Jet-Cooled Spectroscopy of Ortho-Hydroxycyclohexadienyl Radicals

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Jet-Cooled Spectroscopy of Ortho-Hydroxycyclohexadienyl Radicals Callan M. Wilcox, Olha Krechkivska, Klaas Nauta, Timothy W. Schmidt, and Scott H. Kable J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b07603 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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The Journal of Physical Chemistry

Jet-Cooled Spectroscopy of ortho-Hydroxycyclohexadienyl Radicals Callan M. Wilcox, Olha Krechkivska, Klaas Nauta, Timothy W. Schmidt, and Scott H. Kable∗ School of Chemistry, University of New South Wales, New South Wales 2052, Australia E-mail: [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Abstract

The electronic spectra of the ortho-hydroxycyclohexadienyl radical have been observed following the supersonic expansion of the electric discharge products of phenol and water. Hydrogen atoms, split from water, add to the phenol ring at the ortho position, generating syn and anti rotamers with respect to the hydroxyl group. The D1 ← D0 transitions were recorded by resonance-enhanced multiphoton ionisation spectroscopy. The spectrum of each isomer was isolated through hole burning spectroscopy. The assignment and symmetry of the excited state is evaluated through ab initio calculations and is employed to assign each spectrum. Both rotamers are calculated to have a puckered ring in the excited state, leading to C1 symmetry. The spectrum of the anti isomer is assigned well using this symmetry; however the syn isomer is assigned better in the Cs symmetry of the ground state. We use Duschinsky matrices as a tool for the spectroscopist to determine which point group to use when ab initio calculations are ambiguous.

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2

Introduction

Aromatic compounds are released to the atmosphere via many pathways, both anthropogenic and natural. 1,2 They comprise a large fraction of fuels, for example the Australian standard allows for up to 30% aromatics by weight, and they account for as much as 20-30% of the urban volatile organic compound budget to the atmosphere. The atmospheric oxidation of aromatics, such as benzene and its methylated derivatives, leads to enhanced tropospheric ozone concentrations and photochemical smog. 3 Their ability to form secondary organic aerosols (SOA) is an expanding area of recent study, and are predicted to generate significant quantities of SOA within urban environments. 4–8 The reactivity of benzene is relatively low compared to other substituted aromatics, and hence its atmospheric half-life is long, lasting up to several weeks before reaction. 4 Degradation is initiated chiefly by reaction with the hydroxyl radical, which proceeds through two key mechanisms, depending on temperature; 3,9–11

C6 H6 + OH −−→ C6 H6 OH −−→ C6 H5 + H2 O

(1) (2)

At atmospherically relevant temperatures, the addition reaction (1) dominates, forming a C6 H6 OH adduct, the ipso-hydroxycyclohexadienyl radical (i-chdOH). The rest of the atmospheric degradation mechanism is being continually improved. 12 A number of mechanistic studies have been performed on this radical, with intermediate products and reaction channels showing a clear dependence on ambient NO and NO2 concentrations. 4,5,8,13,14 Subsequent reactions involve removal of hydrogen by molecular oxygen abstraction, resulting in formation of phenol + HO2 . 3,9,14–16 The ring breaking and propagating mechanisms of fur-

3



ther oxidation have been studied, and believed to involve the formation of bicyclic species, via O2 addition across the phenol aromatic ring. 2,17 For temperatures above ∼325 K, the chdOH radical becomes unstable and readily decays back to reactants (benzene + OH or phenol + H), while for even higher temperatures (combustion-relevant, T > 500 K) the reaction almost exclusively follows a hydrogen abstraction mechanism, reaction (2), forming phenyl and H2 O. 18 This abstraction channel, accounting for only ∼5% tropospheric benzene oxidation is comparably well understood 19 and its intermediates have been successfully observed and identified in an argon matrix by Mardyukov et al. 20 Clear progress has been made in the understanding of the degradation pathways of benzene and other alkyl substituted aromatics, under different temperature and varied ambient molecular concentrations. However top-down (measuring aerosol concentrations) and bottom-up (productions of aerosols from known compounds) approaches to SOA formations vary by an order of magnitude in their carbon balance. 5,7 In a matrix study of the benzene-hydroxyl complex, Mardyukov et al. observed the formation of ketene products and proposed a new mechanism, involving o-chdOH as an intermediate, as shown in figure 1. 21 As discussed above, in the troposphere the i-chdOH (1i) radical is able to form products (2), usually via O2 abstraction. In the matrix, a complex between phenol and H atom can be trapped and proceed to form the o-chdOH adduct, (1o). Intramolecular hydrogen migration from (1i) to form the (1o) radical was ruled out due to its high-barrier. Since none of the intermediates were directly observed in this study, however, this mechanism remains speculative. It does nonetheless well illustrate the importance of the o-chdOH radical in the chemistry of the benzene + OH/phenol + H system. A recent paper by our group described how hydrogen atom addition to phenol is ortho directed. 22 An absorption spectrum taken of the first excited state revealed a complicated spectrum, containing numerous peaks over an extended range. An unexpectedly small spac-

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2 OH

O -H

+OH +H

H

O

H

C

-H H

OH

H

OH

OH H

-H

3

H H

1i

1o

Figure 1: Possible ring-breaking mechanism from the ipso-hydroxycyclohexadienyl radical. A scheme of ketene formation from an initial OH-addition to benzene yielding 1i. 21 The dotted line emphasises that the hydrogen migration pathway from 1i to 1o is energetically unfavourable. 22 The favored matrixmechanism suggested by Mardyukov is indicated by the blue arrows.

ing between the first two peaks of only 18 cm−1 led us to conclude that the spectrum contained more than a single compound. Further investigation determined that these species were conformers of the same structural isomer, the syn and anti conformers of (1o). No other H addition isomers; ipso, (1i), meta (1m) or para (1p) were detected. In that paper, we reported potential the energy landscape for the torsion of the hydroxyl group. The anti rotamer was found to be the lowest energy configuration, by ∼450 cm−1 compared to syn. The conformers are separated by a barrier of ∼1750 cm−1 (measured from the anti well). The hydrogen addition entrance channel is assumed to be similar for the two rotamers, especially when compared to the entrance channels forming ipso, meta and para hydroxycyclohexadienyls. Kinetically, we would thus expect the formation of both rotamers. Thermodynamically, formation of the lower energy anti configuration would be preferred, and so any system where equilibrium could be reached would favor an increased population in the anti rotamer. Lifetime measurements and ionisations potentials were also reported.

