THERMODYNAMICS OF PEROXYNITRITE AND ITS CO2 ADDUCT

The equilibrium constant, K-3, Of aqueous homolysis of peroxynitrous a cid 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(f)H degrees(ONOO-,aq) = -10.8 k caV mol and the enthalpy of ionic dissociation of ONOOH(aq), Delta H d egrees(1) = 0 kcal/mol, as well as of the entropy of gaseous ONOOH, S degrees(ONOOH,g) = 72 eu. Furthermore we assumed the entropy of hydrat ion of ONOOH, Delta S degrees(2), to be -25 eu, a value closely bracke ted by the hydration entropies of analogous substances. The rate const ant of radical recombination of OH. with NO2. to yield ONOOH, k(-3), w as resimulated from experimental data and found to be ca. 5 x 10(9) M- 1 s(-1) Together with the estimated K-3, this yields the homolysis rat e constant k(3) = 2.5 s(-1). This value is close to 0.5 s(-1), the rat e constant of formation of a reactive intermediate during the isomeriz ation of peroxynitrous acid to nitrate. Our thermodynamic estimate is therefore consistent with substantial amounts of OH. and NO2. free rad icals being formed in this process. The thermodynamic implications for the carbon dioxide/peroxynitrite system are also discussed.