Ab initio calculation of the¹⁷O and¹H NMR parameters for various OH groups: Implications to the speciation and dynamics of dissolved water in silicate glasses

Ab initio molecular orbital calculations have been carried out for silicate, aluminosilicate, and aluminate clusters to study the NMR characteristics of various types of hydroxyls (OH) that are possibly present in hydrous silicate glasses and melts. The clusters have been optimized with the density functional theory (B3LYP/ 6-311+G(2df,p)) and their NMR parameters calculated at HF/6-311+G(2df,p). Our calculations suggest that the ¹⁷O and ²H quadrupolar coupling constants (C_Q^O and C_Q^H) and ¹H chemical shift (δ₁^H) of SiOH, AlOH, and bridging OH's (Si(OH)Al, Al(OH)Al) all show good correlation with the O-H and O-H-O distances. The calculated C_Q^O, C_Q^H, and δ_i^H values agree well with those of the experimental data for OH groups with similar O-H⋯O distances in crystalline phases. Hydroxyls with stronger hydrogen bonding tend to yield smaller C_Q^O and C_Q^H and larger δ_i^H. SiOH and bridging OH's of comparable hydrogen-bonding strengths give similar ¹⁷O and ¹H (²H) NMR parameters. A1OH have a tendency not to form strong AL-O-H-⋯O type hydrogen bonding and, thus, give relatively large C_Q^O and C_Q^H and small δ_i^H. On the basis of these calculation results, together with information for hydrogen-bonding strengths estimated from experimental vibrational spectra and ¹H NMR data, we were able to predict ¹⁷O NMR parameters for hydroxyls in hydrous silicate glasses. The observed ¹⁷O NMR peaks for silica gel and hydrous albite glass, that have been attributed to SiOH, are significantly narrower than expected from C_Q^O, suggesting that at least some of the SiOH, if present, must be nonrigid. The observed broad ¹⁷O NMR peaks for hydrous albite and alkali silicate glasses, originally attributed to molecular H₂O, could equally well be ascribed to rigid hydroxyls with weak hydrogen bonding.