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

The ONIOM method has been used to model the isolated hydroxyl group at the silica surface using the octa-hydrosilasesquioxane 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 vibra-tional stretching frequency, NMR isotropic dð1HÞ and dð29SiÞ chemical shifts and the binding energies of NH3 com-puted 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. 2001 Elsevier Science B.V. All rights reserved. 1