Formation and Decay of Peroxynitrous Acid: A Pulse Radiolysis Study

Article abstract of DOI:10.1021/j100127a016

Peroxynitrous acid and peroxynitrite anion have been studied using pulse radiolysis of nitrite and nitrate solutions.The formation rate constant determined to be k(OH + NO2) = (4.5 +/- 1.0) * 10⁹ M^-1 s^-1, and the rate constant for the OH radical reaction with nitrite is determined to be k(OH +NO2^-) = (6.0 +/- 1.0) * 10⁹ M^-1 s^-1.In nitrate solutions, the competing reaction between OH and NO3^2- is found to have a rate constant of k(OH + NO3^2- = (3.0 +/- 1.0) * 10⁹ M^-1 s^-1.The intermediate species in the nitrate system, NO3^2-, HNO3^-, and H2NO3, decay into NO2 according to the first-order rate constants: (5.6 +/ - 0.5) * 10⁴, (2.0 +/- 0.5) * 10⁵, and (7.0 +/- 2.0) * 10⁵ s^-1, respectively.The rate constants k(H + NO3^-) = (1.0 +/- 0.3) * 10⁷ M^-1 s^-1 and k(H + NO2) = (1.0 +/- 0.2) * 10¹⁰ M^-1 s^-1 were also determined.The pK_a of NOOH is found to be 6.5 +/- 0.1 by absorption measurements, and the maximum extinction coefficient at 240 nm is ε₂₄₀(ONOOH) = 770 +/- 50 M^-1 cm^-1.The decay of peroxynitrous acid is detrmined to proceed through the first-order isomerization of ONOOH to HNO3 according to the rate equation k_obs = k_iso/(1 + K_a/+>) with rate constants k_iso = 1.0 +/- 0.2 s^-1 and K_a = (1.0 +/- 0.3) * 10^-7.A comparison of all available literature values for the pK_a and the decay rate is reported.