5



In this contribution we examine the detailed spectroscopy of these ortho-chdOH isomers and evaluate the theoretical methods and approximations used to assign their spectra. Recently, the Wright group has highlighted the effectiveness of using Duschinsky rotation matrices to enable a consistent labelling scheme of vibrational modes between substituted benzene rings. 23 Their method has also been effective in labelling the vibrational modes of the ground state, S0 , excited state, S1 and cation D0+ vibrations in a consistent manner, preserving the mode labels. 24 Here we also utilize Duschinsky rotation matrices to consistently label the D1 vibrations of the syn and anti conformers, by their respective D0 labels, and extend their use for identification of the symmetry of the excited electronic state.

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3

Experimental

A description of the experimental apparatus for these experiments has been published in more detail previously. 25 Here, we describe the experiments in general terms, including details that are specific to this work. Argon (6-7 bar) was bubbled through a sample of phenol (Aldrich, ≥99%), heated to ∼ 45◦ C, and H2 O, resulting in a seeded gas mix of ∼0.1-1% organic and ∼1% water. The sample was expanded into a differentially-pumped vacuum chamber through a pulsed discharge nozzle. The discharge (1.6 kV, 25 kΩ) is struck for ∼80 - 130 µs to coincide with the latter part of expanding sample. A discharge in argon, with trace amounts of water has been shown to be an effective way of producing H and OH radicals; occurring through collisions with metastable argon. 25 The discharge products were cooled to ∼10 K in a supersonic expansion, and the coldest part of this expansion was passed through a 2 mm skimmer. The molecular beam entered the ionization chamber, where the molecules were interrogated using resonance enhanced multi-photon ionization (REMPI) spectroscopy, which, here, utilised up to three counter-propagating laser beams. Tunable light was provided by Nd:YAG pumped dye lasers. For normal REMPI experiments, the excitation laser, near 550 nm, utilised Coumarin 540A dye. The ionisation laser, at 230 nm, was the frequency doubled output of a dye laser using Coumarin 460 dye. The resulting cations are orthogonally accelerated down the length of a Wiley-McLaren-type time-of-flight mass spectrometer (ToF-MS), detected by a tandem multi-channel plate (MCP) and viewed by an oscilloscope. Custom-written LabView software was used to record the spectra. A wavemeter was used to calibrate laser wavelengths. For hole burning experiments, a third laser (dye laser with Coumarin 540A dye) was introduced around 100 ns before the REMPI lasers, and scanned through the entire absorption spectrum. The hole burning spectrum is recorded as loss of REMPI signal as a function of burn laser wavelength.

7



4

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Theory

The geometries of the syn and anti conformers of o-chdOH radical in the ground electronic state were calculated using density functional theory (DFT) at B3LYP/6-311+G(d,p) level of theory due to its relatively low cost and reasonable accuracy. All calculations were computed utilizing the GAUSSIAN-16 suite of programs. 26 The ground state geometries for both syn and anti conformers have Cs symmetry at this level of theory, as shown in figure 2.

syn ground state

anti excited state

ground state

excited state

Figure 2: Profile and top down views of ground and excited states of syn and anti chdOH radicals, showcasing the geometry change and the near-planar nature of the excited syn geometry. These geometries were optimised at the (TD) – B3LYP/6-311+G(d,p) level of theory.

Time-dependent density functional theory (TD-DFT) was used for calculating the structure and harmonic vibrational frequencies in the excited states (B3LYP/6-311+G(d,p)). The electronic transition is of a double excitation nature, where one electron moves from the HOMO (π) into the SOMO(n), and another from the SOMO(n) into the LUMO (π*). Upon excitation for both conformers, the functionalised part of the framework is prone to puckering, and torsion of the hydroxyl group occurs, resulting in formal C1 symmetry. However, the magnitudes of these changes are quite different for each conformer. The results of these calculations can be seen in figure 2. 8


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The syn conformer, undergoes minimal distortion upon electronic excitation. Excited state calculations carried out without a diffuse function (6-311G(d,p)) yielded a Cs excited state. Upon addition of this diffuse function, however the excited state became C1 geometry. Imposing a symmetry constraint of Cs geometry yielded a single imaginary frequency, corresponding to the CO carbon puckering out-of-plane. We also note that the sp3 hybridised carbon and OH groups rotate and subtly distort to oppose this puckering. The excited state is thus assigned by the software as C1 . For the anti conformer, on the other hand, a significant geometry change occurs involving the hydroxyl group. The oxygen bends out-of-plane by ∼16◦ and the respective H atom increases its dihedral angle to the framework by ∼6.5◦ . The ring-carbon bound to this hydroxyl group also puckers out-of-plane. The vibrational modes that contain these prominent out-of-plane distortions are likely to show significant Franck-Condon activity. Minor out-ofplane distortions of some ring-CH hydrogens also occur as do small in-plane distortions of CC bonds around the ring also occur. Frequencies calculated for both the ground and excited states of the syn and anti isomers can be seen in tables 1 and 2 respectively. Ground state frequencies have been scaled by a factor of 0.9688, suggested by Merrick et al., as very tight convergence criteria were applied to the geometry optimization. 27 The excited state was scaled by the same factor. The largest difference in vibrational frequencies between the conformers lies in the lowest frequency modes, which is helpful in distinguishing between the isomers. The ground state harmonic zero-point energy for the anti rotamer, based on our calculated frequencies, table 1, is higher than that of syn by ∼130 cm−1 , table 2. This has the effect of reducing the energy difference between the rotamers from ∼450 cm−1 to ∼320 cm−1 .

