
As a final topic of discussion, we consider the role of intramolecular vibrational
coupling in the phenyl spectra. In high-resolution IR spectra of sufficiently large gas
phase molecules, isolated transitions which carry the oscillator strength to so called zeroorder “bright states” can be broadened or split into multiplets by unimolecularvibrational coupling to a typically much denser manifold of zero order “dark

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states.”[28, 30, 43-44] When two or more zero order states have similar energies, residual
coupling in the Hamiltonian can mix the zero order states to form eigenstates with both
fractional bright and dark state character. As a result, excitation into an optically brightstate necessarily prepares a coherent superposition of eigenstates, which in turn results
in a time dependent transfer of vibrational energy into nearby states, or, expressed
equivalently in the time domain, intramolecular vibrational redistribution (IVR). The
critical question of interest is therefore whether typical off-diagonal IVR coupling
matrix elements (i.e., Vij = ⟨Ψdark|H|Ψbright⟩) are sufficiently large with respect to theinverse density of vibrational states (1/0(Evib)) for such intramolecular coupling effects
to become evident in the high-resolution spectrum.

Experimentally, the !19

rovibrational spectrum of phenyl radical appears completely assignable and shows noeffects from IVR at sub-Doppler spectroscopic resolution (*!FWHM ≈ 50 MHz = 0.00167
cm-1) under Ne slit discharge expansion conditions. The relevant issue is whether the i)
vibrational state density and ii) coupling matrix elements in phenyl radical are eachsufficiently small to rationalize this behavior.
The vibrational state density of a molecule at a given excitation energy grows
exceptionally rapidly with number of vibrational degrees of freedom, and in particular,
the number of low vibrational modes.[3, 45-50] Due to the presence of 3N-6 = 27 vibrational
normal modes, phenyl radical might be expected to suffer substantial loss of spectral
simplicity from IVR induced fragmentation of isolated eigenstates of the lower orderand largely separable Hamiltonian. Note that such intramolecular vibrational dynamics
does not necessarily imply any true homogeneous broadening of lines beyond the subDoppler linewidth, as could occur for the case of strong predissociative coupling withthe continuum. However, clearly neither scenario is the correct description, which we

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would experience as partial blurring in each of the high-resolution spectral samples(Fig. 3.3 - 3.6). Instead, based on efficient backtracking state counting algorithms by
Kemper et al. and simple harmonic predictions,[48] the vibrational state density persymmetry group in phenyl radical can be readily predicted to be 0(Evib) ( 0.15
states/cm-1 at Evib ~ 3000 cm-1, which is clearly unusually small for a molecule with 11
atoms.

The obvious reason for this is its aromatic C6 ring conformation, whichconstrains proliferation of low frequency bends and internal torsional modes that candramatically increase the density of states in linear hydrocarbon systems and thus theimpact of mode-mode coupling.[51] In a closed C6 ring structure, on the other hand, thelowest frequency vibrational modes are considerably higher than for a linear C6 chain
structure, which considerably decreases the state density at typical CH stretch
frequencies.
To provide some additional quantitative context, there have been systematic
high-resolution IR studies of IVR coupling dynamics for hydrocarbons such as terminal
CH stretch excited substituted acetylenes.[31, 47, 52] By virtue of jet cooling and subDoppler resolution, these studies were able to resolve the true molecular eigenstatesarising from these bright state/dark state interactions in the CH stretch manifold, which
thus permitted coupling matrix elements to be extracted directly from the spectra. A
typical coupling matrix element size was found to be on the order of ⟨Vij⟩ ~ 0.01 cm-1,
which was consistent with an empirical threshold of state density on the order of 0(Evib)~ 102 symmetry selected states per cm-1 for onset of additional spectral complexity due
to IVR mixing.[43, 53] Note that this critical threshold state density is nearly 3 orders ofmagnitude smaller than predicted for phenyl radical at 3000 cm-1 levels of internal
excitation. Thus, if one were to assume a similar range of coupling matrix elements for

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fundamental CH stretch excitation in aromatic ring systems, this would indeed beentirely consistent with the observed lack of IVR induced fragmentation of bright stateswith near resonant dark states in the current phenyl radical spectra.
As a final comment, we briefly consider the future prospects for extending suchhigh-resolution IR studies to even larger polyatomic aromatic hydrocarbon systems.
Fig. 3.8 summarizes vibrational state density predictions (0(Evib)) as a function ofinternal energy for a series of single, double and larger aromatic ring systems, based on
simple harmonic frequency predictions (B3LYP/6-311++g(3df,3pd)) and the Kemper
backtracking algorithm.[33, 48] Given that the arguments here rely on only approximate
vibrational state densities, the choice of density functional theory predictions for thesefrequencies should be entirely adequate. For each of the single aromatic ring species, thedensity of states at fundamental CH stretch energies is roughly 0(Evib) ~ 0.1 states/cm-1,
with the corresponding numbers of 0(Evib) ~ 20 states/cm-1 and 0(Evib) ~1000 states/cm-1
for double (naphthalene) and triple (phenanthrene, anthracene) aromatic ring systems.
Also indicated on this plot is the ~ 102 states/cm-1 threshold density of states empiricallyobtained from high-resolution studies for CH stretch excitation in the terminal
acetylenes.[53] The predictions for the larger ring systems suggest that spectralcongestion due to IVR coupling will be dominant, although results for double aromaticring systems such as naphthalene may be sufficiently sparse to tackle with highresolution methods. However, it is also worth noting that these state densities represent
extremely rapidly growing functions of internal vibrational energy, and thus any such
IVR imposed limits on molecular size in the CH stretch region could be considerably
reduced for excitation at lower frequencies. In any event, the data for each of the single
aromatic ring systems at the fundamental CH stretch excitation are already consistently

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2-3 orders of magnitude below this threshold state density, which clearly bodes well forfurther study of the corresponding radical species via high-resolution IR spectroscopy.

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Fig. 3.8. Estimates of symmetry selected vibrational state density (i.e. scaled topoint group order) for a series of aromatic and polycyclic aromatic hydrocarbon (PAH)molecules, based on simple density functional predictions (B3LYP/6-311++g(3df,3pd))
and an efficient backtracking algorithm by Kemper et al.[33, 48] The vertical arrow
represents the fundamental CH stretch region, while the horizontal band indicates thethreshold region of state density (~ 100 sym selected states/cm-1 ) suggested from highresolution studies to be necessary for promoting IVR coupling between “bright” and“dark” zeroth order levels.[3, 28, 30, 43-44, 54] Color/shape coding for the calculations: single
ring: phenyl (green circles), benzene (red circles), pyrazine (purple circles), pyridazine(grey circles), pyridine (dk red circles), naphthalene (red circles), phenanthrene (cyancircles), anthracene (dk blue circles), pyrene (yellow circles). Note that all the single ring
species are well below this critical state density at ~ 3000 cm-1, which suggests thefeasibility of high-resolution infrared detection for any of the corresponding single ringaromatic radical species under supersonically cooled conditions.

!

74

!

V. Summary and Conclusions
The phenyl radical has been studied with high-resolution infrared spectroscopy
enabling the identification of the ! = 1 ← 0 excitation of the C-H symmetric out-ofphase vibration, !19. This assignment has been unambiguously confirmed via groundstate combination difference band comparison with previously published microwave
data,

[27]

with

a

vibrationalband

origindetermined

to

high

precision(!0 = 3071.8915(4) cm-1) and over 100 spectrally resolved transitions fit to near
experimental uncertainty levels (. = 11 MHz = 0.0004 cm-1). The band origin is in quite
good agreement with matrix isolation studies (!19 = 3072 cm-1) performed by
Friderichsen et al.. This would be consistent with a red shift that is remarkably small(roughly 0.96 cm-1) and more typical of those observed under the more weakly
perturbative conditions of rare gas Ne matrices or even van der Waals complexes.[25, 55-59]
Given the relatively large number of atoms and normal modes in phenyl radical, theobserved IR spectrum is surprisingly clean and lacks any additional structure and/or
broadening due to intramolecular vibrational relaxation. Experimental and theoreticalanalysis of ground and excited state rotational constants help inform on changes inintramolecular bonding due to the half-filled s molecular orbital, with increased CCC
bond angle and decreased bond CC lengths consistent with partial sp vs sp2 characterfor the radical-carbon atom.

Analysis of nuclear spin statistical weights andexperimental inertial defects confirms the vibrationally averaged planarity of phenyl aswell as the 2A1 symmetry of the electronic ground state. The feasibility of such a spectral
analysis constitutes an important step in progress towards high-resolution infrared
spectroscopy of additional aromatic ring hydrocarbon radical systems, which likely
play important roles in both combustion and interstellar chemistry.

!

75

!

References for Chapter 3!

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Chemistry 1991, 95, 2636-2644.
[56] D. C. Clary, C. M. Lovejoy, S. V. Oneil and D. J. Nesbitt. Infrared-spectrum of
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82

Chapter 4
Observation of the a1 C-H Stretching Modes of Phenyl Radical, !1 and !2

Abstract
High-resolution infrared rovibrational transitions of the C−H stretching modes inphenyl radical (C6H5) are investigated in the slit-jet supersonic expansion with subDoppler resolution (roughly 60 MHz). Two new fundamental modes have been studied
that have never before been observed with rovibrational resolution.

These b-typevibrational bands originate from the !1 and !2 mode of phenyl radical. The band origins
are determined to be !1 = 3073.96850(8) cm-1 and !2 = 3062.26480(7) cm-1, which agree towithin 5 cm-1 with the theoretical anharmonic scaling prediction (B3LYP/6311g++(3df,3dp) basis set). The assignments show slightly larger (11 cm-1) red shiftcompared to the corresponding experimental Ar matrix isolation [Friderichsen et al. J.
Am. Chem. Soc., 123, 1977(2001) ]. Intensities for the three bands (!1, !2, and !19) arealso analyzed and the relative intensities agree well with theoretical calculations. The
physical interpretation of the inertial defect and perturbation of the position of the band
origin in the matrix are also discussed.

I. Introduction
The cyclic phenyl radical (C6H5), shown in Fig. 4.1, plays a crucial role incombustion and soot formation processes.[1-2] It is also proposed that the formation of83

the phenyl radical is an early stage in the further processing of polycyclic aromatichydrocarbons (PAH).[3-5]The reaction of the phenyl radical with unsaturatedhydrocarbons is proposed as an elementary step in soot formation in oxygen-poor
environments as well as in the outflow of carbon rich stars in the interstellar medium.[6]Fig. 4.1. Structure of phenyl radical with carbon atom positions labeled. The b
axis is the C2v axis and the c axis is defined out-of-plane.

Studies of the electron spin resonance for phenyl radical in an Ar matrix by Kasai
et al.[7] and Bennett and Thomas[8] have revealed that after cleavage of the C−H bond, theunpaired electron is found to be localized in the nonbonding σ orbital at the C1 position,rather than in the six center π orbital. Initially, lower resolution infrared spectroscopyof phenyl radical had been reported by Pacansky and Bargon[9] in an Ar matrix
environment and only the CH out-of-plane bending mode was assigned, at 710 cm-1.
Matrix isolation Fourier Transform infrared (FTIR) was used to study phenyl radical
along with five deuterated isotopomers, as reported by Ellison and coworker.[10-11]84Linear dichroism was employed to explore the polarization of each vibration and to
distinguish the symmetry for each mode.

The characterization of the electronictransition of phenyl radical had been studied by Ikeda et al.[12] and Radziszewski,[13] andthe results indicated that there are three main electronic transitions of phenyl radical in
the UV-VIS region, 2B1, 2A1, and 2B2 at 510.5 nm, 235.1 nm, and 211.5 nm, respectively.
The 2B1 band was tracked by the Lin group[14-15] to study the kinetic of reactions of thephenyl radical with a series of other species via cavity ring-down laser spectroscopy.
Their recent rotationally resolved spectrum indicated that the lifetime of the 2B1 excitedstate is about 96 ps due to predissociation.[16] The only pure rotational spectroscopy ofphenyl radical was performed by McMahon et al.[17] from a pulsed supersonic beam in
the mm-wave region between 9−40 GHz and 150−330 GHz. The rotational constants forthe 2A1 ground state as well as spin rotation constant (εcc=4.78(2) MHz) were reported
for the first time. Their studies provide the fingerprint information for this primary
candidate in the field of astronomical detection.

Recent work in our group[18] reported on the a-type rovibrational transition in the
C−H stretching mode, !19. The assignment of this C−H mode is based on theoretical
calculations and band type comparison with the Ar matrix results. In addition, theassignment is confirmed by excited state rotational constants, which show elongation
along the a-axis consistent with the displacement vectors of !19. The band origin for thismode was reported as 3071.8907(10) cm-1 in the gas-phase, very close to the Ar matrix
assignment of 3071 cm-1. These experimental results indicated that the !19 vibrationalmode is very minimally perturbed by the inert gas matrix environment.

Theexperimental results of the gas-phase, Ar matrix, and theoretical predictions are85

summarized in Table 4.1. The quantum chemical structure and frequencies for thephenyl radical had been investigated with density functional methods (B3LYP) over aseries of the correlation consistent Dunning basis set function and 6-311g++(3df,3dp)
basis set. The empirical anharmonic scaling factor[19] (0.964) was guided by the previous
study of a series of CH stretching modes of hydrocarbon radical in the gas-phase, whereexperimental results were correlated with theoretical harmonic predictions.

The displacement vectors for each hydrogen atom in the five different C-H
stretch vibrational modes are shown in Fig.4. 2. The excitation of the !1, !2 and !3stretching modes correspond to the generation of the derivative of the dipole moment
along the molecular b axis, as defined in Fig. 4.1. The !19 and !20 modes generate thedipole moment derivative along the a-axis. For the previous study of the !19 band,
theoretical calculations of the out−of−phase motion of the C−H stretching for the orthoposition CH group predicted the integrated IR intensity of 17.1 km mol-1. For the !20
mode, cancelation of the motion at the ortho-position C-H group results in only about
4.0 km mol-1 infrared intensity. For the a1 symmetry of the vibrational modes, such as!1, !2, and !3 modes, the motion of the H atoms that superimpose each other withrespect to the C2v rotation cancel out and generate net zero projection along this a axis.
For the !1 band, the main motion of the hydrogen atom that contributes to the infraredintensity comes from the hydrogen motion at the C4 position. The combination of thethree C-H displacement vectors along the b-axis generates about 3.7 km mol-1 of IR
intensity in the !2 mode.

For the !3 mode, the C−H stretching at the C4 position

diminishes the contribution of the other four C-H motions and should carry lessoscillator strength as an infrared transition.

86!

Fig. 4.2. The displacement vectors for each vibrational mode in the region of
C−H stretching of phenyl radical.

In our previous studies of the infrared transitions of the phenyl radical in theregion of 3068 - 3075 cm-1, only one of these modes was observed it was and assigned as
the excitation of !19, characterized as an a-type band transition. Analysis of the momentof inertia also agrees well with the mm-wave result showing that the phenyl radical
maintains planar geometry. This observation is reasonable since the excitation induces
C-H stretching motion in the molecular plane. Compared with the Ar matrix results
and theoretical intensity predictions, as indicated in Table 4.1, it is promising that weshould be able to detect the other C−H stretching mode in the nearby transition region.
Table 4.1 also shows that the other four modes, except the !3 mode with only 0.8 km

87mol-1, should be detectable in our quantum-shot limit approach infrared spectrometer.
Further, the band type associated with the different symmetry modes should allow forstraightforward band assignments. In this work, we report the successful acquisition of
the other two b-type bands of the C−H stretching mode in addition to the previously
observed !19 band.

The improvements in signal-to-noise ratio (S/N) reveal moredetailed structure not observed previously.

Spectral analysis indicates that theexcitations of the !1 and !2 modes are responsible for these two additional observed
bands. However, there are a few rovibrational lines that cannot be cleanly assigned,thus implying perturbation in the excited vibrational state.

II. Experimental
The details of the sub-Doppler resolution of the infrared spectrometer have been
described in previous work and the[20-21] preceding chapter.

The infrared laser isgenerated by nonlinear difference frequency generation (DFG) using the single mode ofthe dye laser and the Ar+ laser within a thin (10 mm×50 mm) periodically poled LiNbO3(PPLN) crystal. The frequency range available is 2500 to 5000 cm-1, which can easily be
extended with the Ar+ laser lasing at either 488 nm or 514 nm. The crystal is positioned
in a precisely temperature-controlled oven housing, which is tuned during scanning tocomplement the quasi-phase matching conditions. The DFG laser that exits the crystal
is split into two parts, signal and reference beams. These are monitored on separate
N2(l) cooled InSb detectors to achieve fast electronic subtraction of the common-mode
amplitude noise. The signal beam, before being imaged on the detector, traverses 16
times between two mirrors in a Herriot cell configuration downstream of a slit jet (4 cm
× 300 µm) expansion with concentration modulation at 50 kHz resulting from the88electric discharge source. The attenuated modulation signal is decoupled with a lock-in
detection scheme to improve the signal-to-noise ratio. Relative frequency is estimatedto about 8 MHz through interpolation of an optical transfer market cavity with free
spectral range (FSR) of 250 MHz. The absolute frequency is calibrated with respect to
the CH4 R(4) line at 3067.300 cm-1.[22]Phenyl radical is generated through electron associative detachment in theelectric discharge as illustrated in equation 4.1.
C6H5Br + e - → C6H5 + Br

-

(4.1)

The products of the discharge are then adiabatically cooled in a supersonic expansion at
the exit of the discharge source. A dilute sample of the precursor bromobenzene (0.3 %)is prepared in a 70:30 Ne:He mixed carrier gas. The discharge source makes use of a
50 kHz square modulating 500 Volt DC (current = 0.28 A) in a 1.2 ms pulse durationthat is operated at 19 Hz. This instrument achieves signal-to-noise of about 70 for thestrongest transitions of the phenyl radical.

III. Results and Discussion
A. Spectral Observation and Analysis
In our previous studies of the infrared absorption of the C−H stretching mode ofphenyl radical in the same spectral region, only one of these modes was observed (!19)
due to its significant infrared absorption intensity (17.1 km mol-1) and the distinctive
spectroscopic signature of a Q branch. The preliminary results of the assignment andanalysis were reported and discussed in the previous studies.[18]89Improvements insignal-to-noise permitted acquisition of the spectrum shown in Fig. 4.3, which enabled
further analysis to update results for the !19 band, as summarized in our 2013 work[23].

!

0.08
ν19

Absorbance (%)

0.06

Q branch

R branchν1

P branch
P branch

Q branch

0.04
R branch

0.02

0.00
3068

3070

3072

3074

3076

3078

-1

Wavenumber (cm )Fig. 4.3. Rotationally resolved spectrum of the phenyl radical in the region of3068−3078 cm-1. The cluster of lines near 3072 cm-1 is the Q branch of the !19 mode.
Another progression in the rovibrational spectrum is clear at the blue end of this region,
assigned as the R branches of the !1 band. The P and Q branches of the !1 band areinterspersed with the R branch of the !19 mode.
In addition to the observed !19 band, we observe another transition that arises
from the other band on the blue end of the spectrum, near 3076 cm-1. The P branch ofthis progression overlaps with the R branch of the !19 band and was thus difficult to
identify initially. The Q branch of this progression does not stand out as it does withthe !19 progression, indicating that the second observed transition has a different band90

type. More information can be gleaned by comparing the intensities of the two bands;
the second band shows non-negligible absorption intensity, about 70−80 % comparedwith the !19 band. As shown in Table 4.1, there are five C−H stretching modes in the
spectral region accessible with our laser system. The band type of the transition forthese five modes can be classified as either a-type or b-type according to the vibrational
symmetry (a1 and b2).

91

Symmetrymode

6-311g++(3df,3dp)

a1
a1
a1
b2
b2

ν1
ν2
ν3
ν19
ν20

3192.84 [8.6]a3180.11 [3.4]3160.53 [0.8]3182.28 [17.2]
3166.64 [3.9]

Phenyl radical
Anharmonic
Ar matrixc
Predictionb
3077
3065
3046
3067
3052

3086 [3.1]a3072 [0.2]3037 [0.2]3071 [10.6]3060 [0.1]CationGas phasedAr matrixe

3073.96850(8)3062.26480(7)3071.89150(7)3110Table 4.1. The symmetry and band origin frequencies (cm-1) from theoreticalpredictions, gas-phase, and Ar matrix studies of phenyl radical and cation in the regionof C−H stretching frequenciesa.
Numbers in brackets represent intensity in km mol-1b.
The anharmonic prediction is scaled from the theoretical calculation of density
functional B3LYP/6-311g++(3df,3pd) calculations scaled by a factor of 0.964c.
Friderichsen et al. 2001.[11]d.
This work.
e.
Winkler and Sander.[24]Theoretical calculations predict that the !1 vibration is the second strongest C-H
stretching band (8.6 km mol-1) and that !1 has a different band-type than !19. Thisjustifies assigning the new progression as !1. In our survey of this spectral region(± 20 cm-1), there is also a weak band observed to the red of the features in Fig. 4.3. The
transition position of this weak absorption is about 12 cm-1 red-shifted with respect tothe !19 band. The rovibrational lines of this weak band are shown in Fig. 4.4 as well as
the corresponding vibrational displacement vectors of hydrogen motion in !2. The
intensity of this weak band is about an order of magnitude smaller than that for the !1
and !19 bands. In Fig. 4.4 we clearly see weak P and R branches, but no observation of
the Q branch, consistent with a b-type band.

92!

Absorbance (%)

0.02

0.01

ν2

R branch

P branch

0.00
!

3058

3060

3062

3064

3066

-1

Wavenumber (cm )Fig. 4.4. Spectrum of the weak band observed at 3062 cm-1 with thecorresponding vibrational assignment. The rovibrational lines of the P and R branch
are clearly visible but the Q branch is not easily identified. The correspondinghydrogen displacement vectors and the induced dipole moment are also shown in the
top of the figure.
The assignments of rotational structure for the fundamental C−H stretchingassociated with !1 and !19 also are further confirmed via the two line ground state
combination difference based on the result from the mm-wave studies of McMahon
et al.,[17] which enables us to eliminate the possibility of the excitation from a hot bandtransition. Unlike the prominent Q branch in the a-type band transition, the structure of
the Q branch for the b-type band is diffuse, thus not easily identified upon first93examination. Due to the highly oblate asymmetric top nature of phenyl radical, thestrong lines observed in the P and R branches of !1 originate from the overlap between
the two dominant progressions of N$1N ←N0N and N$0N ←N1N following the b-type$$

selection rules, ΔKa ≠ 0, ΔKc ≠ 0. Part of the spectrum is shown in Fig. 4.5 along with therotational assignments and the simulated spectrum below in red.
!
0.05
0.04

919←808
909←818

11111←10010
11011←10110

10110←909
10010←919

Absorbance(%)

0.03
0.02
0.01

*

*

0.00
-0.01
-0.02
-0.03
-0.04
3075.83075.93076.03076.13076.23076.3-1

Wavenumber(cm )Fig. 4.5. A portion of the rotationally resolved rovibrational (black) spectrum inthe vicinity of 3076 cm-1. These features arise from excitation of the C−H stretchingmode !1, with the label of the rotational quantum number (NKaKc). The star ( * ) indicatestransition lines associated with the !19 band. The spectrum shown below in red ispredicted based on the rotational constant and a rotational temperature of 15 K.
The asymmetric splitting of Kc is not clear and cannot be resolved within the subDoppler resolution (roughly 60 MHz) and thus provides no further information about
the upper state structure. However, due to the planarity of phenyl radical (B ≅ 2C), a
94

further overlap structure arises from the ΔN = ΔKa = ± 1 transition out of the levels with
Kc=N, N−2, N−4, etc. Due to this overlap, we cannot eliminate the contribution fromother rotational transitions. Overall, the analysis is first guided by the prediction of thefrequency position with line width convolution, and nuclear spin statisticalweighting (10 for Ka+Kc = even level, and 6 for Ka+Kc = odd level) are also included. The
constants for the ground state are fixed to the previously published values[17] while A’,
B’, C’, and band origin parameters are allowed to vary during the process of the fitting.
The final fitting results for the !1 and !2 bands in this work are summarized in Table 4.2,
with other results from the theoretical calculation and mm-wave studies. The result of
the previous !19 band analysis is also included for comparison.

The excited state

centrifugal distortional constants (ΔN, ΔNK, ΔK) could be tentatively fit but the results forthese parameters are an order of magnitude larger than expected by comparison withthe ground state constants. Instead, these constants are fixed to be the same as theground state values in the fitting process.

95

96

Table 4.2. Spectroscopic parameters of the phenyl radical in the ground state and excited CH stretching modes
from this worka.
Ref .[18]b.
Ref.[17]c.
The centrifugal distortion constants are kept the same as the ground state values.

In this work, the band origins for the !1 and !2 modes are determined for the firsttime in the gas-phase to be 3073.96850(8) cm-1 and 3062.26480(7) cm-1, respectively. In
the Ar matrix study, Friderichsen et al. assigned the band origins for these two modes as
3086 and 3072 cm-1, which are 12 and 10 cm-1 higher in energy than the results in thiswork. These shifts are considerably larger than that of !19, which was 3071.89150(7) cm-1
in the gas-phase compared to 3072 cm-1 in the condensed phase. In an Ar matrix, theradical is entrapped in an inert gas matrix and is often perturbed, which can affect theobserved transition frequencies and infrared intensities. Friderichsen et al.[11] estimatedthat the line widths of the IR band in the matrix spectra about 0.5−0.8 cm 1 due to matrix−

inhomogeneous line broadening. They considered the effect of broadening and matrix
shift and estimated that the deviation from the gas-phase to be about ± 1 % (30 cm-1 for
C-H stretches) for the corresponding transition frequencies.

A recent review by

Jacobs[25] provided a guideline for polyatomic free radicals in Ar matrices, indicatingthat shifts are usually less than 1 % and often occur at lower energy.

Table 4.1 summarizes findings from experimental results (gas-phase and matrix
isolation) and calculations (performed with B3LYP/6-311g++(3df,3dp) basis set
including an anharmonic scaling (0.964) for the ν1,ν2, andν19 bands). One interestingfinding is that the difference bands have different gas-phase to matrix shifts. One
physical interpretation for this observation is provided by examining the displacement
vectors for the different modes. Fig. 4.2 indicates that the vibrational motion of the Hatoms in !1, !2 and !3 tend to expand the molecule’s physical size. In contrast, the H
motion in !19 and !20 does less to affect the effective size of the radical. The shift infrequency could be through an effective increase in a force constant when the H atoms

97

must work against the rigid environment along the specific normal mode, and result inthe blue shift of the transition.

The effective rotational constants are also determined from the fitting process(shown in Table 4.2.) and analysis for the !1 and !2 bands agrees well with the
mm-wave study. The effect for the three molecular constants upon excitation of !1 is
estimated to be ΔA/A= −0.05 %, ΔB/B= −0.18 %, and ΔC/C=−0.08 %, consistent with inplane C−H motion. The behaviors for the !2 band are also listed in Table 4.2. The ΔC/Cof the !19 band is −0.10 %, larger than for the !1 and !2 bands analyzed here (− 0.08 %),indicating a larger change along the C axis, the out of plane rotational axis. Further
information can be extracted from the inertia defect (Δ ≡ Ic−Ia−Ib) for the upper state.
Our result indicates that the vibrational excited states maintain a planar structure, with∆(!1) = 0.009 amu⋅Å2, ∆(!2) = 0.016 amu⋅Å2, and ∆(!19) = 0.089 amu⋅Å2. These are all nonzero but small, similar to the ground vibrationless level (∆ = 0.046 amu⋅ Å 2). The
inertial defects for the three vibrationally excited states also support the assignment ofin-plane vibrational modes. Any mode with significant out-of-plane displacement or
combination bands with out-of-plane motion would have excited state ∆ values that aremuch larger.

In our analysis of the spectrum in this region, there are still a few features thatcannot be assigned. It is plausible that there exists coupling to other vibrational modes
that have no oscillator strength (dark modes). For such a large molecule as phenyl
radical, the density of vibrational state is estimated to be about 5−10 states/cm-1.[18] This
mainly arises from counting the various low-frequency bending modes. The spectrum
98

will become complex due to the mixing of rovibrational levels of the bright and darkstates when the oscillation character is shared by two eigenstates. However, in our
analysis, we do not observe spectroscopic signatures of any given coupling to specificstates.

B. Intensity Analysis
Since we have observed three gas-phase C-H vibrations, !1, !2, and !19, it isinteresting to compare the experimental integrated intensity for the vibrational bands.
Besides the predicted band origin for each mode, the predicted intensity (in km mol-1) isalso included in Table 4.1. The values are !1 = 8.5, !2 = 3.7, !3 = 0.8, !19 = 17.1 and !20 =4.0 km mol-1. As mentioned in the previous section, due to overlap of the dominantfeatures in the P and R branches, extracting the total integrated intensity for each bandis challenging. However, there are still many single isolated rovibrational features thatcan be analyzed.

For a given single isolated rovibrational feature, the integrated line absorption
coefficient is defined as: Si = ∫ ε(ν )dν , which is proportional to the number density of theradical in the initial level, the rotational partition function and the Hönl-London factor.
The explicit formula[26] is given in equation 4.2.
Si = ε(v)dv∫

g g e − Erot / kT ⋅ HL ⋅ µv
8π 2 1=
N A ⋅ v fi ⋅ ns N
3hc 4π ε 0Qrot

2(1 − e− hv fi / kT

)

(4.2)

Here NA is Avogadro’s number, gns and gJ are the nuclear spin statisticalweighting and degeneracy of the rotational level, HL is the Hönl-London factor, and µv

99

is the transition dipole moment for the specific vibrational transition. Here, we adapted
the function for a symmetric top rather than an asymmetric top.

