TIME-RESOLVED AND PRODUCT-RESOLVED PHOTODISSOCIATIONS OF BROMOTOLUENERADICAL CATIONS

Photodissociations of o-, m-, and p-bromotoluene radical cations have been studied in the wavelength range 575-475 nm using Fourier transfor m-ion cyclotron resonance (FT-ICR) mass spectrometry. The parent ions were prepared by charge-transfer reactions of bromotoluenes with tolue ne-d(8) radical cations produced by two-photon ionization of toluene-d (8) at 266 nm. Bromotoluene radical cations dissociate to C7H7+ by los s of Br. The dissociation rates were measured by time-resolved photodi ssociation spectroscopy. Structures of C7H7+ from one-photon dissociat ion were identified by their bimolecular reactivities with toluene-d(8 ). The C7H7+ products from all three isomers were identified as the be nzyl cation. No unreactive tropylium ions were detected within experim ental limits. The rate constants measured in this work were combined w ith the previous photoelectron-photoion-coincidence results to refine activation parameters for the Rice-Ramsperger-Kassel-Marcus rate-energ y curves, k(E), for the low barrier rearrangement process. The activat ion 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 entro py changes for the activation, Delta S-double dagger(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. (C) 1997 American Institute of Physics.