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UBC Theses and Dissertations

Multiple quantum NMR studies of solutes in liquid crystals Rendell, John Charles Thomas


The spectrum of 1,3-dichloro-2-ethenylbenzene partially oriented in nematic liquid crystalline solvents has been analysed using a number of complimentary nuclear magnetic resonance (NMR) techniques. The analysis of this spectrum poses a considerable problem due to its complexity and the uncertain geometry and orientation of the molecule. The technique of multiple quantum NMR (MQNMR) has the capability of tremendously simplifying complex spectra. The multiple quantum spectrum was measured and proved very useful in the analysis but unavoidable resolution difficulties in the MQNMR experiment prevented a complete solution of the problem using this spectrum alone. While the spectrum of 1,3-dichloro-2-ethenylbenzene is dominated by only a few large interactions, the lines are split by relatively small dipolar couplings and the limited resolution available in the multiple quantum spectrum makes the determination of the smaller couplings difficult. To overcome this difficulty a frequency selective excitation of the multiple quantum spectrum was adapted and developed. After testing the experiment on the relatively simple spectrum of 1,1,2-trichloroethane dissolved in a nematic solvent, this selective experiment was applied to the much more complex spectrum of 1,3-dichloro-2-ethenylbenzene where it proved capable of directly measuring the small couplings in the spectrum without interference from any of the larger interactions. This information contributed greatly to the eventual analysis of the spectrum. MQNMR experiments can be very time consuming and as a result the spectroscopist must frequently make do with very limited time domain signals from which the spectrum must be extracted. This creates a number of difficulties when the signals are analysed with the fast Fourier transform (FFT), the standard method of spectral analysis used in NMR. With these problems in mind, the suitability of MQNMR time domain signals for analysis by a method of spectral estimation due to Burg, commonly called the maximum entropy method (MEM), was examined. By testing Burg's MEM with the MQNMR spectra of a number of different solutes partially oriented in nematic phases, it was found to be a useful adjunct to the FFT when dealing with MQNMR interferograms. While some care is required in its application, this method of spectral analysis should find important uses in the estimation of MQNMR spectra. Solution of the spectrum yielded information on the molecular geometry and the orientation of the 1,3-dichloro-2-ethenylbenzene in the nematic solvents used. While an extensive analysis of molecular geometry proved impossible, the information on molecular orientation was examined in terms of two different models. The orientation data shows excellent agreement with a recently developed model for orientation based upon the shape of the solute.

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