Peroxynitrate and Peroxynitrite: A Complete Basis Set Investigation of Similarities and Differences between These NOx Species

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Abstract
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 CO2, 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 NH3, H2S, and H2CCH2 by PNA and PNI are found to be generally similar. The O−O BDE of O2NOOCO2- is predicted to be 26 kcal/mol greater than that of ONOOCO2-. This accounts for the insensitivity of PNA to the presence of CO2. Likewise, the O−O BDE of O2NOOH 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 NO2 that is formed from PNI can relax from the 2B2 excited state to the 2A1 ground state, whereas no such comparable state change occurs with NO3 from PNA. Decomposition of the anions OxNOO- is more complex, with the energetics influenced by solvation. ONOO- can homolyze to yield NO/O2-; however, this pathway represents a thermodynamic “dead end” since the radical pairs generated by homolysis should mostly revert to starting material. However, decomposition of O2NOO- yields the stable products NO2-/3O2, a couple that is more stable than starting material. This may occur either by initial formation of NO2/O2- or NO2-/1O2, 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 O2NOO- decomposition kinetics. This is in agreement with the first of two “kinetically indistinguishable” mechanistic possibilities proposed for O2NOO- decomposition (Goldstein, S.; Czapski, G.; Lind, J.; Merényi, G. Inorg. Chem. 1998, 37, 3943−3947).