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 re al system. For the model system, the B3-LYP/DZP level has been adopted for all calculations. Geometries, OH vibrational stretching frequency, NMR isot ropic delta(H-1) and delta(Si-29) 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 be st results, the cheapest ONIOM(B3-LYP/DZP: AM1) level can be adopted to mod el very large portions of siliceous materials. (C) 2001 Elsevier Science B. V. All rights reserved.