AB INITIO CALCULATIONS ON RING-SHAPED SILICA CLUSTERS

Hartree-Fock SCF calculations have been performed on four ring-shaped clusters, H2nSinO3n, where n ranges from 3 to 6. These clusters consist of a number of n linked SiO4 tetrahedra, with the dangling oxygen bonds saturated with hydrogen atoms. C-nv symmetry was imposed. For each cluster, first a geometry optimisation was performed using the STO3G basis set. This optimisation was followed by a single point calculation using the more flexible 6-31G split-valence basis set. Geometries and total energies of the clusters were compared. Si-O bondlengths are found to decrease as the ringsize increases. The total energy per elementary building unit decreases similarly. The differences found between the rings for n = 4, 5 and 6 are very small. The 3-ring on the other hand is significantly different in both geometry (longer bondlengths, smaller bond angles) and total energy (less stable). These results support the view that the relative stability of aluminium-free zeolite lattices, containing many rings of linked tetrahedra, is not determined by local bonding effects, and will be insensitive to the particular lattice structure.