Effects of Crystalline Environment on Pyrimidine Free-radical Formation: Barbital Crystal Polymorphs and Sodium Barbital

Summary

The effects of crystalline environment on radiation-induced free radicals in a model system of pyrimidines (barbital I, II, IV, and sodium barbital) were investigated. The characteristics of the stable room-temperature radical were determined by e.s.r. spectroscopy. Single-crystal studies were performed on barbital I and IV. The major free radical in all four species is formed by an ethyl abstraction from C₅. The unpaired spin density was 0·4 in all cases. A reorientation of the free radical in the crystal structure was observed in barbital I. An increase in radical yield was observed in sodium barbital (G=0·41) as compared to barbital I and II (G=0·17). Maximum radical concentrations were comparable in sodium barbital and barbital I and slightly lower in barbital II. These radical characteristics, together with the known crystal structures of the model system, indicate the relative importance of intermolecular crystalline environment parameters to solid-state free radical formation.