THEORETICAL-STUDIES OF HYDROGEN-BONDED COMPLEXES OF H3SIO(H)ALH3 WITHOH2, NH3, AND CH4

The hydrogen-bonded complexes ZH ... OH2, ZH ... NH3, and ZH ... CH4 ( where ZH = H3SiO(H)AlH3) have been studied al different levels of ab i nitio molecular orbital theory with the highest level being G2(MP2) th eory. These complexes are models for adsorption of CH4, NH3, and H2O a t acid sites in zeolites. We have also considered complexes where the hydroxyl hydrogen has been transferred, i.e. Z(-) ... HOH2+, Z(-) ... HNH3+, and Z(-) ... HCH4+. Both rigid and relaxed geometries have been considered. The results provide benchmark calculations for future stu dies of molecular interactions at acid sites in zeolites using larger aluminosilicate clusters. The HF/3-21G level of theory systematically overestimates the binding of the complexes by 5-10 kcal mol(-1). When going beyond the HF/6-31G(d) level of theory, electron correlation sta bilizes the complexes while extended basis sets, which include polariz ation and diffuse functions, cause destabilization. Complexation energ ies obtained at the G2(MP2) level are reproduced to better than 1 kcal mol(-1) by the MP2/6-311+G(3df,2p) level of theory, indicating that t he dominant correlation correction is obtained by second-order Moller- Plesset perturbation theory.