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

Abstract

The structure of the antiferroelectric smectic C^*_A 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° angle with the molecular core, and some fragments in the fluorinated part of the achiral terminal chain are in the gauche conformation.