NMR studies of oriented molecules Page: 3 of 372

                                
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NMR STUDIES OF ORIENTED MOLECULES
Steven Williams Sinton
Abstract
The properties of liquid crystalline mesophases have been of
continuing interest in physics and chemistry since the discovery of these
novel compounds. Recently, nuclear magnetic resonance (NMR) spectroscopy
has been extensively used to probe the microscopic nature of liquid
crystal samples. The NMR spectra contain information which is sensitive
to internal molecular parameters and reflect the anisotropic potential
in which the molecules reorient. Fast diffusion and rotational motion
remove the effects of couplings between molecules.
In this work, deuterium and proton magnetic resonance are used in
experiments on a number of compounds which either form liquid crystal
mesophases themselves or are dissolved in a liquid crystal solvent. The
nature of the information available from the spectra and limitations
imposed by assumptions necessary in their analyses are discussed. The
new technique of proton multiple quantum NMR is employed as a means to
simplify complicated spectra without the need for selective isotopic
substitution. In a multiple quantum experiment, the change of the total
magnetic quantum number, M, associated with observed spectral lines may
be any integer allowed by the number of coupled spins; e.g., AM = 0, +1,
...,+N for N coupled spin-1/2 nuclei. This experiment also retains the
higher sensitivity and precision in structural information available from
proton NMR compared with other nuclei. The theory of non-selective
multiple quantum NMR is briefly reviewed. Experimental examples with
benzene dissolved in a liquid crystal are used to demonstrate several