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

The sum-over-states density functional perturbation theory has been used to calculate Si-29 shielding constants and chemical shifts in zeolites. The c alculations were carried out on one-site (1T) Si(OSiH3)(4) and two-site (2T ) R3SiOSiR3 (R=OSiH3) models, including three coordination shells around ea ch site. The Si-29 NMR chemical shifts are shown to be very sensitive to th e local geometry. A linear correlation between chemical shifts and average SiOSi angles has been established, taking into account two different zeolit es, i.e. mazzite and zeolite-beta. The use of 1T models allows the assignme nt of the experimental spectra, whereas that of 2T models, containing event ually four-membered rings, improves considerably the calculation of the abs olute Si-29 chemical shifts, including those of silicon sites in aluminated zeolites. (C) 1999 Elsevier Science B.V. All rights reserved.