Solid phases of bicyclohexyl studied with vibrational spectroscopy and calorimetry

Abstract

Bicyclohexyl, (C₆H₁₁)₂(BCH), is a simple hydrocarbon and is the backbone of a group of compounds that form nematic (and sometimes smectic) liquid crystals. The infrared (IR) spectra of BCH, studied some years ago, have been re-investigated. Low-temperature, and also high-pressure spectra, at room temperature, of the amorphous phase and of three crystalline phases were obtained. Raman spectra of the liquid and of the crystalline solids were also recorded. Bicyclohexyl has a number of phase transitions. They are reversible in terms of temperature and pressure variations, and they were carefully investigated by differential scanning calorimetry (DSC) and by the variable-temperature X-ray powder technique. In the low-temperature and also in the high-pressure crystals, BCH existed in three phases: in phase 1 (274–277 K) all conformers were present; in phase 2 (256.5–274 K) only the equatorial–equatorial anti conformer and in phase 3 (below 256.5 K) the equatorial–equatorial gauche conformer was present. An additional phase transition was observed at 266.5 K by DSC and X-ray methods, but no noticeable changes were observed in the spectra. A single crystal of the high-pressure phase 1 was investigated by polarized radiation, and the order parameters for various bands were determined in the IR region. From these data it is argued that phase 1 could be a smectic liquid crystal rather than a plastic crystalline phase.