Peroxynitrate is formed rapidly during decomposition of peroxynitrite at neutral pH

Abstract

The decomposition of peroxynitrite near neutral pH leads ultimately to the formation of dioxygen and nitritevia an intermediate with an absorbance maximum at 284 nm. The intermediate oxidises I⁻ with a rate constant of (78 ± 8) × 10 M⁻¹s⁻¹ and decays near pH 7 with a rate constant of (0.58 ± 0.02) s⁻¹ at 22 °C, but is longer-lived at lower pH. On the basis of experiments performed with a tandem-quenching flow reactor, we tentatively identify this intermediate as peroxynitric acid, formed during the proposed reaction sequence ONOOH + ONOO⁻→ NO₂⁻ + O₂NOO⁻ + H⁺→ 2 NO₂⁻ + O₂. These products are those expected from a peracid. The rate constant for the first reaction is ca. 3 × 10⁴ M⁻¹ s⁻¹. Part of the dioxygen formed is in the ¹Δ_g state (S. Miyamoto, G. E. Ronsein, T. C. Corréa, G. R. Martinez, M. H. G. Medeiros and P. Di Mascio, Dalton Trans., 2009, DOI: 10.1039/b905560f). The decay of peroxynitrous acid at concentrations higher than 0.1 mM near neutral pH is best described by the simultaneous process of isomerisation (k = 1.2 s⁻¹) and decomposition to peroxynitrate. The rate of formation and the amount of peroxynitrate formed are much larger than can be accounted for by homolysis reactions.