Oxidation of Nitrite by Peroxynitrous Acid

The kinetics of the oxidation of nitrite to nitrate by peroxynitrous acid at pH 5.2 is best described by the rate law k obs = k iso + k ‘[NO 2 - ] + k ‘ ‘[NO 2 - ] 2, in which the peroxynitrous acid isomerization rate constant k iso = (1.10 ± 0.05) s -1, k ‘ = (3.2 ± 0.1) M -1 s -1, and k ‘ ‘ = (4.2 ± 0.3) M -2 s -1, at 25 °C. The ternary reaction may involve initial formation of an adduct between nitrite and peroxynitrite, followed by reaction with a second nitrite to form two nitrite and a nitrate. Ab initio calculations indicate that there is only a small intrinsic barrier to the net transfer of HO + from peroxynitrous acid to the nitrogen atom of nitrite. A similar transfer to either of the two oxygens of nitrite produces the reactants, and would not lead to an increase in the rate of disappearance of peroxynitrous acid, as observed. The low rate constant is most likely due to stringent orientational constraints. Formal transfer of HO + to 15 NO 2 - results in formation of 15 NO 3 -, as experimentally observed. HO + transfer is common to the chemistry of peracids, a family of compounds to which peroxynitrous acid belongs.