Unveiling New Isomers and Rearrangement Routes on the C₇H₈⁺ Potential Energy Surface

Abstract

The unimolecular reactions of C7H8 + 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 C7H8 + 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 C7H8 + species. The isomerization route from the norbornadiene radical cation to the toluene radical cation, which competes with isomerization to the bicyclo[2.2.1]­hepta-2-ene-5-yl-7-ylium radical cation, is identified. Further, this work expands understanding of the C7H8 + 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 C7H8 + species.

Vacuum ultraviolet photoionization and dissociative photoionization of toluene: Experimental and theoretical insights

“…The structure of the C 7 H 7 + ion has been investigated by various methods. 15,25,27,40–47 C 7 H 7 + ion has two important isomers: benzyl (six-membered ring) and tropylium cation (seven-membered ring), 15,25,27,43–46 and toluene ion can generate benzyl and tropylium cation via two competitive pathways. 15,47 From the PIES of C 7 H 7 + (Figure 4(a)), there are two AEs of C 7 H 7 + : the higher AE of C 7 H 7 + is 11.17 ± 0.03 eV, the other is 10.71 ± 0.03 eV.…”

“…Obviously, another C 7 H 7 + ion is tropylium cation (P2) in Figure 4(a), and the formation process for P2 is somewhat complicated, 15,25,27 which involves several TSs and INTs, as shown in Figure 8. First, the hydrogen atom H15 of the methyl group is transferred to the C3 of the benzene ring to form INT1 via TS1.…”

“…The experimental and theoretical studies on the photoionization and dissociative photoionization of toluene have been performed by many groups. 11–33 By using photoionization mass spectrometry (PIMS), Traeger and Mcloughlin 11 and McLoughlin et al 16 obtained the ionization energy (IE) of toluene and the appearance energy (AE) of C 7 H 7 + , which are 8.82 and 10.71 eV, respectively. Schwell et al 12 obtained the IE of the parent molecule and the AEs of the C 7 H 7 + , C 6 H 5 + , C 5 H 6 + , C 5 H 5 + , C 4 H 5 + , C 4 H 4 + and C 4 H 3 + , and estimated the AE of C 7 H 7 + to be 11.3 eV according to the photoelectron photoion coincidence spectroscopy (PEPICO) measurements of Bombach et al 13,14 Torma et al 15 also used the same means to obtain the formation mechanism for the tropylium and benzyl cations from toluene cations.…”

“…But only the theoretical AEs of the C 7 H 7 + (11.5 eV), 22 C 6 H 5 + (13.3 eV) and C 5 H 6 + (13.6 eV) 24 were given, respectively. In particular, the structure of the C 7 H 7 + has been investigated extensively by various means: 15,25–27 Jacovella et al 26 studied electronic spectrum of the tropylium by resonance-enhanced photodissociation wherein the dissociation mechanism of C 7 H 7 + from toluene ion was studied in detail, including interconversion of two parent ions and two isomers of C 7 H 7 ions. 15,25,27 C 5 H 5 + , a secondary product generated from the formation of C 7 H 7 + , was reported, 28–30 but the dissociation mechanism has not been fully understood.…”

“…In particular, the structure of the C 7 H 7 + has been investigated extensively by various means: 15,25–27 Jacovella et al 26 studied electronic spectrum of the tropylium by resonance-enhanced photodissociation wherein the dissociation mechanism of C 7 H 7 + from toluene ion was studied in detail, including interconversion of two parent ions and two isomers of C 7 H 7 ions. 15,25,27 C 5 H 5 + , a secondary product generated from the formation of C 7 H 7 + , was reported, 28–30 but the dissociation mechanism has not been fully understood. For C4 series fragments, the dissociation channel and dissociation mechanism have not been further studied, nor have theoretical analysis been done.…”