Generation, structures, relative energies, and isomerization reactions of C₅H₅⁺ cations

The B3LYP, MP2, and CBS-QB3 quantum chemical methods are used to study the relative energy and isomerization reactions of C₅H₅⁺ cations. Ease of generation of 14 C₅H₅⁺ isomers by ionic dissociation of halide precursors does not correlate well with carbocation stability. The reaction profiles of concerted isomerization of various C₅H₅⁺ cations to six select cations are established along with the respective transition states. The rate coefficients of these processes are estimated by using transition state theory and activation energies computed. The transition states for these six reactions are characterized with regard to position along the isomerization pathway as per Hammond's postulate. The 6 isomerization reactions are combined to yield multi-step conversions of various C₅H₅⁺ species to the lowest energy vinylcyclopropenyl cation 1. Finally, three different routes for obtaining the select cations from C₅H₅Br precursors are profiled and the most favored pathways predicted.