9



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Table 1: Ground and excited state frequency calculations for the syn conformer of the chdOH radical, at the (TD)-B3LYP/6-311+G(d,p) level of theory. Frequencies are labelled according to the Cs point group of the ground state, where the excited state has been reordered according to the Duschinsky matrix (see text). Scaled frequencies (×0.9688) are included for comparison. Ground State

syn

Excited State

Symmetry

Frequency

Scaled Frequency

Frequency

Scaled Frequency

1

a′

3836

3716

3809

3691

OH stretch

2

a

′

3199

3100

3165

3067

CH stretch

3

a′

3179

3080

3206

3106

CH stretch

4

a

′

3176

3077

3218

3118

CH stretch

5

a

′

3157

3059

3191

3092

CH stretch

6

a′

2896

2806

2922

2831

CH2 sym stretch

7

a′

1635

1584

1583

1533

CC stretch

8

a

′

1552

1504

1448

1403

CC stretch

9

a′

1451

1406

1394

1351

CH2 scissors

10

a′

1441

1396

1432

1387

CH ip bend/CC stretch

11

a′

1435

1391

1377

1334

CH ip bend/CC stretch

12

a

′

1362

1320

1333

1291

CH/OH ip bend

13

a

′

1312

1271

1543

1495

CH2 ip bend

14

a′

1238

1200

1235

1196

CO stretch

15

a

′

1193

1156

1174

1138

OH ip bend

16

a

′

1183

1146

1160

1124

CH ip bend

17

a′

1125

1090

1135

1100

CH ip bend

18

a

1006

975

1019

987

ring breathe

19

a′

974

944

961

931

ip ring deformation

20

a′

933

904

929

900

ring breathe

21

a

′

780

756

631

611

ip ring deformation

22

a′

586

567

759

735

ip ring deformation

23

a′

497

481

539

522

ip ring deformation

24

a

392

379

382

371

CO ip bend

25

a′′

2880

2791

2949

2858

CH2 asym stretch

26

a′′

1184

1147

1104

1069

CH2 twist

27

a′′

956

927

718

696

CH oop bend

28

a′′

936

907

795

771

CH2 oop rock

29

a

′′

894

866

944

915

CH oop bend

30

a′′

743

719

517

501

CH oop bend

31

a

′′

661

640

476

461

CH oop bend

32

a

′′

516

500

468

453

oop ring deformation

33

a′′

443

429

205

198


34

a′′

285

277

148

143


35

a′′

156

151

324

314

OH oop bend

36

a

119

116

91

88

CH2 oop rock

′

′

′′

(a) ip = in-plane;

oop = out-of-plane


Mode Description

(a)

Mode #

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Table 2: Ground and excited state frequency calculations for the anti conformer of the chdOH radical, at the (TD)-B3LYP/6-311+G(d,p) level of theory. Frequencies are labelled according to the Cs point group of the ground state, where the excited state has been reordered according to the Duschinsky matrix (see text). Scaled frequencies (×0.9688) are included for comparison. Ground State

anti

Excited State

Mode #

Symmetry

Frequency

Scaled Frequency

Frequency

Scaled Frequency

Mode Description(a)

1

a′

3811

3692

3627

3514

OH stretch

2

a

′

3198

3098

3152

3054

CH stretch

3

a′

3176

3078

3200

3100

CH stretch

4

a

′

3157

3059

3175

3077

CH stretch

5

a

′

3138

3040

3181

3082

CH stretch

6

a

′

2934

2842

2951

2859

CH2 sym stretch

7

a′

1624

1573

1549

1501

CC stretch

8

a

′

1545

1497

1433

1389

CC stretch

9

a

′

1461

1415

1450

1405

CC stretch

10

a′

1449

1404

1359

1317

CH2 scissors

11

a′

1434

1389

1399

1355

CH ip bend

12

a

′

1364

1321

1308

1267

CH ip bend

13

a

′

1316

1275

1575

1526

CH2 ip bend

14

a′

1257

1218

1228

1190

CO stretch

15

a

′

1182

1145

1183

1146

CH ip bend

16

a

′

1174

1138

1157

1121

OH ip bend

17

a′

1127

1092

1143

1108

CH ip bend

18

a

′

1010

978

1024

992

CC stretch

19

a

′

975

945

974

943

ip ring deformation

20

a′

929

900

930

901

ring breathe

21

a

′

783

759

764

740

ring breathe

22

a

′

586

568

549

532

ip ring deformation

23

a

′

500

484

481

466

ip ring deformation

24

a′

397

384

392

380

CO ip bend

25

a

′′

2925

2834

2994

2901

CH2 asym stretch

′′

26

a

1183

1147

1097

1063

CH2 twist

27

a′′

956

926

759

735

CH oop bend

28

a′′

937

908

793

768

CH2 oop rock

29

a

′′

863

836

579

561

CH oop bend

30

a′′

736

713

948

919

CH oop bend

31

a′′

657

637

572

554

CH oop bend

32

a

′′

517

501

460

445


33

a

′′

457

443

299

289


34

a′′

403

391

350

339

OH oop bend

35

a

′′

279

270

178

173


36

a

′′

131

127

50

48

CH2 oop rock

(a) ip = in-plane;

oop = out-of-plane



4.1

Duschinsky Mixing

The ground state frequencies in tables 1 and 2 are labelled according to the Mulliken convention. For a molecule with Cs symmetry, we first label the a′ modes in order of decreasing frequency, followed by the a′′ modes in a similar fashion. The excited state frequencies in the tables, however, appear to be more random in order, and this section will be used to clarify. Upon excitation, the geometry of the syn and anti configurations distort, as seen in figure 2. The frequencies and character of each mode therefore change, impacting the order and description. To label the excited state modes, in the basis of the ground state modes, Duschinsky matrices were constructed, using the FCLab II Software. 28 We have previously used such matrices to assign excited state modes of the 1-phenylpropargyl radical and found they worked favourably. 29 A Duschinsky matrix is constructed by taking scalar product of the mass-corrected displacements of each normal mode in the ground state with each of those in the excited state. This results in a 3N-6 × 3N-6 matrix where all matrix elements represent the overlap integral between modes in the ground and excited states. In figure 3, the matrix elements are coloured according to this scalar product, where 0 is represented by white, 1 by black, and a linear greyscale between the two. In the case where the geometry change is minimal, and all modes in the excited state directly correlate to those in the ground state, a diagonal matrix would be generated. The necessity for a new labelling convention becomes apparent when we consider how the modes change in frequency, and hence order, upon excitation, as shown in figure 3. Here, the modes in the ground and excited state are labelled simply in decreasing frequency order, straight from a GAUSSIAN output file. There exists a large number of off-diagonal elements in figure 3. As a result, the excited state labels become inconsistent with the ground state for the majority of modes. For ′

example, for the anti isomer, the excited state mode ν19 has a near perfect overlap with