-12ln [ S/gns⋅gJ⋅HL ]-13

Trot=11.2(9) K

-14

-15

-16
0.0

2.5

5.0

7.5

10.0

12.5

15.0

-1

Erot(cm )Fig. 4.6. Boltzmann plot of the single isolated lines in the !19 band leading to a!
rotational temperature of about 11.2 K.
The Boltzmann plot, which analyzes single rovibrational absorption features in
the !19 band, is shown in Fig. 4.6. The slope of the Boltzmann plot indicates a rotationaltemperature of 11.2 ± 0.9 K.

A comparison between the experimental intensity

observation and theoretical prediction offers a good test of the quality of the theoreticalmethod and basis set used in the calculation.

In order to avoid the bias of theoverlapping structure in the P and R branches, we first predicted the rovibrational lines
for each band based on the previous fitting rotational constants, rotational temperature(11.2 K), and linewidth convolution (60 MHz). The predicted intensity for each band is100

scaled with the experimental observation to match the whole spectrum.

Then theintegrated band absorption coefficient is obtained by the sum of the individual lines as
seen in equation 4.3
n

S≡

2

∑ S = (8π 3hc ⋅ 4π ε ) N ⋅ v ⋅ µ .
2

i

0

fi

v

(4.3)

i =1

The results are normalized to the strongest !19 band and shown and are summarized inFig. 4.7. Also included in Fig. 4.7 are the intensities reported from the Ar matrix studyof the phenyl radical.[17] Disparities between gas-phase and condensed phase are due toperturbations from the matrix likely causing some fluctuation in absorbance.

Theexperimental observations agree well with the ratio of the theoretical predictions
indicating that the high level of basis set and method in used with density functional
theory provide reliable predictions of the absorbance intensity. The intensity of the !1,!2 and !3 bands is related to the derivative of the dipole moment, which for !1 isdominated by the H atom motion in the C4 position. In the !3 mode, the effect of H atom
motions at the C3, C4 and C5 cancel since they are out-of-phase during the motion.
Unfortunately, we did not observe any signature of the !20 band. The reason may be
due to its weak intensity (3.9 km mol-1).

101

Fig. 4.7. The normalized integrated band intensity for the experimental (gasphase and Ar matrix) observations and the theoretical predictions.

The infrared spectrum of the phenyl cation has been studied by Winkler andSander[24] with Ar matrix isolation techniques. They reported that the corresponding !19band for the cation species is found at 3110 cm-1 in the condensed environment. No
other transitions have been reported for this isolated species in the gas-phase. The
calculated infrared intensity based on the B3LYP/cc-pVDZ method predicted that thismode carries an intensity of 204 km mol-1. The identification of rovibrational transitionsof this cation with sub-Doppler resolution should be feasible using the radical
spectroscopy presented here as a starting point. An open question currently exists
regarding the ground state multiplicity of the phenyl cation; there is some debate
regarding the gap between the 1A1 and 3B1 states.[27] Experimental studies of the phenyl
cation in the gas-phase should be able to shed new light on this issue.

102IV. Summary and Conclusions
The infrared absorption of the rovibrational transitions of the in-plane C−H
stretching mode of the phenyl radical have been investigated in a slit-jet supersonic
expansion. With the improvement of the absorption sensitivity since studying the !19mode, two new b-type bands resulting from C-H stretches have been observed. The
new bands are assigned as !1 and !2 and these assignments are confirmed via two-line
combination difference.

The band origins for these two modes are reported!1 = 3073.96850(8) cm-1and ! 2 = 3062.26480(7) cm-1. This constitutes the first observationof these bands in the gas-phase and the observed values agree well with theoreticalanharmonic predictions accompanying these findings.

Physical interpretations are

given regarding the discrepancy between the results of theoretical prediction, Ar
matrix, and gas-phase, both for band origins and absorbance intensities. The inertial
defects for each are also discussed and indicate planarity of the excited vibrational
states. The band intensities of the !1 !2 and !19 bands have been compared and theresults agree well with theoretical predictions. The capability to measure the infrared
spectroscopic signatures of aromatic radical species offers a promising probe for
identification of the ubiquitous PAHs in the interstellar medium and provides
spectroscopic information relevant for combustion models.

103

References for Chapter 4[1] A. Kazakov and M. Frenklach. Dynamic modeling of soot particle coagulation andaggregation: Implementation with the method of moments and application tohigh-pressure laminar premixed flames. Combustion and Flame 1998, 114, 484-501.
[2] J. A. Miller and C. F. Melius. Kinetic and thermodynamic issues in the formation ofaromatic-compounds in flames of aliphatic fuels. Combustion and Flame 1992, 91,
21-39.
[3] R. I. Kaiser, O. Asvany, Y. T. Lee, H. F. Bettinger, P. V. Schleyer and H. F. Schaefer.
Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon
molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3; X 1A' )formation from reaction of C6H5 (X 2A1) with methylacetylene, CH3CCH (X 1A1)
Journal of Chemical Physics 2000, 112, 4994-5001.
[4] P. Ehrenfreund. Astrophysical chemistry: Molecules on a space odyssey. Science
1999, 283, 1123-1124.
[5] H. Bohm and H. Jander. PAH formation in acetylene-benzene pyrolysis. Physical
Chemistry Chemical Physics 1999, 1, 3775-3781.
[6] E. Herbst. Chemistry in the interstellar-medium. Annual Review of Physical Chemistry
1995, 46, 27-53.
[7] P. H. Kasai, E. Hedaya and E. B. Whipple. Electron spin resonance study of phenyl
radicals isolated in an argon matrix at 4 K. Journal of the American Chemical Society
1969, 91, 4364-4368.
[8] J. E. Bennett, B. Mile and A. Thomas. Electron spin resonance spectrum of phenyl
radical prepared by chemical reaction at 77 K. Chemical Communications 1965, 265267.
[9] J. Pacansky and J. Bargon. Low-temperature photochemical studies on acetylbenzoyl peroxide - observation of methyl and phenyl radicals by matrix-isolation
infrared spectroscopy. Journal of the American Chemical Society 1975, 97, 6896-6897.

104[10] J. G. Radziszewski, M. R. Nimlos, P. R. Winter and G. B. Ellison. Infraredabsorption spectroscopy of the phenyl radical. Journal of the American Chemical
Society 1996, 118, 7400-7401.
[11] A. V. Friderichsen, J. G. Radziszewski, M. R. Nimlos, P. R. Winter, D. C. Dayton, D.
E. David and G. B. Ellison. The infrared spectrum of the matrix-isolated phenyl
radical. Journal of the American Chemical Society 2001, 123, 1977-1988.
[12] N. Ikeda, N. Nakashima and K. Yoshihara. Observation of the ultravioletabsorption spectrum of phenyl radical in the gas-phase. Journal of the American
Chemical Society 1985, 107, 3381-3382.
[13] J. G. Radziszewski. Electronic absorption spectrum of phenyl radical. Chemical
Physics Letters 1999, 301, 565-570.
[14] T. Yu and M. C. Lin. Kinetics of phenyl radical reactions studied by the cavity-ringdown method. Journal of the American Chemical Society 1993, 115, 4371-4372.
[15] T. Yu and M. C. Lin. Kinetics of the C6H5 + O2 Reaction at Low-Temperatures.
Journal of the American Chemical Society 1994, 116, 9571-9576.
[16] K. Freel, M. C. Lin and M. C. Heaven. Cavity ring-down spectroscopy of thephenyl radical in a pulsed discharge supersonic jet expansion Chemical Physics
Letters 2011, 507, 216-220.
[17] R. J. McMahon, M. C. McCarthy, C. A. Gottlieb, J. B. Dudek, J. F. Stanton and P.
Thaddeus. The radio spectrum of the phenyl radical. Astrophysical Journal 2003,
590, L61-L64.
[18] E. N. Sharp, M. A. Roberts and D. J. Nesbitt. Rotationally resolved infrared
spectroscopy of a jet-cooled phenyl radical in the gas phase. Physical Chemistry
Chemical Physics 2008, 10, 6592-6596.
[19] F. Dong, S. Davis and D. J. Nesbitt. Slit discharge IR spectroscopy of a jet-cooled
cyclopropyl radical: Structure and intramolecular tunneling dynamics. Journal of
Physical Chemistry A 2006, 110, 3059-3070.
[20] A. S. Pine. High-resolution methane nu-3-band spectra using a stabilized tunabledifference-frequency laser system. Journal of the Optical Society of America 1976, 66,
97-108.

105

[21] S. Davis, M. Farnik, D. Uy and D. J. Nesbitt. Concentration modulation
spectroscopy with a pulsed slit supersonic discharge expansion source. Chemical
Physics Letters 2001, 344, 23-30.
[22] J. Pliva and A. S. Pine. Analysis of the 3-mu-m bands of benzene. Journal of
Molecular Spectroscopy 1987, 126, 82-98.
[23] G. T. Buckingham, C.-H. Chang and D. J. Nesbitt. High-Resolution Rovibrational
Spectroscopy of Jet-Cooled Phenyl Radical: The !19 Out-of-Phase Symmetric CH
Stretch. The Journal of Physical Chemistry A 2013, 117, 10047-10057.
[24] M. Winkler and W. Sander. Isolation of the phenyl cation in a solid argon matrix.
Angewandte Chemie-International Edition 2000, 39, 2014-2016.
[25] M. E. Jacox. The vibrational-energy levels of small transient molecules isolated inneon and argon matrices. Chemical Physics 1994, 189, 149-170.
[26] P. R. Bunker, P. Jensen, N. R. C. Canada and N. R. C. C. M. P. Program, MolecularSymmetry and Spectroscopy, NRC Research Press, 2006, p. 90-92.
[27] A. Nicolaides, D. M. Smith, F. Jensen and L. Radom. Phenyl radical, cation, andanion. The triplet-singlet gap and higher excited states of the phenyl cation.
Journal of the American Chemical Society 1997, 119, 8083-8088.

106Chapter 5Hydropyrolysis of Ethylbenzene in a Heated Micro-Reactor

Abstract
We have investigated the role that gas-phase hydrogen atoms can play in thepyrolysis of ethylbenzene (C6H5CH2CH3) in a heated micro-reactor.

Methyl nitrite(CH3ONO) thermally decomposes to form formaldehyde (CH2O), nitric oxide (NO),hydrogen atom at reactor temperatures of 800 K and higher. Gas-phase methyl nitrite
can be mixed with ethylbenzene and the pyrolytically formed hydrogen atoms areshown to alter the pyrolysis of ethylbenzene.

Hydrogen mediated pyrolysis, orhydropyrolysis, is studied using both photoionization mass spectrometry and matrix
isolation infrared spectroscopy.

The products observed during hydropyrolysis of

ethylbenzene are styrene (C6H5CH=CH2), toluene, (C6H5CH3), benzene (C6H6), andethylene (CH2=CH2).

I. Introduction
As the world’s energy usage continues to grow, the use of fossil fuels has been
under increasing scrutiny due to its one-directional deposition of carbon from the solid
phase into the gas phase in the form of greenhouse gasses. Alternate fuel sources suchas biofuels provide an attractive alternative since the fuel precursor, plants, remove
carbon from the air rather than from inside the earth. However, biofuel globalization isstill limited in part due to the difficulty of rendering the stored energy in plants
107chemically available and industrially practical. Efforts have been marginally successful
to utilize cellulose based plant matter but utilization of the second largest polymer
component of plants, lignin,[1] has proven elusive due to challenges in liquefaction anddeoxygenation of lignin fuel stock. Discovery of a more efficient de-polymerization
mechanism will be necessary before any lignin derived liquid fuel can become a viableenergy source in our current environment.
To this end, scientists have studied techniques to process lignin from its raw
form into a more useful, monocyclic form.

Lignin is made up of threephenylpropanoids with varying degrees of methoxylation (see Chapter 1).

Thesemonomers combine to form a complex network of cross linkages that make up lignin as
it is found in nature.[1] Previous efforts to process lignocellulosic biomass have explored
fast pyrolysis,[2-3] metal-catalyzed decarboxylation,[4] metal-catalyzed hydrogenation[5]and hydrogenolysis.[6] One of the more successful techniques thus far has been fastpyrolysis, where high pressures (1500-2500 psig) produce pyrolysis oils that require
subsequent upgrading due to their high acidity, low heating value, and high oxygen
content.

Recent work at the Gas Technology Institute has demonstrated a promising newtechnique to convert raw plant matter to gasoline and diesel fuel through a process
called integrated hydropyrolysis and hydroconversion (IH2).[7] In this process, biomass
feedstock is fed into a fluidized catalyst bed under 14–35 bar of hydrogen gas at
temperatures between 350°C and 480°C.

During hydropyrolysis, the biomass isdeoxygenated to less than 1 % oxygen content and carbon that is bound in complex
lignin linkages is converted to liquid phase hydrocarbons that are easily separated from
aqueous phase products. These hydropyrolysis oils are lighter than pyrolysis oils and108show smooth boiling point distributions in the ranges of gasoline, diesel and jet fuel.
These advances are particularly promising because the hydrogen responsible forhydropyrolysis is regenerated in the process, whereby hydrogen acts as a catalyst rather
than a reagent. As this promising research demonstrates a potential future in biofuel
gasification, more work must be done to gain a more detailed understanding of thechemistry that underlies IH2.
Recent work by Vasiliou et al. has begun to elucidate the chemical mechanisms inplay during the hydropyrolysis process.[8] Gas phase benzaldehyde was pyrolyzed in aheated tubular reactor and the fragments were investigated using photoionization mass
spectrometry and matrix isolation spectroscopy. It was found that benzaldehyde reacts
with hydrogen atom to produce benzene and HCO, the latter of which further
decomposes to CO and H, thus establishing a hydrogen atom chain reaction. Fig. 5.1
summarizes the chemical pathways discovered.

109

Fig. 5.1. Schematic of the chemical pathways confirmed for the pyrolysis ofbenzaldehyde (C6H5CHO) mixed with methyl nitrite (CH3ONO). The methyl nitrite candecompose at low temperatures (900 K) in the micro-reactor to produce hydrogen atom.
Hydrogen atom can replace the aldehyde group from benzaldehyde but anotherhydrogen atom is formed in the process, making this process catalytic.
As shown in Fig. 5.1, to confirm that the decomposition was hydrogen atom
mediated, a chemical source of H atom was employed. Methyl nitrite is an attractivehydrogen atom source because the first step of decomposition is facilitated by a very
weak bond dissociation enthalpy.

The CH3O—NO bond can be calculated fromexperimental heats of formation[9-10] to be 42.7 ± 0.6 kcal mol-1, which is sufficiently small
to begin decomposition at temperatures lower than most other thermal decomposition
pathways. This is a convenient source of hydrogen atom for pyrolysis that is available
at lower temperatures than those at which target molecules (in this case benzaldehyde)
begin to decompose.

110

In the absence of a low-temperature hydrogen source, pyrolysis of benzaldehyde
required heating the reactor to sufficiently high temperatures (1600 K) to provide
unbound hydrogen atoms.

Establishing similar mechanisms at lower pyrolysis

temperatures and for a broad array of substituent groups will aid in determining amechanistic description of the hydropyrolysis that converts biomass feedstock into
viable gasoline or diesel fuel sources.
In order to gain a deeper understanding of catalytic hydropyrolysis, a detailed
study of the simplest biofuel molecules has been preformed in our labs, starting with
ethylbenzene(C6H5CH2CH3),

isopropylbenzene (C6H5CH(CH3)2).

n-propylbenzene(C6H5CH2CH2CH3),

andThese alkylbenzene molecules function as

surrogates for the aromatic ring/side-chain bonding found in lignin polymers. To
isolate the effects of hydrogen atoms on the thermal decomposition of these relatively
small molecules, a dilute hydrogen atom source has been developed and characterizedthat allows for the presence of hydrogen atoms at much lower temperatures than thoseused in the study of benzaldehyde.
Pyrolysis reactions of these alkylbenzene molecules in a micro-tubular reactor
are studied using the combination of photoionization mass spectrometry and matrix
isolation Fourier transform infrared spectroscopy to identify the fragments formed as aresult of heating, both in the presence and absence of the hydrogen atom source. In thisway, the products that form as a result of hydropyrolysis can be isolated from those that
occur during the un-catalyzed pyrolysis. The data show that through both hydrogen
atom abstraction and addition to the parent molecule, the catalytic hydrogen atoms
promote chemical pathways that were previously inaccessible until much higher
pyrolysis temperatures. These findings will hopefully be used to modify and optimize111

current hydropyrolysis practices in the conversion of lignin containing biomass tofungible fuel sources in the future.

II. Experimental
Ethylbenzene (C6H5CH2CH3) and methyl nitrite (CH3ONO) are pyrolyzed in aheated SiC micro-reactor as described in Chapter 2. At the exit of the reactor, thepyrolysis products are entrained in a supersonic expansion of either helium or argon
and are then detected using either photoionization mass spectrometry (PIMS) or argon
matrix isolation infrared (IR) spectroscopy. The reactor wall temperature is monitored
using a Type-C thermocouple and temperature is controlled using a variabletransformer to run current through the SiC reactor to cause resistive heating.
Methyl nitrite was synthesized daily for each set of experiment. Sodium nitrite
and methanol were mixed in a round bottom flask and 37 % hydrochloric acid was
slowly added in using a dropping funnel and the reaction flask was cooled in an ice
bath. The gas-phase methyl nitrite was collected in a cold finger submerged in liquid
nitrogen and the pale yellow solid methyl nitrite was purified by a few freeze-pumpthaw cycles. The sample gas cylinder was prepared by removing the cold finger fromthe liquid nitrogen for roughly 1 minute and collecting the vapor-phase methyl nitrite
and diluting with helium or argon. Dilute ethylbenzene samples are prepared simply
by collecting the room temperature vapor pressure above liquid ethylbenzene.
PIMS is performed by tripling the tripled (355 nm) output of an Nd:YAG laser
and intersecting the light with the skimmed output of the heated micro-reactor. The 9th
harmonic is generated by nonlinear sum frequency generation in a stagnant Xe:Ar(pressure ratio 10:90) cell mounted to the PIMS vacuum chamber. Upon ionization by112

10.487 eV photons, cations are accelerated into a commercial time-of-flight spectrometerand PIMS spectra are recorded by monitoring the microchannel plate (MCP) detector
signal over time.

Matrix isolation IR spectra were recorded by seeding the target

molecules in argon and then aiming the output of the reactor at a cryogenically cooled
CsI window maintained at roughly 20 K. Argon condenses onto the window trapping
all dilute species in an inert matrix where local densities of IR absorbers can be built up
without reaction or degradation of the pyrolysis products. After roughly two hours of
dosing onto the CsI window, the reactor assembly is rotated 90° and the sample is
moved into the beam path of a commercial Fourier transform IR spectrometer.

III. Results and Discussion
A. Ethylbenzene Decomposition
The first molecule investigated was ethylbenzene (C6H5CH2CH3), which is thesimplest alkyl benzene after toluene (C6H5CH3). This molecule has a sufficient vapor
pressure at room temperature to make dilute pre-made mixes in helium. The dilution
discussed here is 0.1 % but further dilution shows similar decomposition pathways. A
more recent study on benzyl radical decomposition[11] has made use of ethylbenzene as
the main precursor for pyrolytic production of benzyl radical.
ethylbenzene at temperatures up to 1300 K is shown in Fig. 5.2.

113

The pyrolysis of

Fig. 5.2. Thermal decomposition of dilute ethylbenzene (0.1 % in He) at 900 K,
1200 K and 1300 K. The only major products observed are benzyl radical (m/z 91) andmethyl radical (m/z 15) at 1200 K. Upon heating to 1300 K, a third decomposition
product is observed at m/z 65.
The decomposition of ethylbenzene observed using PIMS is relatively simple fortemperatures up to 1300 K. The first products are benzyl radical and methyl radical,
which is predictable given the weak C6H5CH2—CH3 bond[11] (DH298 = 77.6 ± 0.7
kcal mol-1). At 1300 K a third feature appears at m/z 65 that has been confirmed to be
cyclopentadienyl radical (c-C5H5).
To identify the effects of hydrogen atom on ethylbenzene pyrolysis, methyl
nitrite was added to the premixed sample volume and PIMS was recorded duplicating
all other experimental conditions. Methyl nitrite has been used as pyrolytic hydrogen114

atom source previously[8] but the process was confirmed here. Fig. 5.3 shows the PIMS
spectra of methyl nitrite pyrolysis between 600 K and 1200 K. Pyrolysis is initiated at
800 K as indicated by the feature at m/z 30. Following the chemical diagram in Fig. 5.1,
this is assumed to be nitric oxide (NO). The co-produced fragment methoxy radical(IE(CH3O) = 10.726 ± 0.008 eV)[12] is likely responsible for the small peak at m/z 31 but is
expected to be short lived. Loss of a hydrogen atom forms formaldehyde (m/z 30),
which is very stable but has an ionization energy too large for the 118.2 nm PIMS
(IE(CH2O) = 10.8850 ± 0.0002 eV).[13] At 900 K and 1200 K, a feature emerges at m/z 15,
presumed to be methyl radical (IE(CH3) = 9.8380 ± 0.0004 eV),[14] which indicatesanother chemical pathway is also active. Methyl nitrite can also fragment as shown inequation 5.1.
CH3ONO (+M)  CH3 + ONO(5.1)

This pathway is higher in energy than the hydrogen atom forming route(DH298(CH3—ONO) = 59.1 ± 0.3 kcal mol-1)[9-10] but can explain the observed m/z 15
feature. The other product generated by the reaction in equation 5.1 would be NO2,
which, despite its ionization energy being less than 10.482 eV (IE(ONO) = 9.586 ± 0.002
eV)[15], is not observed, which may be the result of bimolecular reactions or a small
absorption cross section at 10.487 eV. The thermal decomposition of methyl nitrite has
been the topic of recent computational investigation,[16] where equations 5.1 is indicatedas well as direct formation of HNO (m/z 31). For this reason, the small feature at m/z
31 in Fig. 5.3 could be HNO. The potential existence of bimolecular chemistry further
clouds this issue.

For these purposes, however, the formation of hydrogen atom isindicated by its role in hydropyrolysis, i.e, the formation of benzene from115benzaldehyde. The presence or absence of other mechanisms is not relevant to thepresent work but could be the subject of further investigation.

Fig. 5.3. Thermal decomposition of 0.5 % methyl nitrite (CH3ONO) in a pulsed
He micro-reactor. PIMS was used to identify the parent peak at m/z 61 anddecomposition to NO is indicated starting at temperatures of 800 K. By 900 K, no
parent methyl nitrite is observed, although at 1200 K, a small feature at m/z 15 isobserved, indicating a secondary decomposition pathway to form methyl radical (CH3).

B. Hydropyrolysis of Ethylbenzene
When ethylbenzene is pyrolyzed in the presence of methyl nitrite, a few new
product features are observed compared to the simple decomposition shown in Fig. 5.2.

116

These PIMS spectra are shown in Fig. 5.4. By comparing the products in Fig. 5.2 and Fig
5.3 with those found in Fig. 5.4, we conclude that any new features are a result fromreactions between ethylbenzene and hydrogen atom.

Fig. 5.4. A mixture of 0.1 % ethylbenzene and 0.5 % methyl nitrite was pyrolyzed
at 900 K, 1200 K, and 1300 K in a micro-reactor and the products were detected using
118.2 nm PIMS. The features at m/z 78, 92, and 104 are only seen when the two
precursors are mixed and are concluded to be the result of hydropyrolysis.

117

The features in Fig. 5.4 that are not found in Fig. 5.2 or Fig 5.3 are m/z 78, 92, and104, which are each consistent with hydrogen mediated reaction pathways. Assuming asimilar pathway to that deduced in benzaldehyde pyrolysis (Fig. 5.1), hydrogen atom
can add to the ipso position carbon atom in ethylbenzene to form an sp3 hybridized
carbon atom. Formation of this adduct is stabilized by the doubly allylic radical formed
among the remaining five carbon atoms in the ring. This intermediate can regainaromaticity by losing either the hydrogen atom or the ethyl group. The former leads tono observable reaction but the latter will produce benzene and ethyl radical (CH3CH2).
Ethyl radical will likely form ethylene (IE(CH2=CH2) = 10.51268 ± 0.00003 eV),[17] thus
reproducing a hydrogen atom. By replenishing the hydrogen atom, the hydropyrolysis
is catalytic. This mechanism is illustrated in Scheme 5.1.

H
H
H
C
HC

H
C

H

C

HC

CH
C
H

H
H
C

C
H

HC
H

C

CH

C
H

HC
H

H

H
C

H

C

HC

*

H

H

CH

HC

C
H

CH
C
H

H

H

C

H

+

C
H

C
H

H

H
C
+

H

H

Scheme 5.1.

Another result of hydropyrolysis shown in Fig. 5.4 is the creation of a productwith m/z 104. This is assigned to be styrene (IE(C6H5CH=CH2) = 8.464 ± 0.001 eV),[18]the mechanisms for which is shown in Scheme 5.2. Rather than hydrogen addition onto
carbon, this mechanism indicates hydrogen abstraction to form H2 and the resultant
radical eliminates another hydrogen atom to form styrene (m/z 104). Here again, thehydrogen atom is catalytic.

118

H

H
H
H
C

H
C

HC

C

HC

CH
C
H

H
C
H

H

H
H

H

H2

H
C
HC HC

H
C

HC HC

CH CH
C
H

C
H

C

+

C

C
C

C
H

H
C

H

H
C

HH

HH

H2

H

H

C

HC

C

HC

CH

C

H

H

HC
H

C
H

Scheme 5.2.

The intermediate radical in Scheme 5.2 is in fact a benzyl radical with the formulaC6H5-CR2. As will be seen in Chapters 6 and 7 of this work, benzyl radicals have a set ofdecomposition pathways identified. Two products from the fulvenallene pathway[11]are consistent with features in Fig. 5.4, namely m/z 104 (coincident with styrene) andm/z 103. Since m/z 103 is quite small and 104 cannot be disambiguated without
further experiments, these pathways will not be addressed in detail.
A third change observed as a result of methyl nitrite addition is a dramaticincrease in the relative ratio of m/z 92 to 91. Due to the natural abundance of 13C, allfeatures in PIMS spectra corresponding to species containing carbon (m/z = N) have asmall feature at m/z = N+1 that results from roughly 1.08 % of molecules containing asingle 13C atom. In Fig. 5.4, the N+1 feature of m/z 91 at 1300 K is much larger than 13C
natural abundance would imply 3.79 %. This enhancement is significantly larger thanthe experimentally determined standard deviation on 13C peaks (1.02 ± 0.16 %), whichfound by comparing 15 different hydrocarbon molecules studied with our apparatus.
Given the hydrogen atom formed in the reactor, it seems that we are observing radicalradical recombination between H atom and benzyl radical (m/z 91) to form toluene(m/z 92).

Bond forming reactions like this are often too exothermic[11](DH298(C6H5CH2—H) = 89.8 ± 0.6 kcal mol-1) and the chemically activated adduct will
undergo homolytic bond cleavage. In this case, the statistically relevant enhancement
119

H
C

H

C
C

HC

CH
C
H

of m/z 92 indicates that with a sufficiently large recombination partner, the excessenergy can be redistributed into available degrees of freedom and the adduct survives.
To confirm the hydropyrolytic conversion from ethylbenzene to benzene andethylene (Scheme 5.1), it is important to ensure that the products in the PIMS spectra are
the correct isomers.

Further, ethylene cannot be ionized by 118.2 nm light(IE(CH2=CH2) = 10.51268 ± 0.00003 eV)[17] so a complementary technique is required tocorroborate the chemical mechanism. Matrix isolation spectroscopy is ideal for thissince the predicted products are both well studied and stable enough to be prepared in
pre-mixed samples. Dilute benzene and dilute ethylene were both studied individually
so that their IR matrix spectra can be directly compared to the pyrolysis products ofethylbenzene under the same spectroscopic and pyrolytic conditions. Fig. 5.5 showsthat benzene is formed by hydropyrolysis of ethylbenzene.

When ethylbenzene isdecomposed at 1200 K, vibrational bands for benzene are not observed. However,when methyl nitrite is added to the mixture and then decomposed with the same
conditions, three vibrational modes assigned to benzene are observed as indicated withblack arrows.

120

Ar#Matrix#IR#AbsorbanceBenzene 1200K
Ethylbenzene + Methyl Nitrite 1200K
Ethylbenzene 1200K
Methyl Nitrite 1200K;1675#cm###############ν4;1

1483#cm################ν13###(C6H6)######(C6H6);13046#cm################ν12#####(C6H6)#3060 3050 3040 1490

1480

680
Wavenumbers#(cm )
;1

675

670

Fig. 5.5. Matrix isolation IR spectra of gas mixtures flowed through a heatedmicro-reactor. The green trace is from dilute benzene heated to 1200 K, the black trace
is ethylbenzene and methyl nitrite heated to 1200 K, the red trace is ethylbenzene
heated to 1200 K and the blue trace is methyl nitrite heated to 1200 K. Arrows indicate
vibrational modes assigned[19] to benzene that are only seen when ethylbenzene andmethyl nitrite are pyrolyzed together.

