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 (CQO and CQH) and 1H chemical shift (δ1H) 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 CQO, CQH, and δiH 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 CQO and CQH and larger δiH. 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 CQO and CQH and small δiH. 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 CQO, 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry