Structure-reactivity relationships in reactions of alkane radical cations: Study of the proton transfer from alkane radical cations to alkane molecules in γ-irradiated disordered CCl 3F/alkane systems by ESR spectroscopy at cryogenic temperatures

Abstract

A summary is presented of ESR results obtained in γ-irradiated disordered CCl 3F/alkane systems at cryogenic temperatures, with respect to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The nature of the alkyl radicals formed by proton transfer is indicative for the site of proton donation and is derived unambiguously from ESR results by comparison with powder spectra of authentic isomeric alkyl radicals, obtained by γ-irradiation of various chloro and bromoalkanes in perdeuterated cis-decalin. The experiments can be divided into two main classes. (i) Experiments on n-alkane radical cations in the extended all- trans conformation, i.e. ESR results on the system CCl 3F/heptane. The ESR spectrum of γ-irradiated CCl 3F/heptane consists of a triplet due to heptane radical cations in the extended all- trans conformation. In this conformation, the unpaired electron is delocalized over the carbon-carbon σ-bonds as well as the two chain-end carbon-hydrogen bonds that are in the plane of the CC skeleton. Superimposed on the ESR triplet is a low-intensity spectrum due to heptyl radicals, which increases drastically with increasing heptane concentration. The formation of these heptyl radicals can be attributed unambiguously to proton transfer from heptane radical cations to heptane molecules, taking place in small heptane clusters to which positive-hole transfer still occurs efficiently. At the onset of proton transfer with increasing heptane concentration only primary heptyl radicals are present, clearly showing that the proton transfer takes place selectively from a chain-end position, in accordance with the electronic structure of the reacting radical cations. At higher heptane concentration secondary heptyl radicals also appear as a result of intermolecular radical-site transfer, i.e. the nature of the heptyl radicals becomes governed by their thermodynamic stability. (ii) Experiments on n-alkane radical cations in the gauche-at-C 2 conformation, i.e. ESR results on the system CCl 3F/octane. The ESR spectrum of γ-irradiated CCl 3F/octane indicates that octane radical cations are largely in the gauche-at-C 2 conformation in this matrix, with large unpaired-electron (and positive-hole) density on one planar chain-end CH bond and one planar penultimate CH bond at the other side of the radical cation. Careful investigation of ESR spectra with increasing octane concentration clearly reveals that in this case secondary octyl radicals are present from the very onset of proton transfer, in accordance with the electronic structure of the reacting radical cations. The results clearly point to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules and to a strict dependence of the site of proton donation on the electronic structure and conformation of the reacting radical cations.