Thermodynamics of Peroxynitrite and Its CO2 Adduct

The equilibrium constant, K 3, of aqueous homolysis of peroxynitrous acid into hydroxyl and nitrogen dioxide free radicals was estimated to be 5 × 10-10 M. This value was derived from a thermodynamic cycle by use of the experimentally known Δf H°(ONOO-,aq) = −10.8 kcal/mol and the enthalpy of ionic dissociation of ONOOH(aq), ΔH°1 = 0 kcal/mol, as well as of the entropy of gaseous ONOOH, S°(ONOOH,g) = 72 eu. Furthermore we assumed the entropy of hydration of ONOOH, ΔS°2, to be −25 eu, a value closely bracketed by the hydration entropies of analogous substances. The rate constant of radical recombination of OH• with NO2 • to yield ONOOH, k -3, was resimulated from experimental data and found to be ca. 5 × 109 M-1 s-1. Together with the estimated K 3, this yields the homolysis rate constant k 3 = 2.5 s-1. This value is close to 0.5 s-1, the rate constant of formation of a reactive intermediate during the isomerization of peroxynitrous acid to nitrate. Our thermodynamic estimate is therefore consistent with substantial amounts of OH• and NO2 • free radicals being formed in this process. The thermodynamic implications for the carbon dioxide/peroxynitrite system are also discussed.