Spin delocalization in radical cations of oxygen-containing organic compounds as revealed by long-range hyperfine interactions and solvent effects

Abstract

E.s.r. evidence is presented for long-range ¹H hyperfine interactions and thermally reversible solvent effects involving mainly the radical cations of aldehydes and ketones in Freon matrices. Long-range couplings to δ protons are found to be surprisingly large in the cations of acyclic, cyclic and polycyclic carbonyl compounds. The conformational requirements for these large couplings are discussed in terms of facile spin transmission via a trans arrangement of C—C and C—H σ bonds, the assumption of this trans effect providing a rigorous and consistent analysis of the experimental results for both non-rigid and rigid molecular geometries. The matrix perturbation observed at low temperatures is believed to arise from a weak complex formed between the radical cation and the solvent, the hyperfine interaction occurring mainly with a single chlorine nucleus. The reversible loss of the e.s.r. substructure associated with this complex at elevated temperatures is attributed to motional averaging of the chlorine hyperfine anisotropy rather than to dissociation of the complex.