Direct molecular simulation of rovibrational relaxation and chemical reactions in air mixtures

We present direct molecular simulations (DMS) of internal energy relaxation in air mixtures. We extract characteristic times of rotational and vibrational relaxation over a wide temperature range for the diatom-diatom interactions N₂ – N₂, O₂ – O₂, N₂ – O₂ and O₂ – N₂, as well as for diatom-atom interactions N₂ – N, O₂ – O, O₂ – N, etc. These results rely entirely on first-principles calculations, where a set of potential energy surfaces (PESs) constitute the sole model input. Where possible, we compare our DMS results with experimentally derived relaxation times and the standard correlation-based estimates for τ^vib from Millikan & White and Park. Furthermore, we apply the DMS method to the simulation of coupled internal energy relaxation, molecular dissociation and exchange reactions in 5-species air (N₂, N, O₂, O, NO) at high temperature. These calculations incorporate the full set of ab initio potentials currently available from the University of Minnesota computational chemistry group. This setup allows us to study the effect of competing reactions in the consumption of molecular oxygen and nitrogen and simultaneous formation of nitric oxide. Special emphasis is placed on extracting the rate coefficients for the two Zel’dovich exchange reactions during the quasi-steady-state (QSS) dissociation phase. These QSS rate coefficients are then compared to thermal rate coefficients for the same reactions obtained via quasi-classical trajectory (QCT) calculations on the same PESs. Our comparisons strongly suggest that both exchange reactions proceed at their thermal rates, without any vibrational biasing. The same is not true for the dissociation reactions during QSS.