121

Similarly, to confirm the other product from Scheme 5.1 (ethylene), thehydropyrolysis of ethylbenzene is compared to pure ethylene in matrix IR spectra in
Fig. 5.6. After ethyl radical is ejected from the H and ethylbenzene adduct, loss of one
hydrogen atom allows the formation of ethylene, replenishing the catalytic hydrogen
atom that can then react another ethylbenzene molecule. In Fig. 5.6 we can see that the
mixture of ethylbenzene and methyl nitrite heated to 1200 K reproduces two vibrationalmodes of ethylene that are not observed when ethylbenzene or methyl nitrite arepyrolyzed alone.

122

Ar0Matrix0IR0Absorbance

Ethylene 1200K
Ethylbenzene + Methyl Nitrite 1200K
Ethylbenzene 1200K
Methyl Nitrite 1200K

1450

31
'1(
1440 '1( 947$cm
9470cm
1440$$cm
ν1600000000ν7 ν7(
ν12(
(CH
(C
H
)
2 2)(
(C22CH
H2)2)((CH2 2CH001440

1430 960 955 950 945 940
31Wavenumbers0(cm )Fig. 5.6. Matrix isolation IR spectra of gas mixtures flowed through a heatedmicro-reactor. The green trace is from dilute ethylene heated to 1200 K, the black trace
is ethylbenzene and methyl nitrite heated to 1200 K, the red trace is ethylbenzene
heated to 1200 K and the blue trace is methyl nitrite heated to 1200 K. Arrows indicate
vibrational modes assigned[19] to ethylene that are only seen when ethylbenzene andmethyl nitrite are pyrolyzed together.

Matrix IR spectroscopy can also be used to confirm the recombination reaction of
H and benzyl radical. The vibrational spectrum of toluene has been assigned in the IR
spectrum from hydropyrolysis of ethylbenzene by comparing to spectra taken in a neon123matrix as is described in Chapter 7. The precursor cycloheptatriene (C7H8) has beenshown in previous work[20] and in our reactor to isomerize to toluene.

Directcomparison between spectra recorded in argon matrices and neon matrices iscomplicated by the fact that the two carrier gases perturb vibrational band origins bydifferent amounts, often differing by 5 to 10 cm-1. Many strong vibrational features
assigned to toluene from the cycloheptatriene precursor are found in a congested regionof the spectrum in the hydropyrolysis experiment and thus cannot be assigned
unambiguously. Between the Ar and Ne matrices and gas phase assignments fromprevious work,[21] four observed vibrational features can be assigned (see Table 5.1).
One features observed at 1383 cm-1 in the neon spectrum is an overtone band of toluene(!4 = 695 cm-1) that cannot be directly identified in the hydropyrolysis spectra due to astrong nearby absorbance feature originating directly from heated methyl nitrite. Manyother strong features assigned to toluene in the neon spectrum are also observed in thehydropyrolysis spectra but are omitted from Table 5.1 due to absorption features within
5 cm-1 that originate from either precursors or other known products.

Mode

Gas(a)

Neon(b)Argon(c)2 x !4
!19b
!8a
!7b
!4

1384
1472
1611
3032
695

1383
1472
1610
3036
696

1376
1470
1611
3035
congested

Table 5.1. Vibrational band assignments of toluene (a) Experimental values andmode numbering from previous work.[21](b)Present work, cycloheptatriene precursors 1300 K.
(c)
Present work, hydropyrolysis of ethylbenzene using methyl nitrite mixture, reactor at
1200 K.

124

The formation of styrene is also confirmed using IR spectroscopy. The liquidphase and IR and Raman vibrational spectra of styrene were assigned[22] by comparing
the d0, d3, d5 and d8 isotopomers. The 14 strongest features assigned are summarizedin Table 5.2 alongside corresponding features in the hydropyrolysis Ar matrix
spectrum. All 14 of the strongest modes can be identified as a unique feature in thehydropyrolysis spectrum or are in a region too congested with absorptions from othermolecules to make a distinct assignment.

Mode

Liquid(a)Argon(b)!37698congested
!25
776
778
!33
909
909
!31
992
992
!23
1019
1016
!21
1083
1086
!14
1412
1409
!13
1450
1446
!12
1494
congested
!11
1575
1580
!9
1630
congested!63029
congested!43061
congested!33084
congested
Table 5.2. Vibrational band assignments of styrene(a)Experimental values and mode numbering from previous work.[22](b)Present work, hydropyrolysis of ethylbenzene using methyl nitrite mixture, reactor at
1200 K.

IV. Conclusions
The pyrolysis of dilute ethylbenzene in a heated micro-reactor at temperatures
up to 1300 K shows relatively few decomposition products, namely m/z 15, 65, and 91.

125

Cleavage of the ethyl C—C bond forms methyl radical (m/z 15) and benzyl radical(m/z 91) at wall temperatures of 1200 K and further decomposition to m/z 65 isobserved upon heating to 1300 K. Cyclopentadienyl radical is shown to be the fragmentwith m/z 65 (as will be discussed in more detail in Chapter 6) and the co-produced
fragment acetylene (m/z 26) cannot be observed using our 118.2 nm ionization source.
Upon addition of the hydrogen atom source, methyl nitrite, pyrolysis is clearly affected.
Two new fragments are observed, m/z 78 and m/z 104, and we observe an increase in
the signal at m/z 92 in excess of the natural abundance 13C peak for m/z 91. Scheme 5.1
illustrates how hydrogen atom can catalytically produce benzene and ethylene bydisplacing the ethyl side chain from ethylbenzene and the two products are bothconfirmed using matrix isolation IR spectroscopy. Styrene is formed via hydrogen
abstraction of a side-chain hydrogen atom but subsequent H atom loss (regenerating thecatalytic hydrogen atom) forms the carbon double bond of styrene, see Scheme 5.2.
Matrix IR vibrational spectroscopy also confirms the identity of the species at m/z 104
as styrene. Finally, we show that radical-radical recombination occurs between benzyl
radical and hydrogen atom to form toluene. Despite the large exothermicity of thisprocess (D298H(C6H5CH2—H) = 89.8 ± 0.6 kcal mol-1) the chemically activated toluene isable to redistribute energy and exits the nozzle to be detected by both PIMS and matrix
IR. Similar hydropyrolysis experiments have been performed on other substituted
phenyl systems including n-propylbenzene (C6H5CH2CH2CH3), isopropylbenzene(C6H5CH(CH3)2), styrene (C6H5CH=CH2), phenol (C5H6OH), anisole (C6H5OCH3), andbenzyl phenyl ether (C6H5CH2OC6H5).

PIMS spectra for these are shown inAppendix A.

126

References for Chapter 5[1] G. Knothe. Analyzing biodiesel: Standards and other methods. Journal of theAmerican Oil Chemists Society 2006, 83, 823-833.
[2] A. V. Bridgwater, D. Meier and D. Radlein. An overview of fast pyrolysis ofbiomass. Organic Geochemistry 1999, 30, 1479-1493.
[3] A. V. Bridgwater. Review of fast pyrolysis of biomass and product upgrading.
Biomass & Bioenergy 2012, 38, 68-94.
[4] S. Ohyama, K. E. Popp, M. C. Kung and H. H. Kung. Effect of vanadia on theperformance of NiO in vapor-phase oxidative decarboxylation of benzoic acid tophenol. Catalysis Communications 2002, 3, 357-362.
[5] T. Q. Hu, B. R. James, S. J. Rettig and C. L. Lee. Stereoselective hydrogenation oflignin degradation model compounds. Canadian Journal of Chemistry-Revue
Canadienne De Chimie 1997, 75, 1234-1239.
[6] E. Furimsky. Catalytic hydrodeoxygenation. Applied Catalysis a-General 2000, 199,
147-190.
[7] T. L. Marker, L. G. Felix, M. B. Linck and M. J. Roberts. Integrated Hydropyrolysis
and Hydroconversion (IH2) for the Direct Production of Gasoline and Diesel
Fuels or Blending Components from Biomass, Part 1: Proof of Principle Testing.
Environmental Progress & Sustainable Energy 2012, 31, 191-199.
[8] A. K. Vasiliou, J. H. Kim, T. K. Ormond, K. M. Piech, K. N. Urness, A. M. Scheer, D. J.
Robichaud, C. Mukarakate, M. R. Nimlos, J. W. Daily, Q. Guan, H.-H. Carstensen
and G. B. Ellison. Biomass Pyrolysis: Thermal Decomposition Mechanisms ofFurfural and Benzaldehyde Journal of Chemical Physics 2013, 139, 104310.
[9] S. J. Blanksby and G. B. Ellison. Bond dissociation energies of organic molecules.
Accounts of Chemical Research 2003, 36, 255-263.
[10] J. B. Pedley, R. D. Naylor and S. P. Kirby, Thermochemistry of Organic Compounds,
Chapman and Hall, New York, 1986, p. 87-222.

127

[11] G. T. Buckingham, T. K. Ormond, J. P. Porterfield, P. Hemberger, O. Kostko, M.
Ahmed, D. J. Robichaud, M. R. Nimlos, J. W. Daily and G. B. Ellison. The
thermal decomposition of the benzyl radical in a heated micro-reactor. I.
Experimental findings. The Journal of Chemical Physics 2015, 142, 044307.
[12] B. Ruscic and J. Berkowitz. Photoionization mass-spectrometric studies of theisomeric transient species CD2OH and CD3O. Journal of Chemical Physics 1991, 95,
4033-4039.
[13] A. M. Schulenburg, M. Meisinger, P. P. Radi and F. Merkt. The formaldehyde
cation: Rovibrational energy level structure and Coriolis interaction near the
adiabatic ionization threshold. Journal of Molecular Spectroscopy 2008, 250, 44-50.
[14] J. A. Blush, P. Chen, R. T. Wiedmann and M. G. White. Rotationally resolved
threshold photoelectron-spectrum of the methyl radical. Journal of Chemical
Physics 1993, 98, 3557-3559.
[15] K. S. Haber, J. W. Zwanziger, F. X. Campos, R. T. Wiedmann and E. R. Grant.
Direct determination of the adiabatic ionization potential of NO2 bymultiresonant optical absorption. Chemical Physics Letters 1988, 144, 58-64.
[16] R. S. Zhu, P. Raghunath and M. C. Lin. Effect of Roaming Transition States upon
Product Branching in the Thermal Decomposition of CH3NO2. The Journal of
Physical Chemistry A 2013, 117, 7308-7313.
[17] X. Xing, B. Reed, M. K. Bahng and C. Y. Ng. Infrared-vacuum ultraviolet pulsed
field ionization-photoelectron study of C2H4+ using a high-resolution infraredlaser. Journal of Physical Chemistry A 2008, 112, 2572-2578.
[18] J. M. Dyke, H. Ozeki, M. Takahashi, M. C. R. Cockett and K. Kimura. A study ofphenylacetylene and styrene, and their argon complexes PA–Ar and ST–Ar withlaser threshold photoelectron spectroscopy. The Journal of Chemical Physics 1992,
97, 8926-8933.
[19] T. Shimanouchi Tables of Vibrational Frequencies, Consolidated Volume I, NSRDS-NBS
39, 1972.
[20] S. H. Luu, K. Glänzer and J. Troe. Thermal isomerization in shock waves and flash
photolysis of cycloheptatriene, III. Berichte der Bunsengesellschaft für physikalische
Chemie 1975, 79, 855-858.

128

[21] N. Fuson, C. Garrigou-Lagrange and M. L. Josien. Spectre infrarouge et attribution
des vibrations des toluènes C6H5CH8, C6H5CD8 et C6D5CD3. Spectrochimica Acta
1960, 16, 106-127.
[22] D. A. Condirston and J. D. Laposa. Vibrational-spectra of styrene-H8, styrene-D3,
styrene-D5, and styrene-D8. Journal of Molecular Spectroscopy 1976, 63, 466-477.

129

130

!

Chapter 6
The Thermal Decomposition of the Benzyl Radical in a Heated MicroReactor: I. Experimental Findings

Abstract
The pyrolysis of the benzyl radical has been studied in a set of heated microreactors. A combination of photoionization mass spectrometry (PIMS) and matrix
isolation infrared (IR) spectroscopy has been used to identify the decomposition
products. Both benzyl bromide and ethyl benzene have been used as precursors of theparent species, C6H5CH2, as well as a set of isotopically labeled radicals: C6H5CD2,
C6D5CH2, and C6H513CH2. The combination of PIMS and IR spectroscopy has been used
to identify the earliest pyrolysis products from benzyl radical as: C5H4=C=CH2, H atom,
C5H4-C≡CH, C5H5, HC≡CCH2, and HC≡CH. Pyrolysis of the C6H5CD2, C6D5CH2 andC6H513CH2 benzyl radicals produces a set of methyl radicals, cyclopentadienyl radicals,
and benzynes that are not predicted by a fulvenallene pathway. Explicit PIMS searches
for the cycloheptatrienyl radical were unsuccessful; there is no evidence for theisomerization of benzyl and cycloheptatrienyl radicals: C6H5CH2 ⇋ C7H7. These labelingstudies suggest that there must be other thermal decomposition routes for the C6H5CH2
radical that differ from the fulvenallene pathway.

!

131

!
I. Introduction
Benzyl radical, C6H5CH2, is a long-lived combustion species and is an importantsoot-precursor. Reaction with oxygen is quite slow because the resulting adduct,
C6H5CH2OO, is not very stable: C6H5CH2 + O2 ⇋ C6H5CH2OO. Consequently theoxidation of C6H5CH2 is sluggish, which leaves benzyl radicals available to contribute tofurther chemical reactions including soot formation.
This chapter is focused on the thermal decomposition pathways of the benzyl
radical itself. Pyrolysis of benzyl bromide and ethyl benzene in a set of heated microreactors produced samples of C6H5CH2 that were observed to decompose to a complex
set of atoms, radicals, and metastables. A combination of photoionization mass
spectrometry (PIMS) and matrix infrared spectroscopy (IR) has been used to identify
the initial decomposition products.
In 1948 pioneering shock tube measurements[1] decomposed the aromatics
toluene and xylene. About 30 years later, a set of experiments was reported[2] in which
toluene was decomposed in a Knudsen cell. The Knudsen cell was heated to 2100 K andcoupled to an electron impact (EI) mass spectrometer. The resulting mass spectra
identified the presence of C2H2, C3H3, C4H2, C4H4, C5H5, and C6H6 and all species wereassigned as thermal decomposition products of C6H5CH2. Subsequent shock tube
studies were carried out on a variety of aromatic precursors and the decomposition
products were monitored by vacuum ultraviolet (VUV) absorption spectroscopy.[3-5] It
was demonstrated[5] that ethylbenzene decomposes to the radical pair, (C6H5CH2 andCH3) and not to (C6H5CHCH3 plus H). In 1986, Rao and Skinner[6] presented the resultsof shock tube measurements for the decomposition of ethylbenzene-dl0 and ethyl-α,α-

!

132

!
d2-benzene in argon behind incident shock waves at temperatures of 1430-1740 K.
Progress of the reaction was followed by analysis for H and D atoms by atomic
resonance absorption spectroscopy (ARAS). ARAS spectroscopy demonstrated thatbenzyl radicals decompose to hydrogen atoms and other unknown products. Benson
made the imaginative suggestion[6] that the pyrolysis of benzyl radical proceeded byrearrangement to a bicyclic species followed by fragmentation to fulvenallene(C5H4=C=CH2) and H atom. Fulvenallene could further decompose to the fulvenallenyl
radical, C5H4-C≡CH. Rather than Benson’s stepwise rearrangement to fulvenallene,
benzyl could suffer a concerted fragmentation to the cyclopentadienyl radical andvinylidene. Vinylidene (CH2=C:) is known[7-8] to rapidly isomerize to HC≡CH. Finallyclassical

organic

chemistrymight

expectrearrangementofbenzyltothecycloheptatrienyl radical; subsequent C7H7 fragmentation could directly provide
HC≡CH and C5H5. These suggestions are outlined in Fig. 6.1.

!

133

!

Possible Pathways for Benzyl Radical Thermal Decomposition
H
C

HC
HC

H
C

CH 2•

C

HC

CH

•

HC

C
H
benzylm/z 91

C
H

CH
CH
CH

cycloheptatrienylm/z 91

H
H
CH 2

C

C

C

H

C
HC
HC

•

C

HC

CH

HC

CH

CH

HC

CH

fulvenallenyl
m/z 89

H
C
•

HC

CH

C
H
cyclopentadienylm/z 65

fulvenallene
m/z 90+ HC≡CHm/z 26

Fig. 6.1. Benzyl radical is proposed to decompose not through direct bondbreakage but through isomerization(s) reactions. Loss of one hydrogen atom can leadto fulvenallene (C5H4=C=CH2, m/z 90) and a second hydrogen atom loss would
produce the fulvenallenyl radical (C5H4-C≡CH, m/z 89). Isomerization tocycloheptatrienyl radical has also been proposed.

Decomposition of benzyl radical has also been investigated by photochemistry.
Molecular beams of either toluene or cycloheptatriene were irradiated with UV light toform “hot” benzyl radicals that underwent immediate unimolecular decomposition.
Fragmentation products were detected using EI mass spectrometry[9] and the first direct
detection of C7H6 (m/z 90) was reported as the product of C7H7 → C7H6 + H.
Later experiments were reported to pursue Benson’s suggested mechanism.[10-11]
In the work of Jones et al.[12], shock tube measurements of the decomposition of benzyl
!

134

!
bromide were coupled with computational studies to suggest multiple benzyl radical
decomposition mechanisms. Four mechanisms were proposed: 1) formation of thenorbornadienyl radical followed by C5H5 + HCCH formation, 2) ring opening leading tolinear C7H6 + H and further fragmentation, 3) Benson’s isomerization to the bicyclic
intermediate, 6-methylenebicyclo[3.1.0]hex-3-en-2-yl radical that subsequently decomposes to the fulvenallenyl radical (C5H4-C≡CH) + H atom or C5H5 + HCCH and, 4)
isomerization to a seven-membered ring followed by formation of C5H5 + HCCH.
Further computational studies have reported[13] fulvenallene formation as the major
decomposition route for benzyl radical. Density functional theory (DFT) calculations[14]found that hydrogen addition to fulvenallene would result in C5H5 + C2H2. Recently
there has been extensive work[15-19] performed on potential energy surfaces for each of
the species C7H7, C7H6, and C7H5 and their interplay during benzyl radical
decomposition. Because of their likely importance during the decomposition of benzyl,
both fulvenallene and fulvenallenyl radical,[20-21] as well as the cycloheptatrienyl
radical[22] have been detected and identified by photoionization mass spectrometry.
To explore the thermal decomposition of the benzyl radical, we have prepared
C6H5CH2 from both ethylbenzene and benzyl bromide in a heated micro-reactor. By
increasing the temperature of the reactor, pyrolysis of the benzyl radical was observed.
Our goal is to identify the nascent fragmentation products: C6H5CH2 (+ M) → products.
Over the last decade, we have used a set of micro-reactors to examine thepyrolysis of complex organic molecules. These resistively heated silicon carbide (SiC)
reactors are typically 0.5 – 1 mm inner diameter and 2 – 3 cm long. Target molecules aredelivered to the reactor as dilute mixtures (typically 0.1 % or less) in He, Ar, or Ne
buffer gases with entrance pressures of 200 – 300 Torr. The micro-reactors can be

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operated in either pulsed or continuous flow (CW) modes and are heated totemperatures up to 1800 K. Residence times in the micro-reactors are in the range of 25 –150 µsec. Gases exit the hot micro-reactors into a vacuum of 10-6 Torr where all reactions
are quenched. A combination of photoionization mass spectrometry, matrix isolation
infrared spectroscopy, resonance-enhanced multiphoton ionization spectroscopy, andmicrowave spectroscopy has been used to identify all the decomposition products. This
package of spectroscopies is very powerful and enables identification of all products(atoms, radicals, metastables) that are formed in the first 100 µsec of complex fuelpyrolysis.[23-29] The goal of the present study is to produce clean samples of benzyl andto observe the temperature-dependent decomposition of the radical. With care, the setof micro-reactors can be used to confirm the fragmentation products from benzyl
radical and indicate which mechanism(s) are activated first.
The experimental thermochemical and electronic properties of benzyl radical(C6H5CH2, X̃ 2B1) have been compiled in Table 6.1, alongside data for its isomer, thecycloheptatrienyl radical (C7H7, X̃ 2E2"). Experimental information is included for threeprecursors of benzyl radical: toluene, benzyl bromide, and ethylbenzene.

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Experimental Properties of Benzyl and Cycloheptatrienyl Radicals
IE(C6H5CH2, X̃ 2B1)

7.2487 ± 0.0006 eV[30-31]∆fH298(C6H5CH2, X̃ 2B1)

49.7
(208

± 0.6
± 3

kcal mol-1
kJ mol-1)∆rxnH298(C6H5CH2 → HCCH + C5H5)

68
(284

± 2
± 6

kcal mol-1 this workkJ mol-1)∆acidH298(C6H5CH2-H)

382.3
± 0.5
kcal mol-1
(1600
± 2
kJ mol-1)
0.912 ± 0.006 eV

EA(C6H5CH2, X̃ 2B1)

∆isomerizationH298(C6H5CH2, X̃ 2B1 → C7H7, X̃ 2E2")15.3(64
IE(C7H7, X̃ 2E2")

± 0.9
± 4

[32]

[33]

[34]

kcal mol-1 this workkJ mol-1)

6.221 ± 0.006 eV[22, 35]∆fH298(C7H7, X̃ 2E2")

65.0
(272

± 0.7
± 3

kcal mol-1
kJ mol-1)∆rxnH298(C7H7 → HCCH + C5H5)

53
(220

± 2
± 7

kcal mol-1 this workkJ mol-1)± 0.02eV[36]

AE(C7H8 → C7H7+ + H)

9.36

[32]

∆fH298(C5H5, X̃ 2E1”)

63
(264

± 1
± 6

kcal mol-1
kJ mol-1)[37]

DH298(C6H5CH-H)

106
(444

± 4
± 15

kcal mol-1
kJ mol-1)[38-39]DH298(C6H5-CH2)

124.3
(520

± 0.9
± 4

kcal mol-1
kJ mol-1)[32]

DH298(C6H5CH2-H)

89.8
(376

± 0.6
± 3

kcal mol-1
kJ mol-1)[33]DH298(C7H7-H)

73.9
(309

± 0.5
± 2

kcal mol-1
kJ mol-1)

[36]

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DH298(C6H5CH2-Br)

61
(255

± 1
± 4

kcal mol-1
kJ mol-1)[32]

DH298(C6H5CH2-CH3)

77.6
(325

± 0.7
± 3

kcal mol-1
kJ mol-1)[32]

DH298(C6H5-CH2CH3)

102.3
(428

± 0.7
± 3

kcal mol-1
kJ mol-1)[32]

Important Ionization Energies
CH3

9.8380±0.0004

eV

[40]

HC≡CH

11.40081 ±

0.00001eV[41]

HC≡CCH2

8.7006±0.0002

eV

[42]

C5H5

8.4268±0.0005eV[43]

o-C6H4

9.03

±

0.05

eV

[44]

C6H5

8.32

±

0.04

eV

[45]

C5H4-C≡CH

8.19

±

0.02

eV

[20]

C5H4=C=CH2

8.22

±

0.01

eV

[20]

C6H5CH2

7.2487±0.0006eV[30-31]

C7H7

6.221

±

0.006

eV

[22, 35]

C6H5CH2CH3

8.7734±0.0007eV[46]

C6H5CH2Br

8.990

±

0.015

eV

[47]

Table 6.1. Experimental properties of benzyl and cycloheptatrienyl radicals andimportant ionization energies

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II. Experimental
The thermal decomposition of benzyl radical is studied by entraining a dilute
sample of precursor in helium, neon, or argon. The precursors used in this study are
benzyl bromide (C6H5CH2Br) and ethylbenzene (C6H5CH2CH3). Both precursors areavailable from Sigma-Aldrich with reported purities of 98 % and 99 %, respectively, andare liquid at room temperature. Additionally, multiple isotopomers have been
examined

including

C6H5CD2Br,

C6D5CH2Br,

C6H5CD2CD3, C6D5CH2CH3,

and

C6H513CH2CH3, all of which were purchased from CDN Isotopes, with reported isotopic
enrichment of 98 % for the deuterium isotopomers and 99 % for the 13C isotopomer; 13C
NMR was used to confirm that the α-carbon is 98.9 ± 0.1 % 13C. The vapor pressure ofethylbenzene is sufficiently large to prepare static sample volumes of between 0.1 %and 0.01 % ethylbenzene in up to 1500 Torr of noble gas. Benzyl bromide, however,requires flowing pure carrier gas over a sample of liquid precursor.

The benzyl

bromide sample was never heated so as to avoid higher C6H5CH2Br concentrations thatwould trigger bimolecular chemistry. The ratio of the vapor pressure at the sample
temperature to the backing pressure was used to approximate the concentration of
benzyl bromide in the beam. A careful dilution study with ethylbenzene shows that asconcentrations were lowered from 0.1 % to 0.02 %, some peaks in the mass spectra lost
signal intensity more rapidly than other peaks, thus indicating their higher orderdependence on concentration. As a result, all of the PIMS peaks discussed herein are
believed to be solely a result of unimolecular decomposition.
Benzyl radical, its precursors, and its fragmentation products are detected bymeans of PIMS and IR spectroscopy. We have carried out 118.2 nm PIMS with a pulsed

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micro-reactor as well as synchrotron-based PIMS with continuous flow micro-reactors
at the Lawrence Berkeley National Lab’s (LBNL) Advanced Light Source (ALS) in
Berkeley, CA, USA and the Swiss Light Source (SLS) at the Paul Scherrer Institute in
Villigen Switzerland.
PIMS with a pulsed micro-reactor[48] combines the 9th harmonic of a Nd:YAG
laser (118.2 nm or 10.487 eV). The laser is synchronized to a Parker general pulsed valve
backed by a helium-hydrocarbon mixture. The pulsed valve operates at 10 Hz and isopen for about 1 msec. A skimmer is placed about 1 cm from the exit of the SiC reactor
to select only the component of the expansion traveling in the longitudinal direction
with respect to the reactor tube. The internally frequency-tripled output of a Nd:YAG
laser is directed into a cell that is pressurized to 150 Torr with a 10:1 argon:xenon
mixture. This mixture is optimized to promote sum frequency generation that creates
118.2 nm VUV photons, which have sufficient energy to ionize most hydrocarbon
molecules. Generation of 118.2 nm photons by Xe/Ar tripling cells has been studied at
great length.[49] Based on the use of 30 mJ pulse-1 power at 355 nm from the YAG laser
and the established tripling efficiency of 1 x 10-5 in pure xenon[50-51] as well as thetransmission efficiency of the MgF2 lens, we expect to produce roughly 30 nJ pulse-1 of
118.2 nm light. This VUV laser light is directed into a vacuum chamber maintained at
10-7 Torr by a 1200 L/s Pfieffer TPU 1201 P turbomolecular pump and is thenintersected with the skimmed output of the supersonic expansion. After ionization themolecules in the expansion are accelerated into a Jordan reflectron time-of-flight
spectrometer with an MCP detector producing PIMS spectra at 10.487 eV.
Other experiments employ a tunable ionization source that is used with a set ofcontinuous flow micro-reactors to measure photoionization efficiency (PIE) curves.
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These PIMS experiments were conducted at the Advanced Light Source using thebeamline’s 9.0.2 molecular beam end-station. The gas mixture of product molecules
entrained in helium emerging from the hot micro-reactor is formed into a molecularbeam. The skimmed molecular beam is intersected with tunable synchrotron light andthe nascent ions are detected in a Jordan reflectron time-of-flight apparatus. Utilizationof pseudo-CW ionizing radiation requires that voltages on the repeller plate be pulsed
to allow discrete data acquisition. The ALS synchrotron does not easily permit scans
below 7.4 eV so in order to distinguish the isomers, benzyl (7.2 eV) andcycloheptatrienyl radicals (6.2 eV), tunable light down to 6 eV was required. These
photon energies are available at the Swiss Light Source, where tunable synchrotron
light extends to energies as low as 6 eV. During the SLS experiments[52] a continuous
stream of Ar and precursor was expanded through a micro-reactor forming a molecularbeam, which was skimmed. The products were detected applying imaging
photoelectron photoion coincidence (iPEPICO) techniques.[53]Matrix isolation Fourier Transform infrared spectroscopy is a complementary
detection method performed to help identify individual isomers present at the exit of
the reactor. A similar pulse valve and tubular reactor assembly is coupled to a helium
compressor-cooled cesium iodide (CsI) window that is enclosed and pumped to 10-6
Torr during pulse valve operation. Gas mixtures consist of dilute samples of molecularprecursor entrained in neon. Neon is used rather than argon because its smaller atomic
radius and lower electronic polarizability has been shown to create inert solid matricesthat have smaller perturbations than comparable matrices using argon. Gas mixturespulsed through the heated reactor are then directed towards the CsI window that ismaintained at 5.4 K. Gas is pulsed for 1 to 2 hours with the pulse valve opening time

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!
set to 1 msec and a repetition rate of roughly 10 Hz. Repetition rate and opening time
are tuned to maintain a consistent dose rate as measured by upstream pressure decreaseover time. After dosing is completed, the pulse valve assembly can be rotated out of theway to allow the CsI window to be lowered into the beam path of a commercial FTIR,
where between 200 and 1000 scans can be averaged to achieve a single composite
absorbance spectrum from 400 to 4000 cm-1 with 0.25 cm-1 resolution. Spectra taken in asolid neon matrix can then be compared directly to published neon matrix spectra or
carefully assigned using similar spectra taken in argon matrices or in the gas phase toallow vibrational assignments. The band origin assignments in neon matrices show
shifts from the gas-phase values of around 5 cm-1, while argon matrix assignments often
have shifts on the scale of 10 cm-1. A complete list of vibrational assignments formolecules detected during the decomposition of benzyl radical (which was preparedusing ethylbenzene as the precursor) is included in supplementary material (seeAppendix B).
The micro-reactor assembly used for pyrolysis is described in more detailelsewhere[48, 54-58] and consists of a 0.6 mm or 1 mm inner diameter SiC tubular reactor
that can be resistively heated up to 1800 K. Carbon discs are fitted to the outer diameterof the reactor approximately 1 cm apart and these form the contact points for theelectrical circuit used to resistively heat the reactor. A Type C thermocouple with
0.005” diameter wires is tightly attached to the SiC reactor between the two electricalcontacts so that the surface temperature can be monitored directly. Backing pressuresand downstream pressures vary between experiments. Table 6.2 is provided tosummarize the conditions across all experiments performed in this work.
!
!

