Interconversion of Methyltropyl and Xylyl Radicals: A Pathway

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Interconversion of Methyltropyl and Xylyl Radicals: A Pathway Unavailable to the Benzyl-Tropyl Rearrangement Neil James Reilly, Gabriel da Silva, Callan M. Wilcox, Zijun Ge, Damian L. Kokkin, Tyler P. Troy, Klaas Nauta, Scott H. Kable, Michael C. McCarthy, and Timothy W. Schmidt J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11914 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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The Journal of Physical Chemistry

Interconversion of Methyltropyl and Xylyl Radicals: A Pathway Unavailable to the Benzyl-Tropyl Rearrangement Neil J. Reilly,† Gabriel da Silva,‡ Callan M. Wilcox,¶ Zijun Ge,§ Damian L. Kokkin,k Tyler P. Troy,⊥ Klaas Nauta,¶ Scott H. Kable,¶ Michael C. McCarthy,∗,# and Timothy W. Schmidt∗,¶ †Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston MA 02125 ‡Department of Chemical Engineering, The University of Melbourne, Parkville VIC 3010, Australia ¶School of Chemistry, UNSW Sydney, NSW 2052, Australia §School of Chemistry, The University of Sydney, NSW 2006, Australia kDepartment of Chemistry, Marquette University, P.O. Box 1881 Milwaukee, WI 53201-1881 ⊥Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 #Harvard-Smithsonian Center for Astrophysics and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 E-mail: [email protected]; [email protected] Abstract The products of an electrical discharge containing toluene are interrogated using resonance-enhanced multiphoton ionization and laser-induced fluorescence spectro-

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scopies. A previously unreported electronic spectrum recorded at m/z = 105, with a putative origin band at 26053 cm−1 , is assigned to methyltropyl radical, which appears to be a major product of the toluene discharge, plausibly arising from CH insertion. All three o-, m- and p-xylyl isomers are also identified. These isomers are detected in electrical discharges containing various xylenes, where it is also found that interconversion occurs: A discharge of o-xylene produces some m-xylyl; a discharge of m-xylene produces some o-xylyl; and a discharge of p-xylene produces all three isomers. No α-methylbenzyl was detected, but styrene was. These observations are supported by state-of-the-art quantum chemical calculations which reveal an isomerization pathway between methyltropyl and xylyl radicals for which there is no analogue in the canonical tropyl-benzyl isomerization.

Introduction Polycyclic aromatic hydrocarbons play crucial roles in many chemical processes, ranging from combustion, 1,2 to atmospheric pollution, 3–5 to the molecular evolution of cosmic carbon. 6 Although different mechanisms for their assembly and growth have been proposed, including hydrogen abstraction-acetylene addition 7 and Diels-Alder reactions, 8 many gaps in our knowledge persist. Ring-insertion and isomerization reactions are two areas in which additional experiments and calculations might improve our understanding of PAH growth and aromatic decomposition. The complete decomposition chemistry of toluene, for example, is still not fully understood, and several of its decomposition products remain poorly characterized. Tropyl and benzyl radicals are both isomers of C7 H7 , with the latter calculated to be roughly 16 kcal/mol lower in energy. 9 The barrier to interconversion between the two, however, is substantial (91.7 kcal/mol 10–12 ), significantly hampering facile rearrangement under a wide range of conditions. Indeed, in a recent mass spectrometric study of the flash pyrolysis of tropyl radical in a heated micro-reactor, it was found that tropyl does not isomerize to

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benzyl at reactor temperatures up to 1600 K. 13 Similarly, tropylium cation is a six-electron aromatic system which is 7 kcal/mol more stable than benzylium cation, 14 but it also has a sizable barrier to isomerization (72 kcal/mol above tropylium 10 ). Tropyl radical is of interest as a prototypical system for the study of ring-expansion isomerization reactions, while benzyl radical is a resonantly-stabilized radical (RSR), which, owing to its thermodynamic stability 15 compared to other transient radicals, may have a high steady-state abundance in many environments. For the reasons outlined above it is perhaps unsurprising that experiments which interrogate the radical products of an electrical discharge starting with toluene yield very little tropyl radical. More surprising is a precursor that does produce this radical isomer in abundance: benzene. In fact, with benzene, there is a strong propensity to form tropyl rather than benzyl radical, 16 despite apparently unfavorable energetics. Molecular beam time-offlight mass spectroscopy of an electrical discharge by He et al. 17 using VUV photoionization at 118 nm confirms the propensity of benzene and toluene to produce higher – rather than lower – mass products. Prominent product mass peaks, corresponding to net addition of CH to the parent, at m/z = 91 (C7 H7+ ) and 105 (C8 H9+ ), respectively, were found for both precursors; analogous measurements with fully deuterated samples confirm these findings. Although the isomeric distribution can not be determined from these studies, the authors conjecture that tropylium-like cations (which could be subsequently neutralized) might be involved in molecule formation. Jet-cooled electrical discharges are widely used sources for the generation and spectroscopic characterization of resonance-stabilized radicals (compiled in a recent review 18 ) and other reactive molecules, including nearly isoenergetic conformational isomers (e.g., cis- and trans-vinyl propargyl radicals 19 ), as well as widely energetically separated structural isomers (e.g., iso-fulminic acid, which lies 84 kcal/mol above fulminic acid 20 ). While no comprehensive model of discharge chemistry exists, the energetics and formation pathways of C7 H7 isomers in an electrical discharge starting with toluene can be plausibly inferred. Unimolecular

