Structures and reactions of radical cations of some prototype alkanes in low temperature solids as studied by ESR spectroscopy

Abstract

The structures and reactions of radical cations of prototype alkanes such as ethane, propane, isobutane, neopentane, etc. produced in SF6 and in other halocarbon matrices irradiated at 4 K have been extensively studied by ESR. The geometrical structures as well as the unpaired electron orbitals of these cations are unequivocally characterized based on the simple symmetry considerations as well as the INDO MO calculations. The ESR hyperfine coupling constants determined for these cations as well as their deuterated homologs give conclusive evidence for the interpretations. The unpaired electron in linear alkane cations is delocalized over the in-plane C–H and C–C σ bonds forming delocalized σ radicals, whereas that in highly branched alkane cations is more confined to one of the C–C bonds forming localized σ radicals. The H1s spin density, giving a large hyperfine coupling in the linear alkane cations, is largely contributed from a direct participation of the C–H bond in the unpaired electron orbital, whereas that in the highly branched alkane cations is due to mainly hyperconjugation. The alkane cations undergo two types of reactions: one is deprotonation to form neutral alkyl radicals in SF6 and in CFCl2CF2Cl and the other is the H2 and CH4 elimination to form olefinic π cations in CFCl3. Based on the matrix effects on the structure of propane and isobutane cations, the site preference in forming alkyl radicals, and other observations, it is concluded that the alkane cations undergo reactions through the breakage of the bond in which the unpaired electron is highly populated.