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

Abstract

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.