Time-resolved photodissociation of toluene ion

Abstract

Time-resolved photodissociation (TRPD) was observed for toluene ion at wavelengths from 450 to 480 nm, and for toluene- d 8 ion for wavelengths from 430 to 459 nm. Dissociation was found to be slow, with unimolecular rate constants in the range 920–3600 s −1 for toluene ion, and 200–1000 s −1 for toluene- d 8 ion (decreasing with increasing wavelength). Rate—energy curves for both species were drawn using Rice—Ramsperger—Kassel—Marcus (RRKM) fitting, where the RRKM parameters were found to be E 0 = 2.11 (2.13) eV, and Δ S‡(1000K) = − 11.7 (−12.4) cal K −1 for toluene (toluene- d 8) ions. These kinetic parameters are very similar to those previously found for the analogous dissociation of methylnaphthalene ions. A fast dissociation component was also observed, contributing about 30% to the total amount of dissociation, and was attributed largely to the presence of cycloheptatriene ion in the parent ion population. TRPD of authentic cycloheptatriene ions showed their dissociation to be fast (> 5 × 10 4 s −1. These results neither supported nor ruled out a very slow dissociation to tropylium ion with E 0 much less than 2.1 eV, as suggested by several photoionization studies.