The yield of hydroxyl radical from the decomposition of peroxynitrous acid

Mechanistic predictions, based upon the assumption that free (OH)-O-. and (NO2)-N-. 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 IrCl63- oxidation by ONOOH were independent of their concentrations in submillimolar range. A 55 +/- 7% Fe(CN)(6)(3-) yield and 25 +/- 3% IrCl62- were measured at pH 5.7, 22 degrees C. A yields ratio close to 2 is expected, because, of the two radicals produced, only (OH)-O-. can rapidly oxidize IrCl63-. The competition kinetic studies demonstrated that the relative reactivities of the oxidizing intermediate generated by ONOOH toward IrCl63- and NO2- were identical with the reactivities of the "authentic" (OH)-O-. radical generated by pulse radiolysis. It is concluded that ONOOH decomposes via its peroxo bond homolysis producing a pair of discrete (OH)-O-. and . NO2 radicals with 28 +/- 4% yield. A bimolecular reaction between ONOOH and Fe(CN)64- 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-)].