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decomposition in glow discharges is driven by collisions with electrons, which have an average kinetic energy of several eV (∼50 kcal/mol); 21 and with metastable carrier gas atoms, which lie several eV above ground (e.g., ∼11.5 eV for the 3p5 4s1 metastable state of Ar). 22 Because the weakest bond in toluene is the C-H bond on the methyl group (i.e. ∼ 90 kcal/mol, or roughly 10-15% weaker compared to C-H bonds on the aromatic ring), bond rupture preferentially occurs at this location, yielding benzyl radical and a hydrogen atom. 23 Because there is insufficient energy prior to dissociation or following benzyl formation to surmount the large isomerization barrier, this pathway leads to very little tropyl radical. The use of substituted toluenes, such as benzyl chloride (C6 H5 CH2 Cl), which has an even weaker C-Cl bond (e.g. 70 kcal mol 24 ), to produce benzyl radical provides strong supporting evidence for this simple formation route. In the infrared study of Satink et al., 25 for example, benzyl radical was generated by these means, while a seven-membered ring (cycloheptatriene) precursor was needed for tropyl radical. Cavallotti et al. 11 investigated the C7 H7 potential energy surface (PES) using ab initio methods and concluded that the main benzyl decomposition pathway ultimately produces fulvenallene plus a H atom; this mechanism is now well accepted, 13,26–30 and the subsequent reaction chemistry of fulvenallene has since been substantially developed. 8,9,31–36 The production of C7 H7 isomers in a benzene discharge is more speculative, but several inferences can be drawn. Because CH radical, along with C2 and C3 , is a prominent decomposition product, 37 and experiments have shown that reaction of CH with benzene is rapid (approaching the encounter rate 38 ), both energized tropyl and benzyl radicals can be formed. The exothermicity of the benzyl channel is so great (∼ 110 kcal/mol, from standard thermochemistry), however, that this isomer can readily rearrange to tropyl or form dehydrogenated hydrocarbons such as fulvenallene and ortho-benzyne by H and CH3 loss; 30,32 there are fewer pathways to relax energetic tropyl radical because it has less excess energy (95 kcal/mol), and furthermore, it can not as easily isomerize to the benzyl isomer owing to the high barrier. For these reasons, plausibly some fraction of energetic tropyl radical

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is cooled by collisions and ultimately trapped in the deep potential well on the millisecond timescale of the supersonic expansion. By comparison, the chemistry of the C8 H9 isomeric system, formed most simply by CH addition to toluene, is far less clear, owing to a combination of factors, which include the larger number of stable isomers, the relatively few measurement techniques that are able to distinguish between these isomers, and the paucity of good theoretical calculations of the PES and thermochemistry. While fragmentary, the picture that has emerged from the work to date is that four methyl benzyl radicals (o-, m-, and p-isomers and α-methylbenzyl) are comparable in energy, 39 but at least two other isomers lie within about 15 kcal/mol, the 2-phenylethyl and methyltropyl radicals (Fig. 1). Although the electronic spectra of the three xylyl radicals have been the subject of several previous studies, only very recently was the electronic spectrum of α-methylbenzyl radical reported, 40 and nothing is known about the optical spectroscopy of the methyltropyl radical. With respect to the xylyl radicals, excitation spectra to the first excited state (A˜ ← X) have previously been reported in the visible region by Fukushima and Obi, 41 and Miller and co-workers. 42 Jet-cooled emission spectra of all three isomers in a corona discharge were published by Selco and Carrick; 43 the origins of the o-, m- and p-isomers lie at 21345.6 cm−1 , 21486.0 cm−1 , and 21699.6 cm−1 , respectively. Because the origin transition of α-methylbenzyl radical also lies nearby (at 21887 cm−1 ), 40 the relative abundances of these four isomers in discharges beginning with various precursors can be conveniently and rapidly assessed using action spectroscopies in the 470 − 450 nm region. The purpose of this paper is to describe gas-phase spectroscopic measurements in combination with new theoretical calculations that primarily focus on the formation and rearrangement pathways of the C8 H9 isomers in an electrical discharge starting from toluene. Both laser-induced fluorescence (LIF) and resonant two-color two-photon ionization (R2C2PI) spectroscopy in combination with time-of-flight detection were used to investigate the discharge products. Theoretical calculations of the potential energy surface and thermochem-

