Isomerization of 1,6-dimethylcyclohexadienyl cations obtained in the gas phase from insertion of ortho-tolylium ions into methane at 1-3 atm pressure. Does it involve excited or thermal species?

1,6-Dimethylcyclohexadienyl cations have been conveniently produced in the gas phase, without any contamination from their isomers, by insertion of stable ortho-tolylium ions, obtained from selective F--transfer from ortho-fluorotoluene to radiolytic SiF3+, into methane. The relative extent of competing isomerization pathways occurring in the 1,6-dimethylcyclohexadienyl cation thus formed, i.e. methyl and hydrogen 1,2-shifts, has been measured as a function of the temperature (40-150 degrees C) and pressure (1-3 atm) of the bulk gas (SiF4). At any given temperature, the strong, negative pressure dependence of the relative extent of methyl versus hydrogen 1,2-transfer reveals that the latter isomerization channel involves a vibrationally excited 1,6-dimethylcyclohexadienyl intermediate. This observation is discussed in the framework of the more general problem of the specific activation mode, either electrostatic or thermal, operating in gas-phase ionic processes at different pressures.