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 RC6H4XC6H4R ' 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 C6H5XC6H5 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 degrees, in the liquid crystal are larger than those in the gas phase for tolan and tl ans-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.