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NMR studies of solutes in nematic liquid crystals: understanding the nematogens Ter Beek, Leon Christiaan


NMR spectroscopy and dynamics of small solute molecules dissolved in several nematic liquid crystals provided insights toward a better understanding of nematogens. Expressions for all possible spin-relaxation rates for dideuterium are given. Exper- imentally obtained Zeeman relaxation rates in several liquid crystals were fitted to the expression for the Zeeman relaxation rate, and correlation times were thus obtained. From our study of the dynamics of dideuterium in nematic liquid crystals it was also found that ortho and para dideuterium have different relaxation rates due to the inter- ference of the dipolar and quadrupolar coupling constants and their spectral densities. The Zeeman relaxation rates of dideuterium show no liquid crystal solvent effect. Three different solute molecules with a single bond of rotation have been investigated in a special zero-electric field gradient nematic mixture: furfural, 2-chlorobenzaldehyde, and 2,2’-dithiophene. Dipolar coupling constants measured from NMR experiments are reported for these solutes and discussed in terms of the order parameters and conformer probabilities. In the course of the analysis a new geometry for 2,2’-dithiophene has been found. The spectrum of propane has been analyzed with the use of Multiple-Quantum NMR. Dipolar coupling constants of ethane, propane, benzene, 1 ,3,5-trifluorobenzene, hexaflu- orobenzene, and 1 ,3,5-trichlorobenzene as an internal reference for liquid crystal orient- tation, as solutes in three different nematic liquid crystals, were obtained from NMR spectra. The results are discussed in terms of orientation and solute—solvent interactions and especially the long-range molecular quadrupole moment—mean electric field gradi- ent interaction. A method for obtaining molecular quadrupole moments of solutes in nematic liquid crystals is discussed. The molecular quadrupole moment—mean electric field gradient interaction contributes to the orientation potential of these small alkanes and benzenes and will contribute to the orientation of alkyl chains and aromatic cores of nematogens as well.

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