New NMR techniques for the study of quadrupolar nuclei

The recently-developed multiple-quantum MAS (MQMAS) NMR experiment allows the acquisition of high-resolution, isotropic spectra of half-integer quadrupolar nuclei through the removal of the second-order quadrupolar broadening. Much interest has been generated in the combination of this high-resolution experiment with methods for obtaining structural information, such as cross-polarization. This technique, used with great success in the NMR of spin I = 1/2 nuclei, provides information on the spatial proximity of atoms. However, cross-polarization involving quadrupolar nuclei is an inherently more complex process, the mechanism of which is poorly understood. This thesis is concerned with further developments in the high-resolution NMR of quadrupolar nuclei in solids using the MQMAS experiment and, in particular, with novel techniques involving the combination of MQMAS with cross-polarization. The application of cross-polarization to quadrupolar nuclei is considered and the novel possibility of cross-polarization directly to the multiple-quantum coherences of a quadrupolar nucleus in a powdered sample is proposed. A theoretical and experimental investigation of both single- and multiple-quantum cross-polarization in static samples is undertaken. The intensity and lineshape variations observed in cross-polarized NMR spectra are modelled by computer simulations and the relationship between cross-polarization and quadrupolar nutation is demonstrated. The effects of MAS upon the cross-polarization process are then discussed in the light of these findings using both simulations and experimental methods. The combination of direct multiple-quantum cross-polarization with MQMAS in novel two-dimensional techniques is demonstrated, allowing spectral editing of the high-resolution NMR spectra of quadrupolar nuclei. Examples of cross-polarization to triple-, five- and seven-quantum coherences of 23Na, 17O,27Al and 45Sc are shown. Additional two-dimensional techniques involving cross-polarization to single-quantum coherences are also discussed. The MQMAS and cross-polarized MQMAS experiments are then applied to systems of geochemical and industrial interest. An 17O NMR study of silicate minerals and an 27Al NMR study of amorphous alumino silicates are undertaken, and the ability of these techniques to provide additional insights into the nature of these materials is investigated. (author)