142

!
!
CarrierGas

Flow

Ionization
SourceBacking
Pressure

OutletPressure

ReactorGeometry

Photoionization
Mass Spectrometry Heliumin Colorado

Pulsed118.2 nm(10.487 eV)
Laser

1500 Torr

2 x 10-6
Torr

I.D. 1 mm
HeatedLength: 15 mm

Photoionization
Mass Spectrometry Heliumat the ALS

Continuous

TunableSynchrotron

50 Torr2 x 10-5
Torr

I.D. 0.6 mm
HeatedLength: 10 mm

Photoionization
Mass Spectrometry Argon
at the SLS

Continuous

TunableSynchrotron

50 Torr

5 x 10-6
Torr

I.D. 1 mm
HeatedLength: 15 mm

Matrix Isolation IR
Spectroscopy inColorado

Pulsednone

1000 Torr

2 x 10-6
Torr

I.D. 1 mm
HeatedLength: 10 mm

Neon

Table!6.2.!Summary!of!Experimental!Conditions

Recent computational fluid dynamics (CFD) simulations[59] have dramaticallyclarified the nature of the pyrolysis in the micro-reactors. These devices operate in a
steady-state (CW) mode when using synchrotron photons (LBNL Advanced Light
Source and Paul Scherrer Institute Swiss Light Source) or in a pulsed mode for 10.487
eV PIMS and IR studies. During a CW experiment, the volumetric flow rate iscontrolled using a mass flow controller, and the pressure is measured just upstream of
the reactor and in the downstream vacuum chambers. The upstream pressure typicallyranges from 10 Torr to 400 Torr depending on the heating temperature and flow rate. In
the CW case, an interesting result of these simulations was the discovery of a “sweetspot”, which emerges from the interaction of the rising gas temperature and the rapidly
falling gas pressure, as will be discussed later in the paper. For a pulsed reactor, theflow rate is determined by the time-dependent geometry of the valve pintle and orifice,which unfortunately is not well known.

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The characteristics of CW and pulsed flows are quite different. In steady-statelaminar flow, the time-dependent terms in the Navier-Stokes equation drop out of theequation. In our situation the geometry is fixed and the mass flow rate is controlled,thus making CFD numerical solutions tractable[59] and detailed flow field information
attainable. Given a reaction mechanism and kinetic parameters, one can include
chemistry in the simulations, predicting, for example, the overall conversion percentageof the reactant. As a result, simulations can be used to test reaction mechanisms. In
pulsed flow, the geometry is varying in time due to motion of the pintle within thevalve and the instantaneous mass flow rate is thus unknown. Furthermore, the duty
cycle is such that the tubular reactor is completely purged of gas between pulses
resulting in a highly complex transient flow that transitions from collisionless tocollision-dominated over the course of a few milliseconds. This is a formidable problem
to model and is the subject of current research.

III. Results
We have chosen benzyl bromide and ethylbenzene as convenient precursors of
the benzyl radical.
C6H5CH2Br (+ M) → C6H5CH2 + Br

(6.1)

C6H5CH2CH3 (+ M) → C6H5CH2 + CH3(6.2)

Table 6.1 shows that ∆rxnH298(1) is 61 ± 1 kcal mol-1 (260 ± 4 kJ mol-1) while ∆rxnH298(2) is77.6 ± 0.7 kcal mol-1 (325 ± 3 kJ mol-1).

Because of the differing thermochemistry

between reactions (6.1) and (6.2), benzyl radicals are observed at lower decomposition
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!
temperatures when using C6H5CH2Br than with C6H5CH2CH3. However the higher
vapor pressure of ethylbenzene makes sample preparation comparatively simpler. An
important experimental difficulty associated with benzyl bromide is dissociative
ionization. The IE of C6H5CH2Br is 9.0 eV (see Table 6.1) and so 10.5 eV (118.2 nm) PIMS
is 1.5 eV above the ionization threshold. Consequently the ion resulting from irradiating
C6H5CH2Br with 118.2 nm, [C6H5CH2Br+]* (m/z 170/172), commonly fragments to
C6H5CH2+ (m/z 91) and Br. This dissociative ionization process obscures the m/z 91
signal that results from thermally formed benzyl radicals. While the IE(C6H5CH2CH3) is
8.8 eV (see Table 6.1), very few of the C6H5CH2CH3+ (m/z 106) cations subsequently
fragment, which produces cleaner PIMS spectra. Synchrotron radiation is especially
useful for taking PIMS spectra of benzyl bromide because the ionization source can belowered to just above the ionization energy of the precursor, 9 eV.

A. Decomposition of Benzyl-d0 Radical
Fig. 6.2 shows the 118.2 nm PIMS spectrum that results from pyrolysis of a dilute
sample of C6H5CH2CH3 (0.02 % in He). The bottom scan was recorded with the reactor
at 350 K and the spectrum only shows ions resulting from the parent at m/z 106 and an
appropriate isotope peak at m/z 107. There is a small feature at m/z 91 resulting from
dissociative ionization of the C6H5CH2CH3+ cation (C6H5CH2CH3+ → C6H5CH2+ + CH3).

!

145

!

Fig. 6.2. Dilute ethylbenzene (C6H5CH2CH3) heated in a pulsed helium tubular
micro-reactor studied using PIMS at four different reactor temperatures, 350 K, 1300 K,
1400 K, and 1500 K. At 1300 K (shown in red), the large peak at m/z 91 is benzyl
radical, along with the associated fragment methyl radical (m/z 15).
Some
decomposition to m/z 65 and 39 is observed. Further heating to 1400 K and 1500 K
shows significant destruction of benzyl radical and a host of fragment peaks. The y-axis
is arbitrarily scaled to allow easier comparison between temperatures.

Pyrolysis of ethylbenzene is more likely to occur by reaction (6.2) but formationof the methylbenzyl radical, C6H5CH-CH3, is also a possibility. In a hot micro-reactor,
the methylbenzyl radical will rapidly decompose to styrene and H atoms.
C6H5CHCH3 → H + C6H5CH=CH2(6.3)

In Fig. 6.2, heating the reactor to 1300 K triggers the decomposition of ethylbenzene to
produce the benzyl (m/z 91) and methyl radicals (m/z 15). There is no evidence for thepresence of styrene, C6H5CH=CH2, m/z 104, which indicates there is no formation of
the C6H5CH-CH3 radical.

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Once the benzyl-d0 radical is produced, it begins to fragment. At 1300 K,
pyrolysis of C6H5CH2 is underway and ions at m/z 39, 41, 51, and 65, appear. Raising
the micro-reactor temperature to 1400 K leads to the appearance of ions at m/z 63, 64,
89, and 90. The pyrolysis scheme in Fig. 6.1 suggests that an assignment for thesefeatures is fulvenallene (C5H4=C=CH2), m/z 90, and the fulvenallenyl radical(C5H4-C≡CH), m/z 89. The IE(C5H4=C=CH2) is 8.22 eV while IE(C5H4-C≡CH) is 8.19 eV(see Table 6.1) so the 118.2 nm VUV laser can ionize both. The strong band at m/z 65 iscyclopentadienyl with IE(C5H5) of 8.4 eV (see Table 6.1), m/z 39 is likely propargyl
radical with IE(HCCCH2) of 8.7 eV, and m/z 15 is methyl radical, IE(CH3) of 9.8 eV. The
weak features at m/z 41 are very likely CH2CHCH2+ and m/z 51 is probably protonateddiacetylene, (HC≡C-C≡CH, H+).
The top trace in Fig. 6.2 is pyrolysis of ethylbenzene at 1500 K and it is apparent
that new chemistry is taking place. At 1500 K benzyl radical (m/z 91) is largely
consumed and new features at m/z 40, 50, 52, 62, 63, 64, 76, and 77 grow in. Possible
assignments for some of the new species are CH3C≡CH (m/z 40), HC≡C-C≡CH (m/z50), HC≡C-CH=CH2 (m/z 52), o-benzyne (o-C6H4, m/z 76), and phenyl (C6H5, m/z 77).
The vexing problem is to apportion the source of these products. They could be new,higher temperature decomposition products of C6H5CH2 or they could be fragmentation
products from the pyrolysis of fulvenallene (C5H4=C=CH2), fulvenallenyl radical(C5H4-C≡CH), or cyclopentadienyl radical (C5H5).

!

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Fig. 6.3.
Matrix isolation IR absorbance spectra of dilute ethylbenzene(C6H5CH2H3) in a pulsed neon reactor at 300 K (thin line) and 1500 K (thick black line).
At 300 K, the only peak in this frequency window is from the precursor, ethylbenzene.
Upon heating to 1500 K, peaks grow in for fulvenallene, benzyl radical and acetylene.

To confirm many of these assignments, we turn to IR spectroscopy and PIE
measurements. Fig. 6.3 is a matrix IR spectrum of the results of pyrolysis of a 0.05 %mixture of C6H5CH2CH3 in a pulsed Ne micro-reactor. The spectrum at the bottom is acontrol scan at 300 K. Heating C6H5CH2CH3 to 1500 K produces the spectrum shown
above. In previous work the infrared spectra of fulvenallene was reported[60] and 22
fundamentals and 2 overtone bands were assigned.

The five strongest vibrational

bands of fulvenallene have been identified in our Ne matrix isolation spectrum taken

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148

!
during benzyl radical decomposition. These bands include ν4, ν11, ν22, ν28, and ν9, the lastof which is diagnostic of a >C=C=C< allenic stretch. The IR spectrum in Fig. 6.3
demonstrates the presence of ν9(C5H4=C=CH2) and ν5(C5H4=C=CH2) as well asν7(C6H5CH2). The intense ν5(HC≡CH) bend is also present and is consistent with thescheme in Fig. 6.1. The CH stretching region of the IR spectrum is shown in Fig. 6.4. The
presence of acetylene is again confirmed by observation of ν3(HC≡CH) and its DarlingDennison Resonance. The intense ν1(HCCCH2) band of propargyl radical[61] is detected
as well. Fig. 6.4 also has a number of strong CH bands around the acetylenic stretchingregion, 3330 – 3340 cm-1, that are marked by bullets (•). These IR transitions have notyet been assigned since the candidate -C≡C-H molecules have not yet been assigned. In
addition to IR identification of C5H4=C=CH2, C6H5CH2, HCCCH2, and HC≡CH, we havetentative IR evidence for the C5H5 radical. One complication is that the two vibrations of
C5H5 with the strongest IR intensities have been assigned[62-63] to within 5 cm-1 of intense
bands of benzyl radical,[62] making exact identification impossible at our spectral
resolution. However, other fundamentals of the C5H5 radical, ν2, ν3, ν6, and ν14, havebeen observed following the pyrolysis of C6H5CH2CH3. Many of the vibrational modes
that are assigned in this study are collected together in the supplementary material (seeAppendix B).

!

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Fig. 6.4. The acetylenic stretch region of the vibrational spectrum is shown forthe decomposition of ethylbenzene (C6H5CH2CH3) in pulsed neon at 300 K (thin line)
and 1500 K (thick black line). Peaks for propargyl radical (ν1) and acetylene (ν3) are
shown, and the Darling-Denison splitting of ν3(HCCH) is evident. Multiple peaks inthis region remain unassigned. Molecular assignments are collected in the
Supplementary IR table (see Appendix B). Many candidates exist but the vibrational
spectra for these molecules have not been measured. In the right panel, the absorbanceis multiplied by three to highlight the assigned peaks.

In addition to IR spectroscopy, the measurement of PIE curves can be used to
identify products. Fig. 6.5 shows the PIE(m/z 76) and PIE(m/z 65) that result when a
0.2 % mixture of C6H5CH2CH3 in a CW He reactor is thermally decomposed at 1500 K.
The photoionization efficiency curve for m/z 76 has an ionization threshold that is
consistent with that[44] of o-benzyne, 9.03 ± 0.05 eV. Likewise the PIE(m/z 65) is
consistent with the measured[43] IE(C5H5) of 8.4268 ± 0.0005 eV; the PIE(m/z 65) inFig. 6.5 shows good agreement with the measured[64] photoionization efficiency curvefor the cyclopentadienyl radical, C5H5.

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Fig. 6.5. Photoionization efficiency curves (PIE) for m/z 76 (left) and m/z 65(right) measured during the 1500 K decomposition of 0.2 % ethylbenzene (C6H5CH2CH3)
in a CW helium reactor. The ionization energy[44] of ortho-benzyne (C6H4) is included
and the signal at m/z 76 shows good agreement with this value. Published IE values ofbenzyne and cyclopentadienyl radical are collected in Table 6.1. To confirm that m/z 65
is cyclopentadienyl radical, the authentic[64] PIE of C5H5 is overlaid with theexperimental trace.

Figs. 6.6 and 6.7 illustrate important experimental complications. As mentioned
in the introduction, these heated micro-reactors are complex, nonlinear devices
predicted[59] to have a reaction “sweet-spot.” This results from the interaction of therapidly rising gas temperature and the falling gas pressure. Consequently kinetic
properties of the reactor will be very sensitive to changes in gas pressure, flow rates,
nature of the buffer gas, dimensions of the reactor, and potentially other factors thathave not yet been considered.

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In Figs. 6.6 and 6.7, benzyl bromide has been used as a source of the C6H5CH2
radical because the thermochemistry [in reaction (6.1) and Table 6.1] implies ease of
decomposition of C6H5CH2Br; consequently benzyl bromide provides a lower
temperature source of benzyl radical. In Fig. 6.6 a CW SiC micro-reactor is used with ahigh flow rate of He (right panel) as well as a low flow rate (left panel). It is apparent
that reactors with different flow rates decompose the C6H5CH2Br sample to different
extents. The synchrotron at the ALS is operating at 9.4 eV so the signals in the PIMS
should not arise from dissociative ionization. The low flow rate scan (5 sccm) in Fig. 6.6
is consistent with the 1300 K scan in Fig. 6.2, which uses C6H5CH2CH3 as a benzyl source
in a pulsed He reactor with 118.2 nm PIMS. Signals corresponding to fulvenallene (m/z90), fulvenallenyl (m/z 89), cyclopentadienyl (m/z 65), and propargyl (m/z 39) areevident. However the right panel in Fig. 6.6 shows that when the micro-reactor isoperated at higher flow (100 sccm) conditions, C5H4=C=CH2 (m/z 90) is present but
only weak signals at m/z 65 or m/z 39 are detected. The two scans in Fig. 6.6 were
measured under the same experimental conditions except for the flow rate. It appears
that different chemistry occurs in the CW reactor with different flow rates even though
the wall temperature is the same.

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Fig. 6.6. PIMS spectra recorded during the decomposition of benzyl bromide at
1200 K (red) and 1300 K (black) in a CW reactor. Helium gas was flowed over the liquidsample of benzyl bromide at two different mass flow rate settings, 5 sccm and 100 sccm.
Aside from the different flow rates, the scans are otherwise identical and peaks at m/z
39 and 65 clearly depend dramatically on the flow rate setting. All spectra shown arenormalized relative to the peak at m/z 91.

Changing the buffer gas from helium to argon can also lead to variability inexperimental findings. Fig. 6.7 shows the pyrolysis of C6H5CH2Br entrained in Ar in a
CW SiC micro-reactor measured at the Swiss Light Source. The left panel in Fig. 6.7
demonstrates that the initial decomposition products of C6H5CH2 at 1150 K in a CW Ar
source are C5H4=C=CH2 (m/z 90) and C5H4-C≡CH (m/z 89). The decomposition schemein Fig. 6.1 indicates that C5H4-C≡CH originates from C5H4=C=CH2 via loss of a H atom.
The right panel in Fig. 6.7 uses the SLS synchrotron at 13.8 eV to photoionize hydrogen
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atom and it is apparent that there are H atoms present at 1150 K. Figs 6.6 and 6.7 imply
that differences in operational conditions (temperature, pressure) or carrier gas (He, Ar)
are very important to the product distributions of thermal fragmentation.

Fig. 6.7. Thermal decomposition of benzyl bromide in a CW argon reactor
measured at the tunable synchrotron light source at the SLS. The left panel shows theformation of m/z 90 (C5H4=C=CH2) and 89 (C5H4-C≡CH ) during the 1150 K
decomposition of benzyl radical (m/z 91). The photon energy was set to 9.0 eV, justhigh enough to ionize these products. The right panel shows the appearance ofhydrogen atom produced (m/z 1) at the same temperature (C6H5CH2 → C5H4=C=CH2 +H). To ionize hydrogen atom, 13.8 eV photons were used.

Recent CFD simulations[59] are currently limited to CW reactors but are able toprovide insight into the comparison between experiments using Ar or He as a carriergas. Given similar conditions, a reactor using He will exhibit considerably hotter
temperature profiles along the reactor length, showing both higher peak temperatureand more rapid heating. Furthermore, the residence time is a function of the mass flow
rate. Both of these flow dependencies will alter the decomposition environment present
and may explain the differences indicated by Figs. 6.6 and 6.7. Since decomposition
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pathways have differing temperatures dependence, varying the reactor environment
may dramatically alter the decomposition products.
B. Decomposition of Benzyl-d2 Radical
Fig. 6.8 shows the result of pyrolysis of C6H5CD2CD3; the predictedfragmentation of C6H5CD2 is presented in the supplementary figure (see Appendix B),
Fig. B.1. The signals at m/z 111 and m/z 112 are present from photoionization of
C6H5CD2CD3 and its natural abundance isotope peak. Small peaks at m/z 93(dissociative ionization) and an impurity peak at m/z 110 are also present. At 1300 K,
the decomposition of C6H5CD2CD3 is demonstrated by the presence of CD3 (m/z 18) andC6H5CD2 (m/z 93). Complications with the scheme in Fig. 6.1 are immediately evident.
This scheme would predict C6H5CD2 to lose H atom and produce fulvenallene-d2 (m/z
92) followed by loss of D atom to generate fulvenallenyl radical (m/z 90). Fig. 6.1 would
also predict formation of cyclopentadienyl radical-d0 (m/z 65) plus DCCD (m/z 28).
Fig. 6.8 clearly shows the presence of the unexpected peak m/z 91 as well as formationof intense signals at m/z 66 and 67 in addition to the expected peak at m/z 65. Alsopresent are ions at m/z (39, 40, 41, 42, and 43). As the micro-reactor is heated to 1500 K,
numerous unexpected species appear. Among these new peaks at 1500 K is m/z 17,
CHD2.

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Fig. 6.8. Dilute ethylbenzene-d5 (C6H5CD2CD3, m/z 111) heated in a pulsed
helium tubular micro-reactor studied using PIMS at three different reactor
temperatures, 300 K, 1300 K, and 1500 K. At 1300 K (shown in red), the large peak at
m/z 93 is benzyl radical-d2, along with the counter fragment methyl radical-d3 (m/z18). Some decomposition is observed resulting in cyclopentadienyl radical (m/z 65 is
d0, 66 is d1, and 67 is d2) and 39, 40 and 41 (presumed to be propargyl radical) are
observed. At 1500 K we see peaks at 90, 91, and 92, 50, 51, and 52, and a triplet at 76, 77,
and 78. Comparison to Fig 6.2 (the d0 case) is useful. The y-axis is arbitrarily scaled toallow easy comparison between temperatures.

A potential factor for many of these new peaks is the fact that radical-radical
processes in these micro-reactors have been observed earlier. In a study of the pyrolysis
of isotopically substituted acetaldehydes,[65] rapid isotope exchanges were observed:
CD3 (m/z 18) + H ⇋ CHD2 (m/z 17) + D. Likewise in an earlier study of the pyrolysis offuran,[66] it was observed that H atoms attacked CH3C≡CH to produce CH3 radicals(m/z 15) and HCCH. Careful dilution of the furan sample to 0.01 % (or lower) in He
was required[66] to suppress this H atom chemistry. The reaction scheme in Fig. 6.1

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predicts the formation of H atoms and confirmation of this is found in Fig. 6.7, whichshows PIMS detection of H atoms following pyrolysis of C6H5CH2.
One explanation of the unexpected sets of peaks in Fig. 6.8 could be rapid
radical/radical bimolecular chemistry. Many radical/radical reactions are very fast.[67]C5H5 (m/z 65) + D ⇋ [C5H5D]* ⇋ C5H4D (m/z 66) + H(6.4)

To find more direct evidence for H/D atom chemistry, we have examined the chemistryof D atoms reacting with cyclopentadienyl radicals. In an earlier study[25] of thepyrolysis of benzaldehyde, methyl nitrite-d3 was used as a chemical source of D atoms.
CD3ONO (+ M) → D + CD2O + NO

(6.5)

Additionally, anisole (C6H5OCH3) is known[28] to be a convenient source of C5H5 radicals
so we can test for rapid D atom reactions with cyclopentadienyl radical. Fig. 6.9 shows
the results of this examination. The thin trace is from thermal decomposition of anisole
at 1300 K.
C6H5OCH3 (+ M) → [C6H5O] + CH3 → C5H5 (m/z 65) + CO + CH3 (m/z 15)

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(6.6)

!

Fig. 6.9. The thin black trace is the PIMS spectrum of C6H5OCH3 decompositionat 1300 K yielding cyclopentadienyl radical (C5H5, m/z 65) observed with a small
natural abundance 13C peak at m/z 66. Shown above is the outcome of mixing 0.02 %C6H5OCH3 with 0.25 % CD3ONO, a known D atom source, in pulsed helium. We canclearly see significant incorporation of D atom up to C5D3H2 due to bimolecular radicalradical reactions (C5H5 + D → C5DH5* → C5DH4 + H). Note the monotonic decrease insignal observed with increasing deuteration events.
The thicker trace in Fig. 6.9 is the result of pyrolysis of a mixture of 0.02 % C6H5OCH3
and 0.25 % CD3ONO in a pulsed He reactor at 1300 K. Since pyrolysis of CD3ONO
produces CD2O (m/z 32) plus NO (m/z 30) and D atoms, radical-radical reactions
ensue that lead to scrambling of the cyclopentadienyl radical.
C5H5 (m/z 65) + D ⇋ C5DH4 (m/z 66) + H

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(6.7)

!
In addition to C5DH4, Fig. 6.9 demonstrates that D atom chemistry can exchange
multiple H atoms to produce both the C5D2H3 and C5D3H2 radicals as well.
This set of complementary data helps in the interpretation of the H/D ratios inpeaks in Fig. 6.8. In the 1300 K trace for C6H5CD2CD3, instead of one peak at m/z 65(assigned as C5H5 in Fig 6.2) features at 65, 66, and 67 are observed; in Fig. 6.8 the peaks
at 66 and 67 are surprisingly intense. Decomposition of C6H5CD2 via Fig. 6.1 predicts
formation of C5H5 only. It may seem initially attractive to assign m/z 66 and 67 in
Fig. 6.8 to H/D scrambling but the intensities are inconsistent with this process.
Bimolecular scrambling produces monotonically decreasing intensity for subsequent
deuteration of C5H5, as evident in Fig 6.9. Due to the qualitative differences between
Fig 6.8 and Fig 6.9, it seems likely that the intensities for C5H4D (m/z 66) and C5H3D2(m/z 67) result from different pathways than those shown in Fig. 6.1 and not simple
bimolecular H/D scrambling.
Additional indication that our experiments follow unimolecular decomposition
is found in the absence of styrene formation. If hydrogen atoms were present in large
enough densities to facilitate bimolecular chemistry, a likely target would be H-atom
abstraction from ethylbenzene of the α–carbon, yielding methylbenzyl radical (C6H5CHCH3), which would rapidly undergo hydrogen atom loss to produce styrene, see
reaction (6.3). This process was observed during the decomposition of ethylbenzene inshock tubes and was commonly observed when bimolecular reactions dominate.[68]Since no C6H5-CH=CH2 (m/z 104) is observed in the PIMS spectra (Fig. 6.2), it isunlikely that H atom chemistry is prevalent under the operating conditions of the
micro-reactor.

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D. Decomposition of Benzyl-d5 Radical
Fig. 6.10 shows the results of pyrolysis of a 0.02 % mixture of C6D5CH2CH3 in apulsed micro-reactor; the predicted fragmentation for C6D5CH2 is shown in the
supplementary figure (see Appendix B), Fig. B.2. The initial trace at 350 K shows theparent peak for C6D5CH2CH3 at m/z 111. The small feature at m/z 110 reveals a slight
impurity (C6HD4CH2CH3) of the benzyl-d5 precursor as well as a small amount ofdissociation ionization producing C6D5CH2+ at m/z 96. At 1300 K a large portion of
C6D5CH2CH3 decomposes to generate C6D5CH2 (m/z 96) as well as the methyl radical,
m/z 15. Instead of the expected feature for C5D5 (m/z 70) there are additional signals at
m/z 65, 68, and 69 similar to Fig 6.8. There are also a number of peaks at (m/z 40, 41,
42, 44, and 45) as well as (m/z 52 and 53) which remain unassigned. As the reactor isheated to 1400 K, benzyl-d5 radical pyrolyzes to fragments at m/z 92, 93, 94, and 95. A
decomposition scheme based on Fig. 6.1 predicts that C6D5CH2 (m/z 96) generates
fulvenallene-d4 (m/z 94) and the fulvenallenyl radical-d4 (m/z 93); the production ofradicals at m/z 95 is not anticipated. Additionally, a number of new, unexpected peaks
grow in that are comparable to those found during benzyl-d2 radical decomposition.

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Fig. 6.10. Dilute ethylbenzene-d5 (C6D5CH2CH3, m/z 111) heated in a pulsed
helium tubular micro-reactor studied using PIMS at three different reactor
temperatures, 350 K, 1300 K, and 1400 K. The peaks here are to be compared to the d0
case and the d2 shown in Fig 6.2 and Fig 6.8, respectively. At 1300 K (shown in red), thelarge peak at m/z 96 is benzyl radical-d5, along with the counter fragment methyl
radical (m/z 15). At 1300 K we see some decomposition to cyclopentadienyl radical(m/z 68 is d3, 69 is d4, and 70 is d5) and 40, 41, and 42 are observed (likely isotopomers
of propargyl radical). Further heating to 1400 K shows significant destruction of benzyl
radical and the appearance of a set of new peaks from m/z 65 – 70 and 92 – 95, now
assumed to be isotopomers of C5H4=C=CH2 and C5H4-C≡CH. The y-axis is arbitrarily
scaled to allow easy comparison between temperatures.

D. Decomposition of 13C-Benzyl Radical, C6H513CH2
To circumvent the complications with H/D atom chemistry entirely, 13C wasused as an isotopic label. Shown in Fig. 6.11 is the 118.2 nm PIMS of the products ofthermal fragmentation of 0.02 % C6H513CH2CH3 in He in a pulsed SiC micro-reactor; thedecomposition predictions for C6H513CH2 are shown in the supplementary figure (seeAppendix B), Fig. B.3. The bottom scan at 300 K reveals the parent peak for
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C6H513CH2CH3 at m/z 107. There is a trace impurity in the 13C sample, C6H512CH2CH3 at
m/z 106, and a small amount of dissociative ionization, C6H513CH2+, at m/z 92. Heating
the sample to 1300 K induces the formation of 13C-benzyl radical (C6H513CH2 at m/z 92)
along with its co-product CH3 (m/z 15). Upon formation the 13C-benzyl radical begins
decomposition to 13C-fulvenallene (m/z 91) and the 13C-fulvenallenyl radical (m/z 90).
In stark contrast to expectations from Fig. 6.1, not only 12C5H5 (m/z 65) is present but
also 13C5H5 (m/z 66). There is also a group of products present at m/z 39, 40, 41, and 42.
Further heating of C6H513CH2CH3 to 1500 K leads to other peaks that are not predicted
by Fig. 6.1. The peak at m/z 16 is certainly 13CH3. Likewise a cluster of peaks at m/z (50,51, and 52) appears as well as the set at m/z (76, 77, and 78). Based on measured PIE
curves, we assign m/z 76 as o-C6H4 and m/z 78 as benzene; m/z 77 is likely the phenyl
radical, C6H5. Many of these features are inconsistent with the mechanisms depicted inFig. 6.1. From these PIMS scans we notice that m/z 76, 77, and 78 are still present andshow no sign of 13C incorporation. This is important to note because benzene has beenshown[28] to form from dimerization of propargyl radicals, (HCCCH2 + HCCCH2), andfrom reactions of methyl radical with the cyclopentadienyl radical (CH3 + C5H5). If
these pathways were the source of the benzene observed, then the 13C:12C ratio would
match that of the two radicals reacting. Both 13(HCCCH2) and 13(C5H5) are present in thereactor so the absence of peaks at m/z 79 and 80 indicates that, however benzene isformed, it does so without 13C atom incorporation.

