Free Radicals in Pyrimidines: ESR of X-Irradiated Single Crystals of Barbital (5,5-Diethylbarbituric Acid)

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Abstract
X-irradiated single crystals of one polymorph of 5,5-diethylbarbituric acid show ESR spectra characteristic of a radical formed by abstraction of an ethyl group from C(5) of the pyrimidine ring. The principal values of hyperfine coupling are AA=13.2, AB=11.4, AC=10.1 G, and the principal values of the spectroscopic splitting factor are gu=2.0052, gv=2.0041, and gw=2.0023. The spin density on C(5) was determined to be approximately 0.4 with the remainder delocalized onto O(4) and O(6). All barbituric acid derivatives studied previously have formed radicals via a variety of mechanisms which also leave the major site of unpaired electron density at C(5). This is also a preferred site for other pyrimidines. The unpaired spin density on C(5) in the barbituric acid derivatives shows a large dependence on the substituent group on C(5), with values of 0.4 for ethyl, 0.5 for hydroxyl, and 0.7 for hydrogen.