Spin trapping with 5, 5-dimethylpyrroline-N-oxide (DMPO) of various radicals was carried out in aqueous solutions and the ESR parameters for the spin adduct radicals were determined. Comparison of hyperfine coupling constants obtained in aqueous solutions and in benzene revealed that considerable solvent effect exists. For the radical obtained by the addition of a hydroxyl radical to DMPO, the hyperfine coupling constant was found to depend on the basicity of the solution. This phenomenon was analyzed in terms of the acid dissociation equilibrium of the hydroxyl proton of the adduct radical, and the pK value was determined to be 12.96.
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Kirino et al. (1981) studied this question.