Vertical decorative line
Research Center for Complex Molecular Systems and Biomolecules
Center for Biomolecules and Complex Molecular Systems   
Supported by the Ministry of Education, Czech Republic      
Decorative element
Decorative element
Decorative element Research | Papers | People | Events | Archive | Teach | Contact   
Decorative element
Empty gif
Empty space gif.
Abstract of the article
Empty gif Empty gif
A simple correlation between average T-O-T angles and 27Al MAS NMR chemical shifts does not hold in high-silica zeolites.

Kucera, J.; Nachtigall, P., Microporous and Mesoporous Materials, 85 [3] 279 - 283 (2005)

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.


<< Go back
Empty space gif.
Empty gif
Decorative element