Acetylene in nematic liquid crystals: A vibrational analysis of the observed dipolar couplings

Abstract

Experimental dipole couplings for several isotopically substituted modifications of acetylene dissolved in the nematic liquid crystals 1132 (Merck ZLI 1132), EBBA [N-(4-ethoxybenzylidene)-2, 6-dideutero-4-n-butylaniline] and a 55 wt. % 1132/EBBA mixture are obtained from NMR measurements. The dipolar couplings are calculated using a model for the solvent–solute interaction. The interaction is taken to be of second rank tensorial form and the effects of the coupling between the vibrations and rotations are taken into account. The calculated values are in good agreement with the experimental results although some discrepancies exist. These discrepancies are discussed in terms of a possible contribution to the observed carbon–carbon coupling from the anisotropy in the indirect coupling. The results show that the interaction between the vibrations and rotations of the solute plays an important role in determining the observed couplings and that specific interactions with the liquid crystal need not be invoked to explain the results. Previous studies have shown that molecular hydrogen experiences an external electric field gradient due to its liquid crystal environment and that in 55 wt. % 1132/EBBA the value of this field gradient is zero at 301.4 K. It has also been shown that the interaction between this field gradient and the solute molecular quadrupole moment dominates the ordering of molecular hydrogen in liquid crystals, such as 1132 and EBBA, where the field gradient is large. A qualitative analysis indicates that this interaction is important for the ordering of acetylene and that other mechanisms also play a role.