Unveiling New Isomers and Rearrangement Routes on the C7H8+

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Unveiling New Isomers and Rearrangement Routes on the CH Potential Energy Surface 7

8+

Ugo Jacovella, Gabriel da Silva, and Evan John Bieske J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10642 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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The Journal of Physical Chemistry

Unveiling New Isomers and Rearrangement Routes on the C7H+ 8 Potential Energy Surface Ugo Jacovella,† Gabriel da Silva,‡ and Evan J. Bieske∗,† †School of Chemistry, The University of Melbourne, Victoria 3010, Australia ‡Department of Chemical Engineering, The University of Melbourne, Victoria 3010, Australia E-mail: [email protected]

Abstract The unimolecular reactions of C7 H+ 8 radical cations are among those most studied by mass spectrometry, especially the rearrangement of toluene and cycloheptatriene molecular ions, which are directly connected to the formation of benzylium and tropylium cations. This study reveals important new isomers and isomerization pathways on the C7 H+ 8 potential energy surface, through the application of gas-phase electronic photodissociation spectroscopy in conjunction with ab initio calculations. Presented are the first gas-phase vibrationally resolved electronic spectra of the o-isotoluene, norcaradiene, bicyclo[3.2.0]hepta-2,6-diene radical cations, and ring-opened products from cyclic C7 H+ 8 species. The isomerization route from the norbornadiene radical cation to the toluene radical cation, which competes with isomerization to the bicyclo[2.2.1]hepta2-ene-5-yl-7-ylium radical cation is identified. Further, this work expands understanding of the C7 H+ 8 potential energy surface by connecting spiro[2.4]hepta-4,6-diene and acyclic 1,2,4,6-heptatetraene radical cations, and confirms the important role of the o-isotoluene radical cation in the interconversion pathways of C7 H+ 8 species.

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Introduction The gas-phase interconversion of toluene (1+ ) and cycloheptatriene (5+ ) radical cations is of fundamental interest in organic chemistry, and is perhaps the most studied unimolecular reaction in all of mass spectrometry. 1–22 Interest in the rearrangement routes of the C7 H+ 8 isomers has also been motivated by a desire to understand the formation of two C7 H+ 7 isomers, namely the intriguing tropylium (Tr+ ) and benzylium (Bz+ ) cations. 8,23–33 The chemistry of + these C7 H+ 7 species is intimately linked to the C7 H8 potential energy surface, as the internal + + energy required to drive the C7 H+ 8 interconversion from 1 , which is the most stable C7 H8

isomer, to 5+ is only slightly less than the energy required to lose an H atom and form Tr+ or Bz+ . The important species involved in the interconversion of 1+ and 5+ are shown in Scheme 1.

Scheme 1: Important species involved in interconversion of 1+ and 5+ . Information on the C7 H+ 8 interconversion routes stems from three main sources: (i) matrixisolation and gas-phase spectroscopy studies, 3,7,9,12,16,20 (ii) mass spectrometry, 1,2,4,6,10,11,13,15,17–19,21,26 and (iii) theoretical investigations. 8,21,22,24,27–29 The matrix-isolation studies point to the central importance of the o-isotoluene cation (3+ ), and possibly the norcaradiene cation (4+ ), in the interconversion routes of the C7 H+ 8 isomers. The mass spectrometry studies generally agree that the 1+ /5+ rearrangement mechanism is a multi-step process involving 2+ , 3+ and 4+ . They also reveal that the product ratio of Tr+ /Bz+ is strongly correlated to the energy initially deposited into the system. Moreover, by comparing results obtained by different experimental techniques it is clear that the fragmentation reactions of 1+ and 5+ exhibit

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different temporal behavior. 27,28 All these studies are supported by theoretical calculations which have revealed multi-step mechanisms for 1+ /5+ interconversion. 8,21,22,24,27–29 Experimental and theoretical studies have reached the conclusion that two different pathways contribute to interconversion between 1+ and 5+ (see Scheme 2).

