Electron-spin-resonance Spectroscopy of Radiation-induced Free Radicals in Irradiated Single Crystals of Thymine Monohydrate

SummaryThe molecular structures of free radicals formed in irradiated single crystals of thymine monohydrate were investigated by ESR spectroscopy. Two different types of radicals were observed, their relative contribution in the spectra depending on the experimental conditions. One is the previously known 5,6-dihydro-5-thymyl radical, the conformation of which is discussed briefly. It is proposed that the additional radical is formed by loss of a hydrogen atom from the methyl group, leaving an unpaired electron in a 2pz orbital of the methyl carbon atom. The hyperfine spectrum of this radical originates from the interaction of the unpaired electron with the two remaining α-protons of the methyl group and, via resonance delocalization, with the α-proton attached to carbon atom C6 of the thymine molecule. The hyperfine tensors of these couplings are given together with the g-tensor of this radical.