Radiation-induced free radicals in solid 5-halouracil derivatives: single crystals of 1-methyl-5-bromouracil.

Single crystals of the 5-halogen substituted uracil derivative 1-methyl-5-bromouracil have been irradiated at 77 and 295 K and studied by electron spin resonance spectroscopy. At 295 K a hydrogen abstraction radical (I) from the methyl group was observed, together with a modified 5-yl radical (II). The latter species seem to be formed by addition of a non-interacting residue (probably CH3) to C6. At 77 K, radical I is observed together with two bromine-centered species. One of these (III) exhibits only bromine hyperfine interaction (maximum splitting of 515 G) and is assigned to a complexed cation radical. The third 77 K resonance could be due to a beta-bromine radical R'-C5HBr-C6H-R'. A computer program able to calculate powder spectra of radicals with dominant halogen interaction is described. The results are discussed in relation to radiation damage processes in the isomorphous crystals of 1-methylthymine.