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

Abstract

Ab initio NMR gauge-including atomic orbital (GIAO) calculations were used to constrain assignments of resonances in 27 Al 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 27 Al NMR peak positions in good agreement with the experimental data, suggesting that any or all of the species AlF 4 - , AlF 3 O(SiH 3 ) - , AlF 2 O 2 (SiH 3 ) 2 - , and AlFO 3 (SiH 3 ) 3 - may be present. Three of the investigated 5-fold coordinated species AlF 5 2- , AlF 3 O 2 (SiH 3 ) 2 2- , and AlF 2 O 3 (SiH 3 ) 3 2- fit the experimental requirements well, whereas the remaining 5-fold coordinated species that were tested [AlF 4 O(SiH 3 ) 2- , and AlFO 4 (SiH 3 ) 4 2- ] did not.