Modeling physisorption with the ONIOM method: the case of NH3 at the isolated hydroxyl group of the silica surface

The ONIOM method has been used to model the isolated hydroxyl group at the silica surface using the octahydrosilasesquioxane cluster. Three different bi-layer clusters have been defined, using five levels of theory for the real system. For the model system, the B3-LYP/DZP level has been adopted for all calculations. Geometries, OH vibrational stretching frequency, NMR isotropic δ(1H) and δ(29Si) chemical shifts and the binding energies of NH3 computed with the ONIOM approach have been compared with the full ab initio results. Whereas the ONIOM(B3-LYP/DZP : HF/3-21G) level gives the best results, the cheapest ONIOM(B3-LYP/DZP : AM1) level can be adopted to model very large portions of siliceous materials.