Can O&dbd;NOOH Undergo Homolysis?

Recent thermodynamic calculations of Merényi and Lind [(1997) Chem. Res. Toxicol. <BO>10</BO>, 1216−1220] suggest that O&dbd;NOOH can undergo homolysis to form the hydroxyl radical and nitrogen dioxide. This result is based in part on our statement that the enthalpy of ionization of O&dbd;NOOH is close to zero [Koppenol et al. (1992) Chem. Res. Toxicol. <BO>5</BO>, 834−842]. As the ionization of O&dbd;NOOH is sensitive to the milieu and the rate of isomerization (to nitrate) to the total concentration of O&dbd;NOOH and O&dbd;NOO^- [Kissner et al. (1997) Chem. Res. Toxicol. <BO>10</BO>, 1285−1292], we reinvestigated the temperature dependence of the ionization constant and determined a ΔH° of 4 ± 2 kcal mol^-1. This results in a standard Gibbs energy of homolysis of 16 kcal mol^-1 and a rate of homolysis of 1 × 10^-2 s^-1. Given the uncertainty in the Gibbs energy of homolysis, upper and lower rates are 1 × 10^-4 and 0.6 s^-1, slower than the rate of isomerization, 1.2 s^-1 at 25 °C. The recombination of the homolysis products NO<INF>2</INF>^• and HO^• is known to lead to mainly peroxynitrous acid. If one assumes that a few percent of the recombinations lead to nitrate instead, then the rate of homolysis must be much higher than the rate of isomerization. We conclude therefore that homolysis is unlikely.