The Pentamethylbenzene Cation

Abstract

A RADICAL having electron spin resonance spectra in accord with expectation for the cation of pentamethylbenzene (I; X =H) has been obtained by dissolution of hexamethylbenzene in concentrated oleums. The well-resolved spectrum has been satisfactorily analysed in terms of two sets of equivalent methyl groups which give rise to two nearly equal septets having hyperfine coupling constants of 10.3 and 10.85 gauss, and a further weakly interacting methyl group which gives a quartet with a splitting of 0.56 gauss. No splitting from the single ring proton could be detected. This result is reminiscent of the results for a radical formed under similar conditions from durene, which was identified as the durene cation. Again, no coupling to the ring protons could be detected, but four equivalent, strongly coupled methyl groups gave a hyperfine splitting of 11.0 gauss¹. This spectrum was interpreted¹ in terms of a major splitting of the symmetric and antisymmetric π-molecular orbitals, with the unpaired electron largely in the latter. The present results confirm this assignment, but the very small interaction with the ring proton is more difficult to understand in view of the clearly resolved quartet from the fifth methyl group.