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NMR studies and computer simulations of solutes in nematic liquid crystals

University of British Columbia

This thesis is concerned with the orientational and conformational behaviour of molecules partially oriented in a nematic liquid crystal. We have studied these systems experimentally using Multiple-Quantum NMR spectroscopy, and computationally using the Monte Carlo simulation method. An important goal of the experimental component of this thesis was the investigation of the usefulness of applying Multiple-Quantum NMR spectroscopy as an aid for the analysis of complex one-quantum NMR spectra of oriented solutes. The technique was applied to biphenylene and butane. For the eight-spin molecule biphenylene, the analysis of the six-quantum and seven-quantum spectra was shown to be sufficient to provide a simple solution of the one-quantum spectrum. However, it was necessary to reduce the number of fitting parameters by fitting the proton geometrical coordinates and molecular order order parameters instead of the dipolar coupling constants. An analysis of the dipolar coupling constants was used to determine the vibrationally averaged molecular structure. An analysis of the seven-quantum and eight-quantum spectra of the ten-spin molecule butane provided an excellent prediction of the one-quantum spectrum, which could then be solved trivially. The dipolar coupling constants were analyzed to study the conformational behaviour of butane. The trans-gauche energy difference was determined to be in the range of 2.1-3.0 kJ/mol. This is significantly less than the gas phase value and indicates that the condensed environment enhances the gauc/ze-conformer probability. Further, the conformational biasing was primarily a result of the isotropic component of the solute-solvent interaction; the anisotropy of the nematic solvent has only minor effects. Finally, the analysis of the couplings involved the use of mean-field models to describe orientational ordering for each conformer. Several models were able to provide an adequate description of orientational ordering as determined by the ability to fit the dipolar coupling constants. Monte Carlo computer simulations were used to investigate the mechanisms for orientational ordering of solutes in nematics and test several empirical and theoretical meanfield models of ordering. The importance of shape anisotropy and electrostatic interactions were studied. Solute and solvent molecular shapes were approximated by hard ellipsoids. Some simulations incorporated the interaction between point quadrupoles placed at the centres of the ellipsoids. In the purely hard-core systems, orientational or-der parameters and orientational distribution functions were calculated for a collection of different solutes under under a variety of conditions. Several empirical models were used to analyze the data. Fitting parameter values were quantitatively very similar to values obtained from previous fits to experimental data. This result clearly demonstrates the importance of anisotropic short-range repulsive forces for orientational ordering in nemat-ics and firmly establishes the connection between these various molecular-shape models with these interactions. The quadrupolar systems were used to investigate a mean-field model in which an interaction between the solute molecular quadrupole moment with an average electric-field gradient provides an orientational ordering mechanism. Simula-tions indicate that the electric-field gradient is highly dependent on the properties of the solute, contrary to some experimental evidence. Further, a mean-field theory developed to describe this model was found to provide a qualitatively correct but quantitatively imprecise prediction of orientational ordering.

Thesis/Dissertation

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2009-03-17

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http://hdl.handle.net/2429/6146

Physics

University of British Columbia

UBCV

DSpace