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Spectroscopic and kinetic properties of the radical zwitterion and related intermediates in the one-electron oxidation of p-aminobenzoic acid
Tripathi,Su, Yali
, p. 2235 - 2244 (2007/10/03)
The time-resolved resonance Raman spectra and acid-base properties of the transient prepared on the submicrosecond time scale by N3. oxidation of aqueous p-aminobenzoic acid (PABA) in near-neutral solutions identify it as the charge-neutral 4--O2C-aniline.+ radical. This zwitterion intermediate undergoes slow thermal dissociation (K ~ 2.4 x 103 s-1) by intramolecular electron transfer and CO2 elimination. It reacts with common bases, such as OH-, N3-, and HPO22- ions, at the rate constants of 1.9 x 1010, 2.7 x 106, and 2.2 x 108 M-1 s-1, respectively, and converts into the nondissociative anilino radical form (pK(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 x 105 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 x 108 M-1 s-1, to form the 4-HO2C-aniline cation radical (pK(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--O2C-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.
