Kinetic isotope effect and extended pH-dependent studies of the oxidation of hydroxylamine by hexachloroiridate(IV): ET and PCET.
The oxidation of hydroxylamine by [IrCl6]2- has been studied spectrophotometrically in deoxygenated aqueous solutions in the range of pH 4-9 at 25 degrees C. The reaction is catalyzed by Cu2+, Fe2+, and impurities of aquochloroiridium complexes. Oxalate is a very effective inhibitor of catalysis by copper and iron ions. With excess hydroxylamine, the reaction follows pseudo-first-order kinetics, and the stoichiometric ratio (DeltanIr(IV)/Deltanhydroxylamine) is 1.05 at pH 5.9. Over the pH range 4.2-8.8, the empirical rate law is -d[IrCl(6)2-]/dt=k[IrCl6(2-)][NH2OH]tot, with k=k1Ka1/([H+]+Ka1)+k'Ka1/([H+]([H+]+Ka1)), where Ka1 is the dissociation constant of NH3OH+. Least-squares fitting yields k1=(17.05+/-0.47) M-1 s(-1) and k'=(2.59+/-0.09)x10(-6) s(-1) at ionic strength of 0.1 M (adjusted by NaClO4) and 25 degrees C. The kinetic isotope effects (KIE) (kH/kD) for k1 and k' are 4.4 and 9.8, correspondingly. A mechanism is inferred in which k1 corresponds to concerted proton-coupled electron transfer (PCET) and k' corresponds to electron transfer from NH2O-. In this mechanism, the large KIE for k' is due almost entirely to the equilibrium isotope effect for the pKa of NH2OH.
Makarycheva-Mikhailova
Anastasia V
AV
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, USA.
Stanbury
David M
DM
McKee
Michael L
ML