Kinetics of Nitric Oxide Autoxidation in Aqueous Solution in the Absence and Presence of Various Reductants. The Nature of the Oxidizing Intermediates

The reaction of ·NO with O₂has been investigated in the absence and presence of various reductants in aqueous solution by the stopped-flow method. The measured yield of nitrite during the autoxidation of ·NO is in agreement with reaction 1: 4•NO + O₂ + 2H₂O → 4NO₂^- + 4H⁺. Kinetic studies show that the rate law is — d[O₂]/ dt = k₁[·NO]^-2[O2^]with k₁= 2.9 × 10⁶M^-2 s^-1 at 22 °C, which was unaffected by pH and by the presence of either ferrocyanide or ABTS, which are oxidized in this system to ferricyanide and ABTS⁺, respectively. Under limiting concentrations of ·NO, the rate law is -d[·NO]/dt = 4k₁[·NO]²[O₂], and it is reduced to 2k₁[·NO]²[O₂] in the presence of ferrocyanide or ABTS. The results of the competitive kinetic studies in the presence of ferrocyanide demonstrate that the oxidizing intermediate is ·NO₂, whereas in the presence of ABTS both ONOONO, which is the precursor of ·NO₂, and ·NO₂ are the oxidizing entities. The effect of pH and phosphate buffer concentrations on the oxidation yields and rates suggests the formation of N₂O₃ in these systems, as both OH^- and HPO₄^2- enhance the rate of hydrolysis of N₂O₃. All data were consistent with the formation of ONOONO, ·NO₂, and N₂O₃ as oxidizing intermediates during the autoxidation of ·NO. The role of each of these intermediates in the oxidation process depends on the relative rates of the reactions of these intermediates with the substrate and those of the different steps of the autoxidation process. A detailed mechanism for the autoxidation of ·NO is given and discussed.