Modeling 29Si Chemical Shift in Crystalline and Amorphous Silicas

Abstract

Using quantum-chemical calculations on a series of Q^4 silicate clusters, (H_3SiO)_3--Si--O--Si--(OSiH_3)_3, we examined the effects of changing Si-O-Si angle, Si-O distance, and O-Si-Si-O dihedral angle on the Si-29 isotropic chemical shift. We found a strong linear dependence of the chemical shift on the bridging oxygen s-character with variation in Si-O-Si angle. Furthermore, we propose a more accurate functional form for the mean bridging oxygen s-character in terms of the mean Si-O-Si angle. Through the use of principal component analysis we were able to obtain a reliable model predicting the Si-29 isotropic chemical shifts of Q^4 sites using strictly mean values of the Si-O-Si angle and Si-O distance, making model useful to amorphous and crystalline materials alike. This model is cross-validated (10-fold) using experimental Si-29 chemical shift data from 13 different crystalline silicas, with a total of 60 distinct Q^4 sites. Our chemical shift model not only gives a more accurate prediction of experiment of the Si-29 chemical shift of Q^4 sites, but it does so by requiring only two structural parameters, i.e., mean angle and distance. In contrast, previously existing models require 8 parameters, i.e., all four individual angles and four individual distances around a Q^4 site.