The radicals induced in single crystals of thymine monohydrate by ionizing radiation have been studied by ESR. Both at 77°K and at room temperature three different radicals are formed. The spectral changes observed at different temperatures are due to variations in the relative yields of these three radicals. In addition to the previously known 5-thymyl (5,6-dihydrothymin-5-yl) radical, two other radicals have been identified. The hyperfine splitting tensors and the directions of the principal axes show that for both radicals the unpaired spin density is mainly in a$p_{\pi}$ orbital on C6. One radical has hyperfine coupling to one α and one β proton; the other radical has coupling to one α proton only. The radicals are the 6-thymyl (5,6-dihydrothymin-6-yl) radical formed when a hydrogen atom is added to$C_{5}$, and a radical in which a hydrogen atom is added to the oxygen atom bonded to C4. The latter radical is presumably formed by electron capture with subsequent proton tunneling in the hydrogen bond to the water molecule.

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