Dependence on conformation of the site of proton transfer from alkane radical cations. Nature of the octyl radicals formed by proton transfer from octane radical cations to octane molecules in CCl3F matrices at 77 K

Abstract

A study has been made by EPR spectrometry of octane radical cations and octyl radicals formed by γ-irradiation of octane at various concentrations in CCl₃F. The EPR spectrum at very low concentration (⩽0.5 mol%) is essentially due to octane radical cations, which are largely in the gauche-at-C-2 conformation with large unpaired-electron density on one in-plane chain-end hydrogen and one in-plane hydrogen attached to C-2. Superimposed on the radical cation spectrum, a low intensity spectrum due to octyl radicals appears above 0.5 mol%. The signal intensity and relative contribution to the paramagnetic absorption of this spectrum increase very strongly with increasing concentration of octane. It is observed that at low octane concentration both primary and secondary octyl radicals are present in irradiated CCl₃F–octane systems, and this is true from the very first appearance of octyl radicals in such systems. At higher concentration, secondary octyl radicals become very much the dominant neutral radical species. The results indicate that: (i) primary and secondary octyl radicals are formed by proton transfer from octane radical cations (in the gauche-at-C-2 conformation) to octane molecules; (ii) with increasing concentration of octane in CCl₃F, the size of octane aggregates increases gradually from two-molecule to higher-order clusters, resulting in intermolecular radical site transfer with transformation of primary into secondary octyl radicals. The results confirm that the nature of the alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules is related to the structure of the semi-occupied molecular orbital of the parent cation.