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istry using the G3SX composite theoretical methodology were performed concurrently. In addition to a report of the electronic spectrum of the methyltropyl radical, which apparently has not been observed prior to this work, jet-cooled spectra of the o-, p-, and m-xylyl radicals have also been measured in the visible region. From the observed spectra, we conclude that methyltropyl radical is likely the dominant stable C8 H9 isomer, plausibly formed by CH addition to toluene, and that rearrangement of this radical contributes to formation of the three xylyl isomers that are also observed in the same discharge expansion. These observations and the apparent absence of α-methylbenzyl radical under the same conditions can be rationalized based on arguments of energetics and reaction barriers from the theoretical potential energy surface.

Experimental Details The electronic spectra of various C8 H9 isomers were studied using a combination of LIF (in Sydney) and R2C2PI (in both Sydney and Cambridge) spectroscopies; details of the individual spectrometers have been described elsewhere. 37,44,45 In the R2C2PI experiments, methyltropyl and the o-, m-, and p-xylyl radicals were produced in an electrical discharge through ∼ 1% toluene in argon, in which the discharge was struck during the early stages of supersonic expansion into a large vacuum chamber. The discharge nozzles used in both laboratories are similar to a design that has been described previously. 46 In brief, they comprise a pinhole solenoid valve with a series of annular teflon insulators and copper (or stainless steel) electrodes fixed to the valve faceplate. The two electrodes are separated by a distance of ∼ 1 cm and the diameter of the expansion channel increases gradually from 0.5 mm at the valve orifice to ∼ 3 mm at the terminus of the outer electrode, which facilitates cooling of the discharge products. A pulsed high voltage of a few tens of microseconds duration (1 kV through a current-limiting resistance of 5 kΩ) was applied to the inner electrode, where the higher gas density (in contrast to the outer electrode) favours molecular weight growth

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chemistry. In R2C2PI measurements, the central part of the expansion passed through a 2 mm diameter skimmer, situated approximately 50 mm downstream from the discharge nozzle, into a second, differentially-pumped chamber where it was then probed between two extraction grids of a time-of-flight (ToF) mass spectrometer by tunable radiation from a Nd:YAGpumped dye laser. For the xylyl radicals, spectra were recorded between 420 and 480 nm, and the resulting electronically excited molecules were subsequently ionized using the fourth harmonic (266 nm) of a Nd:YAG laser, or with the 248 nm output of a KrF excimer laser. The power of the 266 nm (or 248 nm) radiation was adjusted so that minimal non-resonant signal was observed. For methyltropyl radical, spectra were recorded between 345 and 390 nm, and the ionization wavelength was 248 nm. Pressures in the source and ToF chambers when the nozzle was pulsing were of order 10−5 and 10−6 , respectively. Following these experiments, LIF measurements were undertaken to disentangle the observed spectra of the xylyl radicals and examine relative abundances of the three isomers. In this experiment, the jet-cooled products of the xylene discharges were expanded into a vacuum chamber through a pulsed nozzle in which the stagnation pressure behind the valve was maintained at 5 bar; the approximate pressure in the chamber with the nozzle pulsing was 5 x 10−5 Torr. The expansion was intercepted by radiation from an excimer-pumped dye laser ∼ 50 mm downstream of the nozzle orifice. Fluorescence was imaged onto the slits of a small monochromator and detected by a photomultiplier tube. The monochromator was used in low resolution mode (band-pass of ∼ 20 nm), primarily as a band-pass filter to reduce scattered laser light. Application of a short discharge pulse is advantageous in fluorescence experiments, because it reduces background emission by metastable carrier gas atoms. As well as this, discharges of xylenes (among other precursors) have been used to observe emission spectra of xylyl radicals. 43 Thus, some fraction of the background (i.e., not laser-induced) emission from the discharge is due to emission from electronically excited states of the radicals of interest. It is therefore important to terminate the discharge pulse

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sufficiently in advance of laser interrogation that the excited xylyl radicals have returned to their ground electronic states. In both the LIF and R2C2PI experiments, the repetition rate of the discharge nozzle source and laser systems was 10 Hz, and the absolute vacuum wavenumber of the dye lasers was calibrated using an external wavemeter. Owing to slight differences in the expansion conditions between the R2C2PI and LIF experiments, before any xylyl LIF spectra were recorded, care was taken to adjust the nozzle tension and other experimental parameters (e.g., discharge voltage and current) so that the LIF spectrum recorded with a toluene discharge closely matched that observed by R2C2PI. Then, the toluene sample was replaced with o-xylene and the experiment repeated to determine which peaks arise primarily from o-xylyl radical. This process was subsequently repeated using m- and p-xylene. The vapour pressures of the three xylene precursors are similar (approximately 6 Torr at room temperature), corresponding to a seed ratio of ∼ 0.3% in our experiments. To minimize crosscontamination between measurements, the discharge nozzle was carefully cleaned, gas lines were purged, and a new sample reservoir was used for each precursor. Each xylene precursor was examined by 1 H NMR spectroscopy; no impurities were found.

