An Electron Spin Resonance Investigation of the Radiolysis of Hexamethylbenzene and Hexamethyl(Dewar)Benzene

Exposure of hexamethyl(Dewar)benzene (HMD) to γ-rays in a solid CFC1₃ matrix at 77K generated the corresponding radical cation (HMD^+•) as shown by the observation of hyperfine coupling (A = 9·5 G) to the protons of four equivalent methyl groups: thus the ²B₂ state (I) of the ion is established. On annealing to 140 K, ring-opening occurred to give the hexamethylbenzene cation (HMB^+•). When CF₂ClCFCl₂ was used as the matrix material, a completely different spectrum was produced, consisting of seven lines with a common spacing of 14 G. Results obtained using different batches of the freon, and from careful annealing and concentration studies, establish that the low-field line is not part of the remaining six-line spectrum which can be assigned to the allylic radical (III), formed by deprotonation of the parent cation. γ-Irradiation of HMD in 2- methyltetrahydro-furan (MeTHF) followed by annealing gave the ψ_s^* Jahn-Teller distorted form of the (ring-opened) HMB^-• radical anion [A(6H) = 9 G] and so the parent anion is unstable, as is the cation. This was confirmed by an identical spectrum being obtained after the irradiation of an authentic sample of HMB in MeTHF. γ-Irradiation of pure HMD gave rise to a spectrum that can be analysed in terms of (III) and the endo H-atom adduct of HMD (IV). Hexamethylbenzene (HMB) on γ-irradiation in solid CFC1₃ gave the parent radical cation (HMB^+•) At 130 K, this has a 6·7 G splitting and correct binomial intensities for 18 equivalent protons; on cooling to 77 K, the cation distorts so that a ψ_A type SOMO is adopted with four strongly coupled (A = 11·5G) and two weakly coupled (A = −2·8G) methyl groups. γ-Irradiation of pure HMB gave only broad features which we ascribe to a mixture of pentamethylbenzyl (VIII) and hexamethylcyclohexadienyl (IX) radicals, in which proton hyperfine structure is unresolved due to a range of restricted conformations being adopted by various methyl groups because of steric hindrance between them.