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 pK_a 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.