Effect of chain length on aggregation of n-alkanes in CCl 3F matrices at 77 K. Further ESR evidence for the occurence of hydrogen and/or proton transfer between higher alkanes and their cations

Abstract

After γ-irradiation of hexane and decane at low concentration in CCl 3F at 77 K only the ESR spectrum of the corresponding radical cations is observed. At higher concentrations (from about 3 mol% hexane and 0.5 mol% decane), the spectra of alkyl radicals also appear. The signal intensity and relative contribution of these alkyl radicals to the observed ESR spectra increases with increasing alkane solute concentration. In contrast, alkane radical cations but no alkyl radicals are observed after irradiation of hexane and decane in CCl 3CF 3 and other matrices at concentrations where alkyl radicals are already quite prominent in CCl 3F. This contrast is especially pronounced in the case of decane, the signal intensity and relative contribution of alkyl radicals in the ESR spectrum of irradiated CCl 3F-decane systems being much higher than in irradiated CCl 3F-hexane systems. Most of the alkane radical cations observed possess the extended structure, resulting in a triplet with substructure for hexane and a broad singlet for decane, but different conformers are also observed, viz. for hexane in CCl 3CF 3 and for decane in CCl 3F. The results provide conclusive evidence for the occurence of hydrogen and/or proton transfer between C 6 (only weakly) and C 10 (very pronounced) n-alkanes and their cations. They show further that in CCl 3F at 77 K alkanes are present as small aggregates to which hole transfer still occurs efficiently and, in conjuction with other data, indicate that the extent of such aggregation increases with increasing chain length of the alkane solute.