AB INITIO CALCULATION OF 1H, 17O, 27AL AND 29SI NMR PARAMETERS, VIBRATIONAL FREQUENCIES AND BONDING ENERGETICS IN HYDROUS SILICA AND NA-ALUMINOSILICATE GLASSES

Abstract

Ab initio, molecular orbital (MO) calculations were performed on model systems of SiO2, NaAlSi3O8 (albite), H2O-SiO2 and H2O-NaAlSi3O8 glasses. Model nuclear magnetic resonance (NMR) isotropic chemical shifts (δiso) for 1H, 17O, 27Al and 29Si are consistent with experimental data for the SiO2, NaAlSi3O8, H2O-SiO2 systems where structural interpretations of the NMR peak assignments are accepted. For H2O-NaSi3AlO8 glass, controversy has surrounded the interpretation of NMR and infrared (IR) spectra. Calculated δiso1H, δiso17O, δiso27Al and δiso29Si are consistent with the interpretation of Kohn et al. (1992) that Si-(OH)-Al linkages are responsible for the observed peaks in hydrous Na-aluminosilicate glasses. In addition, a theoretical vibrational frequency associated with the Kohn et al. (1992) model agrees well with the observed shoulder near 900 cm−1 in the IR and Raman spectra of hydrous albite glasses. MO calculations suggest that breaking this Si-(OH)-Al linkage requires ∼+56 to +82 kJ/mol which is comparable to the activation energies for viscous flow in hydrous aluminosilicate melts.