Ab initio studies of possible fluorine-bearing four- and fivefold coordinated Al species in aluminosilicate glasses

Ab initio NMR gauge-including atomic orbital (GIAO) calculations were used to constrain assignments of resonances in ²⁷ AI 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 AlF₅^2-, AlF₃O₂ (SiH₃)₂^2-, and AlF₂O₃ (SiH₃)₃^2- fit the experimental requirements well, whereas the remaining 5-fold coordinated species that were tested [AlF₄O(SiH₃)^2-, and AlFO₄ (SiH₃)₄^2-] did not.