12


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syn

anti

1.0

0.8

Ground State

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

ν’’18 ν’’16

0.4

ν’19 ν’19

0.2

0.0

Excited State

Excited State

Figure 3: Duschinsky matrices for the syn isomer (left) and the anti isomer (right). Both the ground and excited state modes are labelled according to the Mulliken convention assuming a C1 point group. Matrix elements are shown in a linear greyscale, where white represents 0 and black represents 1. ′′

ground state mode ν16 , indicated by the blue lines, inferring that these two different labels represent the same molecular motion, but are of a different relative frequency order. This ′

′′

is also apparent for the syn isomer, where ν19 has a near perfect overlap with ν18 . Other than this direct mode relabelling, the purple boxes emphasise a binary or ternary mode ′′

′′

′

′

relabelling, e.g. ν6 , ν7 → ν7 , ν6 for both isomers, indicating the labels of these modes should be swapped for the excited state. Further, the modes within green boxes represent an extensive scrambling, where no ground state mode label is correctly represented by its corresponding excited state label. To rectify these labelling inconsistencies, and to label similar vibrations with the same label, we have rearranged the excited state frequency order, to best form a diagonal matrix. This maps modes with similar character in the excited state onto the same label in the ground state and makes the spectral labelling much more consistent. In order to prevent confusion when talking about the original C1 excited state labels the prefix ‘or-’ will be used. Before the excited state is dealt with, the ground state modes are reordered according to

13



Page 14 of 33

Mulliken convention for a molecule with Cs symmetry. Boxes emphasising the ground state symmetry labels have been drawn on the matrices in figure 4, which will be discussed later. For each matrix in figure 4, the excited state column headings list both the original unordered (C1 ) labelling from figure 3, and the final labelling after diagonalization. This labelling convention ensures that when discussing a vibrational mode in the excited state, we can draw direct comparison with the same (or most similar) mode in the ground state. It should be noted that non-zero off-diagonal elements do not represent a coupling between these two modes upon electronic excitation. Rather, they signify the excited state normal mode contains character of more than one ground state normal mode. The reordered Duschinsky matrix for the syn isomer is displayed on the left in figure 4. A quick comparison between the original (C1 ) and final (Cs ) mode labels emphasises the need for a reordered matrix, as only three mode labels, (ν1 , ν3 and ν36 ), have been retained. Two ′

′

modes, ν25 and ν32 , are calculated to contain significant character of ground state modes with different symmetry, which will be discussed later. Apart from these two modes, the symmetry of the remainder of modes is preserved upon excitation. From this plot, we can see that the majority of excited state modes can be mapped clearly onto the ground state basis. In several cases, however, this mapping is not evident with two excited state modes calculated to contain a large component of the same ground state mode. For the syn isomer, as seen on the left in figure 4, this occurred three times. ′

′

Excited state modes or-ν18 and or-ν16 originally had their largest matrix element mapping ′′

′

to ground state mode ν17 , shown by the green dotted lines in the left of figure 4; or-ν18 ′′

however had a larger element and was thus matched with ν17 . We then considered the next ′

′′

highest unassigned element of or-ν16 which was that of ν15 , and after confirmation upon ′

′

visual inspection, assigned accordingly. Modes or-ν13 and or-ν12 underwent a similar review, ′

′′

′

indicated by the blue dotted lines, resulting in mode or-ν12 being assigned to ν9 , and or-ν13 ′′

′

′

to mode ν11 . The original modes or-ν9 and or-ν8 also both contained a dominant character ′

′

from ν7 , indicated by the purple dotted lines. As or-ν8 contained the largest character, mode

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Page 15 of 33

syn

anti

a”

1.0

a”

0.8

Ground State

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a’

a’ 0.6

0.4

0.2

0.0

Excited State

Excited State

Figure 4: Duschinsky matrix for the syn (left) and anti (right) isomers. The ground states have been labelled according to traditional Mulliken convention for Cs molecules. The excited state axes exhibit both the original Mulliken convention (as though the excited states were C1 ), and the the new numbering after the matrices were diagonalised with their respective Cs ground state. Matrix elements are shown in a linear greyscale, where white represents 0 and black represents 1. ′

′′

′′

or-ν9 was reassigned by its 3rd highest overlap with ν13 , as the 2nd highest (ν8 ) already had a well-defined diagonal element. In cases like this, the label loses meaning with respect to ground state modes and becomes just a label. Similar to the the syn matrix, only four labels remained unmodified following the rotation of the anti matrix, (here ν1 , ν4 , ν35 and ν36 ), shown in the right of figure 4. Upon ′

′

electronic excitation, 8 of the 12 modes between ν25 - ν36 , which were of a′′ symmetry in the ground state, are calculated to contain character of ground state modes with both a′ and a′′ symmetries. This is significantly more scrambling than observed for the syn isomer and will be discussed later in reference to the excited state symmetries of both isomers. The anti isomer, also contained three elements with non-unique ground states, which can ′

′′

be seen on the right in figure 4. Mode or-ν26 contains almost even character of ν21 (45%) ′′

′

and ν27 (43%), indicated by the green dotted lines. Original mode or-ν25 has a 41% overlap ′′

′′

′

with ν21 , but only 25% with mode ν27 . As such, to maximise the diagonal, modes or-ν26 ′

′′

′′

′

′

and or-ν25 were assigned to modes ν27 and ν21 accordingly. Modes or-ν22 and or-ν27 were

15



′′

′′

paired respectively with modes ν30 and ν29 in a similar fashion, indicated by the purple lines. ′

′

′′

′′

Finally, or-ν18 and or-ν16 were assigned as ν17 and ν15 by their overlap elements, indicated by the blue lines.

