Free Radicals in Pyrimidines: ESR of Xhyphen;Irradiated Single Crystals of Barbital (5,5hyphen;Diethylbarbituric Acid)

摘要

Xhyphen;irradiated single crystals of one polymorph of 5,5hyphen;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 areAAequals;13.2,thinsp;ABequals;11.4,thinsp;ACequals;10.1G, and the principal values of the spectroscopic splitting factor areguequals;2.0052,thinsp;gvequals;2.0041, andgwequals;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 onC(5), with values of 0.4 for ethyl, 0.5 for hydroxyl, and 0.7 for hydrogen.