Free radicals in pyrimidines: ESR of x-irradiated 5-ethylbarbituric acid and its potassium salt

Irradiation of 5‐ethylbarbituric acid yields a free radical formed by hydrogen abstraction from C(5) of the pyrimidine ring. The characteristics of this radical are similar to those of the same radical formed by abstraction of an ethyl group from 5, 5‐diethylbarbituric acid. There is no evidence of an ethyl abstraction radical in this compound, indicating that the abstraction of the hydrogen rather than the ethyl is highly preferred. Irradiation of the potassium salt of 5‐ethylbarbituric acid yields a free radical formed by hydrogen abstraction from the ethyl group. Principal values are as follows: g₁=2.0032, g₂=2.0031, g₃=2.0025, $A_1^{\mathrm{H}}\text{= 26.4\hspace{0.17em}}\mathrm{G}$ , $A_2^{\mathrm{H}}\text{= 16.5\hspace{0.17em}}\mathrm{G}$ , $A_3^{\mathrm{H}}\text{= 10.9\hspace{0.17em}}\mathrm{G}$ , $A_1^{{\mathrm{CH}}_3}\text{= 21.0\hspace{0.17em}}\mathrm{G}$ , $A_2^{{\mathrm{CH}}_3}\text{= 20.5\hspace{0.17em}}\mathrm{G}$ , $A_3^{{\mathrm{CH}}_3}\text{= 19.6\hspace{0.17em}}\mathrm{G}$ . Only one molecule per unit cell was observed magnetically, although the crystal structure would indicate that two or three should be seen. The formation of this hydrogen abstraction radical is surprising because a simple ionization of the parent molecule would yield the very stable radical observed in 5‐ethylbarbituric acid and 5, 5‐diethylbarbituric acid.