Unimolecular isomerizations of C₆H₆^•+ radical cations: a computational study

Eighteen concerted isomerization reactions of various C₆H₆^•+ radical cation (RC) species are studied and found to proceed via well-defined transition states, whose relative positions along the reaction pathway generally agree with Hammond's postulate. From the barrier heights, the rate coefficients of these reactions are estimated by using transition state theory, and the activation energies are computed. Through combination among themselves, these 18 isomerizations yielded 15 multi-step conversion routes of various C₆H₆^•+ species to the lowest energy benzene radical cation isomer 1, which routes are compared.

Use is made of DFT with the B3LYP and M06-2X functionals, along with the CBS-QB3 approach to arrive at better energies. From the barrier heights for each of the concerted reactions, canonical transition state theory was applied to evaluate rate coefficients k over the temperature range 200-500 K. The Arrhenius activation energies were computed using the plot of ln k vs. 1/T.