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Fig. 6.11. Dilute ethylbenzene-13C (C6H513CH2CH3, m/z 107) heated in a pulsed
helium tubular micro-reactor studied using PIMS at three different reactor
temperatures, 300 K, 1300 K, and 1500 K. At 1300 K (shown in red), the large peak at
m/z 92 is benzyl radical-13C along with the counter fragment methyl radical (m/z 15).
Decomposition to cyclopentadienyl radical shows surprising incorporation of 13C at
m/z 66. At 1500 K, 13C isotopomers are observed for propargyl radical (m/z 40) andunexpectedly, methyl radical (m/z 16) but not in the triplet at 76-78, indicating
unimolecular formation of these three peaks from C6H513CH2. The y-axis is arbitrarily
scaled to allow easy comparison between temperatures.

To confirm the identification of the ions at m/z 90 and 91 in Fig. 6.11, PIEs were
measured with the synchrotron at the ALS. Shown in Fig. 6.12 are the PIE curves form/z 90 and m/z 91 measured during the pyrolysis of a dilute mixture of C6H513CH2CH3
in He in a CW SiC micro-reactor. Both of these PIE curves are consistent with theassignment of m/z 90 as the 13C isotopomer of the fulvenallenyl radical (C5H4-C≡CH)
while m/z 91 is identified as 13C-fulvenallene (C5H4=C=CH2). The ionization thresholds
agree with the published[20] values and the known PIE curves as indicated by black

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lines. Fig 6.13 shows a pair of PIE curves from the same C6H513CH2CH3 decomposition
that identify both the m/z 65 and m/z 66 signals as cyclopentadienyl radicals, 12C5H5and (13C)C4H5.

Fig. 6.12. PIEs confirming the identity of m/z 91 as C5H4=C=CH2 and m/z 90 as
C5H4-C≡CH each containing one 13C. Red data points are from ethylbenzene-13C
decomposition at 1500 K in a CW helium reactor. In black are previously reported[21]PIEs for the two molecules with their IE’s labeled on the plots. Y-axis values are scaled
so that signal intensities are approximately equal.

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Fig. 6.13. PIEs confirming the identity of m/z 65 and 66 as cyclopentadienyl
radical (C5H5). Red data points are from ethylbenzene-13C decomposition at 1500 K in a
CW He reactor. In black are the previously reported[64] PIEs for C5H5 with the IE labeledon the plots; 13C substitution should have no effect on the IE. Y-axis values are scaled sothat signal intensities are approximately equal.
Fig. 6.1 illustrates that benzyl radical could isomerize to cycloheptatrienyl
radical, C6H5CH2, X̃ 2B1 (+ M) ⇋ C7H7, X̃ 2E2" and this isomerization is consistent withmany of the ratios of isotopomers detected during the D/H and 13C precursors
decomposition.

Because the ionization energies for both benzyl (7.2 eV) andcycloheptatrienyl (6.2 eV) radicals are known (Table 6.1), PIMS can differentiate these
two isotopomers. Fig. 6.14 shows the PIMS spectra that result from pyrolysis of
C6H5CD2Br in an effusive, CW Ar micro-reactor at 1250 K taken at the SLS. The species
at m/z 93 could be either benzyl (C6H5CD2) or cycloheptatrienyl (C7H5D2). As thesynchrotron is tuned from 7.5 eV down to 7.0 eV and on to 6.8 eV, the m/z 93 signal

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collapses. The scans at 6.8 eV and 7.0 eV in Fig. 6.14 have been multiplied by a factor of
10. We conclude that all of the m/z 93 radicals that result from pyrolysis of benzyl
bromide-d2 at 1250 K in a CW Ar micro-reactor are benzyl radicals (C6H5CD2). No
cycloheptatrienyl radicals (C7H5D2) are present in the CW Ar micro-reactor. It is worth
noting, however, that while the isomerization, C6H5CD2 (+ M) ⇋ C7H5D2, was notobserved in the effusive, CW Ar reactor in Switzerland, different results could be foundwith pulsed reactors or with the use of different carrier gasses.

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Fig. 6.14. PIMS for the decomposition of C6H5CD2Br at 1250 K in a CW Ar reactor
at the SLS. Benzyl radical (C6H5CD2) and cycloheptatrienyl radical (C7H5D2) have thesame mass but IEs that differ by around 1 eV. The signal intensity at m/z 93 (benzyl
radical-d2) drops off significantly at photon energies below the IE of benzyl radical,
indicating that cycloheptatrienyl radical is not present.

IV. Discussion and Conclusions
This chapter describes an experimental study of the pyrolysis of the benzyl
radical carried out in a set of heated micro-reactors. We have used both benzyl bromide
and ethylbenzene as precursors of the parent species, C6H5CH2, as well as a set of

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isotopically labeled radicals: C6H5CD2, C6D5CH2, and C6H513CH2. A combination ofPIMS, PIE measurements, and IR spectroscopy has been used to identify the thermal
decomposition products. Before making any mechanistic assessments there are at least
two serious complications that must be resolved. These are the non-linear properties ofthe micro-reactors and the unknown pyrolysis mechanisms of the nascent metastables
and radicals: fulvenallene, fulvenallenyl radical, and the cyclopentadienyl radical.
The experiments in this chapter made use of four different micro-reactors: apulsed He reactor, a pulsed Ne reactor, a CW He reactor, and a CW Ar reactor. All ofthese reactors are slightly different. Recent computational fluid dynamics (CFD)
simulations[59] have revealed that the heated micro-reactors are complex, non-linear
devices. The chemical reactions in the micro-reactors vary exponentially with the gas
temperature (which is rising) and quadratically with the pressure (which is falling).
Consequently there is only a small region of the micro-reactor in which chemical
reactions occur. The size and location of the “sweet-spot” can vary dramatically withchanges in reactor dimensions (diameter, length), material (SiC, quartz, or Al2O3), thenature of the buffer gas (He, Ne, or Ar), mass flow-rate, mode of operation (pulsed vs.
continuous), or other factors that have not yet been anticipated. Consequently, caremust be taken when comparing results from different micro-reactors.
The products C5H4=C=CH2, H atom, C5H4-C≡CH, C5H5, HCCCH2 and HC≡CH
have all been detected. Consequently we must contend with subsequent thermal
cracking processes of the products: C5H4=C=CH2 and the radicals C5H4-C≡CH and C5H5.
Several earlier experiments (see Fig. 6.5 in ref.[26]) and a computational paper[69] havedemonstrated that the cyclopentadienyl radical thermally decomposes to propargyl andacetylene: C5H5 (+ M) → HCCCH2 and HC≡CH. The dynamics of fulvenallene pyrolysis

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are

even

more

complicated.[16-17]

When

the

bicyclic

molecule

phthalide(1-

isobenzofuranone, C6H4COOCH2) was used as a fulvenallene precursor, pyrolysis in a
CW Ar micro-reactor reported decomposition of C5H4=C=CH2 (m/z 90) to C5H4-C≡CH(m/z 89) and thence to the expected HCCCH2 (m/z 39) and HC≡C-C≡CH (m/z 50).
However Fig. 6.3 in ref. [16] also reveals the presence of HC≡CH (m/z 26), a set of
products at m/z 63-65, as well as (76, 78). In this work, we have also observed many ofthese products but we cannot determine if they result from C6H5CH2 decomposition orfrom C5H5 and C5H4=C=CH2 decomposition. Until the fragmentation of fulvenallene
and the cyclopentadienyl radical are properly understood, it will be difficult to confirm
the products of C6H5CH2 pyrolysis.
Even with these two complexities — the non-linear nature of the reactors and the
obscuring products from cyclopentadienyl and fulvenallene decomposition — there iscompelling isotopic evidence that the fulvenallene-decomposition pathway in Fig. 6.1 is
not the sole route for C6H5CH2 decomposition. The enhanced D/H ratio on C5H5
observed during C6H5CD2CD3 decomposition (Fig. 6.8) reveals an unexpected
incorporation of D into the cyclopentadienyl radical. The deuteration pattern resulting
from bimolecular D-atom chemistry displayed in Fig. 6.9 is not present in C6H5CD2
decomposition. The presence of C5H5 (m/z 65) predicted by Fig. 6.1 is dwarfed bysignals at m/z 66 (C5H4D) and m/z 67 (C5H3D2). Likewise for the case of C6H513CH2 in
Fig. 6.11, intense signals for both 12C5H5 (m/z 65) and 13CC4H5 (m/z 66) are present. The
PIE curves in Fig. 6.13 confirm the identity of both of these peaks as cyclopentadienyl
radicals. The 13C experiments in Fig. 6.11 also clearly show formation of both CH3 (m/z
15) and 13CH3 (m/z 16) radicals at 1500 K during C6H513CH2 decomposition. Fig. 6.11
also shows clear evidence for 12benzyne (m/z 76), 12phenyl radical (m/z 77), and

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12

benzene (m/z 78). None of these species incorporate 13C, thus indicating theirformation is from direct decay of parent. These results are not compatible with thefulvenallene pathway predicted in Fig. 6.1.
The failure to detect cycloheptatrienyl radicals (Fig. 6.14) is puzzling. An obvious
route to scramble 13C into the cyclopentadienyl radical (see Fig. 6.11) is through thecycloheptatrienyl radical, which could then decompose as summarized in reaction 6.8.
C6H513CH2 (+ M) ⇋ (13C)C6H7 → HCCH + (13C)C4H5(6.8)

If there were a low barrier between the cycloheptatrienyl radical and its decomposition
products (acetylene, cyclopentadienyl radical), there might only be a small pool of C7H7
radicals. Consequently detection of C7H7 could be very difficult. As a result, theimplications of Fig. 6.14 are not completely clear at this time.

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electronic transition. Journal of Chemical Physics 2001, 114, 4869-4882.
[64] J. D. Savee, Private communication, 2014[65] A. Vasiliou, K. M. Piech, X. Zhang, M. R. Nimlos, M. Ahmed, O. Kostko, A. Golan,
D. L. Osborn, J. W. Daily, J. F. Stanton and G. B. Ellison. The products of thethermal decomposition of CH3CHO. Journal of Chemical Physics 2011, 135, 1430614311.
[66] K. N. Urness, Q. Guan, A. Golan, J. W. Daily, M. R. Nimlos, J. F. Stanton, M. Ahmed
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2013, 139, 124305-124314[67] D. M. Golden. Yet another look at the reaction CH3 + H + M = CH4 + M.
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MO/statistical theory study. Journal of Computational Chemistry 2000, 21, 415-425.

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Chapter 7
The Thermal Decomposition of the Benzyl Radical in a Heated MicroReactor: II. Pyrolysis of the Tropyl Radical

Abstract
Cycloheptatrienyl (tropyl) radical, C7H7, was cleanly produced in the gas-phaseand subjected to pyrolysis in a set of flash-pyrolysis micro-reactors. The pyrolysis
products resulting from C7H7 were detected and identified by vacuum ultraviolet
photoionization mass spectrometry. Complementary product identification wasprovided by infrared absorption spectroscopy. Pyrolysis pressures in the micro-reactor
were roughly 200 Torr and residence times in the micro-reactors were approximately
100 µsec. At 1100 K, pyrolysis begins and13 the products from pyrolysis of tropyl
radicals are only acetylene and cyclopentadienyl radicals. Tropyl radicals do notisomerize to benzyl radicals at reactor temperatures up to 1600 K. Heating samples of
either cycloheptatriene or norbornadiene never produced tropyl (C7H7) radicals but
rather only benzyl (C6H5CH2). The thermal decomposition of benzyl radicals has been
was reconsidered without participation of tropyl radicals. There are at least three
distinct pathways for pyrolysis of benzyl radical: the Benson Fragmentation, the methyphenyl radical, and the bridgehead norbornadienyl radical. These three pathways
account for the majority of the products detected following pyrolysis of all of theisotopomers: C6H5CH2, C6H5CD2, C6D5CH2, and C6H513CH2.

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I. Introduction
Resonance stabilized radicals are important in combustion processes because
they are precursors to soot formation. Modern transportation fuels include a large
number[1] of aromatics (benzene, toluene, xylenes, alkylbenzenes and the like); see Fig. 3of ref.[2]. Consequently, benzyl radicals (C6H5CH2) are important intermediates in thehigh-temperature oxidation of these fuels. The pyrolysis of the benzyl radical has
recently been examined[3-5] and evidence was found for several complex decomposition
pathways. Fig. 1 is an overview[5] of the products that were detected following thepyrolysis of the C6H5CH2 radical. A pathway for fragmentation of benzyl radical tofulvenallene and the fulvenallenyl radical was suggested[6] by Benson in 1986. An
isomer of the benzyl radical is the cycloheptatrienyl (or tropyl) radical, C7H7. The
relationship of tropyl to the benzyl radical is shown in Fig. 7.1. In the early literature,
the interconversion of benzyl to tropyl (C6H5CH2 ⇋ C7H7) was considered by several
groups.[7-13] However all recent theoretical studies[14-16] find no pathways below 2000 K
for the isomerization of benzyl to tropyl. A photoionization search for the isomerizationof the C6H5CD2 benzyl radical to the C7H5D2 tropyl radical with tunable VUV radiation
found no evidence for tropyl radical formation.[5] To date, there are no experimental
studies of the thermal decomposition of the tropyl radical, C7H7 (+ M) → products.

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Fig. 7.1 The Benson Pathway[6] for the pyrolysis of benzyl radical[5] is shown. A
possible link of the C6H5CH2 radical to the C7H7 radical is indicated.

The focus of this chapter is to generate authentic samples of the tropyl radical
and to examine the pyrolysis pathways: C7H7 (+ M) → products. Earlier photoionization
studies[17] of the C7H7 radical found bitropyl, C7H7–C7H7, to be a convenient thermal
precursor for tropyl. As sources of tropyl radicals, we have investigated the flash!

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pyrolysis of bitropyl, cycloheptatriene, and norbornadiene. The hydrocarbon samples
were diluted in a carrier gas of He or Ar and subjected to pyrolysis in a heated siliconcarbide (SiC) micro-reactor. The micro-reactor was resistively heated to temperatures
up to 1600 K and operated at pressures of roughly 200 Torr. Approximate residence
times[18] in the reactor are 100 µsec after which the gas mixture exits into a vacuum
chamber at a pressure of 10-6 Torr. The resulting pyrolysis products are entrained in amolecular beam and are analyzed by a combination of photoionization mass
spectroscopy (PIMS) and matrix infrared (IR) absorption spectroscopy.
Because of its importance in organic chemistry, atmospheric chemistry, andcombustion processes, the benzyl radical (C6H5CH2, X̃ 2B1) has been extensively studied
and a list of its properties is collected in Table 1 of ref.[5] The tropyl radical (C7H7 X̃ 2E2”)
is not as well characterized experimentally. Because the tropyl radical has no electric
dipole moment, there are no microwave spectra. To date, there are published laser
induced fluorescence (LIF) spectra[9-10] of the tropyl radical, an IR-UV double resonancestudy with a free electron laser,[13] and preliminary helium nanodroplet IR absorption
spectra,[19] but there is no definitive analysis of the infrared spectrum of C7H7. Some of
the available experimental properties of the tropyl radical are summarized in Table 7.1.

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Tropyl and Benzyl Radical Experimental Properties
43.2
(181
65.0
(272

± 0.5
±2
± 0.7
± 3

kcal mol-1
kJ mol-1)
kcal mol-1
kJ mol-1)[20]

DH298(C7H7-H)

73.9
(309

± 0.5
± 2

kcal mol-1
kJ mol-1)[21]∆fH298(C6H5CH3)

12.0
(50.4

± 0.1
± 0.6

kcal mol-1
kJ mol-1)[20]∆fH298(C6H5CH2 , X̃ 2B1)

49.6
(208

± 0.6
± 3

kcal mol-1
kJ mol-1)[20, 22]

DH298(C6H5CH2-H)

89.8
(376

± 0.6
± 3

kcal mol-1
kJ mol-1)[20, 22]∆fH298(norbornadiene, C7H8)

58.8
(246

± 0.7
± 3

kcal mol-1
kJ mol-1)[20]∆fH298(C5H6)

32.1
(134

± 0.4
± 2

kcal mol-1
kJ mol-1)[20]∆fH298(C5H5, X̃ 2E1”)

63
(264

± 1
± 6

kcal mol-1
kJ mol-1)

[23]

83

± 1

kcal mol-1[20, 23]

(348

± 6

kJ mol-1)

DH298(C6H5-H)

112.9
(472

± 0.5
± 2

kcal mol-1
kJ mol-1)[20, 22]∆rxnH298(C7H7 → HC≡CH + C5H5)

53
(220

± 2
± 7

kcal mol-1
kJ mol-1)

[5]

9.36

± 0.02

eV

[21]

15.3
(64

± 0.9
± 4

kcal mol-1
kJ mol-1)[5]∆fH298(C7H8)∆fH298(C7H7, X̃ 2E2")

DH298(C5H5-H)

AE(C7H8 → C7H7+ + H)
∆isomeriz’nH298(C6H5CH2, X̃ 2B1 → C7H7, X̃ 2E2")[20]

Important Ionization Energies
CH3!

9.8380± 0.0004

183

eV

[24]

!

!
[25]

HC≡CH

11.40081 ± 0.00001 eV

HCCCH28.7006eV[26]

CH2CHCH2

8.13146 ± 0.00025 eV[27-28]o-C6H49.03± 0.05

eV

[29]

C5H5

8.4268± 0.0005

eV[30]

C5H4-C≡CH

8.19± 0.02eV[31]

C5H4=C=CH2

8.22± 0.01eV[31]

C7H7, X̃ 2E2"

6.221

± 0.006eV[7, 17]C6H5CH2, X̃ 2B1

7.2487± 0.0006

eV[28, 32]

C6H5CH3

8.8276± 0.0006

eV[33]

C7H8 (cycloheptatriene)

8.29± 0.01eV[34]

C7H8 (2, 5 norbornadiene) ≤ 8.35

± 0.01eV[34]± 0.0002

Table 7.1. Experimental properties of tropyl and benzyl radicals and important
ionization energies.

This chapter will demonstrate that pyrolysis of the tropyl radical only produces
acetylene and the cyclopentadienyl radical. Tropyl radical does not isomerize to benzyl
radical under any conditions observed in this work. As part of the final Discussion, we
will reconsider the thermal cracking of the C6H5CH2 radical in the absence of C7H7. We
present three separate pathways for the pyrolysis of the benzyl radical that seem toaccount for all of the current experimental findings, C6H5CH2 (+ M) → products.

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II. Experimental
A. Heated Micro-Reactor Pyrolysis Source
To carry out flash-pyrolysis of target molecules, we employ a resistively heatedsilicon carbide (SiC) reactor with an inner diameter (I.D.) of either 1.0 mm or 0.6 mm
and a relatively short heated length, 10 to 15 mm. This geometry along with the present
flow conditions allows[18] for short residence times of around 100 #s. At the exit of thereactor the gas is cooled as it expands into a vacuum chamber pumped to 10-4 to 10-6
Torr, where reactive collisions cease. To monitor the temperature of the reactor, a TypeC thermocouple is attached to the reactor’s outer wall using tantalum wire as described
previously.[35] The fluid mechanics of the heated micro-reactors were the subject of a
recent detailed computational fluid dynamics investigation[18] that modeled the pressure
and temperature in the reactor at wall temperatures up to 1600 K. The findings in thisstudy show that for a given set of reactor conditions and kinetic parameters,
decomposition occurs within a small “sweet-spot” that can be as small as only a few
mm of the total heated length. Another key finding was that the temperature, pressure,and thus decomposition rates can vary dramatically upon varying the experimental
parameters, including: 1) carrier gas (argon, neon, and helium are all commonly used),2) upstream and downstream pressure, and 3) mass flow rate, which can be controlledeither by use of commercial mass flow controllers or with a pulsed valve. Therefore
when comparing results from different experimental techniques, some variation is
expected for the onset temperature of decomposition and/or the product ratios ofcompeting decomposition pathways.

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B. Photoionization Mass Spectroscopy
i. Pulsed Vacuum Ultraviolet Radiation
The 355 nm output light of a commercial Nd:YAG laser (Spectra Physics Pro-23010) is focused into a tripling cell filled to 150 Torr with a 10:1 mixture of argon:xenon,
which produces 118.2 nm (10.487 eV) photons. At the exit of the tripling cell is a MgF2lens used to focus the 118.2 nm light into the interaction region of the Jordan time-offlight spectrometer.

This tripling process has been well-studied[36] and has anefficiency[37-38] of around 1 × 10-5. Additional losses are introduced by incomplete
transmission of 118.2 nm light through the Mg2F lens and intentional off-axis alignment
through the lens to spatially separate 118.2 nm light from the remaining 355 nm lightthat exits the tripling cell, which can cause unwanted multiphoton ionization. As aresult, laser powers of 30 mJ pulse-1 at 355 nm yield roughly 30 nJ pulse-1 of 118.2 nm.
The region where the ionizing radiation and molecular beam are intersected ismaintained at 10-7 Torr by a turbomolecular pump. Any molecule with ionizationenergy less than 10.487 eV is ionized and the resulting ions are accelerated into areflectron time-of-flight spectrometer. In order to sustain sufficiently low pressure to
maintain collisionless conditions, the gas flow must be pulsed with a Parker generalvalve operating at 10 Hz with 1 ms opening time. Backing pressures behind the pulsed
valve are typically 2000 Torr of helium carrier gas and the pressure downstream of thereactor is typically 10-6 Torr, maintained by an 11-inch diffusion pump. Spectra shownin this work are the result of signal averaging of 1000 scans.

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ii. Continuous Vacuum Ultraviolet Radiation
A similar experimental setup for the micro-reactor scheme is used at theChemical Dynamics Beamline endstation (9.0.2) at the Advanced Light Source (ALS) in
the Lawrence Berkeley National Laboratory. Using the synchrotron light for PIMS adds
the experimental degree of freedom — tunable photon energy.[39] The energy of thesynchrotron photons can be tuned between 7.4 eV and 30 eV, sufficiently high to ionize
all species produced in the heated reactor. By measuring ion current at a single mass-tocharge ratio (m/z) while varying the photon energy and normalizing to the measured
VUV power, it is possible to record photoionization spectra, which can be used to
identify individual isomers that may be present. Signal averaging is used to increasesignal-to-noise ratio; typical spectra are the composite of between 5 x 104 and 2 x 105
sweeps. To control the flow of gas through the reactor, an MKS mass flow controller isused along with a slightly smaller SiC reactor with an I.D. of 0.6 mm. Typical flow
conditions include setting the flow controller to 200 standard cm3 min-1 (SCCM) backed
by 5 atm of He. This yields pressures of 100 Torr between the mass flow controller andentrance of the reactor and roughly 10-4 Torr at the exit of the nozzle. With a 1 mmskimmer, the pressure in the ionization chamber can be maintained at roughly 5 x 10-6
Torr by a large turbomolecular pump.

C. Matrix Isolation Fourier Transform Infrared Spectroscopy
Since all polyatomic molecules possess characteristic vibrational spectra, IR
spectroscopy is an excellent technique for identifying molecular species and serves as
valuable complement to the PIMS experiments. The matrix IR spectrometer has been
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!

described before.[40-41] Briefly, a 1 mm I.D. SiC reactor is coupled to a pulsed valve
assembly very similar to the PIMS system described above. The upstream pressure is
between 800 and 1000 Torr, and with the pulsed valve operating with a roughly 1 ms
opening time and 15 Hz repetition rate, the pressure at the reactor exit is typically 10-6
Torr, which is maintained with a small turbomolecular pump. The output of the reactor
impinges on a cryogenically cooled CsI window 4 cm downstream that is maintained at
5 K by a helium cryostat. The carrier gas for this experiment is neon, which condenses
into a solid matrix upon colliding with the 5 K window. A standard matrix is formed byflowing 200 Torr of sample mixture from a roughly 3 L gas manifold, which leads to adeposition time of around one hour. After dosing is complete, the reactor assembly isrotated 90º out of the way and the CsI window is lowered into the beampath of acommercial Fourier transform IR spectrometer (Nicolet 6700). The MCT-A detector hasa spectral range of 4000 to 600 cm-1 and is operated with 0.25 cm-1 resolution, withtypical spectra constructed by averaging 500 scans. Although the neon matrix is
chemically inert, it does affect the vibrational spectra of entrained molecules. Averagematrix shifts are on the order of 1 to 5 cm-1 from the unperturbed, gas-phase values.

D. Sample Preparation
Pyrolysis of bitropyl (7,7’-bi(1,3,5-cycloheptatriene) is a good source of the tropyl
radical.[17] Cleavage of the C-C bond produces two tropyl radicals.
C7H7–C7H7 (+ M) → C7H7 + C7H7

(7.1)

The bond energy, DH298(C7H7-C7H7), was measured[42] to be 43 ± 1 kcal mol-1 (1.86 ± 0.05eV). Bitropyl is solid at room temperature and was purchased from Sigma-Aldrich and
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used without further purification. Another tropyl precursor, cycloheptatriene (C7H8),
was explored. The C-H bond strength[21] of cycloheptatriene is 74 kcal mol-1 and this
implies that C7H8 could be an attractive thermal precursor for tropyl radical; see Table
7.1. Cycloheptatriene is a liquid at room temperature and was purchased from SigmaAldrich with a purity of 95 % and used without further purification. A final potential
precursor for tropyl radical to be considered is 2,5-norbornadiene. This hydrocarbon
was purchased from Sigma-Aldrich with 98 % purity and was used without further
purification. All of the pyrolysis experiments use dilute gas samples with either heliumor neon as the carrier gas. Previous work with SiC reactors[5, 43-44] has shown thatbimolecular chemistry can obscure decomposition spectra so samples were made as
dilute as possible while maintaining sufficient signal-to-noise. Typical dilutions are 0.1 –0.01 %. For some experiments the dilution is difficult to control because the solid
bitropyl precursor must be heated to 50 – 65 °C to achieve sufficient vapor pressure,depending on the gas flow rate and amount of sample surface area present. At the ALS,
the sample was heated in a 1 cm I.D. glass test tube and He carrier gas was directed
over the surface before traveling downstream to the reactor. The matrix IR experiments
make use of a 1 mm I.D. glass vial that is inserted directly behind the pulse valve,
where it is then heated. Pyrolysis appears to be unimolecular since no change inproducts was observed over a range of sample temperatures.

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III. Results and Discussion
A. Photoionization Mass Spectrometry
Fig. 7.2 shows the 9.5 eV PIMS that results from a dilute sample of bitropyl
heated to 600 K in a continuous flow micro-reactor. Earlier threshold photoelectron
spectra of bitropyl reported[17] ions at both m/z 91 and 182 with VUV photons of 8.7 eV.
The 9.5 eV spectrum in Fig. 7.2 shows signals at m/z 91 and 104 only; there are noparent ions at m/z 182 indicating that the bitropyl sample is completely dissociatively
ionized.
C7H7–C7H7 + 9.5 eV → [C7H7–C7H7]+ → products(7.2)

The m/z 91 ion is C7H7+ while the smaller feature at m/z 104 remains unidentified. The
results in Fig. 7.2 are consistent with earlier PIMS studies[42] that revealed theappearance energy for the C7H7+ ion from bitropyl was 8.09 ± 0.05 eV. The extensive
dissociative ionization in Fig. 7.2 results from the fragmentation of the [C7H7–C7H7]+
cation into a pair of exceptionally stable products: X̃ 2E2” C7H7 and X̃+ 1A1’ C7H7+, which
is an aromatic cation.

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Fig. 7.2. The 9.5 eV PIMS spectra of bitropyl (C7H7-C7H7 m/z 182) pyrolyzed in acontinuous flow micro-reactor heated to 600 K. The absence of a signal at m/z 182
indicates that the bitropyl sample is dissociatively ionized. The feature at 104 is theresult of dissociative ionization of the precursor and is probably [styrene]+.

Fig. 7.3 shows the PIMS spectra that result as bitropyl is heated in the microreactor up to 1500 K. As the sample is heated to 1100 K, the 11.8 eV PIMS reveals the

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appearance of a peak at m/z 26 (circled in red) that is shown to be HCCH+ by PIE
spectra. The signal at m/z 65 is assigned to C5H5+ via PIE spectra at reactor
temperatures of 1100 K and higher. At temperature below 1100 K, signals at m/z 65
arise from dissociative ionization. Since the ionization energy of acetylene is so large(11.4 eV, see Table 7.1), signal at m/z 26 will only arise from photoionization ofthermally produced acetylene and not from dissociative ionization. When molecules are
ionized with excess photon energy, cations are formed with internal energy that canfragment to a daughter ion and a neutral fragment(s). By recording PIMS at 11.8 eV, weconclude that the feature at m/z 65 results from dissociative ionization whenever the
co-produced fragment, HC≡CH, is not detected. At 1100 K and hotter, the spectra imply
thermal cracking of tropyl to acetylene and cyclopentadienyl.
C7H7 X̃ 2E2” (+ M) → HC≡CH + C5H5 X̃ 2E1”

(7.3)

As the reactor temperature is increased to 1500 K, the pyrolysis of tropyl radical
is complete and m/z 91 is no longer present. At 1400 K, a 10.0 eV PIMS signal at m/z 39
is detected that is assigned to propargyl radical; the IE(HCCCH2) is 8.7 eV (see Table
7.1). In earlier studies,[5, 45-46] it was observed that at 1300 K the cyclopentadienyl radical
thermally dissociates to propargyl radical and acetylene.
C5H5 (+ M) → HC≡CH + HCCCH2

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Fig. 7.3. PIMS spectra of bitropyl (C7H7-C7H7) recorded in a continuous flow
micro-reactor at temperatures of: 400 K, 1100 K, 1200 K, 1300 K, 1400 K, and 1500 K.
Pyrolysis of tropyl (C7H7 (+ M) → HC≡CH + C5H5) commences at 1100 K as indicated by
the small signal for HC≡CH+ at m/z 26 (denoted with a red circle).

