A comparison between the radicals formed by radiation in barbituric acid and its 2-thio derivative

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Abstract
The radicals formed by radiation in 2-thiobarbituric acid single crystals have been studied by E.S.R. spectroscopy. The main radical is formed by abstraction of one of the protons bonded to C5. The electronic structure of this radical has been compared to that of the related radical formed in ordinary barbituric acid. Both the experimental data as well as the MO calculations show that when oxygen is substituted by sulphur the isotropic proton splitting is reduced and the g-value anisotropy increased. This is explained by a transfer of unpaired spin density from C5 to the sulphur atom.