Time- and product-resolved photodissociations of bromotoluene radical cations

Abstract

Photodissociations of o-, m-, and p-bromotoluene radical cations have been studied in the wavelength range 575–475 nm using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. The parent ions were prepared by charge-transfer reactions of bromotoluenes with toluene-d8 radical cations produced by two-photon ionization of toluene-d8 at 266 nm. Bromotoluene radical cations dissociate to C7H7+ by loss of Br. The dissociation rates were measured by time-resolved photodissociation spectroscopy. Structures of C7H7+ from one-photon dissociation were identified by their bimolecular reactivities with toluene-d8. The C7H7+ products from all three isomers were identified as the benzyl cation. No unreactive tropylium ions were detected within experimental limits. The rate constants measured in this work were combined with the previous photoelectron-photoion-coincidence results to refine activation parameters for the Rice–Ramsperger–Kassel–Marcus rate-energy curves, k(E), for the low barrier rearrangement process. The activation barriers are estimated to be 1.66, 1.80, and 1.78 eV for the o-, m-, and p-bromotoluene radical cations, respectively, whereas the entropy changes for the activation, ΔS‡(1000 K), are −9.6, −7.2, and −5.6 eu., respectively. The mechanism of the rearrangement process is presented to account for the predominant formation of the benzyl cation.