The 1,2-hydride shift barrier in the phenyl cation. An ab initio study.

Recently, kinetic methods involving tritium decay in labeled molecules have provided the first experimental evidence for a gas phase automerization of the phenyl cation. We have now carried out accurate molecular orbital computations to check out the feasibility of such a mechanism. These calculations include adequate basis sets with polarization functions and the effects of electron correlation. The transition state has been well characterized and a very high barrier of ~50 kcal/mol is calculated for the reaction. The high barrier results for the poor overlap of the molecular orbitals involved in forming the three-center bond in the transition state. These results suggest that unless experimentally generated phenyl cations have a great deal of excess energy, automerization by a 1,2-hydride shift mechanism is unlikely and other mechanisms have to be considered to explain the experimental observations.