Generation of C7H7+ cations with isomerization reactions

Abstract

The B3LYP and MP2 methods are used to study isomerization reactions of 15 C7H7+ cations in gas phase, along with an MP2 study of the generation of 34 C7H7+ isomers by ionic dissociation of their C7H7Br precursors. Ionic dissociation is endothermic, where dissociation facility does not correlate well with carbocation stability, though various physicochemical factors facilitating dissociation facility correlate fairly well among themselves. The 15 C7H7+ cations studied for isomerization include the benzyl, o-tolyl, m-tolyl, p-tolyl and 7-norbornadienyl cations, besides hypothetical higher energy species. 16 select concerted isomerizations are considered, of which all but one proceed via well-defined transition states. 1,2 and 1,3 hydride shifts involve larger activation barriers than 1,2 CC sigma bond shifts. By combinations among themselves, these one-step reactions lead to 7 multi-step schemes for conversion of various C7H7+ cations to the most stable tropyl cation isomer. Some conversions involve alternative isomerization routes, of which the energetically more feasible one is identified. Transition states located are described with regard to their relative positions along the reaction coordinate in accordance with the Hammond postulate linking transition state geometry to reaction energetics.