Free radicals in pyrimidines. An ESR-study of electron-irradiated single crystals of 5-nitro-6-methyluracil at 770K

Single crystals of 5-nitro-6-methyluracil have been irradiated at 77°K and studied at this temperature with the electron spin resonance technique. The resonance pattern yielding a triplet of doublets was ascribed to hyperfine interactions between the unpaired electron and a nitrogen and a proton, respectively. It is suggested that the radical responsible for this resonance is formed by abstraction of a hydrogen atom from N(1) leaving the unpaired spin density mainly in the molecular σ-core. In this σ-radical N(1) is responsible for the triplet and the hydrogen atom bound to N(3) for the doublet. A small methyl group splitting as well as INDO molecular orbital calculations for a number of different radical candidates were essential for this interpretation. The radical seems to exist in two slightly different conformations.