Formation of Peroxynitrite from the Oxidation of Hydrogen Peroxide by Nitrosonium Ion (NO⁺): A Pulse Radiolysis Study

The rate constant of the reaction of ·N₃ with ·NO has been determined to be (4.4 ± 0.5) × 10⁹ M^-1 s^-1 using the pulse radiolysis technique at pH 7.6-8.3 and 21 °C. The reaction of ·N₃ with ·NO takes place through an innersphere electron-transfer mechanism yielding N₃NO as an intermediate, which subsequently decays to N₂O and N₂. Peroxynitrite was formed when H₂O₂ was added to the ·N₃/·NO system at pH 5.8-8.3. The maximum yield of peroxynitrite, which was obtained at [H₂O₂] > 0.2 M, was ∼34% of the initially produced ·N₃, indicating that N₃NO does not react directly with H₂O₂. We conclude that, in the presence of high concentrations of H₂O₂, ∼64% of the N₃NO decomposes into N₂O and N₂, whereas the remaining 34% yields NO⁺1" or H₂NO₂⁺, which subsequently reacts with H₂O₂ to form peroxynitrite. The comparison of our kinetic results with those obtained previously in the H⁺/HNO₂/H₂O₂ system shows that the nitrosating species in both systems differ. As H₂NO₂⁺ is the precursor of NO⁺ in the H⁺/HNO₂/H₂O₂ system, we conclude that the reactive intermediate in our system is most probably NO⁺. From the dependence of the yield of peroxynitrite on [H₂O₂], the ratio between the rate constants of the reactions of NO⁺ with H₂O₂ and H₂O was determined to be 65 M^-1. These rate constants were estimated to be k₇ > 3 × 10⁸ and k_-4[H₂O] > 4.6 × 10⁶ s^-1, respectively.