ESR spectra and structures of radical cations of some branched alkanes: β-proton couplings in C–C σ cations

Abstract

The systematic ESR studies of radical cations of five kinds of methyl substituted butanes show that the unpaired electron is rather confined to one of the C–C σ bonds in contrast to the linear alkane cations, in which the unpaired electron delocalizes over the entire σ molecular chain. Analyses of the origin of the trans C–Hβ proton couplings indicate that spin transfer due to hyperconjugation plays an important role in these branched alkane cations. It is also shown that the hyperconjugative effect in these branched alkane cations is two times higher than that in the neutral alkyl π radicals. The large difference of the front and back lobe interactions in the β-proton couplings in these C–C σ radicals is attributable to the bent structure of the radical carbon atom. The front lobe interaction is about one-half of that of the back lobe interaction as is the case of vinyl radicals.