CRYSTALLOGRAPHIC SITE DISCRIMINATION IN ALUMINOPHOSPHATE MOLECULAR-SIEVES BY SOLID-STATE NMR - INFLUENCE OF THE FIELD-DEPENDENT ISOTROPIC 2ND-ORDER QUADRUPOLAR SHIFT ON HIGH-RESOLUTION AL-27 DOR NMR

The application of double-rotation (DOR) NMR in relatively low magnetic fields proves to be particularly advantageous to enhance the dispersion of the isotropic second-order shifts of quadrupolar nuclei located in different crystallographic sites. As an illustrative example, the low-field high-resolution Al-27 DOR NMR spectrum of the very large pore aluminophosphate VPI-5 is shown; the data fully support the recently reported crystal structure refinement and demonstrate the importance of the isotropic second-order quadrupolar shift for crystallographic site discrimination.