Computational Details Calculated pathways for interconversion from methyltropyl to the three xylyl radicals and the α-methylbenzyl radical, and for subsequent H loss to generate xylylenes and styrene, were calculated using the G3SX composite model chemistry. 47 The G3SX method uses B3LYP/631G(2df,p) optimized structures, along with single-point wavefunction theory energy determinations at the HF through to QCISD(T) levels of theory, with incrementally smaller basis sets. Reported energies are at 0 K and include the vibrational zero point energy, with heats of formation obtained from atomization calculations. The G3SX energies are expected to be accurate to within 1 kcal/mol, on average. Minima and transition states possessed zero and

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one imaginary frequencies, respectively, and intrinsic reaction coordinate scans were used to confirm transition state connectivity. All calculations were carried out within the Gaussian 09 program suite. 48

Figure 1: Structures of several low-lying C8 H9 isomers. Standard enthalpies of formation have been calculated at the G3SX level of theory.

m/z 91 #

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wavenumber (cm-1) Figure 2: The R2P2CI spectrum at m/z = 91 (top) and m/z = 105 (bottom) obtained from a toluene discharge. The carriers are attributed to tropyl and methyltropyl radical, respectively. The sharp feature at 26053 cm−1 in the m/z = 105 channel is the putative origin band. The features marked ‘#’ are likely vibronic bands of xylyl isomers. Arrows connect features that are qualitatively similar in the two spectra, supporting our assertion that the carrier of the newly reported m/z = 105 spectrum is the methyltropyl radical; the additional complexity in the methyltropyl spectrum is conjectured to arise from transitions involving the methyl group.

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Results Non-resonant ionization mass spectra of a toluene discharge using 248 nm ionization radiation contain prominent peaks at only two odd m/z channels, 91 and 105, which could arise from radical species. Both benzyl and tropyl isomers are consistent with m/z = 91, while the peak at 105 most likely corresponds to one or more isomers with the elemental formula C8 H9 . In contrast to electron impact studies of toluene carried out at low concentration (approximately 0.1%), 49 which found evidence for fragment ions such as C5 H5+ , C4 H3+ , and C5 H3+ , no corresponding fragmentation products were observed in our studies, a possible indication that recombination to produce higher mass species occurs rapidly in our discharge source. If correct, these mass peaks should become more pronounced at lower toluene concentrations (the concentration in our experiments was close to 1%), while those corresponding to larger aromatic species, including styrene, indene, and fluorene that were observed in the original mass spectrum, should diminish in intensity. Although the m/z = 91 channel is fairly weak in our toluene discharge, subsequent R2C2PI experiments reveal that some tropyl radical is produced along with benzyl radical. These two isomers are easily distinguished by their optical spectra: they possess quite different chromophores, and consequently their origin bands are widely separated in wavelength; that for benzyl radical is around 454.5 nm, while tropyl is near 375 nm. The origin band derived from our measurements is at ∼26575 cm−1 , in good agreement with the published value of 26572 ± 4 cm−1 for tropyl. 16,50 Because the m/z = 105 channel corresponds to CH (13 amu) addition to toluene (92 amu), isomers with both tropylic and benzylic chromophores are possible. One possible incorporation of CH, via cycloaddition and subsequent ring enlargement, would lead to the methyltropyl radical, a species whose optical spectrum has not yet been reported, but which, because it contains a seven-membered cyclic π-system, should absorb (like tropyl) at wavelengths shorter than 400 nm. Motivated by the presence of some tropyl radical as the carrier of m/z = 91 in a benzene discharge, the R2C2PI spectrum of the m/z = 105 channel in a 10


