A high-resolution O-17 and Si-29 NMR study of zeolite siliceous ferrieriteand ab initio calculations of NMR parameters

High resolution O-17 NMR spectra of siliceous ferrierite (Sil-FER) have bee n collected and the Si-29 and O-17 isotropic chemical shifts and the electr ic field gradients of oxygen have been calculated from first principles. Th e theoretical Si-29 MAS NMR spectrum is found to be in excellent quantitati ve agreement with the experimentally determined spectrum, and is extremely sensitive to the accuracy of the structure used for the calculations, thus providing a method for assessing the quality of the structure determination . However, theoretical predictions of the chemical shifts, quadrupolar coup ling constants and asymmetry parameters show only qualitative agreement wit h the experimental O-17 NMR spectra obtained by Double Rotation (DOR) and m ultiple quantum magic angle spinning (MQMAS) as the spectra are much more c omplex (10 peaks within a shift range of less than 15 ppm, and the quadrupo lar coupling constants only differ by 0.4 MHz) and hence higher accuracy is required from the shift calculations (>0.5 ppm), which is currently not po ssible. These findings also demonstrate the current limitations of the expe rimental techniques and show that no simple correlation appears to exist be tween the zeolite structure, such as the Si-O-Si bond angles or lengths, an d the O-17 NMR parameters.