PARAMETERS AND THE INTERMOLECULAR POTENTIAL

The NMR of orientationally ordered solutes in liquid-crystalline solvents gives accurate values of order parameters, and these in turn can be interpreted in terms of the anisotropic intermolecular potential that causes the orientational ordering [1,2]. A wealth of experimental evidence leads to the conclusion that a single mechanism is incapable of describing solute orientational order in different liquid crystals. We shall demonstrate that experimental results can be rationalized in terms of two and only two independent second-rank Maier-Saupe like mechanisms whose relative importance varies with liquid-crystal solvent [3]. There is strong indication that the dominant mechanism involves short-range interactions that depend on solute size and shape. The second mechanism is less certain, and various possibilities will be discussed with the favourite involving the molecular quadrupole. Nematic liquid crystals exhibit orientational order while smectic liquid crystals exhibit, in addition, positional order. Obtaining information from NMR experiments about the positional order is challenging. We have applied the ideas of Kobayashi / McMillan theory for smectic ordering to the results obtained from a collection of solutes in several liquid-crystal solvents that exhibit both a smectic A phase and a higher-temperature nematic phas