The partial averaging of second rank properties of molecules dissolved in liquid crystals and having internal degrees of freedom is discussed in terms of equilibrium statistical mechanics. It is shown that the dependence of the partial average on orientational order is determined only by the nature of the intermolecular potential and not, it is argued, by the relative rates of internal and external motion. This result applies to any form of internal motion and the implications of the theory are discussed for internal motion between discrete molecular conformations, free internal rotation and vibrational motion. An explanation is proposed for the observed, finite dipolar and quadrupolar couplings of nuclei in tetrahedral molecules. This implies that the phenomenon is not necessarily a consequence of a distortion of these molecules in the anisotropic solvent, but does require a dependence of the orientational order on the vibrational state.

The effect of internal motion on the orientational order parameters for liquid crystalline systems

Effects of interaction between molecular internal motion and reorientation on NMR of anisotropic liquids

The rα structure of [1-13C]benzene by NMR of oriented molecules: A study of the possible precision and of solvent effects

A general molecular theory for the description of the orientation of small molecules in anisotropic environments is presented Assuming a second-order tensorial interaction between some solute property βij and the anisotropy in a liquid-crystal “field” Fij rigid and non-rigid contributions to the dipolar and quadrupolar couplings observable by NMR are evaluated The behaviour of the solutes hydrogen, methane and their deuterated analogues in nematic phases can be described by special cases of this general theory and excellent agreement between observed and calculated anisotropic couplings is obtained The present results suggest that the coupling between the solute molecular quadrupole moment and the anisotropy in the liquid-crystal electric field gradient plays a significant role in the orienting process in the case of hydrogen

On the Orientation Mechanism of Small Molecules in Liquid-Crystalline Environments

1H and 2H NMR results are presented for methane and its deuterated analogs dissolved in various nematic liquid crystals. Anisotropic couplings observed in CHD3, CH2D2, and CH3D arise partly from the small deviation from tetrahedral symmetry due to the deuterium substitution, and partly from nonrigid molecule effects which give rise to anisotropic couplings in CH4 and CD4. Excellent agreement is obtained with a simple model based on separability and additivity of these rigid and nonrigid molecule contributions. Order parameters for the rigid molecule contribution are positive along the longest and negative along the shortest molecular directions.

A.D Buckingham

Papers

High-resolution nuclear magnetic resonance spectra of tetrahedral molecules and ions in anisotropic environments