Thermodynamics of peroxynitrite and its CO2 adduct

The equilibrium constant, K3, of aqueous homolysis of peroxynitrous acid into hydroxyl and nitrogen dioxide free radicals was estimated to be 5 x 10(-10) M. This value was derived from a thermodynamic cycle by use of the experimentally known delta fH degree(ONOO-,aq) = -10.8 kcal/mol and the enthalpy of ionic dissociation of ONOOH(aq), delta H degree 1 = 0 kcal/mol, as well as of the entropy of gaseous ONOOH, S degree(ONOOH,g) = 72 eu. Furthermore we assumed the entropy of hydration of ONOOH, delta S degree 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 x 10(9) M-1 s-1. Together with the estimated K3, this yields the homolysis rate constant k3 = 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.