ESR Studies of the Cation Radicals Produced by SbCl5 Oxidation of Some Biphenyls with ortho- and para-Dialkyl Substituents

Abstract

Abstract The cation radicals of some alkylbiphenyls such as 4,4′-bitolyl, 4,4′-di-t-butylbiphenyl, and 2,2′, 4,4′, 6,6′-bimesityl, were generated by SbCl5 oxidation in CH2Cl2. The ESR observation was carried out, and the Q, values of the methyl and t-butyl were estimated based on either the theoretical spin density or the experimentally-determined spin density from the 13C-hyperfine splitting. The Q, value of the methyl in the cation radical was found to be 1.5 times that for the anion radical. The t-butyl groups of the cation radical also showed a much larger Q value than that for the anion radical. The perturbation of the spin density due to the steric hindrance was investigated with reference to the prediction of the McLachlan’s MO calculations carried out for the hindered biphenyl. The experimentally-determined spin density derived from the Q value established for the para-derivative can be adequately interpreted on the assumption that the planar character of the biphenyl would be much reduced, contrary to the tendency found for the anion radicals. This effect suggests that a different delocalization of the unpaired electron may be expected between the cation and the anion radical: that is, the unpaired electron brings about a fair bonding on the central 1–1′ bond in the anion radical, but an antibonding for the cation radical, because the odd π-orbital of the cation has its node between the central 1–1′ bond.