The radical pairs in irradiated organic solids as studied by EPR

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Abstract
The experimental data concerning the pairwise generation of paramagnetic defects (radicals) for irradiation o organic solids are reviewed and discussed. Two main mechanisms of pair formation are shown to be of importance: (1) the disintegration of molecules into heavy fragments remaining inside the ‘cage’ and (2) valence or charge transfer between the nearest molecules following the scheme: . Analysis of EPR signals allows us to calculate the distances separating the radical sites as well as their relative orientations. In the case of mechanism (1) the inter-radical distances are exactly fixed and correspond to that of undamaged molecules, sometimes not only in molecular crystals but even in the frozen solutions. In the case (2) the transfer mechanism permits the generation of a number of pairs differing by distance and orientations, and they are indeed observed, but the radiation yields of different pairs are very much different. The kinetic study of radiation-induced accumulation of pairs of the type (1) and isolated radicals suggests a simple scheme of radical formation through pairs as intermediate species: and allows us to calculate the probabilities of escape from the cage and rate constants for radical reactions ‘in the cage’. For the pairs of type (2) in γ-irradiated dimethylglyoxime the kinetics of transition between the different pairs has been studied yielding information concerning the H-atom transfer from undamaged C:NOH groups to radical C:NO sites located at certain points of the crystalline cell. Some new unexpected effects observed in the course of investigation of radical pairs are also discussed in brief. The results obtained are believed to demonstrate that the radical pair investigation is a new and interesting way to study radiation-induced defects and post-irradiation chemical reactions in solids.