5

Results and Discussion

Figure 5 (upper trace) shows a REMPI spectrum of the m/z 95 compound created from the discharge of phenol and water seeded in argon. The excitation laser wavelength was scanned between 18150 and 19250 cm−1 , whilst the ionization laser was fixed near 43500 cm−1 . This spectrum has been assigned previously to both the syn and anti conformers of the o-chdOH radical, with the electronic origin of the anti conformer at 18200 cm−1 and the syn conformer at 18218 cm−1 . 22 The difference between the origin peak intensities of syn and anti conformers is ∼1:3 respectively. In our previous paper, 22 the oscillator strength of the anti transition was calculated to be about twice as strong as syn. Therefore, assuming that the Franck-Condon factors of the origin transitions are similar, we estimate relative population of the conformers in the molecular beam to be 1:1.5 for syn:anti. As discussed in the theory section, the zeropoint energy of the anti conformer lies 320 cm−1 lower than syn with a 1750 cm−1 barrier from the anti well. A direct count of the harmonic density of states at an energy corresponding to the top of the barrier, using the frequencies in tables 1 and 2, reveals a higher density for the anti conformer over syn by 1.5:1. Therefore the observed relative populations are consistent with the radical being formed from H + phenol, falling into two wells that lie ∼ 121 kJ mol−1 (10,100 cm−1 ) below the entrance channel barrier. 22 In cooling, the new radicals stabilise into the syn and anti wells according to the density of the states at the top of the syn - anti barrier, and are frozen in their respective structures. Hole burning spectroscopy was employed to untangle the contributions of the two conformers. The excitation laser was tuned to the 18200 cm−1 transition for the anti isomer,

16


Page 16 of 33

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavenumber (cm-1) 18200

18400

18600

19000

18800

19200

H O H H

a'' vibrations

000

0

3410 3610

3420

2410

a' vibrations

100

200

3320 3410 3510

3310 3410

300

400

2310 3310 3410

2210 2420

2310

500

600

700

1810

800

900

1000

Relative Wavenumber (cm-1) Figure 5: (Upper) REMPI spectrum of ortho-chdOH radical. (Lower) Hole burning (depletion) spectrum measured while monitoring the REMPI signal 18219 cm−1 . The spectrum is assigned to the syn conformer. Vibrational assignments are indicated in blue font for a′ vibrations and black font for a′′ vibrations.

and a third laser, timed ∼100 ns prior to the REMPI lasers, was scanned across the same spectral range. All transitions with a ground state in common with the 18200 cm−1 peak appear as a depletion in the REMPI signal. The hole burning experiment was repeated for the peak at 18218 cm−1 for the syn isomer. These two experiments give rise to the depletion traces, reflected at the bottom, in figures 5 and 6, (figure 6 is shown later in the paper). The parent peak intensity was depleted by at most 40%.

17



Page 18 of 33

In assigning the spectra, we give priority to peaks in the REMPI spectrum; the signal-tonoise is better, and intensities are more reliable. In all cases, peaks in the REMPI spectrum have counterparts in the depletion spectrum. However there are instances where apparent peaks appear in the depletion spectrum with no REMPI counterpart, which might arise from other experimental artefacts. Any peak that appears solely in one of the hole burning experiments is labelled ‘?’ and not assigned. The hole burning experiment attributed just 12 peaks (including the origin) of the m/z 95 spectrum to the syn conformer. This is far fewer than were attributed to the anti conformer (the remaining lines, justified in figure 6). As described in the theory section, the symmetry of the excited syn conformer was sensitive to the inclusion of a diffuse functional in the calculations. By an evaluation of the Duschinsky matrices, in figure 4, we can investigate the preservation of symmetry upon excitation, to explore the possibility that a higher symmetry is responsible for the fewer peaks, due to symmetry selection rules. As mentioned previously, the syn conformer undergoes a very small geometric distortion upon excitation, when calculated at the 6-311+G(d,p) level, breaking its plane of symmetry. In figure 4, boxes of symmetry are drawn to help us understand how well the symmetry is preserved upon excitation. For the syn conformer, left in the figure, we can see that almost every matrix element lies within the two boxes, indicating that they retain the same symmetry character of their respective ground state mode. There are a couple of significant non-zero matrix elements that require further evaluation. ′′

′′

The ground state symmetric (ν6 ) and asymmetric (ν25 ) CH2 scissoring motions show significant mixing. Animation of these modes reveals that the symmetric and anti-symmetric stretches in the planar ground state have become two local CH stretching modes in the distorted excited state. This local mode formation is also apparent for the anti rotamer, and ′′

′′

can be seen in the Duschinsky matrix in figure 4; where the same two modes, ν6 and ν25 , exhibit significant mixing following excitation.

18


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The unscaled local modes for the two CH vibrations in the syn rotamer have frequencies of 2922 cm−1 and 2949 cm−1 . In order to test the coupling of the two modes, each of the hydrogens in the CH2 group was substituted in turn with deuterium, and the calculations were re-run for each of the isotopologues. The two local CH vibrations were calculated to have frequencies of 2921 cm−1 and 2950 cm−1 , which compare well to the CH2 local mode frequencies above, indicating that they are indeed nearly uncoupled. The formation of local modes which break the symmetry of the molecule has been observed previously by Kemp et al. In their studies on meta-disubstituted benzenes, they observed that upon increasing the mass of one substituent, the symmetric and asymmetric stretches of the C-X bonds (where X represents the substituent) eventually became local modes, where only one stretching mode was active, and the other near stationary, and vice versa. 30 ′

Another, smaller, anomaly is mode ν32 , which contains character of both the a′′ mode ′′

ν32 (60%) and a′ mode ν23 (19%). Upon visual inspection, this mode is clearly dominated by the asymmetric out-of-plane ring distortion, along with large amplitudes of CH/CH2 /OH wag. The mode retains some small character of an in-plane elongation/contraction of the ring, and may excite in single quantum. The excited state of the syn conformer therefore preserves the majority of vibrational mode symmetry. We expect the vibronic transitions for this conformer will be subject to Cs selection rules, that is, only transitions of overall a′ symmetry are allowed. The Duschinsky matrix for the anti isomer, (right in figure 4), shows more extensive mixing between the a′ and a′′ modes upon electronic excitation. This is consistent with the larger distortion observed in the excited state geometry. We attribute the larger number of peaks in the anti spectrum to arise from the lowering of symmetry from Cs to C1 and the consequent relaxation of symmetry selection rules. In C1 symmetry, transitions involving all vibrations are allowed.

