Biaxial ordering, self-diffusion, and helix-distortion effects onH2NMR spectral patterns from cholesteric liquid crystals

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Abstract
Deuterium NMR spectral patterns from cholesteric liquid crystals (twisted nematics) for the case where the pitch axis aligns perpendicular to the direction of the magnetic field are evaluated in terms of three quantities or effects: (i) an asymmetry parameter in the quadrupole interaction which has been motionally induced by biaxial ordering of the molecules, (ii) a self-diffusion constant for migration of the molecules along the pitch axis, and (iii) distortion of the cholesteric helix in the presence of the applied magnetic field. Calculated spectral patterns which include the above effects are fitted to experimentally recorded spectral patterns from a selectively deuterated nematic that has been twisted by the addition of a chiral compound. It is demonstrated how values of the asymmetry parameter and the self-diffusion constant can be separately measured and helix-distortion-effects studied.