Solid state NMR methods for the study of oriented systems : New techniques and applications

Abstract

Nuclear Magnetic Resonance was first observed in 1946 by two independent groups; Purcell et al. at Harvard University and Bloch et al. at Stanford University. Since then, NMR spectroscopy has made such phenomenal progress that it is now one of the most powerful spectroscopic techniques used in various branches of science. Of the many systems for which NMR spectroscopy has been extensively used, systems which retain partial order such as liquid crystals, oriented polymers, molecules oriented by electric and magnetic fields form an interesting class of compounds. Their NMR spectra can exhibit features of both the liquid and the solid state such as chemical shift, J couplings, dipolar and quadrupolar couplings and the study of these systems constitutes an interesting and important area. The emphasis of this thesis relates to some aspects of the study of liquid crystals and molecules dissolved in them using solid state NMR methodologies. One of the major emphases of this thesis is the estimation of order parameters of liquid crystals by an improved NMR method. In order to highlight the usefulness of the method described in this thesis a brief review of various NMR methods, which utilize nuclei such as protons, deuterium and carbon-13 and interactions such as chemical shift, dipolar and quadrupolar couplings are presented and their advantages and disadvantages described. This forms chapter two of the thesis. In chapter three an improved method for obtaining dipolar couplings in the case of liquid crystals is applied and its usefulness demonstrated. This method removes the terms which dampen the dipolar oscillations in two-dimensional cross polarization experiments by using the Lee–Goldburg decoupling during the period. This enables C–H dipolar couplings for several carbons and the corresponding order parameters to be obtained for the case of N-(p'-ethoxybenzylidene)-p-n-butylaniline (EBBA) at four different temperatures. The results are compared with the standard experiment. Dipolar couplings determined from the cross polarization experiment have been used along with chemical shifts in the isotropic and the nematic phases for studying the conformation of some novel liquid crystals with lateral chains in the core region and those with a 2-phenyl indazole motif in the core. These results are presented in chapter four. Chapter five describes the director dynamics of a spinning liquid crystal sample as a function of the orientation of the spinning axis which has been studied by observing the deuterium quadrupolar splitting of a deuterated solute molecule. The properties of the liquid crystal under spinning especially close to the magic angle are shown to lead to a useful method for setting the magic angle for solid state NMR studies. This method overcomes some of the drawbacks of other methods and can be used to set the magic angle with the same elegance as the standard KBr method. A nematic liquid crystal, which is reported to have a biaxial phase, has been investigated and the results are presented in the last chapter. Spinning the liquid crystal at a slow speed perpendicular to the static magnetic field provides a powder pattern for the quadrupole interaction for the nucleus in the deuterated molecule dissolved in the liquid crystal. This contains information on the symmetry of the phase, which is inferred from simulations.