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toluene discharge was recorded near 385 nm (Fig. 2). The spectrum in this wavelength region is far more intense than that found for tropyl under the same conditions, but the similarities between the two are striking, strongly suggesting that they indeed possess the same optical chromophore. Further evidence in support of the assignment is provided by the ionization energy (IE). Since two ∼365.5 nm photons were sufficient to observe a strong ion signal (at the wavelength of maximum resonant signal at m/z = 105), the carrier has an IE ≤ 6.79 eV. This upper limit is less than 10% higher than that of tropyl radical (IE = 6.23 eV), 51 presumably owing to the propensity of both to form highly stable aromatic tropylium cations via electron loss; moreover, it is significantly below the IEs of all three xylyl radicals (which all exceed 6.95 eV 52 ) and of the α-methylbenzyl radical (6.835 eV 40 ). On this basis, we attribute the stronger, sharper features in this region to methyltropyl radical, with an origin band at 26053 cm−1 . Attempts to detect fluorescence from the putative origin band were unsuccessful. Other, albeit weaker, features observed in the R2C2PI spectrum are generally characterized by broader linewidths, and plausibly arise from other isomers (indicated with ‘#’ in Fig. 2). Tropyl radical possesses a doubly degenerate ground and first excited state manifold. As such, its spectrum is plagued by complexity engendered by two Jahn-Teller active states. The tropyl spectrum has been analyzed in detail by Miller and co-workers. 50,53 Although the methyl group will break the D7h symmetry of the parent tropyl, the conical intersections will remain. As such, the spectrum is expected to be similar to that of tropyl, but with more complexity conferred by the breaking of degeneracies of vibronic states and thus fewer rigorous selection rules. Further, it is reasonable to anticipate low-frequency activity associated with the methyl group, perhaps including torsional transitions, that is absent from the tropyl spectrum. Qualitatively, this is essentially what is observed in Fig. 2. Although a more detailed analysis is ultimately needed to properly model Jahn-Teller effects and vibronic couplings, our identification of methyltropyl radical as the spectral carrier appears fairly secure at this point.

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toluene discharge m/z 105

xylyl composite

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wavenumber (cm-1) Figure 3: (top) A portion of the R2C2PI spectrum recorded at m/z = 105 in a toluene discharge. (bottom) Synthetic LIF spectrum derived from a 2.8:4.2:1 mixture of area-normalized (o,m,p)-xylyl radical spectra (see text for additional details).

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o-xylene discharge

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wavenumber (cm-1) Figure 4: A portion of the laser-induced fluorescence spectra obtained from discharges of o-, m- and p-xylene.

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To explore the possibility that still other structural isomers of C8 H9 are present in a toluene discharge, R2C2PI spectra of the m/z = 105 channel were also recorded in the wavelength region where isomers with benzylic chromophores tend to absorb (generally between 480 and 450 nm 18 ). As Fig. 3 shows, the spectrum in this region is both rich and complex. Several bands coincide in frequency with those for the origin bands of the xylyls, 43 implying that these isomers are produced as well. To assess the relative abundance of the xylyl radicals and to better understand their possible interconversion, LIF experiments starting with (o,m,p)-xylene instead of toluene were performed. The spectra obtained with each xylene precursor are shown in Fig. 4. It is apparent that more than one xylyl isomer is produced from each xylene precursor. The spectra obtained with both o- and m-xylene, for example, contain common features, and many of these are also found in the spectrum starting from p-xylene. Interestingly, there are features which appear exclusive to the p-xylene discharge, and o-xylene results in significantly more fluorescence signal compared to either m- or p- xylene. Because NMR experiments were used to confirm the high purity of the xylene precursors, and care was taken to minimize cross-contamination, these findings suggest that rearrangement occurs readily in the electrical discharge. Isomerization and rearrangement of xylyl radicals has also been observed in two recent studies. In the first, m-xylyl radical was studied by VUV synchrotron radiation and imaging photoelectron photoion coincidence spectroscopy and quantum chemical calculations by Hemberger et al. 54 In this study, p-xylylene was identified as the dominant stabilized decomposition product of m-xylyl radical, indicating that this radical undergoes complex rearrangement prior to product formation. More recently, the photoelectron spectra of the three isomeric xylyl radicals were reported along with a theoretical C8 H8 potential energy surface. 55 With the advantage that jet-cooled excitation spectra of the three xylyl radicals have been reported previously, 41,42 the spectra obtained from the (o,m,p)-xylene discharges may be

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disentangled to reveal the spectra of the individual (o,m,p)-xylyl radicals. Spectra obtained beginning with only o-, m-, or p- xylene were normalized by area in a common spectral region and treated as unit vectors for the construction of pure xylyl spectra using appropriate linear combinations, guided by the spectra reported in the literature. Indeed, the spectra of the pure xylyl isomers so obtained (Fig. 5) compare very well to those previously reported, giving us confidence in this procedure. Subsequently, with an appropriately weighted sum of the three area-normalized, individual xylyl radical spectra (2.8:4.2:1 for o:m:p), it was also possible to well-reproduce the original toluene discharge R2C2PI spectrum at m/z = 105 in the 460 nm region (Fig. 3). The differing linewidths notwithstanding (owing to differences in R2C2PI vis-a-vis LIF – the former technique involves a skimmed molecular beam, while the latter involves a free-jet expansion), essentially all of the spectral features in Fig. 3 can be accounted for by xylyl radicals. No bands attributable to α-methylbenzyl radical were found in the spectrum displayed in Fig. 3. From the weighting factors used to reconstruct the observed R2C2PI spectrum from individual xylyl radical spectra, we conclude that oand m-xylyl are a factor of several more abundant than p-xylyl in the toluene discharge, which we rationalize on the basis of quantum chemical calculations of their likely formation pathways (vide infra).

