Formation of nitrous oxide from the reaction of peroxynitrite with sodium azide

Abstract

The reaction of peroxynitrite (oxoperoxonitrate) with azide ion (N₃^–), the residual starting material in the peroxynitrite preparation by the ozonolysis of azide, was investigated in phosphate and carbonate buffers. The observed rate constants (k_obs) for the decay of peroxynitrite at pH 4.5 in the presence of azide up to 30 mM are within experimental error identical to the self decomposition rate of peroxynitrite indicating that the reaction is zero order in azide concentration. However, when the concentration of azide is increased beyond 40 mM a slight increase in k_obs is noticed. From the dependence of k_obs on the concentration of azide the second-order rate constants for the reaction of peroxynitrite with azide are determined to be 3.5 and 1.2 M^–1 s^–1 at pH 4.5 and 7.4 respectively. In the presence of added bicarbonate the reaction between peroxynitrite and azide is zero order in azide concentration. The reaction of peroxynitrite with azide led to the production of nitrous oxide (N₂O) in the absence and presence of bicarbonate as identified by GC-MS analysis of the reaction mixture. At a given pH and peroxynitrite concentration, the yield of N₂O increased linearly with an increase in the concentration of azide up to 100 mM and attained saturation beyond that. Under identical conditions, the yield of N₂O obtained in the absence of bicarbonate is 50% more compared to that obtained in the presence of bicarbonate. Based on kinetics and product studies, it is proposed that the reaction of peroxynitrite with azide involves a one-electron oxidation of N₃^– to azide radical (N₃˙) by the activated form of HOONO (HOONO*). Also that the combination between N₃˙ and nitrogen dioxide (NO₂˙) is rapid and leads to nitryl azide (N₃NO₂) as a transient intermediate and precursor for nitrous oxide (N₂O). In the presence of bicarbonate the peroxynitrite–carbon dioxide adduct (ONOOCO₂^–) or the carbonate radical anion (CO₃^˙–) is proposed as a one-electron oxidant towards N₃^– forming N₃˙. The proposed mechanism for N₂O formation from nitryl azide is supported by high level ab initio calculations.