Prediction of 29Si MAS NMR chemical shifts in zeolites using density functional theory

Abstract

The sum-over-states density functional perturbation theory has been used to calculate 29Si shielding constants and chemical shifts in zeolites. The calculations were carried out on one-site (1T) Si(OSiH 3) 4 and two-site (2T) R 3SiOSiR 3 (R=OSiH 3) models, including three coordination shells around each site. The 29Si NMR chemical shifts are shown to be very sensitive to the local geometry. A linear correlation between chemical shifts and average SiOSi angles has been established, taking into account two different zeolites, i.e. mazzite and zeolite-β. The use of 1T models allows the assignment of the experimental spectra, whereas that of 2T models, containing eventually four-membered rings, improves considerably the calculation of the absolute 29Si chemical shifts, including those of silicon sites in aluminated zeolites.