ESR study of the dissociation of hydroxyl protons in hydroxyl radical adducts to pyridinedicarboxylic acids

Abstract

The pK/sub a/ values for OH proton dissociation in the hydroxyl radical adducts to pyridinedicarboxylic acids have been determined to be 13.0-15.2 in the H/sub -/ basicity scale according to in situ radiolysis-steady-state ESR methods. The OH or O/sup -/ radicals were found to add to ortho and meta positions with respect to a ring nitrogen atom in 2,3-, 3,4-, 3,5-, 2,4-, 2,5-, 2,6-, and 1,4-dihydro-4-oxo-2,6-pyridinedicarboxylic acids. The positions of electron-withdrawing carboxyl groups and a ring nitrogen atom had an effect both upon ESR parameters in acid form and upon pK/sub a/ values. Substrate radicals that have a nitrogen atom at the ortho position with respect to the site of OH addition gave larger g values, larger methylenic and ortho proton hyperfine coupling, larger nitrogen hyperfine coupling, and smaller hydroxyl proton hyperfine coupling than those having nitrogen at the meta position. These differences were explained qualitatively. The presence of a ring nitrogen atom affects g factors additively with increments of +0.00046 and -0.00019 for ortho and meta substituents, respectively. Remarkable changes in ESR parameters with basicity were found in the dominant methylenic proton and several ring proton and nitrogen hyperfine coupling constants upon the dissociation of the OH proton. Themore » pK/sub a/ values are lower by 0.4-1.1 units than corresponding hydroxycyclohexadienyl radicals of benzenedicarboxylic acids, because of an electron-withdrawing character of a ring nitrogen atom. The pK/sub a/ values of radicals with carboxyl groups at the ortho position are higher than those of radicals without a carboxyl group at the ortho position, suggesting the existence of specific interactions between the ortho carboxyl and OH or O/sup -/ groups as is the case in hydroxycyclohexadienyl radicals of carboxylated benzenes.« less