Peroxynitrate and peroxynitrite: a complete basis set investigation of similarities and differences between these NOx...

Peroxynitric acid/peroxynitrate (PNA) rivals peroxynitrous acid/peroxynitrite (PNI) in importance as a reactive oxygen species. These species possess similar two-electron oxidative behavior. On the other hand, stark differences exist in the stability of these molecules as a function of pH and in the presence of CO(2), and also in the types of bond homolysis reactions that PNA and PNI may undergo. Using CBS-QB3 theory, we examine these similarities and differences. The activation barriers for two-electron oxidation of NH(3), H(2)S, and H(2)C=CH(2) by PNA and PNI are found to be generally similar. The O-O BDE of O(2)NOOCO(2-) is predicted to be 26 kcal/mol greater than that of ONOOCO(2-). This accounts for the insensitivity of PNA to the presence of CO(2). Likewise, the O-O BDE of O(2)NOOH is predicted to be 19 kcal/mol greater than that of ONOOH, in excellent agreement with experiment. The fundamental principle underlying the large differences in O-O BDEs between PNA and PNI species is that the NO(2) that is formed from PNI can relax from the (2)B(2) excited state to the (2)A(1) ground state, whereas no such comparable state change occurs with NO(3) from PNA. Decomposition of the anions O(x)NOO(-) is more complex, with the energetics influenced by solvation. ONOO(-) can homolyze to yield NO/O(2-); however, this pathway represents a thermodynamic "dead end" since the radical pairs generated by homolysis should mostly revert to starting material. However, decomposition of O(2)NOO(-) yields the stable products NO(2-)/(3)O(2), a couple that is more stable than starting material. This may occur either by initial formation of NO(2)/O(2-) or NO(2-)/(1)O(2), with the latter intermediates thermodynamically favored both in the gas phase and in solution. Given the extremely fast back-reaction of the homolysis products, heterolysis probably dominates the observed O(2)NOO(-) decomposition kinetics. This is in agreement with the first of two "kinetically indistinguishable" mechanistic possibilities proposed for O(2)NOO(-) decomposition (Goldstein, S.; Czapski, G.; Lind, J.; Merényi, G. Inorg. Chem. 1998, 37, 3943-3947).