Deposition of radiation energy in solids as visualized by the distribution, structure and properties of alkyl radicals in γ-irradiated n-alkane single crystals

Abstract

This paper summarizes results obtained earlier from E.S.R. studies of γ-irradiated n-alkane single crystals. It also contains some new experimental results that serve to give a more complete picture of the deposition of radiation energy in solid alkanes. The experiments performed with solid n-alkanes have thus far provided structural data that permit the nature and even the conformation of alkyl radicals to be clearly understood. Two types of radical exist, namely, one where the unpaired electron is located next to the end methyl group and one with the unpaired electron in the interior of the chain. The first type has a conformation which differs from that of the undamaged molecule. Microwave saturation data show that there is a difference in relaxation properties of these radicals which can be understood in terms of a difference in mobility. Relative yield measurements give the distribution of isomeric alkyl, the result differing from that obtained using product analysis in liquids. For protiated n-alkanes n-alkyl is lacking and the 2-alkyl concentration is higher than expected. For deuterated n-alkanes the E.S.R. spectrum is mainly that of radicals with the unpaired electron located in the interior of the carbon chain. This isotope effect is again contrary to observations in liquid n-alkanes. The broad lines observed in protiated alkanes irradiated at 77 K and deuterated alkanes irradiated at 4.2 K are not believed to arise from strong spin-spin interactions. They are thought instead to arise from distorted crystal and radical structures relating to the damage regions of the crystals. Radical pairs exist with different stability, yield and structure. Our estimate, in deuterated alkanes, that as much as 40% of the radicals are formed in pairs or clusters at 4.2 K shows that radical pair formation is an important radiolytic process in solid n-alkanes. Energy transfer from deuterated to protiated molecules has also been experimentally verified. This energy transfer is temperature dependent and occurs efficiently at 77 K, but less efficiently at 4.2 and 273 K. A relationship also exists for deuterated alkanes between the amount of radical pairs formed at 4 K and the long-range energy transfer at 77 K. This can be readily accounted for by an exciton transfer mechanism analogous to that in aromatic crystals. It might also be qualitatively described by a hydrogen abstraction process. In this case, however, the properties of the deuterium atoms are not understood.