Kinetics and mechanism of the formation and decay of peroxynitrous acid in perchloric acid solutions

Abstract

The kinetics of the formation of peroxynitrous acid (HOONO), and its decay to nitrate ion have been studied in perchloric acid solutions (0·01 to 1·0 mol I^–1) by the stopped-flow method. The rate law for the formation process was found to be of the form (a, b, and c are constants)d[HOONO]//_dt=a[H⁺][H₂O₂][HNO₂]//_b+c[H2O2], which is consistent with a mechanism in which NO⁺ is formed as an intermediate. Peroxynitrous acid decays immediately after it forms with a first-order rate constant at 273·7 K, k_d, given by: 10²k_d=(5·27 ± 0·10)+(59·6 ± 0·8)[H⁺] s^–1. At 298·1 K, activation parameters for the acid independent pathway are ΔH‡= 19·6 ± 0·2 kcal mol^–1 and ΔS‡= 7·4 ± 0·7 cal mol^–1 K^–1(1 cal = 4·1868 J), and for the acid dependent pathway are ΔH‡= 14·8 ± 0·2 kcal mol^–1 and ΔS‡= 5·3 ± 0·7 cal mol^–1 K^–1. The spectrum of peroxynitrous acid was recorded using a continuous-flow mixing cell.