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 27Al 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 29Si NMR chemical shifts and average T-O-T angles was confirmed for all-silica ITQ-4. On the contrary, the correlation between 27Al 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 all-silica form than in the hydrated or dehydrated sodium form of zeolite. In addition, the 27Al 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 27Al 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.