Molecular orbital models of silica clusters modified by fluorine

AM1 molecular orbital calculations were conducted for various cluster models of silica and for those of fluorinated silica. Remarkably, the addition of HF to silica clusters gave an F-Si bond forming a trigonal bipyramid geometry around a 5-coordinated Si. The heat of addition reaction of HF was maximum for 4-membered rings and decreased with an increasing number of Si tetrahedra in ring-type silica clusters, while it increased with an increasing number of Si tetrahedra in block-type silica clusters. The addition reaction was strongly dependent on both whether there is sufficient structural flexibility around the Si sites to allow the formation of intramolecular hydrogen bonds with neighbouring silanol groups in the addition of HF and the degree of the strain of the arrangement for [SiO_4/2F]^-. The heat of reaction for the replacement of one hydroxyl group (OH^-) by F^- decreased with increasing number of Si tetrahedra. The replacement reaction was strongly dependent on both the size of the rings and the number of bridging oxygens per Si tetrahedron.