Time-resolved photodissociations of iodotoluene radical cations

Abstract

Unimolecular dissociations of o-, m-, and p-iodotoluene radical cations that yield C7H7+ by loss of an iodine atom have been studied by time-resolved photodissociation (TRPD) spectroscopy using Fourier transform-ion cyclotron resonance mass spectrometry. Iodotoluene radical cations were prepared by the phoionization charge-transfer (PICT) method. The TRPD spectra were obtained in the wavelength range 480-575 run. The rate-energy data from the present work are combined with the previous rate data obtained in a higher internal energy range to extract activation parameters for unimolecular dissociations. For the direct C-I cleavage channel that leads to tolyl cations, the entropy changes for activation are assumed to be identical to that of the vinylic C-I cleavage of the iodobenzene radical cation, and activation barriers are estimated from thermochemical thresholds. For the lowest barrier rearrangement channel that leads to the benzyl cation, the entropy changes are assumed to be identical to those of the corresponding bromotoluene radical cations. Activation barriers for the rearrangement channel are determined to be 1.77, 1.88, and 1.90 eV for o, m, and p isomers, respectively. Both the product structures and activation parameters for the lowest barrier rearrangement process lend support to the McLafferty mechanism that invokes the H-atom migration from the alpha to the ipso- or ortho-carbon site and the subsequent H-atom and/or CH2-group migration on the ring to form benzyl precursors. The effects of halogen substitution (X = Cl, Br, and I) on the rearrangement process are discussed.