Free radicals in pyrimidines: 5-thymyl radical nitrogen splitting in irradiated thymidine.

Single crystals of thymidine irradiated with 4.0 MeV electrons exhibit the well-known eight-line e.s.r. spectrum due to the 5-thymyl radical. A careful analysis of these lines revealed an extra hyperfine splitting, which by a combination of INDO-MO calculations and experiments, was attributed to an interaction between the unpaired electron and the N3-H group.