The inter-conversion of nitrogen oxides and oxy acids on silica surfaces is of major atmospheric importance. As a preliminary step towards rationalising experimental observations, and understanding the mechanisms behind such reactions we have looked at the binding energies of NO2, N2O4, HNO3, HONO and H2O with simple proxies of a silica surface, namely SiH3OH and Si(OH)4 units. The geometries of these molecular clusters were optimised at both HF/6-311+G(d) and B3LYP/6-311+G(d) level of theory. The SCF energies of the species were determined at the HF/6-311++G(3df,2pd) and B3LYP/6-311++G(3df,2pd) level. The values indicate that nitric acid is by far the most strongly bound species, in agreement with experimental observations. It was also found that the dimer N2O4 is significantly more strongly bound to the Si(OH)4 and SiH3OH units than NO2 itself. The vibrational frequencies calculated for the hydrogen-bonded complexes are compared to the experimentally observed frequencies of the adsorbed species where possible
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