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The thermal decomposition of the tropyl radical from 1200 K up to 1500 K isremarkably simple. We observe features at m/z 65 and 26 that are predicted to be
cyclopentadienyl and acetylene, consistent with eq. (7.3) and Fig. 7.1. To confirm theidentity of the feature at m/z 65, the PIE spectrum of m/z 65 was recorded between 8.0
and 10.5 eV; see Fig. 7.4. At 900 K and below, there is no signal for PIE(m/z 65). At
temperatures of 1200 K and above, the PIE for m/z 65 shows a threshold at 8.5 ± 0.1 eV,
consistent with the observed[30] IE(C5H5) of 8.4 eV (see Table 7.1). The PIE(m/z 65) at
1200 K and 1500 K in Fig. 7.4 agree with the PIE of an authentic sample[47] of C5H5. The
assignment of m/z 39 to the propargyl radical in Fig. 7.3 is also confirmed by ameasurement of the PIE(m/z 39). Both the threshold and the shape of the PIE(m/z 39)
match the known[26] IE(HCCCH2) and photoionization cross section[48] of the propargyl
radical.

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Fig. 7.4. PIE (m/z 65) recorded during the thermal decomposition of bitropyl(C7H7-C7H7) in a continuous flow micro-reactor in He at 900 K , 1200 K, and 1500 K. The
red trace is the PIE spectrum of the C5H5 radical recorded previously.[47] The ionizationthreshold[30] for X̃ 2E1” C5H5 is indicated. There is no evidence for cyclopentadienyl
radical being present at 900 K.

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Fig. 7.5 shows the neon matrix IR spectrum of the products of the thermal

decomposition of bitropyl in a pulsed reactor at 1200 K. This vibrational spectrum
confirms HC≡CH as a pyrolysis product of tropyl radical.

Both the PIE(m/z 65)

spectrum in Fig. 7.4 and the IR spectrum in Fig. 7.5 confirm the products in eq. (7.3).

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Bitropyl (C7H7)2 Decomposition in Pulsed Neon

Ne Matrix IR Absorption

HC≡CH
ν3

1200 K

400 K
1100 K Neon

3300

3290

3280

3270

-1

Frequency (cm )Fig. 7.5. Neon matrix absorption IR spectrum of the 1200 K pyrolysis of bitropyl(C7H7-C7H7) in a pulsed micro-reactor is shown in red. The presence of acetylene[49] isdemonstrated by observation of the intense bands of %3(HC≡CH). The black trace is thatof the bitropyl precursor, which is un-pyrolyzed at 400 K. The green trace is the
background spectrum of pure Ne heated to 1100 K.

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Fig. 7.6 is a contrast between the pyrolysis products of ethylbenzene (top) andbitropyl (bottom). Both samples are thermally decomposed in a 0.6 mm I.D., continuous
flow micro-reactor that is heated to 1500 K. The thermal cracking of C6H5CH2CH3
initially generates[5] methyl radicals (m/z 15) and benzyl radicals (m/z 91). Subsequentfragmentation of the C6H5CH2 radical leads to the complex set of products at m/z 26, 39,
50, 63, 65, 76, 78, 89, and 90 that are discussed in ref. [5]. Pyrolysis of bitropyl at thebottom of Fig. 7.6 produces C7H7 (m/z 91) and the fragmentation products from tropyl,
C5H5 (m/z 65) and HC≡CH (m/z 26). Propargyl radical, HCCCH2 (m/z 39), resultsfrom the thermal cracking of the cyclopentadienyl radical, eq. (7.4). The dramaticdifferences between these two PIMS spectra in Fig. 7.6 provide strong evidence that thetropyl radical does not isomerize to the benzyl radical.

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Fig. 7.6. PIMS spectra comparing the thermal decomposition of the benzyl
radical and the tropyl radical. In both experiments the precursors (ethyl benzene andbitropyl) are decomposed in a continuous, 0.6 mm I.D. micro-reactor that is heated to1500 K. No isomerization from the C7H7 radical to the more stable C6H5CH2 is observed,
as indicated by the paucity of benzyl decomposition products found in tropyl
decomposition spectra.

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B. Matrix Isolation Spectroscopy
Because of the extensive dissociative ionization of the tropyl radical precursor,
C7H7–C7H7, the application of PIMS to study tropyl radical pyrolysis is somewhat
limited. Photo-ions of the pyrolysis products will always be partially obscured byfragments of dissociative ionization. Consequently IR spectroscopy is a very usefulcomplementary detection tool.
However the matrix IR spectroscopy of tropyl radical has its own set ofcomplications. There are no definitive vibrational spectra of the C7H7 radical in a matrix
environment. The gas phase IR spectrum was recorded for the tropyl radical (formed ina discharge from cycloheptatriene) with an IR-UV double resonance technique[13] thatused the free electron laser “FELIX”. Several vibrational modes of tropyl were observedin the fingerprint region but none could be assigned. LIF spectroscopy[10] has been used
to identify some of the modes of C7H7, X̃ 2E2”. This study presented a vibrational
analysis of the Ã 2E3" ← X̃ electronic spectrum and several gas phase modes of theground2

E2” state were reported. Unfortunately, because of the nature of LIF

spectroscopy, there is no information available regarding IR intensities for the groundstate C7H7, X̃ 2E2” fundamentals. An important complication for studying tropyl radical
in a neon matrix is the presence of the Jahn –Teller effect, which distorts the tropyl
radical from the D7h surface to the C2v surface. The effect of inert gas matrices on JahnTeller distorted molecules is not easy to predict and is expected to cause perturbations
from any gas-phase fundamentals greater than usual matrix to gas-phase frequencies
shifts.

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In spite of these difficulties, matrix isolation IR spectroscopy is useful as asecondary confirmation for tropyl radical decomposition. The precursor used is bitropyl
and the experiments were performed using neon as the carrier gas. Fig. 7.7 shows 4scans taken at different reactor temperatures for bitropyl decomposition: 400 K, 1000 K,
1100 K, and 1200 K; as well as a scan of pure neon run at 1100 K as a control to rule out
any systematic contaminants. The thin black trace shows an absorption feature fromthe precursor, which has not undergone decomposition at 400 K. This peak issignificantly depleted upon heating to 1000 K (thick black line) and a strong, broadfeature has developed centered at 3056 ± 6 cm-1. Upon heating to 1100 K (thin red trace)and 1200 K (thick red trace), this new peak is depleted as tropyl radical thermallydecomposes. The feature from the bitropyl precursor is completely absent by 1100 K.
This temperature dependence is in qualitative agreement with that observed in Fig. 7.3,
although one could expect minor discrepancies due to differing carrier gas (Ne vs. He)
and flow conditions (pulsed vs. continuous).

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Fig. 7.7. Neon matrix absorbance IR spectra of the pyrolysis of bitropyl (C7H7C7H7) in a pulsed micro-reactor at 400 K (thin black), 1000 K (thick black), 1100 K (thinred), and 1200 K (thick red). The feature at 3030 cm-1 is assigned as the bitropyl
precursor. A new band at 3056 ± 6 cm-1 is assigned to the C7H7 radical. This assignment
is corroborated by the temperature dependence observed here and in the PIMS of Fig.
7.3. The green trace is a background spectrum of Ne heated to 1100 K.

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None of the vibrational features for tropyl radical in the CH stretch region havebeen previously assigned. A recent helium nanodroplet[19] study observed a pair ofvibrational bands at 3052.9 cm-1 and 3057.4 cm-1 that are assigned to the tropyl radical.
The helium nanodroplet spectra agree well with the matrix IR feature illustrated in Fig.
7.7.
Fig. 7.8 shows an additional intense absorption feature appearing in thedecomposition of bitropyl at 1451 cm-1. This feature exhibits similar temperaturedependence to that shown in Fig. 7.7; it is not observed at 400 K, is strongest at 1000 K
and begins to decay at 1100 K and 1200 K.

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Fig. 7.8. Neon matrix absorbance IR spectra of the pyrolysis of bitropyl at 400 K(thin black), 1000 K (thick black), 1100 K (thin red), and 1200 K (thick red). The
absorption feature at 1451 ± 1 cm-1 is assigned to the tropyl radical. The green scan is abackground spectrum of Ne heated to 1100 K.

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Neither the 3056 cm-1 nor the 1451 cm-1 bands were observed in the spectra frombenzyl decomposition,[5] nor were any vibrational assignments for benzyl radical
observed in the tropyl radical decomposition spectra. This provides further
confirmation that the benzyl and tropyl radicals do not interconvert, even at
temperatures where both radicals thermally decompose.
C. Cycloheptatriene or Norbornadiene as Tropyl Precursors?
In addition to bitropyl, cycloheptatriene (C7H8) was considered as a pyrolytic
source of C7H7. Cycloheptatriene has been successfully used as a precursor to prepare
gas-phase tropyl radicals in discharge sources.[8, 10, 13]. However, shock tube studies[50] of
high-temperature pyrolysis of cycloheptatriene demonstrate complete conversion of
C7H8 to C6H5CH3. A gas-phase, stirred-flow reactor was used[51] to explore theisomerizations of norbornadiene, cycloheptatriene, and toluene. No intermediateradicals could be detected in this early study because the reaction products weredetected by gas chromatography with flame ionization detection. Fig. 7.9 is asummary[52] of the interconversions of norbornadiene, cycloheptatriene, and toluene.
Norcaradiene[53] has been predicted as an intermediate in the equilibration of
norbornadieneandcycloheptatriene.

The

first

decomposition

pathwayofnorbornadiene is the retro-Diels-Alder fragmentation to cyclopentadiene and acetylene.
At higher temperatures, norbornadiene, norcaradiene, cycloheptatriene, and toluene allinterconvert, where toluene is the most stable isomer.[51]

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Fig. 7.9. Isomerizations[50-53] of norbornadiene, norcaradiene, cycloheptatriene,
and toluene.

Fig. 7.10 shows the IR spectra of a dilute mixture of 0.05 % cycloheptatriene inneon that was thermally decomposed in a pulsed micro-reactor. As predicted,[50] C7H8
isomerizes to toluene rather than forming tropyl radical. This finding was confirmed byassigning 11 absorption features to gas-phase[54] C6H5CH3. At 300 K the only feature inthis spectral window is due to the precursor, cycloheptatriene, but upon heating, three
new bands emerge. The three peaks are assigned[54] as % 11 at 730 cm-1, % 4 at 695 cm-1, and

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% 18b at 1083 cm-1, which agree well with the gas-phase values: % 11 = 728 cm-1, % 4 = 695
cm-1, and % 18b = 1080 cm-1. At the highest temperature (1500 K), a few small features areobserved that are assigned to C6H5CH2 including % 7 (762 cm-1) and % 13 (1308 cm-1), inagreement with the argon matrix spectrum of benzyl radical.[55]

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Fig. 7.10. The neon matrix absorbance IR spectrum of cycloheptatriene (C7H8)
pyrolysis in a pulsed micro-reactor at 1300 K is shown in thick black. The thin blackline is the absorbance spectrum of cycloheptatriene at 300 K. The 300 K spectrum shows
only one feature belonging to C7H8. Upon heating to 1300 K, the precursor feature has
diminished and new bands are observed. All three are assigned as vibrations oftoluene.[55]

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The isomerization of C7H8 to C6H5CH3 is confirmed by the 118.2 nm PIMS, shownin Fig. 7.11. In this experiment, cycloheptatriene is subjected to pyrolysis in a pulsed
micro-reactor heated to 300 K, 1400 K, and 1600 K. The products shown at 1600 K arenearly identical to those observed from benzyl radical precursors[5] but with a higher
temperature for decomposition onset; compare with top panel of Fig. 7.6.

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Fig. 7.11. PIMS spectra from the pyrolysis of cycloheptatriene in pulsed helium
are shown at three reactor temperatures, 300 K, 1400 K, and 1600 K. At 1400 K the
expected fragmentation of tropyl radical (see Fig. 7.3) is not observed. At 1600 K a set of
products appears that is very similar to those characteristic[5] of benzyl radical
decomposition (see Fig. 7.6).

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We also studied the decomposition of 2,5-norbornadiene (Fig. 7.12). The IE(2,5-

norbornadiene) is less that 8.35 eV (Table 7.1) and the initial scan (400 K) reveals a small
amount of dissociative ionization: norbornadiene + 118.2 nm → C6H5CH2+ (m/z 91) + H
atom. At 1300 K, it is observed that norbornadiene undergoes a retro-Diels-Alderreaction: norbornadiene (+ M) → C5H6 (m/z 66) + HC≡CH (see Fig. 7.9). As the reactor
is heated to 1600 K, it appears that 2,5-norbornadiene isomerize to C6H5CH3, which
thermally dissociates to H atom and benzyl radical. The fragmentation of C6H5CH2 (topscan in Fig. 7.12) is very close to the pyrolysis pattern observed for norbornadiene(bottom Fig. 7.12).

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Fig. 7.12. PIMS spectra from the pyrolysis of norbornadiene in pulsed helium areshown at three reactor temperatures, 400 K, 1300 K, and 1600 K. At 1300 K
norbornadiene undergoes a retro-Diels-Alder fragmentation and produces HC≡CH andC5H6 (m/z 66); bottom scan. At 1600 K a set of products appears that is very similar to
those characteristic[5] of benzyl radical decomposition; top scan uses C6H6CH2CH3 as asource of benzyl radical.

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IV. Conclusions
The results of Figs. 7.3 – 7.8 eliminate any participation of the C7H7 radical fromthe pyrolysis pathways of the C6H5CH2 radical at temperatures up to 1600 K. Tropyl
radical only decomposes to acetylene and the cyclopentadienyl radical, eq. (7.3). Thesefindings are consistent with the failure[5] of tunable PIMS to detect C7H7 radicals as
benzyl undergoes pyrolysis. We conclude this chapter with a reconsideration of thepathways for the thermal fragmentation of benzyl radical based on all experimental
findings.[5-6, 11, 56-61]
The pyrolysis of the benzyl radical is a complex problem. None of the earlyexperimental papers[61-66] describing the pyrolysis of C6H5CH2 were able to identify anyof the thermal fragments except for the H atoms detected by ARAS.[6, 11] The only
organic radicals resulting from pyrolysis of benzyl radical come from a few studies
using heated micro-reactors outfitted with VUV PIMS.[3-5]The pyrolysis of toluene was studied[3] in a quartz flow tube reactor at pressuresof 8 – 15 Torr and temperatures of 1136 – 1507 K. A more recent study[4] of toluene
pyrolysis used a heated alumina (Al2O3) micro-reactor coupled to a VUV PIMS driven
by tunable radiation from a synchrotron. The 248 nm photochemistry of benzyl radicalhas been studied[67] by photofragment translational spectroscopy. Dissociation occurs
following relaxation from the excited state[68] to the ground state, C6H5CH2, X̃ 2B1,
producing benzyl radicals (C6H5CH2)* excited by 5 eV radiation. The benzyl* radicals
fragment to H + C7H6 and CH3 + C6H4 radicals.
As mentioned in the introduction, pyrolysis of either ethylbenzene or benzyl
bromide in a heated SiC micro-reactor[5] revealed an extended set of products that are
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summarized in Fig. 7.1. When C6H513CH2CH3 was heated[5] to 1200 K, the labeled benzyl
radical, C6H513CH2, was produced. Decomposition of C6H513CH2 at 1300 K produced amixture of 12[cyclopentadienyl] and 13[cyclopentadienyl] radicals. One pathway toincorporate the 13C label into the cyclopentadienyl radical would be isomerization of
C6H513CH2 to 13C-labeled C7H7 (see Fig. 7.1). However the use[5] of 6.5 eV photons tosearch for the isomerization C6H5CD2 → C7H5D2 detected no tropyl radicals. This failure
to observe C7H7 radicals did not conclusively prove that tropyl is not present during thethermal cracking of benzyl radical. However, the current results in Figs. 7.3 – 7.8
demonstrates that clean samples of C7H7 radicals can be produced and that they
fragment by a pathway independent of benzyl.
A. Pyrolysis of Benzyl Radical without Tropyl
In all of the experiments to scrutinize the pyrolysis of benzyl radicals[5] or tropyl
radicals (this chapter), care was taken to study dilute samples (0.1 % — 0.02 %hydrocarbons in He or Ne). In the analysis of these results, we assume all fragmentation
products result from unimolecular chemistry; there is no bimolecular chemistry to beanalyzed. Pathways for the unimolecular, thermal decomposition of the benzyl radical
are shown in Figs. 7.13 and 7.14. These schemes are based on the experimental findings
from shock tubes with ARAS detection[6, 11] and heated micro-reactors monitored by
PIMS and IR spectroscopy.[5] Heating the C6H5CH2 radical to 1300 K triggers an“extended” Benson fragmentation. The original suggestion[6] was only to offer pathwaysfor H atom loss and formation of the fulvenallenyl radical, C5H4-C≡CH. Thismechanism[6] also implies formation of the cyclopentadienyl radical and acetylene. The
bicyclic radicals and vinylidene[69-70] (:C=CH2) in Fig. 7.13 will not be stable in the hot

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micro-reactor and are enclosed in brackets. The production of H atoms is known fromboth[6, 11] ARAS and PIMS spectra.[5]

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Fig. 7.13. The “extended” Benson Fragmentation of the benzyl radical, C6H5CH2
X̃ B1. The original suggestion[6] provided pathways for H atom loss and formation of
the fulvenallenyl radical, C5H4-C≡CH. This suggestion also implies formation of thecyclopentadienyl radical and acetylene. The bicyclic radicals and vinylidene (:C=CH2)
will not be stable in the hot micro-reactor and are enclosed in brackets. The tropyl
radical, C7H7, X̃ 2E2”, does not participate.
2

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The

Bensonfragmentation

in

Figs.

7.13

leads

to

formationofthecyclopentadienyl and the fulvenallenyl radicals. (One could view C5H4-C≡CH as a
substituted cyclopentadienyl radical.) As shown in eq. (7.4), C5H5 decomposes toacetylene and propargyl radicals at 1300 K. Likewise the fulvenallenyl radical has been
observed[71-72] to decompose to propargyl radical and diacetylene. This can besummarized.
C5H4-C≡CH (+ M) → HCCCH2 + HC≡C-C≡CH(7.5a)

C5H4-C≡CH (+ M) → HC≡CH + HC≡C-CH-C≡CH(7.5b)

C5H4-C≡CH (+ M) → HC≡CH + HC≡C-C≡C-CH2(7.5c)The presence of the cyclopentadienyl radical and HC≡CH are confirmed by
PIMS, PIE, and IR spectroscopy.[5] PIMS and PIE spectroscopy also detect both
C5H4=C=CH2 and C5H4-C≡CH. The presence of fulvenallene is additionally confirmed
by IR spectroscopy. PIMS and IR spectroscopy clearly identify the propargyl radical,
HCCCH2. PIMS signals at m/z 50 and 63 are consistent with the presence of HC≡CC≡CH and the two radicals, CH2-C≡C-C≡CH and HC≡C-CH-C≡CH. However neither
the PIE nor IR spectra could distinguish between the (CH2-C≡C-C≡CH, HC≡C-CHC≡CH) pair.
The Benson fragmentation predicts loss of H atom from C6H5CH2 to produce apair of isomers, C5H4=C=CH2 and C5H5-C≡CH, both at m/z 90. Recently pyrolysis of
C6H5CD2 in a shock tube with ARAS detection[11] revealed the formation of both H and
D atoms in agreement with this prediction. Pyrolysis of C6H5CD2CD3 in a micro!

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reactor[5] provided further support with the detection of both C5H4=C=CD2 (m/z 92)
and C5H5-C≡CD (m/z 91).
The Benson fragmentation shown in Fig. 7.13 cannot be the complete story forthe thermal cracking of the benzyl radical. PIMS and PIE spectra confirm[5] that heating
C6H513CH2 radicals to 1300 K produces 13CH3, 13C5H5, and o-C6H4; these products areincompatible with Benson’s suggestion.[6] Fig. 7.14 shows two additional pathways forthermal cracking of benzyl radicals that address these products.

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Fig. 7.14. Fragmentation pathways of the benzyl radical, C6H5CH2 X̃ 2B1, areshown. The internal H-atom abstraction from the ring that leads to the formation of the& methyl-phenyl radical will be endothermic[22] by roughly 1 eV. It also requires a
curve-crossing from the 2B1 benzyl (' radical) to the 2A’ methyl-phenyl (s radical).
Decomposition of the & methyl-phenyl radical produces CH3 and o-C6H4. A second
pathway is the isomerization of benzyl to the bridgehead norbornadienyl radical. The &methyl-phenyl radicals, bridgehead norbornadienyl radicals, and 2A’ cyclopentadienyl
radicals will not be stable in the hot micro-reactor and are enclosed in brackets. The
tropyl radical, C7H7 X̃ 2E2”, does not participate.

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The pyrolysis of C6H513CH2 produces[5] both 13CH3 (m/z 16) and o-C6H4 (m/z 76).

These products could be the result of an isomerization of the 13[benzyl] radicals to the [&13

methyl-phenyl] radicals in Fig. 7.14. The isomerization would be initiated by H-

abstraction from the aromatic ring by the –CH2 side chain. Such an abstraction would be
difficult because of a required (' → () curve-crossing (C6H5CH2, 2B1 → C6H4CH3, 2A’). In
addition the internal abstraction will be endothermic by about 1 eV because of thedifference in the C6H5CH2-H and C6H5-H bond energies (see Table 7.1). Formation of the
C6H4CH3, 2A’ radical in Fig. 7.14 will be followed by rapid fragmentation to the methyl
and o-benzyne radicals and both are confirmed to be present. Heating o-C6H4 to hightemperatures is known[73] to trigger fragmentation to HC≡CH and HC≡C-C≡CH. The
pyrolysis of both the C6H5CD2 and C6D5CH2 radicals are consistent with this pathway.[5]Decomposition of C6H5CD2 radicals at 1500 K forms small quantities of the CD2H (m/z
17) radicals while heating C6D5CH2 radicals to 1400 K leads to appearance of CH2D (m/z
16) radicals.
A second pathway for fragmentation of the benzyl radical shown in Fig. 4.14 isisomerization of C6H5CH2 to the bridgehead, norbornadienyl radical. Such a bridgeheadradical would undergo a retro-Diels-Alder fragmentation producing HC≡CH and the2

A’ C5H5 radical. The planar (() 2A’ C5H5 radical is certainly the initial adduct in thereaction[74-75] of propargyl with acetylene and it rapidly isomerizes to the ground state,
cyclopentadienyl radical, C5H5 X̃ 2E1”. A 13[norbornadienyl] radical will automaticallyincorporate the 13C label into the cyclopentadienyl radical, as observed in Fig. 13 ofref. [5].

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The pyrolysis pathways for C6H5CH2 shown in Figs. 7.13 and 7.14 are consistent

with all of the major peaks of the PIMS in Fig. 2 of ref. [5]. The corresponding pathways
for the isotopically substituted benzyl radicals, C6H5CD2, C6D5CH2, and C6H513CH2 arecontained in Figs E.1 – E.6 (see Appendix E). As in the case of the parent benzyl radical,
these predicted C6H5CD2, C6D5CH2, and C6H513CH2 pathways (see Appendix E) can beused to assign the major peaks of the experimental PIMS spectra in Fig. 8, Fig. 10, andFig. 11 of ref. [5].
A recent paper[16] applied metadynamics to the pyrolysis of benzyl. These
calculations suggested that both C5H5 and its isomer, (CH2)2C-C≡CH, are intermediates
in the high temperature pyrolysis of benzyl. The IE(C5H5) is measured to be 8.4 eV (seeTable 7.1) and the measured[5] PIE(m/z 65) has its threshold at 8.4 ± 0.1 eV. The
IE((CH2)2C-C≡CH) is not measured but it is likely less than that of the allyl radical, 8.1
eV (see Table 7.1). The PIE(m/z 65) resulting from the thermal cracking of the C6H5CH2
radical indicates that there is little (or no) (CH2)2C-C≡CH present in the pyrolysis ofbenzyl. It was also predicted by the metadynamics calculations[16] that the starting
C6H5CH2 radicals could isomerize to a pair of isomeric, dimethylene-cyclopentenyl
radicals. The spectroscopic probes of ref. [5] could not confirm the presence of these two
isomers.
There are small features at m/z 41, 51, 64, 77, and 78 present in the PIMS of
C6H5CH2 (Fig. 2 of ref. [5]) that cannot be rationalized by either pathways of Figs. 7.13
and 7.14. The peaks at m/z 77 and 78 are likely the phenyl radical (C6H5) and benzene.
There is a theoretical calculation that[72] predicts a small channel for pyrolysis offulvenallene to C5H4: + HC≡CH. The singlet carbene (C5H4:) is consistent with the peak

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at m/z 64.
There are small bands in the 118.2 nm PIMS spectra of C6H5CD2 and C6D5CH2(Figs. 8 and 10 of ref. [5]) that suggest an interesting subtlety to the & methyl-phenyl
radical in Fig. 7.14. Perhaps there is some equilibration of benzyl radicals with the& methyl-phenyl radicals? If so, then the C6H5CD2 and C6D5CH2 radicals will scramble:
C6H5CD2 ⇋ [C6H4-CHD2] ⇋ C6H4D-CHD. Application of the [Benson fragmentation,
methyl-phenyl, norbornadienyl] pathways to the o-C6H4D-CHD radical leads to theformation of complex set of labeled (fulvenallenes, ethynl cyclpentadienes, andfulvenallenyl) radicals. Fragmentation of these radicals predicts formation of C5H4C≡CH (m/z 89), C5H4D (cyclopentadienyl radical-d1, m/z 66), o-C6H3D (o-benzyne, m/z77), HC≡C-CH-C≡CH (m/z 63), and DC≡C-C≡CD (m/z 52). There are signals in Fig. 8of ref. [5] that are compatible with these radicals. A similar scrambling of benzyl radicald5 (C6D5CH2 ⇋ [C6D4H-CDH2] ⇋ C6D4H-CHD) leads to production of C5H3D-CCH (m/z92), C5D4H (m/z 69), o-C6D3H (m/z 79), and (DC≡C-C≡C-CD2 or DC≡C-CD-C≡CD) m/z
66. Weak features for all of these radicals are present in Fig. 10 of ref. [5].
The three distinct pathways for pyrolysis of benzyl radical in Figs. 7.13 and 7.14
are complicated. Nevertheless they offer a set of predictions for the thermal cracking of
more complex aromatic compounds that are included in surrogate fuel mixtures.
Aviation fuels are mixtures of a very large number of chemical components[76]. The
detailed numerical simulation of real combustion fuels is too difficult to apply to any
fuel that is not a pure component or a mixture of more than a few species. One way tocircumvent this problem is to study a surrogate fuel. Such surrogate fuels should becomprised of a handful of components but be capable of emulating the gas phase
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combustion characteristics of the real fuel of interest. The advantage, of course, is thatthe resulting reaction mechanism is of much more manageable size.
A surrogate fuel for aviation diesels has been proposed[76] that is a mixture of ten
compounds. This mixture includes 6 alkanes (n-decane, n-dodecane, iso-octane, isocetane, and methyl cyclohexane) and 4 aromatic species (toluene, n-propyl benzene,1,
3,5-trimethyl benzene, and 1-methyl naphthalene). Three of these species (C6H5-CH3,
C6H3(CH3)3, and C6H5-CH2CH2H3) would be predicted to pyrolyze to benzyl radicals.
The decomposition pathways for C6H5CH2 in Figs. 7.13 and 7.14 will be relevant to thesesurrogates.

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Reference for Chapter 7[1] J. L. Burger, T. M. Lovestead and T. J. Bruno. Composition of the C-6+ Fraction of
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232

Chapter 8
Tabletop Vacuum Ultraviolet Laser-Based Photoionization Mass
Spectrometry with Photoelectron Photoion Coincidence Detection of
Pyrolysis Products

Abstract
Pyrolysis using a heated micro-reactor is combined with a tabletop vacuum
ultraviolet (VUV) laser and photoelectron photoion coincidence (PEPICO) detector.
This new combination of techniques provides for more powerful photoionization mass
spectrometry (PIMS) by allowing isomer-specific identification of products and spectral
simplification via reduction of unwanted dissociative ionization. These two advantages
over previous PIMS experiments demonstrate a desirable means of studying thepyrolysis of gas phase molecules like combustion precursors and intermediates.

