Free radicals are produced by x-irradiation in substituted barbituric acid derivatives by the selective abstraction of one of two groups on C(5) of the pyrimidine ring, leaving the unpaired electron localized on C(5). An ESR analysis of irradiated single crystals and polycrystalline samples of 5-methylbarbituric acid, 5-hydroxy-5-methylbarbituric acid, 5-hydroxy-5-ethylbarbituric acid and 5-ethyl-5-methylbarbituric acid leads to the following ordering for the ease of abstracting the various groups:$-{\rm H}\gg -{\rm CH}_{2}{\rm CH}_{3}\gg -{\rm CH}_{3}$$-{\rm OH}>-{\rm CH}_{2}{\rm CH}_{3}$$-{\rm OH}>{\rm CH}_{3}$ The abstraction of the ethyl group involves an intermediate radical$-\dot{{\rm C}}{\rm HCH}_{3}$ formed by abstraction of hydrogen from the ethyl. It is postulated that the ethyl group is subsequently removed as ethene. Hyperfine coupling and g-value principal values are presented for those radicals not previously reported.

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