Dissociation of bromo- and iodotoluene molecular ions: A theoretical study

The potential energy surfaces (PESs) for the formation of C₇H₇⁺ from α-, o-, m-, and p-halotoluene (X = Br and I) molecular ions were determined using density functional theory molecular orbital calculations. Based on the PESs, Rice-Ramsperger-Kassel-Marcus model calculations were carried out to predict the dissociation rate constants of the molecular ions, which were then compared with previous experimental results. Kinetic analysis showed that below the threshold for the formation of tolylium ion (Tl⁺), the benzylium ion (Bz⁺) was the main product in the dissociation of all molecular ions investigated. The α-halotoluene ions produced Bz⁺ by direct C-X bond cleavage. The formation of Bz⁺ from o-halotoluene ions occurred mainly through isomerization to the alpha isomers. On the other hand, the halogenated isotoluene ions played important roles as intermediates in the formation of Bz⁺ from the meta and para isomers. The formation of Tl⁺ became important as the energy increases in the dissociations of o-, m-, and p-halotoluene ions, especially in those of o-, m-iodotoluene ions.