Free radicals in pyrimidines: E.S.R. of a ?-irradiated single crystal of 5-nitrouracil

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Abstract
Free radicals are observed in γ-irradiated single crystals of 5-nitrouracil with the unpaired electron showing hyperfine interaction with one nitrogen atom. The principal values of hyperfine coupling are Ax = 22·5 g, Ay = 25·2 g, and Az = 40·0 g, and the principal values of the spectroscopic splitting factor are gu = 2·0117, gv = 2·0064 and gw = 2·0027. The relationship of the directions of the corresponding principal axes to the molecular orientations show that the unpaired electron must be located in an sp 2 orbital on either N(1) or N(5). Considerations of the mechanism of radical formation and comparison to radiation damage in other molecules make the N(1) location seem more probable. The π interaction of the nitro group on C(5) evidently prevents the formulation of free radicals with the unpaired electron on C(5). That carbon atom is the most common location of unpaired electron density in other pyrimidine free radicals.

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