Abstract

Ab initio NMR gauge-including atomic orbital (GIAO) calculations were used to constrain assignments of resonances in 27Al NMR spectra of F-bearing alkali aluminosilicate glasses. The effect of bond angles within the range 126–150° on the chemical shift was investigated using cluster models of next-nearest atoms that are charge balanced by hydrogen atoms. GIAO calculations used geometries obtained through optimization at fixed Al-O-Si bond angles. The calculated peak positions for all of the 4-fold coordinated Al species yielded calculated 27Al NMR peak positions in good agreement with the experimental data, suggesting that any or all of the species AlF4, AlF3O(SiH3), AlF2O2(SiH3)2, and AlFO3(SiH3)3 may be present. Three of the investigated 5-fold coordinated species AlF52−, AlF3O2(SiH3)22−, and AlF2O3(SiH3)32− fit the experimental requirements well, whereas the remaining 5-fold coordinated species that were tested [AlF4O(SiH3)2−, and AlFO4(SiH3)42−] did not.

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