NMR Characteristics of Possible Oxygen Sites in Aluminosilicate Glasses and Melts: An ab Initio Study

Ab initio molecular orbital calculations have been carried out for silicate, aluminosilicate, and aluminate clusters in order to study the NMR characteristics of oxygen sites that are possibly present in the structure of aluminosilicate glasses and melts. Of particular interest are the different types of bridging oxygens (oxygens bonded to two tetrahedrally coordinated Si and/or Al) and tricluster oxygens (oxygens linked to three tetrahedrally coordinated Si and/or Al). The calculated values for the ¹⁷O quadrupolar coupling constants (QCC) of Si-O-Si, Si-O-Al, and Al-O-Al bridging oxygens agree reasonably well with experimental NMR data for similar oxygen sites in crystalline silicates, aluminosilicates, or aluminates, and do not show significant dependence on the basis set and theory (Hartree-Fock vs density functional theory) of calculations. The calculated values for the ¹⁷O isotropic chemical shift (δ_i^O), on the other hand, show large dependence on the basis set and method of calculations, although the relative differences in δ_i^O for the clusters of interest do not vary as much. Our calculations indicate that bridging oxygens give progressively larger ¹⁷O QCC and larger, but overlapping, δ_i^O from Al-O-Al to Si-O-Al to Si-O-Si. Coordination of Si-O-Al or Al-O-Al bridging oxygens to Ca²⁺, a network-modifying cation, tends to decrease their ¹⁷O QCC and increase their δ_i^O, consistent with experimental NMR data. Tricluster oxygens give progressively larger ¹⁷O QCC and larger δ_i^O values from O(Al₃), to O(SiAl₂), to O(Si₂Al), to O(Si₃). The O(Al₃) and O(SiAl₂) tricluster oxygens have ¹⁷O NMR characteristics overlapping with those of Si-O-Al bridging oxygens, and the O(Si₂Al) tricluster oxygen overlapping with those of Si-Q-Si bridging oxygens. Our calculations suggest that it may be difficult to distinguish O(Al₃), O(SiAl₂), or O(Si₂Al) tricluster oxygens from bridging oxygens in aluminosilicate glasses on the basis of ¹⁷O NMR data alone, although it should be possible to distinguish O(Al₃) tricluster oxygen from Al-O-Al bridging oxygen in aluminates using ¹⁷O NMR. The O(Si₃) tricluster have larger ¹⁷O QCC and larger δ_i^O values than all types of bridging oxygens and thus may be unambiguously identified on the basis of ¹⁷O NMR data, if present in the structure of aluminosilicates or silicates.