Scheme 2: Interconversion routes between 1+ and 5+ . Mechanism 1, known as the Hoffman mechanism, 5 involves an initial [1,2]-H shift whereby a methyl hydrogen migrates to the ipso-position on the ring to form the distonic benzenium cation (2+ ), followed by a ring closure to 4+ , finally forming 5+ by ring expansion. Mechanism 2 shows two paths, both involving 3+ . The one directly connecting 1+ and 3+ originally proposed by Dewar and Landman, 8 consists of a [1,3]-H shift, whereby the methyl hydrogen migrates to the ortho-position to form 3+ , followed by an intramolecular cyclization to form the transient distonic radical cation 6+ , which readily undergoes ring expansion to produce 5+ . However, quantum chemical calculations 22 have shown that it is more favorable to first form 2+ and then 3+ than to directly convert 1+ to 3+ . According to ab initio calculations the Hoffman mechanism is energetically slightly preferred over the Dewar-Landman mechanism. Another important unimolecular reaction on the C7 H+ 8 surface is the facile isomerization 3



of quadricyclane radical cation (8+ ) to norbornadiene radical cation (7+ ). This reaction is considered as a benchmark system for one-electron oxidation reactions and therefore has been intensively investigated 34–40 and received special attention due to its potential solarenergy storage capability. 41,42 In several studies of these reactions, 36,38,43,44 additional radical cation species have been experimentally observed such as bicyclo[2.2.1]hepta-2-ene-5-yl-7ylium (BHE, 9+ ), bicyclo[3.2.0]hepta-2,6-diene (BHD, 10+ ) and 5+ . The key isomers are depicted in Scheme 3.

Scheme 3: Key isomers for conversion of quadricyclane radical cation (8+ ) to norbornadiene radical cation (7+ ). Several rearrangement pathways for 8+ to 5+ have been identified through ab initio calculations. 45–48 One interconversion route starts with the formation of 9+ and/or 10+ prior to forming 5+ . The other route, which is the energetically most favorable, is such that rearrangement from 8+ to 5+ occurs via 4+ . To our knowledge, no gas-phase experimental study has been previously conducted to verify these mechanisms. Despite the abundance of experimental and theoretical work on the 1+ to 5+ interconversion in the gas phase, some aspects remain unclear or only speculative. In this work, we present gas-phase vibronically resolved spectra of several C7 H+ 8 isomers recorded by monitoring the photofragmentation of weakly bound C7 H+ 8 –Ar complexes. Different precursors have been used to probe different regions of the potential energy surface. Excess internal energy was provided by the electron bombardment used to generate the ions and the timescale of any interconversion processes in our experiment was a few µs. In the course of the work we investigated parts of the C7 H+ 8 potential energy surface that have been unexplored or overlooked and spectroscopically identified new isomers in the C7 H+ 8 interconversion routes. 4


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Experimental section The electronic spectra of C7 H+ 8 were recorded by resonance-enhanced photodissociation of + C7 H+ 8 –Ar complexes in a tandem mass spectrometer by monitoring C7 H8 photofragments.

The experimental arrangement has been described in detail previously 31,49–51 and will only be briefly outlined here. The C7 H+ 8 ions were produced by intersecting an electron beam with a pulsed supersonic expansion of Ar gas seeded with one of five neutral precursors: norbornadiene (NBD), 2phenylethanol (PhEtOH), cycloheptatriene (CHT), spiro[2.4]hepta-4,6-diene (SPIRO), and toluene (TOL). The C7 H+ 8 –Ar complexes were created through three-body collisions in the supersonic expansion, which restricted C7 H+ 8 interconversion processes to occur in less than a few µs. The target ions (C7 H+ 8 –Ar) were mass-selected by a quadrupole mass filter, deflected through 90◦ by a quadrupole bender, and passed into an octopole ion guide, where they were exposed to light from a tunable optical parametric oscillator (OPO). Photofragmentation occurred when the light frequency was resonant with a vibronic transition of the weakly bound cluster. Photofragment ions were mass selected by a second quadrupole mass filter and collected by an ion detector. An action spectrum representative of the cluster’s absorption spectrum was then obtained by monitoring the Ar loss channel (C7 H+ 8 ions) as a function of the laser wavenumber.

Results and Discussion An overview of the electronic photodissociation spectra of the weakly bound C7 H+ 8 –Ar complexes obtained using different precursors is depicted in Fig. 1. Before discussing possible isomerization routes of the C7 H+ 8 species, we first present the assignments of the isomers responsible for the observed vibronic transitions arising from each precursor. These results are summarized in Table 1. The assignments of the different isomers are discussed in order of ascending wavenumber of the observed transitions. All transitions correspond to either A5



