Ab Initio Study of the Reaction of NO3 with the OH Radical

High level ab initio study of the structure and vibrational spectra of HO2NO2

A high-level ab initio study has been performed on the conformational structure and vibrational spectra of HO(2)NO(2). Calculations carried out with coupled-cluster methods using a series of Pople and Dunning basis sets reveal that there is a significant basis set dependence on the predicted ab initio structure. Higher angular momentum basis sets are shown to be necessary in order to bring the calculated structure into agreement with experimental rotational constants. Harmonic vibrational frequencies of HO(2)NO(2) are computed at the CCSD(T)/aug-cc-pVTZ level of theory while the corresponding vibrational anharmonicities are calculated at the MP2/cc-pVTZ level. In addition, the absorption cross sections of OH stretching overtones in HO(2)NO(2) are calculated using a dipole function computed at the QCISD level of theory and found to be in good agreement with the available experimental data.

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

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).