Molecular Structures of Related Compounds of Mesogens Studied by 1H NMR Using a Liquid Crystal Solvent: Tolan and trans-Azobenzene

Molecular structures of tolan and trans- azobenzene dissolved in nematic liquid-crystal ZLI 1132 have been determined by proton NMR with the aid of theoretical calculations. They are models of the cores of liquid-crystal molecules of the RC 6 H 4 XC 6 H 4 R‘ type, where R and R‘ denote flexible groups and X is the linking unit. Spectral analysis is quite difficult for such a molecule with 10 protons as C 6 H 5 XC 6 H 5 . In the present study, therefore, the initial spectral parameters that allow spectral assignment have been obtained by analyzing integral curves by means of a genetic algorithm. Structural analyses show that both compounds take planar molecular structures in ZLI 1132. The determined rotational barriers at the dihedral angle of phenyl rings, 90°, in the liquid crystal are larger than those in the gas phase for tolan and trans- azobenzene as in the cases of phenyl benzoate and biphenyl. It is reasonable to consider that the liquid-crystal solvent stabilizes the planar or nearly planar structures of solutes.