I. Introduction
Studying pyrolysis in a heated micro-reactor using photoionization mass
spectrometry (PIMS) has advantages including short data acquisition time and theability to detect all species with sufficiently energetic ionizing radiation. PIMS using an
Nd:YAG laser ionization source is a well established technique that makes use of the 9th
harmonicofthelaser’sfundamental! = 118.2 nm (Ephoton = 10.487 eV).

frequency,

which

corresponds

toThis frequency is useful because it is sufficiently

energetic to ionize the overwhelming majority of organic molecules. Using a static gas

233cell filled with a 10:1 mixture of argon and xenon, one can reproducibly andstraightforwardly produce the 9th harmonic of a commercial Nd:YAG laser.
One major disadvantage of 118.2 nm photoionization techniques is that when
ionizing molecules with ionization energies (IE) smaller than 10.487 eV, the excessenergy can be redistributed in the molecule and result in bond breaking within the
newly formed cation. This process is called dissociative ionization and is summarizedin Equation 8.1. In extreme cases, dissociative ionization is so dramatic that none of theparent signal is detected and only fragment peaks survive to reach the detector.[1] In thecases of dissociative ionization, it is challenging to determine if a peak is the result of
thermal decomposition of the parent that exited the micro-reactor or whether it is theresult of dissociative ionization. Secondary confirmation may be required for a given
fragment if dissociative ionization is suspected. If 118.2 nm PIMS were the only meansof detecting pyrolysis products, many conclusions would be equivocal due to thelimitations of the detection mechanisms.
M + ħ#  [M+]*  daughter ion+ + neutral fragment(8.1)

Our research groups commonly use one of two secondary confirmation
techniques before concluding chemical arguments based upon a 118.2 nm PIMS feature.
The first method is matrix isolation IR spectroscopy, which removes the possibility ofdissociative ionization since no ionizing radiation is needed. The second method is toutilize a synchrotron, which provides tunable ionizing light. Lowering the photon
energy decreases the amount of excess energy available to fragment molecules but more
importantly, scanning the photon energy while recording signal for a single mass peakprovides photoionization spectra, which are diagnostic of a given species. If a feature in
118.2 nm PIMS is the result of dissociative ionization, the photoionization spectrum will234

elucidate this.

When a PIMS feature results from a pyrolysis product, thephotoionization spectrum will agree with previously recorded photoionization spectraand ionization energy of that product, if such information is available. In many cases ofdissociative ionization, the ionization onset in a photoionization spectrum differs
substantially from that of the true molecule at that mass. The use of synchrotron
radiation makes a PIMS experiment much more valuable, however, the effort involved
to obtain synchrotron data is also much greater. The researchers must apply for andreceive beam time at a synchrotron facility, travel to the location, construct a pyrolysis
source, and take as much data as possible within their allotted beam time. These trips
are often only a few days long and can only be done a few times per year, making
synchrotron data both costly and challenging to acquire.
A more desirable method to provide dissociative ionization-free PIMS spectra is
to produce vacuum ultraviolet (VUV) light without the need to travel to a synchrotron
facility. Four-wave frequency mixing has been used[2] to study allyl radical (CH2CCH2)
from a micro-reactor. This method has a relatively low duty cycle (less than 100 Hz) so
it is not commonly used for pyrolysis due to the large amount of time required toperform experiments. A recent collaboration with the group of Dr. Margaret Murnane
and Dr. Henry Kapteyn has yielded a novel combination of experimental techniquesthat produces PIMS spectra where dissociative ionization can be identified and thespectra are collected with fast acquisition times yielding significant increase inexperimentally available information.
The ionization source in these experiments is based on a femtosecond Ti:sapphirelaser that can be upconverted to energies in the vacuum ultraviolet (VUV), the range
relevant for molecular ionization. Rather than a standard Wiley-McLaren time-of-flight235

(TOF) mass spectrometry setup,[3] the detection scheme in these experiments is a custom
designed coincidence detector that allows for spatial imaging of the photoelectrons andthe cations formed for each laser pulse.
Photoelectron photoion coincidence (PEPICO) detection has been used before in
conjunction with a heated micro-reactor and synchrotron source.[4] PEPICO is
accomplished in the present experiment by creating a static electric field that is roughly
flat along the 25 cm long TOF axis, which is enforced by equally spaced copper plates
wired in-series with resistors between each.

This electric field scheme responds

differently to dissociative ionization events than do Wiley-McLaren reflectron
spectrometers. Given the specific geometry and varying electric fields found in WileyMcLaren reflectron TOF, the spread in kinetic energy imparted by dissociative
ionization is condensed back into narrow TOF peaks by the time the ions reach thedetector. The static electric field found in the PEPICO detector does not refocus the ionsin time and so dissociative ionization peaks are broad in TOF and PIMS spectra. This
allows for straightforward identification of dissociative ionization peaks in the spectraand disambiguation in the case where thermal and dissociative ionization peaks areboth present.
Two examples where the present experimental setup is especially useful are 1)
cases where two or more isomers are present and 118.2 nm PIMS is unable todistinguish between them and 2) cases where dissociative ionization is strong enough tocauses spurious assignments of thermal decomposition products. To demonstrate theutility of this combination of experimental techniques, we have investigated two sets ofmolecules.

The

first

experiment

shows

that

the

isomers

1,3-butadiene(CH2=CHCH=CH2) and 1-butyne (CH3CH2C≡CH) can be distinguished in PEPICO
236

spectra due to their different IE. The IE of 1,3-butadiene[5] is 9.082 ± 0.004 eV and the IEof 1-butyne[6] is 10.20 ± 0.02 eV. In this experiment, pure samples of each isomer were
flowed through an unheated micro-reactor.

In a mixture of the two samples,

photoelectron spectra clearly resolve the two isomers. The true test of this method is toprepare a precursor that forms both isomers pyrolytically and confirm that both isomers
remain distinguishable.

The second experiment compares a set of pyrolysis

experiments done both with 118.2 nm PIMS and VUV PEPICO experiments. A set ofmolecules that is challenging to study with 118.2 nm PIMS is small alcohols: methanol,
ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol. The OH groups are highly
susceptible to dissociative ionization, as was shown by a comprehensive study[1] ofalcohols using the 9th harmonic of an Nd:YAG ionization source. Here, we show that
due to dissociative ionization, pyrolysis of these alcohols is challenging with 118.2 nm
PIMS but when using VUV and PEPICO, dissociative ionization can be identified andthus the pyrolysis products can be identified.

II. Experimental

In order to combat issues of dissociative ionization, isomer ambiguity, andinability to ionize molecules with large ionization energies, we have coupled the microreactor to a laser system that generates harmonics of 800 nm Ti:sapphire laser light. The
laser system was built in the group of Dr. Margaret Murnane and Dr. Henry Kapteyn
and is modeled after the KMLabs Inc. DragonTM system. The laser system used here isable to perform with 10 kHz repetition rate, which is important due to the low countrates required for coincidence detection.

After amplifying the 30 fs output of a237

Ti:sapphire mode locked oscillator, the power of the IR light is on the order of 20 W.
This is focused into a waveguide made of a fused silica capillary (150 $m I.D. x 2.5 cmlong) in which Xe is flowed, generating odd harmonics in the range of the 3rd harmonic(roughly 4.8 eV) up through the 9th harmonic (roughly 14.4 eV); the technique is capableof higher harmonics, up to at least the 17th harmonic. The highest IE relevant to ourexperiments[7] is 15.4 eV for H2, so only the 9th harmonic and lower are necessary here.
By filtering of higher harmonics with different filter schemes, harmonics can be selectedby making only minor alterations to the system. The laser system was not modified forthese experiments and a more detailed description can be found in a previous
publication and citations therein.[8]The micro-reactor assembly in this experiment can be used without a pulse valve
since

the

sourcechamberispumped

by

a

large

turbomolecularpump(1650 L s-1 Leybold MAG W 2010 C). Roughly 1 cm downstream of the reactor exit is a
300 $m skimmer that separates the source chamber from the ionization chamber andinsures that a narrow molecular beam reaches the laser interaction region. In order tokeep the pressure low enough for the detector, a 50 $m orifice was brazed into thestainless steel tubing just upstream of the reactor. This allows for a backing pressure of
1 to 2 atm, downstream pressures in the source chamber of 10-4 Torr, and ionizationchamber pressures of 10-7 Torr during continuous flow of He or Ar.

The reactor

assembly is shown in Fig. 8.1, which illustrates a few new design features. The alumina
heat shield is now held tightly in place with a home-built, custom fit Swagelok nut. A
small stainless steel disc is included that has electrical feedthroughs installed to help
stabilize the delicate (0.005”) wires leading to the Type-C thermocouple.

238

Fig. 8.1. Continuous flow heated micro-reactor assembly (inset) and larger
cutaway diagram. The 50 $m orifice limits the gas flow so that a pulsed valve is notneeded to maintain appropriate upstream and downstream pressures. New design
features are highlighted that help stabilize the assembly to prevent misalignment andincrease thermocouple lifetime.
The detection mechanism in this experiment also adds important experimental
information. Rather than a standard TOF configuration, the design of the ion optics in
the interaction chamber allows us to detect both the cations and electrons formed froma single laser pulse. The count rate is intentionally attenuated so that only 1 in 10 laser
pulses leads to a photoionization event, which would be challenging to maintainwithout the high repetition rate of the laser system (10 kHz). For a given ionization
event, it is possible to correlate individual cations and their corresponding
photoelectrons.

The detectors for both the cations and the electrons are position

sensitive so given a sufficiently small ionization volume, the distance traveled in the
directions transverse to the flight axis can be used to calculate the kinetic energy of thecharged particles. By plotting only the electrons detected in coincidence with a given239

cation m/z, a photoelectron spectrum (PE) can be recorded. Although the instrument
resolution is limited to about 0.75 eV, features in these photoelectron spectra can be
identified and used as a diagnostic of given isomers. As long as two isomers have IEs
that differ by more than roughly 1 eV, this detection scheme can be used to identify
which isomers are present at a given m/z. Additionally, if a molecule in the molecularbeam dissociatively ionizes, this detector can be used to identify the fragment mass as a
product of dissociative ionization. The photoelectrons detected in coincidence with thedissociative ionization fragment will correspond to the PE spectrum of the parent
molecule and not the fragment. As long as the PE spectra (or simply the IE) of theparent and the fragment are both known, peaks resulting from dissociative ionization
can be removed from the recorded PIMS spectra during post-processing. These aremajor advantages over the Nd:YAG based TOF PIMS instruments that have been theworkhorse of this experimental technique in the past.
III. Results and Discussion
A. Isomer Identification
An ideal proof of concept system for this new VUV based PIMS experiment is tocompare two stable gas-phase isomers by their ionization energy. For this, we compare
two C4H6 isomers, 1-butyne (CH3CH2C≡CH) and 1,3-butadiene (CH2=CHCH=CH2).
This test case does not require heating the micro-reactor since both hydrocarbons arestable and can be entrained in the gas-phase using their vapor pressure at room
temperature. The photoelectron spectrum of 1-butyne is shown in Fig. 8.2, where only
the photoelectron counts recorded in coincidence m/z 54 cations are plotted. The
ionization energy of 1-butyne was previously established[6] to be 10.18 eV, which isroughly reproduced by the PE spectrum. The instrument resolution limits assignments240

to roughly 0.75 eV and some further calibration is needed that results from day-to-day
fluctuation of laser frequency. However, in cases like C4H6 where isomers differ byroughly 1 eV, qualitative assignment is straightforward.

Fig. 8.2. Photoelectron spectrum of 1-butyne flowed through an unheated CW
micro-reactor. A premixed gas sample was prepared of 10 % butyne in 5 atm of Ar.
The PEPICO detector can be used to identify only photoelectrons detected in the samelaser pulse as cations with m/z 54.
The photoelectron spectrum of 1,3-butadiene is shown in the red trace of Fig. 8.3,
which clearly differs from the blue trace of 1-butyne reproduced in Fig 8.3.

The

photoelectron spectrum of a room temperature mixture of the two is included in Fig. 8.3
as well, shown in black. This illustrates that given a mixture of species detected at m/z
54, the presence of either isomer can be identified.

241

Fig. 8.3. Photoelectron spectrum of 1,3-butadiene (red trace), 1-butyne (bluetrace), and a mixture of the two (black trace) flowed through an unheated CW microreactor. For each of the three traces, a 5 atm premixed gas sample was prepared of 10 %butadiene, 10 % butyne, or 10 % of both in Ar. In each case, only photoelectrons
detected in coincidence with cations of m/z 54 are plotted. The intensities of each arearbitrarily scaled to aid in feature identification. The black trace shows good agreementwith butadiene for the peak near 9 eV but above 10 eV the red trace does not reproduce
the mixture’s spectrum. The peak at roughly 10.2 in butyne is required to match the
mixture spectrum.

B. Disentangling Dissociative Ionization in Mass Spectra
The pyrolysis of alcohols is often complicated by the presence of dissociative
ionization peaks that are indistinguishable from true pyrolysis products in 118.2 nm
PIMS experiments. The nature of the PEPICO detector allows for simplification of thisissue.

As an example, the pyrolysis of 2-butanol (CH3CH2CH(OH)CH3) has been

investigated in both 118.2 nm PIMS and VUV PEPICO PIMS experiments. Shown in242

Fig. 8.4 is the PIMS spectrum of dilute 2-butanol flowed through a pulsed He reactor atroom temperature recorded using the 9th harmonic of an Nd:YAG laser at 118.2 nm(10.487 eV). The parent peak is observed at m/z 74 as well as multiple peaks at lower
masses. Since this spectrum was recorded with a room temperature nozzle, none of thedaughter peaks can be attributed to pyrolysis and are instead assigned as dissociative
ionization of the parent. As if often the case with alcohols, significant fragmentation
occurs after ionization. At elevated temperatures these peaks cannot be distinguishedfrom pyrolysis products, causing potential misinterpretation of the PIMS spectra.

243

Fig. 8.4. Room temperature PIMS spectra of dilute (0.07 %) 2-butanol in a pulsed
He micro-reactor. The ionization source is a 118.2 nm (10.487 eV) laser and PIMS isrecorded in a Wiley-McLaren reflectron time-of-flight mass spectrometer. The parent isobserved at m/z 74 but is accompanied by multiple other fragments that are assumed
to originate from dissociative ionization of 2-butanol.

In contrast, 2-butanol was flowed through the CW He reactor coupled to the
VUV PEPICO detector and the dissociative ionization features are immediately
identifiable. Fragmentation imparts kinetic energy into the daughter molecules and in
the smaller, static electric field of the PEPICO detector, the energy spread in dissociative244

ionization fragments results in a broadening of the PIMS features. Wiley-McLarenreflectron TOF instruments use varying electric fields to compensate for this energy
spread and are able to refocus the cation signals into sharp features. As seen in Fig. 8.5,
the dissociative ionization peaks are much broader than the parent peak at m/z 74, andcan be ruled out when considering what species are present in the molecular beam.

Fig. 8.5. Room temperature PIMS spectra of 2 % 2-butanol in a CW He microreactor. The ionization source is an upconverted Ti:sapphire laser yielding 11.2 ± 0.2 eVlight. The PEPICO detection source allows identification of dissociative ionization
fragments. The parent peak at m/z 74 is narrow but all other peaks have beenbroadened by their spread in kinetic energy.

245Upon heating 2-butanol, dissociative ionization becomes a larger problem. If
products are formed at the same mass as dissociative ionization peaks, it is difficult toassign them as thermal products.

As new features appear in the PIMS spectra, they

may either be the result of thermal decomposition of the parent or dissociative
ionization of either the vibrationally excited parent molecules or of the thermal
decomposition products.

Although the supersonic expansion at the reactor’s exit

should cool all fragments, vibrational excitation often persists due to inefficientcooling.[9]Fig 8.6 shows the 118.2 nm PIMS spectra resulting from pyrolysis of 2-

butanol at 1200 K in a pulsed He micro-reactor. Multiple new features are observed
upon heating, the largest of which are m/z 15, 28, 40, 41, 42, 44, 45, 54, 56, and 59.
Depletion of the parent at m/z 74 indicates that thermal decomposition is occurring but
unfortunately, due to the presence of the dissociative ionization signals at room
temperature, features at m/z 44, 45 and 59 cannot be confidently identified as pyrolysis
products.

246

Fig. 8.6. PIMS of 0.07 % 2-butanol in a pulsed He micro-reactor at 1200 K,
recorded with a 118.2 nm ionization source. The features at 44, 45, and 59 are also
observed in room temperature PIMS spectra and are thus challenging to assign.

247

Fig. 8.7. PIMS of 2 % 2-butanol pyrolyzed in CW helium flowed through a
micro-reactor heated with 3 Amps operating at 13 Volts. This is observationallyapproximated to be a reactor wall temperature of 1200 K. The narrow peaks are
attributed to pyrolysis products and the broad peaks (roughly m/z 30, 45, 59 and 72)
are now assigned to dissociative ionization.
Under similar conditions the experiment was repeated using the VUV PEPICO
experiment and the PIMS spectrum is shown in Fig. 8.7. By comparing Fig. 8.6 andFig. 8.7 we now can directly identify which species are pyrolysis products and which
result from dissociative ionization. In Fig 8.6, the features at m/z 45 and 59 result from
dissociative ionization but the remaining features all appear to be thermal products of
2-butanol pyrolysis and must be accounted for when examining detailed decomposition248

pathways.

An experimental detail worth noting here is that the current PEPICO

experimental setup does not allow for direct wall temperature measurements. Instead,the current run through the resistively heated reactor can be directly controlled but thewall temperature must be roughly approximated by comparing product appearance
and precursor depletion. Fig. 8.7 was recorded with current = 3 A and voltage = 13 V,
corresponding to 39 watts of power. Future versions of the experiment can make useof Type-C thermocouples attached directly to the reactor to monitor temperature moreaccurately.

In addition to the PIMS spectra available with this detection scheme,
photoelectron (PE) spectra can also be recorded. By keeping the ion count rate low(roughly one count per 10 laser pulses) it is possible to correlate electrons and cations
from a single ionization event and experimentally resolve the energy of the electron.
For a given cation m/z, the electrons can be plotted against electron kinetic energy toyield low-resolution PE spectra. The instrument resolution combined with the laser
linewidth limit the PE spectra such that the full width at half maximum (FWHM) of thenarrowest features is roughly 0.8 eV but this is sufficient to provide information
regarding the species present at a given m/z.
Shown in Fig. 8.8 is the PE spectrum coincident with cations of m/z 15 that are
produced during the pyrolysis of 2-butanol. Given the composition of the precursor,
the only reasonable assignment for this species is methyl radical (CH3). The frequency
axis electron binding energy (eBE) shown in Fig. 8.8 (and all following PE figures) was
made by assigning the laser energy so that the PE spectrum in Fig 8.8 matches the
known ionization energy[10] of methyl radical (IE(CH3) = 9.8380 ± 0.0004 eV). The laser249

energy is taken to be 11.0 eV, which is within the accepted ± 0.2 eV uncertainty of thelaser.

Fig. 8.8. PE spectrum of the species with m/z 15, assigned to be methyl radical.
Given IE(CH3) = 9.84 eV, the electron binding energy is determined from theexperimentally recorded eKE of the coincident photoelectrons. The spectrum wasrecorded during pyrolysis of 2-butanol in CW He in a heated micro-reactor.

To corroborate this method of determining the frequency axis, the PE spectrum
coincident with the cation with m/z 28 is shown in Fig. 8.9. This is very likely signal
from ethylene (CH2=CH2) given the small number of molecules with a mass of 28 amu
that can be made with C, H and O atoms. The assumed 11.0 eV photon energy agrees
very well with the PE spectrum, as indicated by the black arrow that marks theionization energy of ethylene[11] (IE(CH2=CH2) = 10.51268 ± 0.00003eV).

250

Fig. 8.9. PE spectrum of the species with m/z 28, assigned to be ethylene. The
electron binding energy is determined from the experimentally recorded eKE from m/z
15 but shows good agreement with ethylene. The spectrum was recorded during
pyrolysis of 2-butanol in CW He in a heated micro-reactor.

These PE spectra are especially useful when cation signal can be assigned tomore than one species. In the pyrolysis of 2-butanol, two cases have been examined.
Given the potential set of pyrolysis pathways present, the feature at m/z 56 could eitherbe

1-butene[12](IE(CH2=CHCH2CH3) = 9.55 ± 0.06 eV)(IE(CH3CH=CHCH3) = 9.12 ± 0.02 eV).

or

2-butene[6]Fig. 8.10 shows the PE spectrum of theunidentified feature at m/z 56. Although more detailed calibration of this method isstill required, the PE spectrum shows provocative agreement with the ionizationenergies of the two butene isomers. The wider feature (FWHM = 1.1 eV) appears to beconsistent with a mixture of the two isomers.

251

Fig. 8.10. PE spectrum of the species with m/z 56, cautiously assigned to a
mixture of 1-butene and 2-butene. The black arrows mark the ionization energy of thetwo isomers. The spectrum was recorded during pyrolysis of 2-butanol in CW He in aheated micro-reactor.
The PE spectrum of m/z 44 is also of interest because two isomers are possible
during pyrolysis of 2-butanol, vinyl alcohol (CH2=CHOH) and acetaldehyde(CH3CHO). The isomers are likely distinguishable using PE spectra due to their 1.1 eV
differenceinionizationenergy:[13-14]

IE(CH2=CHOH) ≤ 9.33 ± 0.05 eV.

IE(CH3CHO) = 10.2295 ± 0.0007 eV

andShown in Fig. 8.11 is the PE spectrum coincident

with m/z 44 (red trace) along with the IE of both isomers indicated with black arrows.
To help identify which isomer(s) is present, the room temperature PE spectrum of
acetaldehyde is shown (black trace) over the same spectral region. The enhancement of
the pyrolysis spectrum towards the red end of Fig. 8.11 also indicates the presence ofvinyl alcohol, which has the lower ionization energy.

252

Fig. 8.11. PE spectrum of the species coincident with m/z 44, potentially assigned
to a mixture of acetaldehyde and vinyl alcohol. The black arrows mark the ionizationenergies of the two isomers. The spectrum was recorded during pyrolysis of 2-butanol
in CW He in a heated micro-reactor.

IV. Conclusions
A new combination of VUV laser, PEPICO detector, and heated micro-reactor
has been assembled that allows more informative PIMS spectra to be taken. In contrast
to tunable synchrotron-based ionization sources, this system offers tabletop operation
yet still provides isomer specific information and can be used to disentangle
dissociative ionization features from pyrolysis products. The PEPICO detector permits
measuring photoelectron spectra that allow assignment of fragments based on their
ionization energy and/or photoelectron spectra.

In the pyrolysis of 2-butanol,

dissociative ionization can be identified in PIMS spectra and individual thermal
decomposition fragments have been identified directly.

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Appendix A

Hydropyrolysis of Other Substituted Aromatic Species

I. n-propylbenzene (C6H5CH2CH2CH3)
II. Isopropylbenzene (C6H5CH(CH3)2)
III. Styrene (C6H5CH=CH2)
IV. Phenol (C5H6OH)
V. Anisole (C6H5OCH3)
VI. Benzyl phenyl ether (C6H5CH2OC6H5)281

I. n-propylbenzene (C6H5CH2CH2CH3)

Fig. A.1. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of npropylbenzene (m/z 120) at reactor temperatures up to 1300 K. Pulsed helium carriergas was prepared with 0.1% precursor. The main products are assigned as methyl
radical (m/z 15), cyclopentadienyl radical (m/z 65), a weak signal of benzene (m/z 78),
benzyl radical (m/z 91), and toluene (m/z 92).

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Fig. A.2. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of 0.1%
n-propylbenzene (m/z 120) mixed with 0.5% methyl nitrite (m/z 61) at reactor
temperatures up to 1300 K in a pulsed helium carrier gas. The hydropyrolysis products
are assigned to be an enhancement of methyl radical (m/z 15), an enhancement ofbenzene (m/z 78), an enhancement of toluene (m/z 92), and the appearance of styrene(m/z 104).

283

II. Isopropylbenzene (C6H5CH(CH3)2)

Fig. A.3. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of
0.02% isopropylbenzene (m/z 120) shown in black the pyrolysis of a mixture of 0.02%
isopropylbenzene with 0.5% methyl nitrite (m/z 61) at reactor temperature of 1100 K ina pulsed helium carrier gas. In the pyrolysis of isopropylbenzene, the largest
decomposition feature is found at m/z 105, which could result from loss of methyl
radical. This process would yield a resonance stabilized benzyl-like radical(C6H5CHCH3). Interestingly, there is no evidence of hydrogen loss that would result in
another resonance stabilized radical, C6H5C(CH3)2. The hydropyrolysis products are
assigned to be the appearance of benzene (m/z 78), the enhancement of styrene (m/z104) and the appearance of a feature at 118, likely !-methylstyrene (C6H5C(CH3)=CH2).
The peak at 109 is believed to be either a contaminant or the dissociative ionization of
the parent. The group of peaks between m/z 40 and 46 are unassigned.

284

III. Styrene (C6H5CH=CH2)

Fig. A.4. Overview of the PIMS spectra recorded with 118.2 nm radiation of thepyrolysis of 0.1% styrene (m/z 104) shown in black the pyrolysis of a mixture of 0.1%
isopropylbenzene with 0.5% methyl nitrite (m/z 61) at reactor temperatures up to 1300
K in a pulsed helium carrier gas. The pyrolysis of styrene shows no decomposition atthese temperatures. The hydropyrolysis products are assigned to be the appearance ofbenzene (m/z 78), the appearance of m/z 92 that is likely toluene, and the appearanceof a feature at 102, likely phenylacetylene C6H5C≡CH).

285

Fig. A.5. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of 0.1%
styrene (m/z 104) shown in black the pyrolysis of a mixture of 0.1% isopropylbenzene
with 0.5% methyl nitrite (m/z 61) at reactor temperatures up to 1300 K in a pulsed
helium carrier gas. The vertical axis range is decreased help identify the weak features
within the mass-to-charge range of m/z 70 to m/z 110. Here we see very weak featuresin the 1300 K pyrolysis of styrene at m/z 102 (likely phenylacetylene, C6H5C≡CH) andm/z 78, likely benzene. A few new weak features are observed in the hydropyrolysis
spectra, namely m/z 76 (likely ortho-benzyne), m/z 90 and 91, potentially fulvenallene(C5H4C=CH2) and benzyl radical (C6H5CH2). We also observe a small peak at m/z 106
at reactor temperatures 1000 K and 1200 K. This is consistent with two subsequent
additions of hydrogen atom, which has not been observed before in a micro-reactor
pyrolysis experiment. Hydrogen addition to the terminal carbon of the side-chain
double bond will be facilitated by the formation of a benzyl-like radical that would beresonant with the aromatic ring. This stable adduct could survive and react with asecond hydrogen atom to form ethylbenzene (C6H5CH2CH3).

286

IV. Phenol (C5H6OH)

Fig. A.6. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of 0.1%
phenol (m/z 94) shown in bottom trace and the pyrolysis of a mixture of 0.1% phenol
with 0.5% methyl nitrite (m/z 61) at reactor temperature of 1500 K in a pulsed helium
carrier gas. In the pyrolysis of phenol, the decomposition features are m/z 39(propargyl radical), m/z 65 (cyclopentadienyl radical), and m/z 66 (cyclopentadiene).
The only hydropyrolysis product is observed with the appearance of benzene (m/z 78).

287

V. Anisole (C6H5OCH3)

Fig. A.7. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of 0.1%
anisole (m/z 108) at reactor temperatures up to 1300 K in a pulsed helium carrier gas.
In the pyrolysis of phenol, the decomposition features are m/z 39 (propargyl radical),m/z 65 (cyclopentadienyl radical), m/z 93 (phenoxy radical, C6H5O) and a weak feature
at m/z 94, likely phenol.

288

Fig. A.8. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of 0.1%
anisole (m/z 108) mixed with 0.5% methyl nitrite (m/z 61) at reactor temperatures up
to 1300 K in a pulsed helium carrier gas. By comparing with Fig. A.7 we can conclude
that the hydropyrolysis products are m/z 66 (cyclopentadiene), m/z 78 (benzene), an
depletion of m/z 93 and enhancement of m/z 94, which we conclude results fromhydrogen addition to phenoxy radical to form phenol, and a small peak at m/z 106,
which remains unassigned at this time.

289

VI. Benzyl phenyl ether (C6H5CH2OC6H5)

Fig. A.9. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of adilute but unknown concentration of benzyl phenyl ether (m/z 184) at reactor
temperatures up to 1300 K in a pulsed helium carrier gas. The pyrolysis of benzyl
phenyl ether appears to be the sum of benzyl radical decomposition and phenol
decomposition, implying that the main pathway is initial cleavage of the C-O bond thatbridges the two rings. The main products are m/z 39 (propargyl radical), m/z 65(cyclopentadienyl radical), m/z 66 (cyclopentadiene), m/z 89 (fulvenallenyl radical,
C5H4C≡CH), m/z 90 (fulvenallene, C5H4C=CH2), m/z 91 (benzyl radical), m/z 92(toluene), m/z 93 (phenoxy radical, C6H5O), and m/z 94 (C6H5OH). The weak feature
at 106 is likely a contaminant of ethylbenzene (C6H5CH2CH3) and the weak feature atm/z 156 remains unassigned but may be the result of dissociative ionization.

290

Fig. A.10. PIMS spectra recorded with 118.2 nm radiation of the pyrolysis of adilute but unknown concentration of benzyl phenyl ether (m/z 184) mixed with 0.5%
methyl nitrite (m/z 61) at reactor temperatures up to 1300 K in a pulsed helium carriergas. By comparing with Fig. A.9 we can conclude that the hydropyrolysis products are
m/z 52 (unassigned), an enhancement of m/z 66 (cyclopentadiene), and the appearanceof m/z 78 (benzene). There is also a weak feature at m/z 122, which is also unassigned.
There is a strong enhancement of m/z 94 at 1300 K compared to that observed in Fig.
A.9. This could be the result of hydrogen atom addition to phenoxy radical to form
phenol, which is then more resistant to thermal decomposition. At 1300 K will
decompose to CO and cyclopentadiene but phenol is stable until heating to 1500 K. A
similar effect is observed with m/z 92, which is likely hydrogen addition to benzyl
radical to form toluene, which is also more resistant to decomposition.

