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

The rate constant of homolysis of peroxynitrite, ONOO-, into O-2(.-) 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 degrees(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 degrees(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.