Discussion The present work appears to confirm mass spectroscopic studies of He and Sulkes 17 that CH insertion/addition reactions may play an important role in the initial steps of the breakdown of toluene, and more generally, in aromatic species. From an analysis of the discharge products of toluene by R2C2PI, we find evidence for the spectroscopic detection of the hitherto unidentified methyltropyl radical, specifically implicating 7-membered carbon ring products in decomposition. Detection of tropyl radical starting from benzene using a similar discharge source to our own by Pino et al. 16 is consistent with these findings. Reactions of CH with

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H CH3

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H

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wavenumber (cm-1) Figure 5: Laser-induced fluorescence spectra of o-, m- and p-xylyl radicals inferred from decomposition of spectra recorded for discharges of o-, m- and p-xylene (Fig. 4.

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small alkenes have been well-studied (for a recent review, see ref. 56 ) and are generally fast and barrierless, proceeding either by addition of CH to a single carbon, or by cycloaddition to yield a 3-carbon atom intermediate, with an apparent preference for the latter pathway. 57 Concerning cyclic alkenes, pyridine has been unambiguously identified by photoionization mass spectrometry as the product of the reaction of pyrrole with CH, ostensibly arising from cycloaddition of CH and subsequent ring-enlargement; 58 and, although the product was not spectroscopically identified, the reaction of CH with benzene has been measured to proceed at nearly the encounter rate. 38 To firmly establish that CH is a discharge product from benzene in our experiments, two-dimensional fluorescence spectroscopy has also been undertaken in the Sydney laboratory. In addition to C2 , C3 , and phenylpropargyl (C9 H7 ), 37 strong lines of CH were found, confirming that this radical is indeed present in high abundance. Because xylyl radicals are formed in the toluene discharge, understanding their decomposition and interconversion is necessary to develop a more complete model of toluene decomposition. Being RSRs, it is well-established that xylyl radicals play a pivotal role in the decomposition of xylenes. Due to their unusual stability, the currently accepted model for the decomposition of xylyl radicals in combustion is via loss of a second hydrogen to form xylylenes, as opposed to oxidation. 59 The relative stability of m-xylyl radical towards decomposition (81.3 kcal/mol compared to 74.1 kcal/mol and 70.5 kcal/mol for the ortho and para isomers, respectively 60 ) has been linked to the slower rate of combustion of m-xylene compared to o- and p-xylenes. 59 The stability of m-xylyl radical has, in turn, been linked to the high-energy barriers which need to be overcome to form m-xylylene, 61 which is a biradical triplet in its ground state. 62 The present detection of four C8 H9 radical isomers raises the possibility that methyltropyl radical is a source of the three xylyl radical isomers detected in our toluene discharge. This motivated us to conduct theoretical calculations to develop a potential energy surface connecting these four key C8 H9 isomers (Fig. 6). Like tropyl radical, once stabilized, methyltropyl radical is a highly stable isomer on the C8 H9 surface, with quite significant barriers to

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rearrangement. Compared to the equivalent pathway from tropyl to benzyl radical, however, the barrier from methyltropyl to any of the o- and p-xylyl radicals or to α-methylbenzyl (the latter in Fig. S1 in the Supporting Information) is approximately 5-10 kcal/mol lower in energy, owing to a 1, 2-hydrogen shift of the methyl hydrogen (barrier heights are indicated in Fig. 6). Once this barrier is surmounted, the hydrogen can facilely move within the C7 ring, enabling the formation of different benzylic isomers. Furthermore, barriers for decomposition of methyltropyl radical, to products such as cyclopentadienyl radical + methylacetylene and cyclohexadienyl radical + acetylene (Fig. S2), again requires barriers greater than those for rearrangement to benzylic radical isomers. A still higher isomerization barrier between tropyl and benzyl radicals is qualitatively consistent with the experimental findings of Pino et al., 16 who identified substantially more tropyl radical compared to benzyl radical in a benzene discharge. These authors also noted that tropyl was not made in a toluene discharge, although it was observed in low abundance in the present investigation. The present calculations also provide insight into the relative abundance of the three xylyl isomers. Although the barriers to rearrangement from methyltropyl radical to each xylyl are essentially identical, with the exception of the m-isomer, as shown in Fig. 6, the stabilities of the corresponding xylylenes vary significantly. 55 Formation of p-xylylene, for example, is exothermic by nearly 12 kcal/mol relative to the highest barrier for rearrangement from methyltropyl, while it is closer to thermoneutral for o-xylylene (−5.6 kcal/mol); for mxylylene, rearrangement is distinctly endothermic by more than 8 kcal/mol. 39,55 The higher barrier to formation of m-xylyl (by an additional ∼ 7 kcal/mol) compared to the o- or pisomers arises because rearrangement cannot undergo a 1, 2-hydrogen shift in the initial step, and instead must go through the tropyl to benzyl-type mechanism. The trend in exothermocities suggests that p-xylyl should be less abundant than either the o- or misomers, both of which have much higher exit channel barriers to H-loss. Our observations support the basic thrust of this argument: an electrical discharge of o-xylene produces o- and m-xylyl but no detectable p-xylyl, while m-xylene produces m- and o-xylyl but no p-xylyl;

