Free radical formation in the peroxynitrous acid (ONOOH)/peroxynitrite (ONOO-) system

The rate constant of homolysis of peroxynitrite, ONOO-, into O2- and NO was determined to be 0.017 s-1 at 20 degrees C. In combination with other experimental data taken from the literature, this value yields the Gibbs free energy of formation of ONOO-, delta f G o(ONOO-) = 16.6 kcal/mol. On the basis of this result, we conclude that peroxynitrous acid homolyzes to yield nitrogen dioxide (NO2) and hydroxyl (OH) free radicals and derive delta f G o(ONOOH) = 7.7 kcal/mol. The rate constant of the reaction between NO and ONOO- was found to be 5 x 10(-)2 M-1 s-1 at most. The implications of the two homolysis reactions are discussed.