NMR determination of smectic ordering of probe molecules

The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, tau, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of tau indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.