The Yield of Hydroxyl Radical from the Decomposition of Peroxynitrous Acid

Mechanistic predictions, based upon the assumption that free • OH and • NO 2 radicals are formed as intermediates during the ONOOH decay, were tested using inorganic radical scavengers. Both the rates and the yields of Fe(CN) 6 4- and IrCl 6 3- oxidation by ONOOH were independent of their concentrations in submillimolar range. A 55 ± 7% Fe(CN) 6 3- yield and 25 ± 3% IrCl 6 2- were measured at pH 5.7, 22 °C. A yields ratio close to 2 is expected, because, of the two radicals produced, only • OH can rapidly oxidize IrCl 6 3- . The competition kinetic studies demonstrated that the relative reactivities of the oxidizing intermediate generated by ONOOH toward IrCl 6 3- and NO 2 - were identical with the reactivities of the “authentic” • OH radical generated by pulse radiolysis. It is concluded that ONOOH decomposes via its peroxo bond homolysis producing a pair of discrete • OH and • NO 2 radicals with 28 ± 4% yield. A bimolecular reaction between ONOOH and Fe(CN) 6 4- with the rate constant (8.2 ± 0.4) M - 1 s - 1 significantly increases both oxidation yield and rate at high [Fe(CN) 6 4- ].