Si-F bonding in aluminosilicate glasses: Inferences from ab initio NMR calculations

Although it has been well accepted that F-Al linkages dominate in F-bearing alkali aluminosilicate glasses, the possibility of F-Si linkages remains unresolved. Recent F-19 NMR results have been interpreted as indicative of not only the presence of the Al-F-Na(n) linkages but also of the presence of Si-F-Na(n) linkages. High level ab: initio NMR calculations were performed to investigate the nature of possible species contributing to the F-19 spectra. The B3LYP/6-31G* level was used to optimize structures and a scaling technique applied to the calculation of NMR properties using both HF and B3LYP methods at the 6-311+G(2df,p) level. The applicability of this scaling method was checked by comparing calculated results against experimental data on several crystalline materials the results are improved over HF or DFT methods alone. The calculation results show that none of the F-Al species investigated can produce the -147 ppm peak of Schaller et al. (1992). However, several species in which F is bonded to fourfold-, fivefold-, and sixfold-coordinated Si produce a F-19 chemical shift of approximately -147 ppm. These results verify the assignments of this peak by Zeng and Stebbins (2000) to tetrahedral Si-F and NaSiF6-like species, and rule out many other possible species. Several additional species such as a fivefold coordinated Si-F species may further contribute to the -147 ppm F-19 peak.