Figure 1: Overview of the photodissociation electronic spectra of C7 H+ 8 –Ar using the precursors shown on the left. The transitions are detected by monitoring the C7 H+ 8 photoproducts from C7 H+ –Ar as a function of laser wavenumber. 8 type transitions, which consist of promotion of an electron from the highest doubly occupied molecular orbital to the singly occupied molecular orbital (SOMO) or B-type transitions, which involve promotion of an electron from the SOMO to unoccupied molecular orbitals (in our case to the lowest unoccupied molecular orbital (LUMO)). Therefore the assignments for transitions which have not been observed previously are based on qualitative arguments founded on similarities with the π system chromophore of conjugated and nonconjugated diene and polyene radical cations. 52–54

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Table 1: Observed transition energies for C7 H+ 8 cations for each precursor and corresponding assignments. Wavenumber given in parentheses denotes vibrational intervals from the respective origin band. Wavenumber / cm−1 Precursora 15 655 TOL, NBD, CHT, SPIRO, PhEtOH 16 215 (560) TOL, NBD, CHT, SPIRO, PhEtOH 16 390 (735) TOL, NBD, CHT, SPIRO, PhEtOH 23 580 CHT, SPIRO + BHE (10 ) 16 825 NBD BHE (10+ ) 17 410 (585) NBD heptatetraene (12+ )c 17 555 NBD, PhEtOH, SPIRO heptatetraene (12+ )c 17 665 NBD, PhEtOH, SPIRO + c heptatetraene (12 ) 17 825 NBD, PhEtOH, SPIRO + c heptatetraene (12 ) 18 010 NBD, PhEtOH, SPIRO heptatetraene (12+ )c 18 225 NBD, PhEtOH, SPIRO + c heptatetraene (12 ) 18 325 NBD, PhEtOH, SPIRO + norcaradiene (4 ) 19 545 TOL, NBD, SPIRO, PhEtOH 20 060 (515) TOL, NBD, SPIRO, PhEtOH 20 600 (1055) TOL, NBD, SPIRO, PhEtOH 21 105 (1560) TOL, NBD, SPIRO, PhEtOH + b cycloheptatriene (6 ) 21 050 CHT + b toluene (1 ) 23 070 TOL, NBD, PhEtOH Precursors: norbornadiene (NBD), 2-phenylethanol (PhEtOH), cycloheptatriene (CHT), spiro[2.4]hepta-4,6-diene (SPIRO), and toluene (TOL). b Band maximum. c Tentative assignment to steroisomers of the 1,2,4,6-heptatetraene radical cation. Assignments o-isotoluene (3+ )

a

Results An expanded view of the lower-wavenumber region showing the corresponding C7 H+ 8 assignments is presented in Fig. 2. The first three sharp bands at 15 655, 16 215 and 16 390 cm−1 appearing in all spectra agree with previous matrix-isolation spectroscopy studies 12,16 and correspond to transitions of 3+ . This is therefore the first observation of vibrationally resolved electronic transitions in the gas phase of 3+ . The next two sharp transitions, at 16 825 and 17 410 cm−1 , are present only when using NBD as a precursor. Those features are assigned to 10+ , based on absorption spectra of radical cations with a similar chromophore and the fact that these transitions are observable only starting with NBD (see discussion about isomerization routes below). The band at

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Figure 2: Expanded view of the lower wavenumber region of the photodissociation electronic spectrum of C7 H+ 8 –Ar obtained using the precursors shown on the left. 16 825 cm−1 was previously assigned to 3+ , 12 however from the comparison of the band intensities between the spectra recorded with different precursors, it is clear that this transition cannot belong to species 3+ . The spectrum obtained using NBD as a precursor then shows a group of sharp, resolved features (17 555 to 18 325 cm−1 ) which also appear as an unresolved envelope in the spectra recorded with PhEtOH and SPIRO as precursors. The spacings between these bands is 100 – 200 cm−1 , making it unlikely that they are vibrational progressions arising from just

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one isomer. The absorption region of these features corresponds to the chromophore of a conjugated triene with one ring, or an acyclic polyene, and we make a tentative assignment as 1,2,4,6-heptatetraene, 12+ (vide infra). The series of closely spaced bands could be associated with the presence of several stereoisomers (either rotational or geometric) as is the case, for instance, with the hexatriene radical cation. 55–57 Another sharp band system is observed in all spectra except when using CHT as a precursor. These bands observed at 19 545, 20 060, 20 600 and 21 105 cm−1 are assigned to 4+ . This assignment is supported again by observed transitions in similar systems and by a matrix-isolation spectroscopy study, 12 where a transition observed at 19 940 cm−1 was tentatively assigned as the origin band of 4+ . We next consider the higher wavenumber region of the spectra, for which an expanded view with the corresponding C7 H+ 8 assignments is shown in Fig. 3. The region from 20 000 to 25 000 cm−1 is dominated by two strong, broad features with maxima at 21 050 and 23 070 cm−1 corresponding to 5+ and 1+ , respectively. The broad, intense band of 5+ is only observed starting from CHT, whereas that of 1+ is observed in spectra starting with NBD, PhEtOH and TOL. The reasons for these assignments are outlined in the discussion section below. The isolated band at 23 580 cm−1 in the spectra of CHT and SPIRO is assigned to the transition to the next excited state of 3+ based on previous works. 12,16 This transition is overlapped by the strong toluene cation transition, which explains why it is only observable in the spectra starting with CHT and SPIRO precursors, although the first transition is observed in all spectra.

