Comparative X-ray Diffraction Study of Two Liquid Crystalline Compounds with Chiral Centers Based on (S)-(+)-2-Octanol and (S)-(+)-3-Octanol

The structure of the antiferroelectric smectic C phase is investigated for two liquid crystals with almost identical molecular structures, except for chiral centers. The X-ray diffraction results determined the crystal unit cell parameters, smectic layer spacing, average distance within layers, and correlation length of the short-range positional order. The coefficients of thermal expansion are determined for the crystal phases. The molecular modeling with the semi-empirical PM7 method and density functional theory calculations with the def2TZVPP basis set and B3LYP-D3(BJ) functional are applied to determine the tilt angle of molecules from the experimental smectic layer spacing. The most probable conformations are then selected based on a comparison with the tilt angle measured by the electro optic method, known from the previous results. In the most suitable molecular models, the chiral chain makes an approximately 90 degrees angle with the molecular core, and some fragments in the fluorinated part of the achiral terminal chain are in the gauche conformation.