Mechanism and Thermochemistry of Peroxynitrite Decomposition in Water

The Journal of Physical Chemistry A1999Vol. 103(29), pp. 5685–5691

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Gábor Merényi, Johan Lind, Sara Goldstein, Gidon Czapski

Abstract

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, kd = (0.36−1.4) × 10-5 s-1, which is four times as high as the rate constant of homolysis into •NO2 and 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 Ea = 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.

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Abstract

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