Discussion A theoretical potential energy surface, shown in Fig. 4, has been developed to aid in the + interpretation of the C7 H+ 8 spectra. Here all energies are relative to 1 , the global minimum.

This surface includes the major pathways identified in previous theoretical studies, along 9



Figure 3: Expanded view of the higher wavenumber region of the photodissociation electronic spectrum of C7 H+ 8 –Ar using several precursors. with a number of important new reaction steps and isomers reported here for the first time. Bold arrows are used in Fig. 4 to indicate pathways from 1+ with energies below that for dissociation to Tr+ + H. Note that a number of reaction pathways with prohibitively high barriers were identified during the course of this study, and are omitted from the potential energy surface shown. Moreover, this surface focuses on pathways connecting the + experimentally observed C7 H+ 8 species, and is not meant as a comprehensive search for C7 H8

structures such as has been carried out for the sec-butyl cation. 58 Calculations were performed using the composite G3X-K theoretical method, which has 10


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been developed for accurate thermochemical kinetics, and is expected to provide barrier heights with an average uncertainty of less than 0.7 kcal/mol. 59 All calculations were carried out using Gaussian 16. 60 Optimized structures for C7 H+ 8 minima and transition states are provided as Supporting Information. Electronic energies are also listed as Supporting Information, along with energies at the M06-2X/6-31G(2df,p) level. The mean unsigned deviation between the G3X-K and M06-2X energies is 1.9 kcal/mol. The Supporting Information also includes reaction energies for H atom loss in each of the C7 H+ 8 isomers included in the potential energy surface, providing an indication of the potential for C7 H+ 7 carbocation formation. A striking result of this and prior studies is that o-isotoluene (3+ ) is a common inter+ mediate in C7 H+ 8 rearrangements. Unsurprisingly PhEtOH is a good 3 precursor, via the

McLafferty rearrangement, 4,7,11 but we also observe this species from the precursors TOL, NBD, CHT, and SPIRO. Fig. 4 shows that the SPIRO cation 11+ can isomerize to 3+ via a series of rearrangements with highest barrier of 17.9 kcal/mol. Moreover, this reaction channel does not require 1+ or 5+ as intermediates and transpires at below the H dissociation thresholds to Bz+ and Tr+ . Starting at 1+ or 5+ we also see that these cations can isomerize to 3+ at below their dissociation thresholds, explaining their ability to form this isomer. Finally, we also observe 3+ from NBD in a process first involving the isomerization of 7+ to 2+ , where the forward barrier height is 27.5 kcal/mol. This would provide the + C7 H+ 8 cation population with sufficient energy to isomerize to 3 , some fraction of which

survives on the timsescale of the experiment. However, both pathways leading from 7+ proceed through transition states at above the dissociation energy of 1+ and 5+ , and the observation of isomerization products from the norbornadiene precursor appears to require some stabilization of transient intermediates. Note that NBD also resulted in a strong C7 H+ 7 signal, as also witnessed by Dunbar and Fu, 3 consistent with stabilized isomer formation being a minor channel. Of the four precursors that result in the formation of 3+ , all but TOL and CHT also yield

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Figure 4: Theoretical potential energy surface for C7 H+ 8 isomerization and dissociation. All + energies are relative to the toluene cation 1 , in kcal/mol, calculated at the G3X-K level of theory. Bold arrows indicate pathways connected to 1+ with energies below the dissociation threshold to Tr+ + H. the conjugated polyene species observed between 17 555 and 18 325 cm−1 . As discussed, these bands likely arise from a set of related rotational or geometric isomers, and we are unable to make a definitive assignment here. Likely carriers of these signals, however, are the open-chain 1,2,4,6-heptatetraene cation stereoisomers. As shown in Fig. 4, 3+ can directly ring open to produce a 1,2,4,6-heptatetraene cation, where the transition state energy is 59.6 kcal/mol relative to the toluene cation global minima. The most stable "all trans" form (shown in Fig. 4), 12+ , sits at 41.0 kcal/mol above the toluene cation (but also with

