The Structure of Sulfur Radicals. An ESR Study of Irradiated Sulfur-Containing Purines and Pyrimidines

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Abstract
Single crystals of 2-thiouracil were irradiated and studied at 77.degree. K by electron-spin resonance spectroscopy. The dominant radical trapped exhibits a resonance pattern consisting of a doublet of triplets. It is ascribed to an RS.cntdot. radical formed by a net H abstraction from N1. A similar RS.cntdot. radical is observed in several S-containing purines and pyrimidines. This resonance exhibits a very large g-value anisotropy (gmax of about 2.17). The cation or the deprotonated cation with the main part of the unpaired spin density localized to S is responsible for this resonance. A 2nd S-centered radical is commonly observed in these compounds. Its resonance exhibits g values of .apprx. 2.060, 2.025, and 2.002. A comparison of the ESR parameters with the crystal and molecular structures makes it reasonable to suggest that the perthiyl radical RSS.cntdot. is the most plausible candidate responsible for this resonance. No experimental evidence indicates that the 2 types of S radicals are connected.