Quantum chemical calculations on the stability of different conformations of silicate building block structures in relevance to zeolite synthesis

The relative influence of various geometric parameters such as bond lengths , bond angles and dihedral angles on the energetics of silicate structures is studied. The influence of dihedral angles, in particular, on the preferr ed conformation and energy of silicate building blocks is highlighted. The possibility of formation of ring structures and the diameters of the rings will depend on these Si-O-Si-O and O-Si-O-Si dihedral angles. Different con formations were obtained for a dimer cluster model by varying the two types of dihedral angles and their relative stability was studied using quantum chemical calculations. The study of variation of the dihedral angles on the stability of silicate structures is further extended to larger cluster mod els. The importance of dihedral angles is discussed in the context of forma tion of silicate building blocks during the synthesis of zeolites. The comb inations of Si-O-Si-O and O-Si-O-Si dihedral angles, such as 0 degrees and 60 degrees, 120 degrees and 300 degrees etc., leading to staggered adjacent SiO4 groups are energetically preferred. The energetic preference of the s taggered structures over eclipsed structures is an order of magnitude highe r for pentamers cluster models than for dimers cluster models. (C) 2001 Els evier Science B.V. All rights reserved.