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Figure 6: Calculated pathways for interconversion from methyltropyl to the three xylyl radicals. The calculated exothermicity of the CH + toluene entrance channel, relative to methyltropyl, is 95.4 kcal/mol. Each pathway also includes the barrier for entry into the C8 H8 surface through the xylylene analogues. 55 Pathways were calculated using the G3SX composite model chemistry. 47 Energies are reported in kcal/mol.

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p-xylene produces all three isomers. The absence of α-methylbenzyl radical can be understood by an analogous argument, supported by the theoretical calculations. As indicated in Fig. S1 of the SI, stabilization of this isomer is unfavorable energetically from methyltropyl because subsequent H-loss to form styrene, a highly stable product, is calculated to be exothermic by almost 30 kcal/mol, and there are no appreciable barriers along this reaction pathway. A prominent peak at m/z 104 was in fact detected when the non-resonant mass spectrum of a toluene discharge was recorded using 248 nm radiation during our calibration experiments. The R2C2PI spectrum recorded near 280 nm is in very good agreement with those previously reported for styrene, 63 confirming that this isomer is also present in a toluene discharge. Importantly, the process with lowest overall barrier for rearrangement and decomposition of methyltropyl radical following the initial 1,2 H-shift (including several not proceeding through xylyl or α-methylbenzyl radicals - see Figures S2-S4)) leads to styrene; this may lead to a point of departure in molecular weight growth schemes proceeding via tropyl vs. methyltropyl. Indeed, it is possible to conjecture that combustion pathways that conventionally go through tropyl could be significantly different when a methyl group is added. For example, while the cyclopentadienyl + acetylene reaction is a growth step that stops at tropyl, 29 the cyclopentadienyl + methylacetylene or methylcyclopentadienyl + acetylene reactions might proceed facilely to methyltropyl decomposition to produce styrene – a substituted aromatic species that can participate in hydrogen abstraction–acetylene addition chemistry.

Conclusions Using mass-selective laser spectroscopy, the C8 H9 radical products from a toluene discharge have been studied in the near-UV and visible wavelength regions, with the goal of better understanding structural isomerization of species that plausibly result from CH insertion. Although a detailed spectral assignment awaits analysis of quite complicated Jahn-Teller

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effects, strong spectroscopic evidence has been found for detection of methyltropyl radical. The three xylyl isomers are also produced under the same experimental conditions, but αmethylbenzyl radical is not detected. The relative abundance of each methyl benzyl isomer in the toluene discharge has been estimated by analogous studies using the corresponding xylene parent. Theoretical calculations starting with methyltropyl radical provide a qualitative framework for rationalizing the observed abundances of the xylyls as well as the absence of α-methylbenzyl, thereby suggesting that this seven-membered radical ring may play a central role in the decomposition of toluene. Further studies of other aromatic precursors are needed to confirm the generality of ring-expansion reactions that putatively proceed by CH insertion, as well as the importance of tropyl-like intermediates.

Acknowledgement This research was supported under the Australian Research Council’s Discovery Projects funding scheme (DP120102559). M.C.M. thanks the Harvard Club of Australia for financial support for an extended visit to the University of Sydney. T.W.S. and G.S. acknowledge the Australian Research Council for Future Fellowships (FT130100177 and FT131010304, respectively).

Supporting Information Available Potential energy surfaces for: interconversion of methyltropyl and styrene, and decomposition of methyltropyl into: acetylene + methylcyclopentadienyl, and methylacetylene + cyclopentadienyl; cyclooctatetraene, cyclohexadienyl + acetylene, and vinylcyclohexadienyl; and benzocyclobutene and 2-phenylethyl radical. This material is available free of charge via the Internet at http://pubs.acs.org.

