Free Radicals in Pyrimidines: A Single Crystal of Dihydro-6-methyl-uracil Irradiated at 295°K

Summary

Single crystals of dihydro-6-methyl-uracil (DH6MU) grown from a D₂O solution were irradiated with electrons and the induced radicals studied by E.S.R. At room temperature, two different radicals were identified and their E.S.R. parameters studied in detail. The radicals were formed by abstraction of a hydrogen atom from C₅ and C₆, respectively, leaving the unpaired spin density mainly to a 2p_z orbital on the same carbon atoms. The radical formed by abstraction of the C₆ hydrogen atom exhibits a relatively isotropic β-proton splitting. This radical has been observed in previous experiments on powder samples. The single crystal observations, however, show a previously-unknown nitrogen splitting for this radical. The observation is supported by INDO molecular orbital calculations. The other radical, which has not been observed previously, is quite similar to a radical (the 6-thymyl radical) found recently in the thymine derivatives.