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A general molecular theory for the description of the orientation of small molecules in anisotropic environments is presented. Assuming a second-order tensorial interaction between some solute property p,, and the anisotropy in a liquid-crystal "field" F,,, rigid and non-rigid contributions to the dipolar and quadrupolar couplings observable by NMR are evaluated. The behaviour of the solutes hydrogen, methane and their deuterated analogues in nematic phases can be described by special cases of this general theory and excellent agreement between observed and calculated anisotropic couplings is obtained. The present results suggest that the coupling between the solute molecular quadrupole moment and the anisotropy in the liquid-crystal electric field gradient plays a significant role in the orienting process in the case of hydrogen.

Proton magnetic resonance (1H NMR) has been used to compare the local orientational order of acyl chains in phospholipid bilayers of multilamellar and small sonicated vesicular membranes of dipalmitoyllecithin (DPL) at 50 degrees C and egg yolk lecithin (EYL) at 31 degrees C. The orientational order of the multilamellar systems was characterized using deuterium magnetic resonance order parameters and 1H NMR second moments. 1H NMR line shapes in the vesicle samples were calculated using vesicle size distributions, determined directly using electron microscopy, and a theory of motional narrowing, which takes into account the symmetry properties of the bilayer systems. The predicted non-Lorentzian line shapes and widths were found to be in good agreement with experimental results, indicating that the local orientational order (called "packing" by many workers) in the bilayers of small vesicles and in multilamellar membranes is substantially the same. This results was found to be true not only for the largest 1H NMR line associated with the nonterminal methylene protons but also for the resolved 1H NMR lines due to the alpha-CH2 and the terminal CH3 positions on the acyl chain. Analysis of the vesicle 1H NMR spectra of EYL taken with different medium viscosities yielded a value of approximately 4 X 10(-8) cm2 s-1 for the lateral diffusion constant of the phospholipid molecules at 31 degrees C.

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Nuclear spin relaxation in a hexagonal lyotropic liquid crystal J. Chem. Phys. 95, 6945 (1991); 10.1063/1.461506Nuclear magnetic resonance determination of the CF2 order parameter tensor in a lyotropic liquid crystal

Articles you may be interested inMild hydration of didecyldimethylammonium chloride modified DNA by 1H-nuclear magnetic resonance and by sorption isotherm J. Appl. Phys. 113, 044702 (2013); 10.1063/1.4789011 Ultrasound-order director fluctuations interaction in nematic liquid crystals: A nuclear magnetic resonance relaxometry study J. Chem. Phys. 118, 9037 (2003); 10.1063/1.1566735Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxationIn this study we use multiple-quantum 1 H-NMR spectroscopy to study butane, the simplest flexible alkane, dissolved in a nematic solvent. An analysis of the highly accurate 1 H dipolar coupling constants gives important information about conformational and orientational behavior, including the trans-gauche energy difference, E tg , and the conformer probabilities and order parameters. An essential component of the analysis involves the use of mean-field models to describe the orientational ordering of solutes in a nematic solvent. Several models were found to adequately describe the molecular ordering, including the chord model of Photinos et al. ͓D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 ͑1990͔͒ and recent versions of a model proposed by Burnell and co-workers ͓D. S. Zimmerman and E. E. Burnell, Mol. Phys. 78, 687 ͑1993͔͒. It was found that E tg lies in the range 2.1-3.0 kJ/mol, which is significantly below most experimental estimates of the gas-phase value. An attempt to describe more realistically the conformational states by including torsional fluctuations about the rotational isomeric states did not significantly improve the quality of the fits or alter the results. Finally, the anisotropic component of the solute-solvent interaction was found to perturb only marginally the conformational probabilities from the isotropic values.