Ab initio calculations on ring-shaped silica clusters

Hartree-Fock SCF calcns. have been performed on four ring-shaped clusters, H2nSinO3n, where n ranges from 3 to 6. These clusters consist of a no. of n linked SiO4 tetrahedra. For each cluster, 1st a geometry optimization was performed using the STO3G basis set. This optimization was followed by a single point calcn. using the more flexible 6-31G split-valence basis set. Geometries and total energies of the clusters were compared. Si-O bond lengths decrease as the ring size 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 bond lengths, smaller bond angles) and total energy (less stable). These results support the view that the relative stability of aluminum-free zeolite lattices, contg. many rings of linked tetrahedra, is not detd. by local bonding effects, and will be insensitive to the particular lattice structure