19



5.1

Page 20 of 33

The Syn Conformer

As discussed above, by an evaluation of the Duschinsky matrix, the structure of the excited state of the syn conformer is effectively of Cs symmetry. We therefore start by assigning the vibrational structure in the spectrum in the Cs point group, where a′ modes are Franck-Condon active and a′′ modes appear only as even overtones and combinations. The calculated and scaled frequencies for the syn isomer, along with their respective symmetries are displayed in table 1. The electronic origin for this transition, 000 , is observed at 18218 cm−1 , and this transition was used as the ‘burn’ transition for the depletion spectrum, shown in the lower panel of figure 5. The assignments are displayed in table 3. Table 3: Excitation frequencies for the Syn conformer, relative to the origin transition (18218 cm−1 ), their assignments and comparison with theory. All units are in wavenumbers (cm−1 ). The theoretical values for the a′′ modes are not strictly from the TD-B3LYP output, see text.

Observed

Assignment

Symmetry Label

Theory

∆T −O

0

000

215

3410 3610

a′′ ⊗ a′′

231

16

274

3420

2a′′

286

12

356

3310 3410

a′′ ⊗ a′′

348

-8

373

2410

a′

371

-2

421

3320

2a′′

396

-25

461

3410 3510

a′′ ⊗ a′′

451

-10

522

2310

a′

522

0

731

2210

a′

735

4

734

2420

2a′

742

8

873

2310 3310 3410

a′ ⊗ a′′ ⊗ a′′

878

5

988

1810

a′

987

-1

20


Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


′

We begin our assignment with the a′ modes. The lowest frequency a′ mode, ν24 , is the in-plane C-C-O bend, calculated at 371 cm−1 , which is an excellent match with a mediumstrong transition displaced +373 cm−1 from the origin transition of the syn conformer and therefore assigned as such. We can then assign the next lowest a′ transition, 2310 , predicted and observed at +522 cm−1 . There is a doublet at +731 and +734 cm−1 . One of these is likely ′

to involve the ring-breathing mode, ν22 , calculated to have an excited state frequency of 735 cm−1 . The other band likely contains the overtone 2420 . In the harmonic limit, this overtone would appear at twice its fundamental, +742 cm−1 . This overtone is uncharacteristically strong for an origin dominated spectra, and likely forms a Fermi resonance with mode 2210 . If the overtone is indeed anharmonic, this reduction in the frequency is reasonable. We therefore assign the lower frequency component at +731 cm−1 to 2210 and +734 cm−1 to 2420 , although the equal intensity of the two components suggests almost complete mixing of the two wavefunctions. Finally, the ring breathe and CH bend of 1810 , predicted at 987 cm−1 is ′

′

observed just 1 cm−1 higher. Single quantum transitions in modes ν19 - ν21 are not observed. ′

′

The asymmetric, out-of-plane modes, ν33 - ν36 , are all involved in the change in geometry from the ground to excited state, and many exhibit Franck-Condon activity as overtones and combinations. None of these modes are assigned as a single quantum transition. This reaffirms our evaluation from the Duschinsky matrix analysis, which reinforces that the excited state has Cs symmetry, as the transitions of the syn conformer adhere to the corresponding Cs selection rules. The next step is to determine the low frequency a′′ overtones, in order to derive their fundamental frequency from assignments of combination bands. These revised frequencies are then used when determining the position of combination bands, rather than the original TDB3LYP values. The band observed at +274 cm−1 is assigned to the overtone 3420 . Assuming ′

that the mode is harmonic, the frequency of the fundamental vibration, ν34 , is determined to be 137 cm−1 , some 6 cm−1 below the theoretically predicted value. The most appropriate assignment for the band at +421 cm−1 is the overtone 3320 , representing a large amplitude

21



Page 22 of 33

′

out-of-plane ring distortion. Again, this implies that the ν33 fundamental vibration has a frequency of ∼211 cm−1 , which is somewhat larger than the theoretically predicted value of 198 cm−1 . We now turn to the other low frequency unassigned bands, at +215 cm−1 , +356 cm−1 and +461 cm−1 . The first of these, at +215 cm−1 can only be assigned to the combination ′

3410 3610 , calculated to be 16 cm−1 higher in frequency, at 231 cm−1 . As ν34 was determined ′

to be 137 cm−1 , this infers that ν38 = 78 cm−1 , which is 10 cm−1 lower than predicted. The band observed +356 cm−1 from the origin is not readily assigned. The closest assignment is the combination 3310 3410 . The fundamental frequencies of both modes were derived above, with frequencies of 137 cm−1 and 211 cm−1 respectively. The combination band would therefore lie at 348 cm−1 in the harmonic limit, indicating this combination has a strong positive anharmonicity. However, we have no better assignment. Finally, the band at +461 cm−1 is assigned to the combination 3410 3510 , predicted at 451 cm−1 . As 3410 has ′

been determined to be 137 cm−1 , this implies that ν35 = 324 cm−1 in the harmonic limit, 10 cm−1 higher than calculated. The only band remaining unassigned in the spectrum is at +873 cm−1 from the origin transition. This band cannot be assigned to any binary combinations, however it assigns well to the 2310 progression on 3310 3410 at 878 cm−1 , only 5 cm−1 higher than observed. This is the only band where multiple quanta of out-of-plane modes (3310 and 3410 ) are coupled to an in-plane mode (2310 ), involving a ring contraction and elongation. Overall there is an excellent agreement between the experimentally observed a′ modes and the scaled theoretical values, with a mean absolute deviation (MAD) of only 1.8 cm−1 (see table 4). The a′′ modes, however, are not as well predicted by this level of theory, with a MAD of 9.6 cm−1 . The shortfalls of TD-B3LYP in calculating out-of-plane modes accurately were also observed by Krechkivska et al. during their studies on protonated naphthalene. They compared their level of theory with the RI-CC2 method used by Alata et al. for the same cation and found the latter significantly superior in that case. 31,32

