Electron spin resonance study of radiation-produced radical pairs in single crystals of 1-methyluracil

Abstract

Five different types of radical pairs produced in single crystals of 1-methyl-uracil by irradiation with X-rays were identified. Partial differentiation between pairs was achieved by utilizing different stabilities versus ultraviolet illumination or annealing, and variation of relative yields with temperature. The composition of pairs was derived from hyperfine couplings, the orientation from the maxima of dipolar couplings. These were found to agree with the directional cosines obtained from the crystal structure. Further evidence was provided by half-field spectra (Δm _S=2). The zero-field splitting constant D was determined for five pairs, the value of E for one pair. Second-order effects observed at 9·5 GHz and at 35 GHz have been fully analysed for two pairs by application of the appropriate theory which permitted to determine the absolute sign of D. Two pairs are homogeneously composed of hydrogen abstraction radicals 1-methyleneuracil. These are located two layers apart for one pair with the link normal to the molecular planes (D/gβ = -107·5 G), and within one plane for the other (-224·0 G). The three remaining pairs consist heterogeneously of an abstraction radical and a hydrogen addition radical 1-methyl-5,6-dihydrouracil-6-yl. Two of these are intraplanar (-180·0 G; -129·0 G), and the other one located in neighbouring layers (-183·0 G).