Benzyl, 9-fluorenyl and diphenylmethyl cations: structures and relative stabilities based on hydride transfer reactions

Abstract

Ab initio molecular orbital calculations are reported at HF/6-31G(d,p) and MP2(fc)/6-31G(d) for the diphenylmethyl ( I), 9-fluorenyl ( II) and benzyl ( III) cations, and for the hydrocarbons diphenylmethane ( IV), fluorene ( V) and toluene ( VI). Structure optimisations have been performed at both levels of theory and all critical points were characterised by harmonic frequency calculations at the Hartree-Fock level. Based on hydride transfer reactions, one phenyl group is calculated to stabilise the methyl cation by 74.3 kcal mol −1, and a second phenyl adds a further stabilisation of 25.1 kcal mol −1. The 9-fluorenyl cation is intermediate in stability between the diphenylmethyl and benzyl cations, but is closer to the diphenylmethyl cation.