22


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Table 4: Comparison between single quanta observed/determined frequencies and TD-B3LYP theory, including the mean absolute deviation (MAD) for each of the a′ and a′′ modes, and the total. All units are in wavenumbers (cm−1 )

a′ mode

Observed

ν24

′

373

371

-2

ν23

′

522

522

0

ν22

′

731

734

4

′

988

987

-1

ν33

MADa′

1.8

ν18

TD-B3LYP ∆T −O

a′′ mode

Observed

TD-B3LYP ∆T −O

ν36

′

88

78

10

ν35

′

324

314

-10

ν34

′

137

143

6

′

210.5

198

-12.5

MADa′′

9.6

MADT OT

5.7

Reviewing the discrepancy between the out-of-plane combinations and the observed bands reveals that the a′′ modes appear to be very anharmonic. This spectroscopic evidence of anharmonicity agrees with the implications of the theoretical study, where different levels of theory and basis set size would converge to either planar and non-planar configurations, implying a very flat potential. The out-of-plane potential is likely anharmonic, or even of a pseudo-double minimum character. Introduction of a quartic term in the potential gives rise to alternating spacings in the vibrational coordinates and to positive anharmonicities, as observed. This is intimated in the experimental data, however there are not enough observed transitions to create an empirical fit of the potential. The ability of theory to successfully account for every peak within the hole burning spectrum ensures that this spectrum has been correctly assigned as the syn conformer of the ortho complex.

5.2

The Anti Conformer

The TD-DFT calculations, and an evaluation of the Duschinsky matrix for the excited state of the anti conformer, indicate C1 symmetry. With no symmetry, single quantum transitions

23



Page 24 of 33

of all in- and out-of-plane modes are allowed. The electronic origin, 000 , is observed at 18200 cm−1 , and this transition was used as the ‘burn’ transition for the depletion spectrum, shown in the lower trace of figure 6. The recorded spectrum spans ∼1000 cm−1 and the calculated and scaled frequencies are displayed in table 2. We begin our assignment of the anti spectrum, as for the syn, by assigning the single quantum fundamental transitions, identified in figure 6 in the red font. These (and all) assignments for the anti isomer are displayed in table 5. The first band observed in the spectrum is at +168 cm−1 , which can only be assigned to the single quantum a′′ transition involving the out-of-plane CO bend and CH2 rock, 3510 , predicted at 173 cm−1 . The two peaks, at +281 cm−1 and +326 cm−1 are readily assigned to transitions in the out-of-plane modes 3310 and 3410, calculated at 289 cm−1 and 339 cm−1 respectively. These three transitions all contained a large overlap with ground state modes of a′′ character, see figure 4. The observation of these transitions in single quantum validate the theoretical evaluation of an excited state with C1 symmetry, contrary to that of the syn isomer. Considering the reduced symmetry in the excited state, single quantum transitions of the lowest frequency a′ and a′′ modes can be assigned to observed peaks based on proximity to ′

′

the theoretical calculations. Indeed, transitions in each of the a′ modes from ν24 - ν18 can ′

′

′

be readily assigned in this way. The out-of-plane a′′ modes ν35 - ν31 and ν27 are similarly ′

observed and assigned accordingly. The assignment of ν27 may at first look out of place, ′

′

however we recall that this is just a labelling convention and mode ν27 and ν21 are near ′′

′′

equivalent in their elements of ground state modes ν21 and ν27 . This is evidenced in the spectrum, as both bands are observed in single quantum only 7 cm−1 apart. The peak observed at +226 cm−1 must now be discussed for purposes of future assignment. It cannot be accounted for by a single quantum transition nor overtone. This peak ′

can only be assigned to the combination of the transitions 3510 and 3610 . As ν35 has been ′

determined above to be 168 cm−1 , this implies that the frequency of ν36 is ∼58 cm−1 in the ′

harmonic limit, some 10 cm−1 higher than theory. Although ν36 is not observed in single

24


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavenumber (cm-1) 18200

18400

18600

18800

19000

19200

H O H H

?

?

?

?

? 3402

3401 3601 3501 3601

3501

3502 3601

3201 3601 2201 3601 2301 3601

2402

3201 3301

2401 3501 3601 2401 3301

3301 3401 2401 3201 2301 2201

3202

2302

2701 3601 2201 3401

2001 1901

2701 2101

2701 3301

1801

000

0

100

200

300

400

500

600

700

800

900

1000

Relative Wavenumber (cm-1) Figure 6: (Upper) REMPI spectrum of ortho-chdOH radical. (Lower) Hole burning (depletion) spectrum measured while monitoring the REMPI signal 18200 cm−1 . The spectrum is assigned to the anti conformer. Vibrational assignments are indicated in red font for fundamental vibrations and black font for overtones and combination bands. The ‘?’ symbol denotes a depleted band that has no corresponding line in REMPI experiments; any coincidence is merely an artefact of the compacted scale.



