Investigation of Al–O–Si bond angle in glass by 27Al 3Q-MAS NMR and molecular dynamics

The Al–O–Si bond angles were investigated in sodium aluminosilicate glasses with variable calcium content by two complementary methods. We developed the spectrum inversion for the 27Al MQ-MAS to infer quantitative distributions of isotropic chemical shifts, enabling us to estimate an angular distribution. The experimental data were then compared with molecular dynamics simulations. In both cases, the Al–O–Si angle was observed to decrease as the calcium concentration increased. Structural analysis accounts for this effect by the gradually diminishing distance – and therefore the increasing interaction – between the network modifiers and oxygen atoms near aluminum. A similar but less intense effect is predicted for the Si–O–Si angles.