Unambiguous Description of the Oxygen Environment in Multicomponent Aluminosilicate Glasses from ¹⁷O Solid State NMR Computational Spectroscopy

Classical molecular dynamics simulations, density functional theory calculations, and spin-effective Hamiltonians have been used to simulate the ¹⁷O MAS and 3QMAS NMR spectra of Ca–Na silicate and aluminosilicate glasses and melts employed as simplified models for basaltic, andesitic, and rhyolitic magmas. The direct comparison of the theoretical NMR spectra of molecular dynamics derived structural models with the experimental counterparts available in the literature has allowed the investigation of the nature of nonframework cation mixing and the extent of intermixing among framework units in Na–Ca aluminosilicate glasses. In particular, in agreement with previous experimental evidence, the results show a nonrandom distribution of the network-modifying Ca and Na in soda-lime glasses with the prevalence of dissimilar Na–Ca pairs around nonbridging oxygens. The oxygen sites are not completely resolved in the MAS spectra of the aluminosilicate glasses. On the contrary, in the ¹⁷O 3QMAS spectra the multiple oxygen sites, in particular the Si–O–Si, Al–O–Al, Al–O–Si, and the nonbridging oxygen peaks, are distinguishable. The small amount of Al–O–Al sites found in the investigated glasses reveals that the Al avoidance rule is not respected in amorphous solids. The Si–O–Al sites are surrounded by Na ions, which play a preferential role as a charge-balancing cation, while Ca can act as a network-modifying cation. Finally, correlations between the structural characteristic and the values of the NMR parameters have been attempted with the aim of helping the interpretation of NMR spectra of glasses with similar compositions.