Spectroscopic and Kinetic Properties of the Radical Zwitterion and Related Intermediates in the One-Electron Oxidation ofp-Aminobenzoic Acid

The time-resolved resonance Raman spectra and acid−base properties of the transient prepared on the submicrosecond time scale by N 3 • oxidation of aqueous p -aminobenzoic acid (PABA) in near-neutral solutions identify it as the charge-neutral 4- - O 2 C-aniline • + radical. This zwitterion intermediate undergoes slow thermal dissociation ( k ∼ 2.4 × 10 3 s - 1 ) by intramolecular electron transfer and CO 2 elimination. It reacts with common bases, such as OH -, N 3 -, and HPO 4 2 - ions, at the rate constants of 1.9 × 10 10, 2.7 × 10 6, and 2.2 × 10 8 M - 1 s - 1, respectively, and converts into the nondissociative anilino radical form (p K a 6.7). In the • OH oxidation, formation of the zwitterion radical occurs via OH adducts (hydroxycyclohexadienyl radicals) of p -aminobenzoate anion, at a rate of 1.4 × 10 5 s - 1 which is independent of the base concentration. In strongly acidic solutions the OH adduct of p -aminobenzoic acid reacts with H +, at a rate constant of 4.7 × 10 8 M - 1 s - 1, to form the 4-HO 2 C-aniline cation radical (p K a 1.1). The benzidine cation radical, observed as one of the transient secondary products in Raman experiments, results from the reactions of p -aminophenyl radical produced on dissociation of the 4- - O 2 C-aniline • + radical. The marked pH dependence of the nature and yields of the photoproducts of p -aminobenzoic acid, observed in several recent studies, is explained in terms of the dissociative properties of the zwitterion radical and its base-catalyzed conversion into the anilino radical. A two-state model has been developed for the explanation of the intramolecular electron transfer and bond dissociation in the radical intermediates with structurally stable ground electronic state.