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(36) Ramphal, I. A.; Shapero, M.; Haibach-Morris, C.; Neumark, D. M. Photodissociation dynamics of fulvenallene and the fulvenallenyl radical at 248 and 193 nm. Phys. Chem. Chem. Phys. 2017, 19, 29305–29314. (37) Reilly, N. J.; Kokkin, D. L.; Nakajima, M.; Nauta, K.; Kable, S. H.; Schmidt, T. W. Spectroscopic observation of the resonance-stabilized 1-phenylpropargyl radical. J. Am. Chem. Soc. 2008, 130, 3137–3142. (38) Hamon, S.; Picard, S. D. L.; Canosa, A.; Rowe, B. R.; Smith, I. W. M. Low temperature measurements of the rate of association to benzene dimers in helium. J. Chem. Phys. 2000, 112, 4506–4516. (39) da Silva, G.; Moore, E. E.; Bozzelli, J. W. Decomposition of methylbenzyl radicals in the pyrolysis and oxidation of xylenes. J. Phys. Chem. A 2009, 113, 10264–10278. (40) Kidwell, N. M.; Reilly, N. J.; Nebgen, B.; Mehta-Hurt, D. N.; Hoehn, R. D.; Kokkin, D. L.; McCarthy, M. C.; Slipchenko, L. V.; Zwier, T. S. Jet-cooled spectroscopy of the alpha-methylbenzyl radical: probing the state-dependent effects of methyl rocking against a radical site. J. Phys. Chem. A 2013, 117, 13465–13480. (41) Fukushima, M.; Obi, K. Jet spectroscopy and excited state dynamics of benzyl and substituted benzyl radicals. J. Chem. Phys. 1990, 93, 8488–8497. (42) Lin, T.-Y. D.; Tan, X.-Q.; Cerny, T. M.; Williamson, J. M.; Cullin, D. W.; Miller, T. A. High-resolution fluorescence excitation spectra of jet-cooled benzyl and p-methylbenzyl radicals. Chem. Phys. 1992, 167, 203 – 214. (43) Selco, J.; Carrick, P. Jet-Cooled Emission Spectra of the Xylyl Radicals. J. Mol. Spectrosc. 1995, 173, 277 – 295. (44) Kokkin, D.; Reilly, N.; McCarthy, M.; Stanton, J. Experimental and theoretical investigation of the electronic transition of CuSH. J. Mol. Spectrosc. 2011, 268, 23–27. 26


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(54) Hemberger, P.; Trevitt, A. J.; Ross, E.; da Silva, G. Direct observation of para-xylylene as the decomposition product of the meta-xylyl radical using VUV synchrotron radiation. J. Phys. Chem. Lett. 2013, 4, 2546–2550. (55) Hemberger, P.; Trevitt, A. J.; Gerber, T.; Ross, E.; da Silva, G. Isomer-specific product detection of gas-phase xylyl radical rearrangement and decomposition using VUV synchrotron photoionization. J. Phys. Chem. A 2014, 118, 3593–3604. (56) Trevitt, A. J.; Goulay, F. Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection. Phys. Chem. Chem. Phys. 2016, 18, 5867–5882. (57) Goulay, F.; Trevitt, A. J.; Meloni, G.; Selby, T. M.; Osborn, D. L.; Taatjes, C. A.; Vereecken, L.; Leone, S. R. Cyclic versus linear isomers produced by reaction of the methylidyne radical (CH) with small unsaturated hydrocarbons. J. Am. Chem. Soc. 2009, 131, 993–1005. (58) Soorkia, S.; Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Trevitt, A. J.; Wilson, K. R.; Leone, S. R. Direct detection of pyridine formation by the reaction of CH (CD) with pyrrole: a ring expansion reaction. Phys. Chem. Chem. Phys. 2010, 12, 8750–8758. (59) da Silva, G.; Moore, E.; Bozzelli, J. Decomposition of methylbenzyl radicals in the pyrolysis and oxidation of xylenes. J. Phys. Chem. A 2009, 113, 10264–10278. (60) Fernandes, R. X.; Gebert, A.; Hippler, H. The pyrolysis of 2-, 3-, and 4-methylbenzyl radicals behind shock waves. Proc. Combust. Inst. 2002, 29, 1337 – 1343. (61) Johnston, R.; Farrell, J. Laminar burning velocities and markstein lengths of aromatics at elevated temperature and pressure. Proc. Combust. Inst. 2005, 30, 217–224. (62) Steglich, M.; Custodis, V. B. F.; Trevitt, A. J.; da Silva, G.; Bodi, A.; Hemberger, P.

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Photoelectron spectrum and energetics of the meta-xylylene diradical. J. Am. Chem. Soc. 2017, 139, 14348–14351. (63) G¨ uthe, F.; Ding, H.; Pino, T.; Maier, J. Diagnosis of a benzene discharge with a massselective spectroscopic technique. Chem. Phys. 2001, 269, 347–355.

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