291

292

Appendix B

Supplementary Material for Chapter 6

Fig. B.1. Benzyl radical-d2 is proposed to decompose not through direct bond breakage
but through isomerization(s) reaction. Loss of one H atom can lead to fulvenallene(C5H4=C=CD2,m/z 92) and loss of a D atom would produce the fulvenallenyl radical(C5H4—C≡CD, m/z 90).

293

Fig. B.2. Benzyl radical-d5 is proposed to decompose not through direct bond breakage
but through isomerization(s) reaction. Loss of one D atom can lead to fulvenallene(C5D4=C=CH2,m/z 94) and loss of a H atom would produce the fulvenallenyl radical(C5D4—C≡CH, m/z 93).

294

Fig. B.3. . 13C labeled benzyl radical is proposed to decompose not through direct bondbreakage but through isomerization(s) reaction. Loss of one H atom can lead tofulvenallene (C5H4=C=13CH2,m/z 91) and a second H atom loss would produce thefulvenallenyl radical (C5H4—C≡ CH, m/z 90).
13295

Decomposition of
Ethylbenzene at 1500 K in a
Neon Matrix (this work)
Mode

Propargyl Radical (CH2CCH)

a1

b1

b2

Description!(cm-1)

Propargyl Radical (CH2CCH)
1 CH2CC-H stretch
2 sym H2C-CCH stretch
3 CH2C=CH - CH2-CCH stretch
4 H2-CCH scissors
5 CH2C=CH + CH2-CCH stretch
6 H2C-CCH umbrella
7 CH2CC-H out-of-plane bend
8 CH2-C=CH out-of-plane bend
9 asym H2C-CCH stretch
10 H2C-CCH + CH2-C=CH in-plane bend
11 CH2CC-H in-plane bend
12 CH2-C=CH in-plane bend
2*7 CH2CC-H out-of-plane bend

3321

687.3

Acetylene(HCCH)

Acetylene (HCCH)

MethylRadical (CH3)

Methyl Acetylene(CH3CCH)a1(z)e(x,y)

a1'
a2"
e'
e'

3297.03296.33284.03281.8732.1"u+ CH stretch"u+ CH stretch"u+ CH stretch"u+ CH stretch#u CH Bend
Methyl Acetylene (CH3CCH)
1 C-H stretch
2 CH3 sym stretch
3 C=C stretch
4 CH3 sym def
5 C-C stretch
6 CH3 deg stretch
7 CH3 def def
8 CH3 rock9 C-H bend
10 CCC bend
Methyl Radical (CH3)1 CH stretch
2 umbrella mode
3 CH stretch
4 deformation

Various Experimental Findings!(cm-1)

Intensity or
Absorbance
Ratio

Argon Matrix1
3308.5 ± 0.05
1003028.3 ± 0.06
2
1935.4 ± 0.441440.4 ± 0.5
2±11061.6 ± 0.8
7±1686.6 ± 0.4
44 ± 4483.6 ± 0.5
63 ± 71016.7 ± 0.4
620 ± 22
33 ± 4972.6~8Neon Matrix2
3297.1
1.27783296.4
1.088
3284
1.78
3281.81.05
731.99>6
Argon Matrix1
3322.9926.6

630.4 and 628

630.6 and 628.9
Neon Matrix3

618.8
3170

296

617
3162
1396

VS
WM
W

Decomposition of
Ethylbenzene at 1500 K in a
Neon Matrix (this work)
Mode

Benzyl Radical (C6H5CH2)

Cyclopentadieneyl Radical (C5H5)o - Benzyne (cyc-C6H4)

a1

b1

Description!(cm-1)o - Benzyne (cyc-C6H4)
1 CH stretch
2 CH stretch
3 C≡C stretch
4 mixed5 mixed
6 CH deformation
7 ring stretch
8 ring stretch
9 ring deformation
14 CH wag
15 CH wag
16 ring torsion
17 CH stretch
18 CH stretch
19 ring stretch
20 mixed
21 mixed
22 mixed
23 ring deformation
24 ring deformation

737.8

473.0

Cyclopentadieneyl Radical (C5H5)
a1'
a2'
a2
e1'

e1
e2'

e2

A1
A1
A1
A1
A1
B1
B1
B1
B1
B2
B2
B2
B2
B2

Various Experimental Findings!(cm-1)

Intensity or
Absorbance
Ratio

Neon Matrix4
3094
WM
3071
W
1846
W
1415
VW
1271
W
1055
WM
1039
M
982
WM
589
VW
838
VW
737
S
388
WM
3086
M
3049
VW
1451
M
1394
WM
1307
VW
1094
W
849
MS
472
VS
Gas Phase5 & Argon Matrix6

2
3
4
5
6
7
8
10
11
12

1072.61242
overlap
1385.3overlap

13
14

576.4

Benzyl Radical (C6H5CH2)
1 CH stretch
2 C-CH2 stretch
3 CH2 deformation
4 CH2 deformation
5 CH deformation
6 ring deformation
7 ring deformation
8 CH2 wag
9 CH2 wag
10 CH2 wag
11 CH2 a-stretch
12 ring stretch
13 ring stretch
14 CH deformation

3071.5
1408

762
668.5
3118
1448

297

1071 51244 5661 63079 61383 61001 5766 51320 51041 5
872 5

S
WM
M

861 5
576 5
Argon Matrix7
3069
W
1469
M
1409
M
1264
M
1015
W
882
W
762
VS
710.9
W
667
S
465
S
3111
W
1446
M
1305
W
948.1
W

Decomposition of
Ethylbenzene at 1500 K in a
Neon Matrix (this work)

Fulvenallene(C5H4=C=CH2)

Mode

Description

A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A1
A2
A2
A2
A2
B1
B1
B1
B1
B1
B1
B1
B1
B1
B1
B2
B2
B2
B2

Fulvenallene (C5H4=C=CH2)
1 CH stretch
2 CH stretch
3 CH2 sym. stretch
4 C-C sym. stretch
5 CH bend
6 CH bend
7 CH2 sym scissors
8 C=C=C sym stretch
9 C=C=C asymmetric stretch
10 C-C stretch
11 C-C-C stretch
12 ring breathing
13 CH2 twist
14 CH bend
15 CH bend
16 ring bend
17 CH stretch
18 CH stretch
19 C-C asym stretch
20 CH bend
21 CH bend
22 CH2 wag
23 C=C=C bend
24 C=C=C wag
25 C-C-C stretch
26 ring bend
27 CH2 asym stretch
28 CH bend
29 CH bend
30 CH2 rockB2

31 C=C=C bend

B2
B2

32 C=C=C rock33 ring bend!(cm-1)

Various Experimental Findings!(cm-1)

Intensity or
AbsorbanceRatio

Gas Phase8

1488

1929
885.9

840

755

298

3135
3092
2966
1486
1362
1320
1432
1078
1935
985
886

M
M
W
MS
W
W
W
W
S
VW
MS

3122
3071
1655

W
W
VW

1170
844

VW
MS

810
530
3010
754
733
604

VW
VW
VW
S
W
M

Decomposition of
Ethylbenzene at 1500 K in a
Neon Matrix (this work)

Ethylbenzene (C6H5-CH2CH3)

Mode

Description!(cm-1)

Various Experimental Findings!(cm-1)Ethylbenzene (C6H5CH2CH3)
a' fundamentala" fundamental∂(CCC), a' fundamentala" fundamental
295 + 158 = 453 (A")
a' fundamental
a' fundamental
a' fundamental
a' fundamental
399 + 314 = 713
2 x 365 = 730 (A')
a' fundamental
a' fundamentala" fundamental; CH2 rocka' fundamental
404 + 486 = 890 (A")
a' fundamental
C-C stretch a' fundamental
a' fundamental
CH3 rock, a' fundamental

1025.6

295
365
404
455
486
556
624
697 (gas)722
735
752 (gas)773 (gas)
787
840
887
904 (gas)
967 (gas)1003
1026 (gas)a' fundamental

1032

1033 (gas)

1066

1064

Intensity or
AbsorbanceRatio

Gas Phase9 & Liquid Phase9

699
752.5
773.3

a" fundamental
CH3 rock, a" fundamental
404 + 696 = 1100 (A")
a" fundamental
a' fundamental
a' fundamental
CH2 wag and twist, a' + a" fundamentals

S
S
M

W
W
M
M

1091
1111
1156
1179
1245

904 + 404 = 1308 (A")

1310

2a" fundamentals
CH3 sym. Deformation a' fundamentals
314 + 1091 = 1405 (A')
CH2 antisym def. + CH2 bend

1450

2a' + a" fundamentals

1457

a" fundamental
a' fundamentala" fundamental
a' fundamental
745 + 904 = 1649 (A')
840 + 904 = 1744 (A")
985 + 787 = 1772 (A"); 1003 + 771 = 1774
(A'); 1-31 + 745 = 1776 (A')
840 + 964 = 1804 (A')
904 + 964 = 1868 (A")
1037 + 840 = 1877 (A"); 1179 + 696 = 1875
(A')
904 + 985 = 1889 (A')
964 + 985 = 1949 (A")
CH3 and CH2 sym stretch (2a')
2 x 1456 = 2912 (A')

1469.9
1500

299

1614
1630

1326 (gas)1379 (gas)1422 (gas)1460 (gas)
1464
(gas)
1587
1613
1653
1745

M
M
W
S

S

1788

1776

1801
1866

1800 (gas)
1866

M

1883

1880 (gas)M

18901960 (gas)2886 (gas)M
S

Decomposition of
Ethylbenzene at 1500 K in a
Neon Matrix (this work)
Mode

Description!(cm-1)

Various Experimental Findings!(cm-1)

Intensity or
AbsorbanceRatio

2943 (gas)2974 (gas)3039 (gas)3053 (gas)

S
S
S
S

3075 (gas)S

Diacetylene (HCCCCH)

Ethylbenzene
continuted

Ethylbenzene (C6H5CH2CH3) Continued
CH3 antisym stretch (a')
CH3 and CH2 antisym st. (a' + a")a" fundamental
a' fundamental
a' fundamental
a' and a" fundamentals
Diacetylene (HCCCH)
1 CH stretch
2 C"C stretch
3 C-C stretch
4 CH stretch
5 C"C stretch
6 CH bend
7 CCC bend
8 CH bend
9 CCC bend

3075

Gas Phase10(overlap)

3329
2020

VS
M

(overlap)630

VS

VS = Very strong
S = Strong
MS = Medium strong
M = Medium
WM = Medium Weak
W = Weak
VW = Very Weak(overlap)= Peak is too close to neighboring peak to be uniquely assigned
References
1 E. B. Jochnowitz, X. Zhang, M. R. Nimlos, M. E. Varner, J. F. Stanton, G. B. Ellison, J. Phys. Chem. A 109, 3812 (2005)
2 Direct study of acetylene peformed in a neon matrix in our group. Assingments compared to Shimanouchi, T., Tables of
Molecular Vibrational Frequencies Consolidated Volume I, National Bureau of Standards, 1972, 1-160.
3 A. Snelson, J. Phys. Chem. 74, 537 (1970)
4 J. G. Radziszewski, B. A. Hess Jr., R. Zahradnik, J. Am. Chem. Soc. 114, 52 (1992)
5 B. E. Applegate, A. J. Bezant, T. A. Miller, J. Chem. Phys. 114, 4869 (2001)
6 V. A. Korolev, O. M. Nefedov, Russ. Chem. Bull. 42, 1436 (1993)
7 E. G. Baskir, A. K. Maltsev, V. A. Korolev, V. N. Khabashesku, O. M. Nefedov, Russ. Chem. Bull. 42, 1438 (1993)
8 C. L. Angell, J. Mol. Struct. 10, 265 (1971)
9 J. H. S. Green, Spectochim. Acta 18, 39 (1962)
10 Shimanouchi, T., Tables of Molecular Vibrational Frequencies Consolidated Volume I, National Bureau of Standards,
1972, 1-160.

Table B.1. Summary of all known vibrational modes for molecules anticipated to
be found in the decomposition of benzyl radical from an ethylbenzene precursor.
The features assigned in the decomposition spectra that are consistent with known
vibrational mode are indicated in the fourth column, “Decomposition of
Ethylbenzene at 1500 K in a Neon Matrix (this work)”.

300

!

!

Appendix C
Decomposition mechanisms for C5 Ring Radicals

Fig. C.1. Proposed decomposition mechanism for cyclopentadienyl radical
to form propargyl radical and acetylene.

!

301

!

!

Fig. C.2. Proposed decomposition mechanism for !,"-d2 cyclopentadienyl
radical to form propargyl radical and acetylene with all H/D combinations thatcan be produced.

!

302

!

!

Fig. C.3. Proposed decomposition mechanism for !,#-d2 cyclopentadienyl
radical to form propargyl radical and acetylene with all H/D combinations thatcan be produced.

!

303

!

!

Fig. C.4. Proposed decomposition mechanism for 13C labeled
cyclopentadienyl radical to form propargyl radical and acetylene with all 13C/12C
combinations that can be produced.

!

304

!

!

Fig. C.5. Proposed decomposition mechanism for fulvenallenyl radical toform acetylene, propargyl radical, diacetylene, HC≡C-CH-C≡CH radical andHC≡C-CC≡CH2 radical using similar decomposition patters as those found for
cyclopentadienyl radical decomposition.

!

305

!

!

Fig. C.6. Proposed decomposition mechanism for fulvenallenyl radical-d1
to form acetylene, propargyl radical, diacetylene, HC≡C-CH-C≡CH radical andHC≡C-CC≡CH2 radical with all H/D combinations that can be produced.
Mechanisms are outlined using similar decomposition patters to those found in
cyclopentadienyl radical decomposition.

!

306

!

!

Fig. C.7. Proposed decomposition mechanism for fulvenallenyl radical-d4
to form acetylene, propargyl radical, diacetylene, HC≡C-CH-C≡CH radical andHC≡C-CC≡CH2 radical with all H/D combinations that can be produced.
Mechanisms are outlined using similar decomposition patters to those found in
cyclopentadienyl radical decomposition.

!

307

!

!

!

308

Appendix D
Additional Data for Benzyl Radical Pyrolysis

Fig. D.1. PIMS spectra comparing the effect of concentration on the
product ratios in ethylbenzene pyrolysis with a pulse micro-reactor. The lower
trace was recorded at 0.025 % ethylbenzene in helium, the middle at 0.05%, andthe top trace at 0.1%. The peaks that gain intensity as concentration increases areconcluded to be the result of bimolecular chemistry.

!

309

Fig. D.2. PIMS spectra of 0.1 % benzyl bromide-d2 pyrolysis recorded witha 118.2 nm ionization source and a pulse He flow. At 1500 K we observe threedifferent isotopomers of methyl radical: m/z 15 (CH3), m/z 16 (CH2D), and m/z
17 (CHD2). This may indicate either i) that ! methyl-phenyl radical equilibrates
with benzyl radical and can exchange H and D atoms multiple times or ii) thatmethyl radical-d2 reacts with hydrogen atoms and can exchange H and D atoms
to effectively scramble the H/D ratio.

!

310

Appendix E
Supplemental Figures for Chapter 7

Fig. E.1. The “extended” Benson Fragmentation of benzyl radical-d2. The original
pathway suggested H atom loss and formation of the fulvenallenyl radical (C5H4C≡CD). Thissuggestion also implies formation of the cyclopentadienyl radical and acetylene. The bicyclic
radicals and vinylidene (:C=CD2) will not be stable in the hot micro-reactor and are enclosed
in brackets. The tropyl radical (C7H7) does not participate.

311

Fig. E.2. Fragmentation pathways of the benzyl radical-d2 are shown. The internal Hatom abstraction from the ring that leads to the formation of the ! methyl-phenyl radical will
be endothermic by roughly 1 eV. It also requires a curve-crossing from the 2B1benzyl radical
to the 2A’ methyl-phenyl (" radical). Decomposition of the ! methyl-phenyl radical produces
CHD2 and o-C6H4. A second pathway is the isomerization of benzyl to the bridgehead
norbornadienyl radical. The ! methyl-phenyl radicals, bridgehead norbornadienyl radicals,
and cyclopentadienyl radicals will not be stable in the hot micro-reactor and are enclosed inbrackets.

312

Fig.
Fig. E.3. The “extended” Benson Fragmentation of benzyl radical-d5. The original
pathway suggested D atom loss and formation of the fulvenallenyl radical (C5D4C≡CH). Thissuggestion also implies formation of the cyclopentadienyl radical and acetylene. The bicyclic
radicals and vinylidene (:C=CH2) will not be stable in the hot micro-reactor and are enclosed
in brackets. The tropyl radical (C7H7) does not participate.

313

Fig. E.4. Fragmentation pathways of the benzyl radical-d5 are shown. The internal Datom abstraction from the ring that leads to the formation of the ! methyl-phenyl radical will
be endothermic by roughly 1 eV. It also requires a curve-crossing from the 2B1benzyl radical
to the 2A’ methyl-phenyl (" radical). Decomposition of the ! methyl-phenyl radical produces
CDH2 and o-C6D4. A second pathway is the isomerization of benzyl to the bridgehead
norbornadienyl radical. The ! methyl-phenyl radicals, bridgehead norbornadienyl radicals,
and cyclopentadienyl radicals will not be stable in the hot micro-reactor and are enclosed inbrackets.

314

Fig. E.5. The “extended” Benson Fragmentation of 13C labeled benzyl radical. The
original pathway suggested H atom loss and formation of the fulvenallenyl radical(C5H4C≡13CH). This suggestion also implies formation of the cyclopentadienyl radical andacetylene. The bicyclic radicals and vinylidene (:C=13CH2) will not be stable in the hot microreactor and are enclosed in brackets. The tropyl radical (C7H7) does not participate.

315

Fig. E.6. Fragmentation pathways of the 13C labeled benzyl radical are shown. Theinternal H-atom abstraction from the ring that leads to the formation of the ! methyl-phenyl
radical will be endothermic by roughly 1 eV. It also requires a curve-crossing from the2
B1benzyl radical to the 2A’ methyl-phenyl (" radical). Decomposition of the ! methyl-phenyl
radical produces 13CH3 and o-C6H4. A second pathway is the isomerization of benzyl to thebridgehead norbornadienyl radical. The ! methyl-phenyl radicals, bridgehead
norbornadienyl radicals, and cyclopentadienyl radicals will not be stable in the hot microreactor and are enclosed in brackets.

316Appendix F

Resonance Stabilization Energy

To attempt to quantify the relative electron delocalization stabilization ofresonance stabilized hydrocarbon radicals, we compiled a table that uses relative
bend strengths as an indicator of radical stability.

Resonance stabilization isimportant in combustion because in order for the unpaired electron to react withanother molecule, the localization of the electron to form a new bond requires
losing the delocalization benefit.

In an attempt to quantify this effect, we

assembled previously reported experimental measurements[1-21] that allow thecomparison of different bond dissociation enthalpies. By comparing resonancestabilized radical products and radicals with localized electrons, we approximate
the energetic benefit of electron delocalization.
findings.

Table F.1 summarizes these

The bond dissociation enthalpy at 298 K (D298) is calculated as thedifference of the heat of formation (∆fH298) of the radical products and the parent
species, see equation (1).
∆fH298(R) + ∆fH298(H) - ∆fH298(RH) = D298(R-H)(1)

The bond dissociation enthalpy at 0 K (D0) is calculated by relating the appearance
potential (AP) of a cation from the parent and the ionization of the radical (IE) assummarized in equation (2).
AP(R+ ← RH) – IE(R) = D0(R-H)

317

(2)

The radicals of interest were compared to two or three similar radicals where theunpaired electron is localized on a specific carbon atom.
Table F.1 indicates that even though benzyl is energetically more stable thantropyl radical, the calculated resonance energy of tropyl is more than twice that ofbenzyl radical (24.8 kcal mol-1 for tropyl versus 11.4 kcal mol-1 for benzyl). All theradicals investigated in Table F.1 have significant resonance stabilization of at least 11.5
kcal mol-1 (roughly 0.5 eV).

This implies that these hydrocarbon radicals will be

relatively resistant to oxidative addition of O2 because such reactions will be roughly 0.5
eV less exothermic than those with equivalent radicals that lack electron delocalization.

318

Cyclopentadienyl
RadicalTropyl Radical

C6H5CH2-CH2C6H5
CH3CH2-CH2CH3

65.2
88.0

± 0.7
± 0.6

C6H5CH2-H
CH3CH2-H

89.8
101.1

± 0.6
± 0.4

C7H7-H(CH3)2CH-H

73.9
98.6

± 0.9
± 0.4

Resonance
EnergyD0(kcal/mol)
77
102

11.4

Resonance
Energy

± 2
± 1.2

± 0.9

12.5

1,2,3

± 1.0
12,13

± 1.2
± 1.7

1,2

25.2

Allyl Radical

±

2
1,2,3

15.4

± 1.5

C5H5-H(CH3)2CD-D

80
97

±
±

16,17

12
1

15,18

17
C5H5-H(CD3)2CH-H

H2C=CH-CH2--CH2-CH=CH2
CH3CH2-CH2CH3

61.6
88.0

± 1.0
± 0.6

HC"C-CH2-H
CH3CH2-H

88.9
101.1

± 0.2
± 0.4

H2C=C=CH-H
CH3CH2-H

87.6
101.1

± 0.3
± 0.4

14,15

1,3,5

80
97

±
±

86
104.5

± 2
± 1.9

12.9

± 0.8

±

12
16

12
2

15,19

17

18.1

±

±

12
1,3,6

20,21

1,2,3

10,22

3
1,6
1,2,3

13.2
Propargyl Radical

2.3

± 1.4
± 0.4

± 0.7
± 0.4

9,10

± 0.7

42.8
93.2

88.2
101.1

1,2

1,3,4

24.8

H2C=CH-CH2-H
CH3CH2-H

7,8

1,2,3

C7H7-C7H7(CH3)2CH-CH(CH3)2
83.3
98.6

1,2

1,2,3

11.4

C5H5-H(CH3)2CH-H±

0K

Benzyl Radical

DH298(kcal/mol)

References

0K

298 K

298 K

± 1.2
1,3,6
1,2,3

12.2

± 0.5
1,3,6
1,2,3

13.5

± 0.5

Table F.1. Resonance energies of hydrocarbon radicals calculated fromexperimental values at 298 K and 0 K.

319

References for Appendix F[1] J. B. Pedley, R. D. Naylor and S. P. Kirby, Thermochemistry of Organic Compounds,
Chapman and Hall, New York, 1986, p. 87-222.
[2] S. J. Blanksby and G. B. Ellison. Bond dissociation energies of organic molecules.
Accounts of Chemical Research 2003, 36, 255-263.
[3] L. V. Gurvich, I. V. Veyts, C. B. Alcock and V. S. Iorish, Thermodynamic Properties ofIndividual Substances, Hemisphere, New York City, 1991, p. 42-43.
[4] J. C. Traeger and R. G. McLoughlin. Photo-ionization study of energetics of C7H7+
ion formed from C7H8 precursors. International Journal of Mass Spectrometry and
Ion Processes 1978, 27, 319-333.
[5] T. Ichino, S. W. Wren, K. M. Vogelhuber, A. J. Gianola, W. C. Lineberger and J. F.
Stanton. The vibronic level structure of the cyclopentadienyl radical. Journal ofChemical Physics 2008, 129, 084310.
[6] M. S. Robinson, M. L. Polak, V. M. Bierbaum, C. H. DePuy and W. C. Lineberger.
Experimental Studies of Allene, Methylacetylene, and the Propargyl Radical Bond-Dissociation Energies, Gas-Phase Acidities, and Ion-Molecule Chemistry.
Journal of the American Chemical Society 1995, 117, 6766-6778.
[7] F. W. McLafferty, T. Wachs, C. Lifshitz, G. Innorta and P. Irving. Substituent effects
in unimolecular ion decompositions. XV. Mechanistic interpretations and thequasi-equilibrium theory. Journal of the American Chemical Society 1970, 92, 68676880.
[8] G. C. Eiden, F. Weinhold and J. C. Weisshaar. Photoelectron-spectroscopy of freeradicals with cm-1 resolution - the benzyl cation. Journal of Chemical Physics 1991,
95, 8665-8668.
[9] I. Omura. Mass Spectra at Low Ionizing Voltage and Bond Dissociation Energies of
Molecular Ions from Hydrocarbons. Bulletin of the Chemical Society of Japan 1961,
34, 1227-1233.
[10] B. Ruscic, J. Berkowitz, L. A. Curtiss and J. A. Pople. The ethyl radical:
Photoionization and theoretical studies. The Journal of Chemical Physics 1989, 91,
114-121.
320

[11] F. A. Elder and A. C. Parr. Photoionization of cycloheptatrienyl radical. Journal ofChemical Physics 1969, 50, 1027-1028.
[12] K. H. Fischer, P. Hemberger, A. Bodi and I. Fischer. Photoionisation of the tropyl
radical. Beilstein Journal of Organic Chemistry 2013, 9, 681-688.
[13] B. Steiner, C. F. Giese and M. G. Inghram. Photoionization of Alkanes. Dissociationof Excited Molecular Ions. The Journal of Chemical Physics 1961, 34, 189-220.
[14] F. A. Houle and J. L. Beauchamp. Photoelectron spectroscopy of methyl, ethyl,
isopropyl, and tert-butyl radicals. Implications for the thermochemistry andstructures of the radicals and their corresponding carbonium ions. Journal of theAmerican Chemical Society 1979, 101, 4067-4074.
[15] A. G. Harrison, P. Haynes, S. McLean and F. Meyer. The Mass Spectra of MethylSubstituted Cyclopentadienes. Journal of the American Chemical Society 1965, 87,
5099-5105.
[16] H. J. Wörner and F. Merkt. Diradicals, antiaromaticity, and the pseudo-Jahn-Teller
effect: Electronic and rovibronic structures of the cyclopentadienyl cation. Journalof Chemical Physics 2007, 127, 034303.
[17] J. M. Williams and W. H. Hamill. Ionization Potentials of Molecules and Free
Radicals and Appearance Potentials by Electron Impact in the Mass
Spectrometer. The Journal of Chemical Physics 1968, 49, 4467-4477.
[18] C. Lifshitz and M. Shapiro. Isotope Effects on Metastable Transitions. II.
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[19] F. P. Lossing. Free Radicals by Mass Spectrometry. XLIII. Ionization Potentials andIonic Heats of Formation for Vinyl, Allyl, and Benzyl Radicals. Canadian Journal ofChemistry 1971, 49, 357-362.
[20] F. A. Houle and J. L. Beauchamp. Detection and investigation of allyl and benzyl
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butane with mass analysis. The Journal of Chemical Physics 1967, 47, 2921-2933.

321

322

!

!

Appendix G
Energy Levels and Symmetries of the Cyclic CnHn Molecules (n = 3 to 7)

D7h$

E3”$
E2”$
E1”$

2E ”1ground1state$
2

A2”$

D6h$

B1g$
E2u$
E1g$

1A 1ground1state$
1g

A2u$

D5h$

E2”$
E1”$

2E ”1ground1state$
1

A2”$

D4h$

B2u$
Eg$

3E 1ground1state$
g

A2u$

D3h$

E”$
A2”$

2E”1ground1state$

Fig. G.1. Chemical drawings, energy levels, electron occupation, and symmetries of theground state for the five smallest cyclic CnHn molecules: cyclopropenyl radical (C3H3),
cyclobutadiene (C4H4), cyclopentadienyl radical (C5H5), benzene (C6H6), andcycloheptatrienyl (tropyl)radical (C7H7)

!

323

!

!

E3”$

D7h$

E2”$

2E ”1ground1state$
2

E1”$

A2”$
Fig. G. 2. Cartoon drawings of the 7 !-electron molecular orbitals of tropyl radical.
Black text on the cartoons indicates carbon atom number (1 through 7) followed by sign
and intensity of the wave function at that location.

!

324

!

!

B1g$

E2u$
D6h$

E1g$
1A 1ground1state$
1g

A2u$

Fig. G. 3. Cartoon drawings of the 6 !-electron molecular orbitals of benzene. Black text
on the cartoons indicates carbon atom number (1 through 6) followed by the sign andintensity of the wavefunction at that location.

!

325

!

!

E2”$
D5h$
E1”$

2E ”1ground1state$
1

A2”$

Fig. G. 4. Cartoon drawings of the 5 !-electron molecular orbitals of cyclopentadienyl
radical. Black text on the cartoons indicates carbon atom number (1 through 5)
followed by the sign and intensity of the wave function at that location.

!

326

!

!

B2u$

D4h$

Eg$

3E 1ground1state$g

A2u$

Fig. G. 5. Cartoon drawings of the 4 !-electron molecular orbitals of cyclobutadiene.
Black text on the cartoons indicates carbon atom number (1 through 4) followed by thesign and intensity of the wave function at that location.

!

327

!

!

E”$

D3h$

2E”1ground1state$

A2”$

Fig. G. 6. Cartoon drawings of the 3 !-electron molecular orbitals of cyclopropenyl
radical. Black text on the cartoons indicates carbon atom number (1 through 3)
followed by the sign and intensity of the wave function at that location.

Orbital symmetries were determined using guidance from the book Chemical
Applications of Group Theory, 3rd Edition.[1]Figures G.2, G.3, G.4, G.5, and G.6 were generated using the Simple Hückel MolecularOrbital Theory Calculator (SHMO).[2]

!

328

!

!

References for Appendix G[1] F. A. Cotton, Chemical applications of group theory, 3rd Edition, John Wiley & Sons, 1990,
p. 150-155.
[2] A. Rauk, Simple Huckel Molecular Orbital Program, Java, Applet, 2010
http://www.ucalgary.ca/rauk/shmo!

329