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appreciably higher entropy), with another three stereoisomers lying within 3.4 kcal/mol of it. A further four forms of 12+ sit yet higher in energy, at between 5.4 and 10.5 kcal/mol above the all trans isomer. The inability of 1+ and 5+ to form 12+ is explained by their dissociation at the required energies on the timescale of the experiment. The three precursors that lead to purported formation of the 1,2,4,6-heptatetraene cations are also observed to form the norcaradiene cation, 4+ . TOL also forms this species, but it is not detected starting with CHT. A set of products included in Fig. 4 are only accessible via 5+ and 7+ , although are not expected from the former given that key isomerization barriers sit above the Tr+ + H threshold. This includes 10+ , which can form from 7+ via ring opening to a five-membered ring distonic radical cation followed by ring closing to this bicyclic species. The barrier for initial ring opening is competitive with that for six-membered ring formation (2+ ), whereas the subsequent transition state to produce 10+ is similar in energy to the barrier for dissociation to C5 H+ 5 + acetylene and for isomerization to a series of stable vinyl-cyclopentadiene (13+ , 14+ ) and methylfulvene (15+ ) radical cations. Of these, 13+ is the most stable at 11.5 kcal/mol above 1+ - indeed it is the most stable C7 H+ 8 cation behind toluene and ortho-/paraisotoluene - and has not been previously reported. This species also has a conjugated triene structure and may contribute to the absorption bands detected at 17 555 to 18 325 cm−1 . Finally, it is of interest to note that we do not appear to observe interconversion of the toluene and cycloheptatriene cations in these experiments, with CHT being the only precursor that yields 5+ . This is consistent with isomerization barriers that approach the H dissociation threshold for 5+ , and reveals that cations formed with sufficient energy to rearrange to 5+ tend to further dissociate to tropylium cations within the timescale of the electron impact ionization stage of the instrument. Moreover, the only C7 H+ 8 isomer other than 5+ produced from the precursor CHT is 3+ , further highlighting the fragility of 5+ . It is somewhat unexpected that CHT forms 3+ and not 1+ , although it is possible that the later is present in small quantities, with its spectral signature obscured by the broad 5+

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band. In contrast to 5+ , the toluene cation is formed by a number of precursors, namely NBD, PhEtOH, and of course TOL, exemplifying its higher stability and dissociation barrier compared to its seven-membered ring counterpart.

Conclusions Photodissociation spectroscopy of weakly bound C7 H+ 8 –Ar complexes has been used to identify different C7 H+ 8 isomers formed following electron impact ionization of norbornadiene, phenylethanol, cycloheptatriene, spiro[2.4]hepta-4,6-diene, and toluene. The spectra reveal the presence of species which have not been detected in the gas phase such as norcaradiene and bicyclo[3.2.0]hepta-2,6-diene radical cations. Vibronic transitions associated with the o-isotoluene radical cation are observed in spectra recorded with all five different precursors, revealing that this species plays a key role in the isomerization route of the C7 H+ 8 isomers and might be an important intermediate in combustion and interstellar chemistry. Furthermore, this paper connects the previous work carried out on the interconversion of toluene ↔ cycloheptatriene radical cations and norbornadiene ↔ cycloheptatriene radical cations. It also reveals new sections of the C7 H+ 8 potential energy surface, explaining how the spiro[2.4]hepta-4,6-diene radical cation connects to the o-isotoluene radical cation as well as the C7 H+ 8 ring-opening mechanism via the o-isotoluene radical cation. Ultimately, this study indicates that the C7 H+ 8 potential energy surface is more complex than previously thought, with the discovery of new energetically low-lying isomers and interconversion pathways. These features need to be considered for global understanding of C7 H+ 8 isomerization and subsequent fragmentation to benzylium and tropylium radical cations.

Acknowledgement This work was supported by the Australian Research Council Discovery Project (DP150101427, DP160100474) and Future Fellowship (FT130101304) schemes. U. Jacovella acknowledges 14


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support from the Swiss National Science Foundation (P2EZP2_178429).

Supporting Information Available Optimized structures and electronic energies for C7 H+ 8 minima and transition states. Reaction enthalpies for H atom loss in C7 H+ 8 isomers. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Unveiling New Isomers and Rearrangement Routes on the C7H8+

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