The reaction mechanism of ozone with the NO and NO2 oxides

Ab initio calculations using Hartree Fock (HF), density functional theory (DFT), complete active space self-consistent field (CASSCF), second order Moller-Plesset perturbation theory (MP2) and coupled cluster with singles and doubles substitutions (CCSD) quantum chemistry models have been made on all of the four reactions in which the mono-nitrogen oxides react with ozone to generate higher oxidation states culminating in N2O5. The relative reaction energies were determined and all reactions were found to be exothermic. Potential energy surfaces of the O3-NO and O3-NO2 reactions were modeled using the HF, MP2, DFT and CASSCF methods and the presence of a transition state was indicated in the HF, MP2 and CASSCF calculations but not in the DFT models. The MP2 level of theory was further applied with three different basis function sets and finer potential energy scan resolution and the activation energies, Ea, determined. For the O3-NO reaction there was reasonably good correlation with the experimental Arrhenius activation energy of 1.2 kcal/mol. However, for the O3-NO2 reaction the calculated activation energy was a factor of 4 higher than the experimental Arrhenius activation energy of 2.2 kcal/mol.