Peroxynitrous acid decomposes via homolysis: Evidence from high-pressure pulse radiolysis

The effect of pressure on the decomposition rate of peroxynitrous acid was studied using the pulse radiolysis technique. Peroxynitrous acid was produced at pH 4.1 by irradiation of (i) aerated solution containing 0.15 M formate and 3 mM nitrite and (ii) deaerated solutions containing 3 mM nitrate and 4 mM phosphate. The volume of activation for the decomposition of ONOOH in these systems was determined to be 10.3 ± 1.5 cm3 mol-1. The activation volume at pH 5.6 using the second system was found to be 10.5 ± 1.1 cm3 mol-1. The activation volume changes upon increasing the pH as pressure affects the pKa values of ONOOH and H2PO4-, which equal 6.6 and 7.2 (I1 = 0) at ambient pressure, respectively. We therefore conclude that the decomposition of ONOOH at pH 4.1 is characterized by a significant positive volume of activation, ΔV‡ = 10.3 ± 1.5 cm3 mol-1, suggesting a bond breakage process that is not accompanied by charge creation, namely, homolysis of ONOOH into •NO2 and •OH. The volume of the ionization of ONOOH has been determined from the dependence of ΔV‡ on the pH to be -7.2 ± 2.1 cm3 mol-1.