Page 26 of 33

Table 5: Excitation frequencies for the Anti conformer, relative to the origin transition (18200 cm−1 ), their assignments and comparison with theory, including the mean absolute deviation (MAD) for the fundamental assignments. All units are in wavenumbers (cm−1 ). Observed

Fundamental Assignments

0

000

168

3510

Combinations & Overtones

3510 3610

226

Theory

∆T −O

173

5

221

-5

281

3310

289

7

326

3410

339

13

368

2410

380

12

372

3410 3610

384

12

403

3520 3610

394

-9

442

3210

445

3

467

2310

466

-1

513

3210 3610

500

-13

522

2310 3610

525

3

532

-1

533

2210

579

2210 3610

591

12

599

2410 3510 3610

594

-5

655

2410 3310

649

-6

657

3420

652

-5

723

3210 3310

723

0

733

2710

735

2

740

2110

740

0

742

2420

736

-6

793

2710 3610

791

-2

860

3410 2210

859

-1

879

3220

884

5

888

2010

901

13

939

1910

943

4

934

-6

992

-11

1014

-3

MAD (fundamentals)

6.0

2320

940 1003 1017

1810 2710 3310


Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


quantum, it seems to readily build upon other a′ and a′′ transitions, including 3510 , 3410 , 3210 , 2310 , 2210 and 2710 . The band at +599 cm−1 also contains 3610 , combined with 2410 and 3510 . The ‘theory’ value for all combinations and overtones in table 5 is taken as the direct harmonic addition of the corresponding observed frequencies for the fundamental transitions, rather than values from the Gaussian calculations. The mean absolute deviation for all fundamental frequencies of the anti conformer is 6.0 cm−1 . This is very similar to the 5.7 cm−1 for all modes of the syn isomer. With all peaks accounted for, we believe that this spectrum can be successfully assigned as the anti conformer of the ortho complex. Finally, the Duschinsky matrices were used initially as a guide for assignment of the spectra. They provided a clear, visual way to inspect whether the point group of the molecule changed following electronic excitation. They guided the assignment process by preserving the Cs symmetry for the syn isomer, but relaxing the symmetry to C1 for the anti isomer. The spectra were successfully assigned using this strategy. In reverse, the success of the Duschinsky matrices and the subsequent spectroscopic assignments reinforces that the nuclear framework of the syn isomer is planar, or at least pseudo-planar with a barrier to planarity below the zero-point energy. The Duschinsky matrix approach might provide a general way to determine whether a molecule is functionally changing symmetry in an electronic transition, especially in circumstances where theory is ambiguous.

6

Conclusions

A REMPI scheme has been recorded for the hydrogen addition to the ortho position of phenol. Two rotamers were uncovered, where the hydroxyl group is situated syn and anti to the ortho CH2 . The spectra span some ∼1000 cm−1 and through the use of hole burning spectroscopy, was able to be separated into an isomeric dependence. The origin transition lies at 18200 cm−1 for the anti conformer, and 18218 cm−1 for the syn conformer. All peaks recorded within the REMPI scan were able to be assigned to one of the two rotamers. In their ground

27



state, both conformers are of Cs symmetry. Upon excitation, the anti conformer undergoes a geometric change to C1 , whilst the syn conformer retains an almost planar Cs nature and the spectra were assigned accordingly. This change in geometry and symmetry is validated by an evaluation of the Duschinsky matrices for each isomer’s excitation. The MAD for the anti fundamental assignments was 6.0 cm−1 . For the syn rotamer, the a′ and a′′ assignments were made with MADs of 1.8 and 9.6 cm−1 respectively. Whilst the relatively cheap TDB3LYP/6-311+G(d,p) level of theory was largely successful in calculating the excited state frequencies, it was clearly more accurate for the in-plane vibrations than out-of-plane. The observation of both rotamers under our discharge conditions implies both will also be present in atmospheric and combustion environments. We expect the two rotamers to have a similar reactivity, in energetic environments; however it will be interesting to see whether their relative abundance and reactivity is changed under cold matrix-isolated conditions, where the relative orientation of reactants plays a larger role. We hope the current work will aid in detecting the cdhOH radicals in chemically relevant environments and further elucidate the mechanism of the benzene + OH reactions.

28


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(8) Borrás, E.; Tortajada-Genaro, L. A. Secondary Organic Aerosol Formation from the Photo-Oxidation of Benzene. Atmospheric Environment 2012, 47, 154 – 163. (9) Lay, T. H.; Bozzelli, J. W.; Seinfeld, J. H. Atmospheric Photochemical Oxidation of Benzene: Benzene + OH and the Benzene−OH Adduct (Hydroxyl-2,4-cyclohexadienyl) + O2 . The Journal of Physical Chemistry 1996, 100, 6543–6554. (10) Ghigo, G.; Tonachini, G. Benzene Oxidation in the Troposphere. Theoretical Investigation on the Possible Competition of Three Postulated Reaction Channels. Journal of the American Chemical Society 1998, 120, 6753–6757. (11) Chen, C.-C.; Bozzelli, J. W.; Farrell, J. T. Thermochemical Properties, Pathway, and Kinetic Analysis on the Reactions of Benzene with OH: An Elementary Reaction Mechanism. The Journal of Physical Chemistry A 2004, 108, 4632–4652. (12) Wang, L.; Wu, R.; Xu, C. Atmospheric Oxidation Mechanism of Benzene. Fates of Alkoxy Radical Intermediates and Revised Mechanism. The Journal of Physical Chemistry A 2013, 117, 14163–14168. (13) Volkamer, R.; Klotz, B.; Barnes, I.; Imamura, T.; Wirtz, K.; Washida, N.; Becker, K. H.; Platt, U. OH-Initiated Oxidation of Benzene Part I. Phenol Formation under Atmospheric Conditions. Physical Chemistry Chemical Physics 2002, 4, 1598–1610. (14) Klotz, B.; Volkamer, R.; Hurley, M. D.; Sulbaek Andersen, M. P.; Nielsen, O. J.; Barnes, I.; Imamura, T.; Wirtz, K.; Becker, K.-H.; Platt, U. et al. OH-Initiated Oxidation of Benzene Part II.Influence of Elevated NO Concentrations. Physical Chemistry Chemical Physics 2002, 4, 4399–4411. (15) Berndt, T.; Boge, O. Formation of Phenol and Carbonyls from the Atmospheric Reaction of OH Radicals with Benzene. Physical Chemistry Chemical Physics 2006, 8, 1205–1214.

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TOC Graphic 000

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O H H

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Jet-Cooled Spectroscopy of Ortho-Hydroxycyclohexadienyl Radicals

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