Mechanism and Thermochemistry of Peroxynitrite Decomposition in Water

The formation of ONOOH/ONOO^- in water has been studied using the pulse radiolysis of nitrite and nitrate solutions. The overall rate constant of the reaction of ^̇OH with ^̇NO₂ was determined to be (1.0 ± 0.2) × 10¹⁰ M^-1 s^-1. This reaction generates almost equal amounts of ONOOH and NO₃^- + H⁺. The overall rate constant of the reaction of ^̇NO₂ with O^̇- was determined to be (3-4) × 10⁹ M^-1 s^-1. From published thermodynamic data the equilibrium constant of homolysis of ONOO^- into ^̇NO₂ and O^̇- is calculated to be (5.9 ± 2.9) × 10^-16 M, and hence the calculated rate constant of homolysis of ONOO^-into ^̇NO₂ and O^̇- is (0.9-3.5) × 10^-6 s^-1. The rate constants for ONOO^- decomposition at pH 13 and 14 (25 °C) were determined to be 1.3 × 10^-5 and 1.1 × 10^-5 s^-1, respectively, and the yield of NO₂^- in this process was found to be ca. 50%. On the assumption that ca. 1/2 of ONOO^- decomposes via homolysis into ^̇NO₂ and O^̇-, a limiting rate constant for the decomposition of ONOO^- can be predicted at sufficiently high pH, k_d = (0.36-1.4) × 10^-5 s^-1, which is four times as high as the rate constant of homolysis into ^̇NO₂and O^̇-. Both results are in excellent agreement with the homolysis model. The activation parameters for the decomposition of ONOO^- at pH 14 were determined to be A = 8 × 10¹⁰ s^-1 and E_a = 21.7 kcal/mol. The relatively low A value suggests a high degree of solvent organization in the transition state. The mechanisms of homolysis of ONOOH and ONOO^- are compared and discussed in detail.