The angular, temperature, and frequency dependences of the proton spin-lattice relaxation rate Til in the smectic A phase of TBBA have been determined. An analysis is made of the possible relaxation mechanisms in smectic A systems. The results seem to show that the angular dependence of Til is produced by the modulation of the intramolecular dipolar interactions dUe to the coupling between selfdiffusion and local "director" fluctuations. The low temperature smectic phases of TBBA have been as well investigated. The decrease in the self-diffusion constant due to two-dimensional translational ordering at the smectic C--+smectic H transition is shown to produce a dramatic increase in Tl and decrease in TID' The rotation of the chain segments abruptly slows down on going from smectic H to smectic VI and discontinuously freezes out on going from smectic VI to smectic VII.
The anisotropy of the translational self-diffusion tensor in the smectic-A (0\\)/0j_) = 0.3) and smectic-C phases of terephthal-fet5~4-w-butylaniline has been determined by multipulse NMR. The experimental results are interpreted in terms of a pseudolattice model with anisotropic potential barriers which seems to provide a better description of the physical situation than the "two-dimensional-liquid" model.
NMR studies of xenon gas dissolved in the liquid crystal ZLI 1132 confined to submicron cylindrical cavities are reported. Spectra taken as a function of temperature yield a clear indication of the nematic to isotropic phase transition of the confined liquid crystals. In the nematic phase at 21 "C, the resonance line of dissolved 129Xe exhibits a chemical shift anisotropy of 15 ppm due to a random distribution of director axes in the plane perpendicular to the long axis of the cylinder. The anisotropy and temperature dependence of the confined system are compared to control experiments that use the bulk liquid crystal. The quadrupolar splitting observed in the I3'Xe NMR spectrum of the confined liquid crystalline solution of xenon gas is slightly greater than that found in the bulk. Two-dimensional exchange NMR demonstrates that the xenon atoms probe different average liquid crystal directors within a single cavity on a 20 ms time scale and that interpore exchange occurs on a time scale of 400 ms. The exchange data indicate that changes in the orientation of the director within individual cavities occur on a length scale of about 2 pm.
IntroductionLiquid crystals are technologically important due to their widespread use in displays (LCDs) and their potential applicability in nonlinear optical devices.'-3 Liquid crystalline phases exhibit long-range molecular orientational order, in contrast to normal liquids which lack positional and orientational order. Devices based on liquid crystals are able to exploit the anisotropy of the medium for the control of alignment and switching of the director axis formed by the liquid crystalline phases. Characterization of the orientational order and the factors which govern it are therefore of considerable scientific and commercial interest.Nuclear magnetic resonance (NMR) is useful for studying liquid crystals4 due to the sensitivity of the NMR spectrum to orientational order. Anisotropic interactions such as chemical shift anisotropy, quadrupolar interactions, and magnetic dipolar couplings make it possible to measure the degree of orientational order. However, spectra can be extremely complicated and intractable for abundant spins in multiple sites, e.g., protons in the liquid crystal molecules. A common simplifying approach is to study small probe molecules dissolved in the liquid crystal or to use liquid crystal molecules which have been isotopically labeled at specific sites. For solute molecules within a liquid crystalline environment, the ordering of the solute by dispersive and steric forces can be described by an order tensor that relates the average alignment of the solute molecular frame to the liquid crystal director frame.5 A particularly simple example is the case of an atom dissolved in a liquid crystal solvent; in this case the principal axis system of the probe is completely determined by the liquid crystal director field. The use of xenon as a microscopic probe of various materials has rapidly developed since the pioneering work in the early
The observed frequency dispersions of the proton spin-lattice relaxation rate T,' in the smectic phases of TBBA in the \05_10 8 Hz region have been analyzed in terms of the order fluctuation. self-diffusion, and rotational contributions to T,'. In the smectic A and smectic C phases the main rate determining contributions are order fluctuations and fast self-diffusion, whereas in the smectic Hand smectic VI phases fast rotations and slow translation self-diffusion determine T ,I.
Motionally averaged proton-proton dipolar couplings measured by nuclear magnetic resonance (NMR) spectroscopy can provide information about the conformations and orientations sampled by partially oriented molecules. In this study, the measured dipolar couplings between pairs of protons on n-hexane dissolved in a nematic liquid crystal solvent are used as constraints in a Monte Carlo sampling of the conformations and orientations of n-hexane. Rotation about each carbon-carbon bond in the molecule is modeled by the complete sinusoidal torsional potential of Ryckaert and Bellemans rather than by the three-state rotational isomeric states (RIS) model that has been used in previous studies. Comparison of the results of the simulations using the Ryckaert-Bellemans potential and the RIS model indicates little difference in the values of the adjustable parameters and the quality of the fits to the experimental data. The primary difference between the models appears in the calculated conformer probability distributions for n-hexane, highlighting the importance of the exact shape of the torsional potential used to model carbon-carbon bond rotation in organic molecules.
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