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

High resolution O-17 NMR spectra of siliceous ferrierite (Sil-FER) have been collected and the Si-29 and O-17 isotropic chemical shifts and the electric field gradients of oxygen have been calculated from first principles. The theoretical Si-29 MAS NMR spectrum is found to be in excellent quantitative 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 coupling constants and asymmetry parameters show only qualitative agreement with the experimental O-17 NMR spectra obtained by Double Rotation (DOR) and multiple quantum magic angle spinning (MQMAS) as the spectra are much more complex (10 peaks within a shift range of less than 15 ppm, and the quadrupolar 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 possible. These findings also demonstrate the current limitations of the experimental techniques and show that no simple correlation appears to exist between the zeolite structure, such as the Si-O-Si bond angles or lengths, and the O-17 NMR parameters.