A simple correlation between average T-O-T angles and 27Al NMR chemical shifts does not hold in high-silica zeolites

The dependence of the Al-27 NMR shielding on the framework aluminum position and on the charge compensating cation coordination and hydration was investigated for MCM-58 zeolite. NMR shielding constants were calculated with the large cluster models (about 20 framework tetrahedra) at the geometries optimized at the periodic DFT level, employing the PW91 exchange-correlation functional. A simple linear correlation between Si-29 NMR chemical shifts and average T-O-T angles was confirmed for all-silica ITQ-4. On the contrary, the correlation between Al-27 NMR chemical shifts and average T-O-T angles (obtained from the diffraction experiments) does not hold. The ordering of the average T-O-T angles for individual T-sites is different in the allsilica form than in the hydrated or dehydrated sodium form of zeolite. In addition, the Al-27 NMR shielding does not depend only on the average T-O-T angle but it also depends on the extraframework cation coordination and hydration. Calculated and experimental data are in very good agreement. It is concluded that the experimental Al-27 NMR spectra of high-silica zeolites can be interpreted based on the theoretical modeling and a combination of experimental and theoretical work may bring information about the framework Al atom localization. (c) 2005 Elsevier Inc. All rights reserved.