Reduction potential of the tert-butylperoxyl radical in aqueous solutions

Rate constants for oxidation of tert-butyl hydroperoxide anion (t-BuO2-) by several oxidants were determined by pulse radiolysis. Rapid oxidation was found with N-3 . (4.4 x 10(8) L mol(-1) s(-1)) and . O- (2.6 x 10(8) L mol(-1) s(-1)), but others (Br-2(.-), I-2(.-), ClO2., CO3.-, C6H5N(CH3)(2)(.+)) reacted much more slowly. Since the reduction potentials of these oxidants are much higher than that estimated for t-BuO2., the observed rate constants suggest that the self-exchange rate for t-BuO2./t-BuO2- is very slow and that it would be difficult to establish rapid equilibrium by pulse radiolysis with a suitable redox pair. Therefore, to determine the reduction potential for this peroxyl radical, the forward and reverse rate constants for reaction 6, t-BuO2. + Fe(CN)(6)(4-)reversible arrow t-BuO2- + Fe(CN)(6)(3-), were measured independently. The forward rate constant was measured by kinetic spectrophotometric pulse radiolysis in N2O-saturated solutions containing high concentrations of N-3(-) (1 mol L-1), t-BuO2- (0.4 mol L-1), and varying concentrations of Fe(CN)(6)(4-) (0.012-0.036 mol L-1) at pH 13.7. The rate constant k(6) was found to be 2.5 x 10(2) L mol(-1) s(-1). Such a low rate constant was possible to measure by pulse radiolysis only because the self-decay of t-BuO2. is very slow compared to those of other peroxyl radicals. The reverse reaction rate constant was measured by mixing the reactants and following the disappearance of Fe(CN)(6)(3-) in the presence of spin traps to remove the peroxyl radicals from the equilibrium. A rate constant of k(-6) = 1.4 x 10(-2) L mol(-1) s(-1) was derived. By taking the reduction potential of Fe(CN)(6)(3-)/Fe(CN)(6)(4-) as 0.459 V vs NHE for the conditions used in these experiments, we calculate E(t-BuO2./t-BuO2-) = 0.71 V and E(t-BuO2./t-BuO2H) = 1.05 V at pH 7 and 1.47 V at pH 0. The rate constants for oxidation of Fe(CN)(6)(4-) and other compounds by various peroxyl radicals were then utilized for the estimation of the reduction potentials of these peroxyl radicals.