Radicals present during photolysis of solutions of 3,5-pyridinedicarboxylic acid and its anions have been studied by the ESR method near room temperature. Spectra were analyzed to obtain parameters for four radicals resulting from reduction of the parent compounds. Radicals I, II, and III, respectively, result from hydrogen atom attachment to the nitrogen atom of the parent dicarboxylic acid, its monoanion, and its dianion. Radical IV results from hydrogen atom attachment to one of the carbon atoms bonded to the nitrogen atom of the parent dianion. Conditions and mechanisms of formation are discussed. The pK value for <tex-math>${\rm I}\rightleftarrows {\rm II}+{\rm H}^{+}$</tex-math> is about 3.4 and that for <tex-math>${\rm II}\rightleftarrows {\rm III}+{\rm H}^{+}$</tex-math> is about 4.3 in aqueous solution. Evidence is given for the switching of the carboxylic acid proton of II between its two equivalent sites.

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