On the applicability of the rotational diffusion model in liquid crystalline solvents. A nuclear magnetic relaxation study of toluene

The model of rotational diffusion in the presence of an orienting potential has been adapted to the case of an asymmetric top molecule in an orienting potential that is defined by two ordering parameters. It has been used to describe the relaxation by the quadrupolar mechanism of the deuterons in toluene, dissolved in «Phase V». No set of diffusion constants could be found, which simultaneously reproduce all spectral densities that were derived from the measured relaxation rates. Only very approximate agreement could be obtained with sets of diffusion constants in which two constants are of the same order of magnitude and the diffusion about the axis in the plane of the benzene ring and perpendicular to the axis of the methylgroup, is slower by an order of magnitude. The anisotropy in the viscosity of the solution has not explicitly been taken into account, but it has been indicated how this might be done. At this stage, therefore, no definitive conclusions can be drawn, but clearly the application of the rotational diffusion model in liquid crystalline solutions should be considered with care