The determination of absolute values of the orientation parameters of partially oriented solute molecules

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Abstract
A method is described for determining the absolute signs of orientation parameters from measurements of the anisotropy of the dielectric constant of partially oriented solutions of polar molecules in nematic solvents. It is shown that the orientation polarizability due to permanent dipoles of a partially oriented system depends upon the first power of one orientation parameter if the molecule has C 2v or higher symmetry. Since the rotational correlation time for the long axis of a nematogen in the nematic phase is comparatively long, it is possible to make dielectric measurements at a frequency such that the dipolar contribution to the polarizability of the solvent is only from the components of its dipole moment perpendicular to its long axis. This has the effect of reducing the dielectric constant of a nematogen from the liquid to the nematic state, enabling substantial increments of the dielectric constant due to polar solutes to be measured. Under these circumstances, the magnitude and sign of the orientation parameter of the solute may be obtained. The value for methyl cyanide is shown to be in good agreement with that obtained directly from nuclear magnetic resonance experiments. The implication of the sign of the parameter on nuclear magnetic resonance quantities is discussed, as is the potential of the method for probing the nature of the internal field in the solution.