An investigation of NO3 as a possible intermediate in the oxidation of nitric oxide

The lowest potential energy surfaces of NO3 have been investigated using CASSCF and contracted CI methods, to determine if NO3 is an intermediate in the oxidation of nitric oxide. This reaction is observed to be termolecular where an intermediate is almost certainly formed. The calculations, which were performed in C2v symmetry, show that the form of NO3 which has a near D3h geometry should not be possible to reach from ground state NO and O2. The energy of this isomer of NO3 is calculated to be 25 kcal/mol higher in energy than separated NO and O2. Furthermore, NO + O2 is separated from NO3 by very high barriers. The C2v constrained barrier is as high as 145 kcal/mol, which makes it very unlikely that NO3 could be formed even along less symmetric pathways. The origin of the barriers are analyzed, and it is shown that a special type of mechanism is involved for the state with the lowest barrier. In this mechanism, which was observed recently also for NiH2, the occupation in each symmetry changes only by one unit in each step of the reaction. The only isomer of NO3 which could be formed is a weak van der Waals complex with a very long bond distance of 6.5 a.u. N2O2 does, however, in this respect seem to be a more likely intermediate with its shorter bond distance of 4.5 a.u. Calculations around the D3h equilibrium further show that 2B2 is the ground state of this isomer of NO3 with 2A2 and 2B1 slightly higher in energy.