Vibrational Spectra of Sterol and Non-sterol Cholesterogens

Abstract Vibrational spectra of sterol cholesterogens, cholesteryl nonanoate (CN), cholesteryl linoleate (CL) and cholesteryl linolenate (CLn) and the non-sterol cholesterogen(+)-2-methyl butyl-(p-(p-methoxy benzylidene) amino) cinnamate (MBMBAC) are discussed. The infrared (IR) spectra of CN, indicates that the intense skeletal in-plane vibrations of (C˭C) bond and bending vibrations due to methyl groups in the cholesterol ring system in the smectic A phase are responsible for the formation of spherulites in the solid phase, during cooling. Vibrational spectra of CL and CLn show that an increase in the unsaturation endows greater rigidity to part of the chain. IR spectra of CL confirms the smectic polymorphism exhibited by this compound (above and below 21°C) and also show that both liquid crystalline phases have smectic A like structure. Vibrational spectra of MBMBAC, obtained in different liquid crystalline phases exhibited by the compound are similar. Appearance of the band corresponding to the asymmetric vibrations due to the asymmetric centre in the supercooled smectic B phase of the compound even at a very low temperature, shows that these vibrations are effective in twisting the molecules in each layer, thus forming a twisted smectic B phase at lower temperatures. Greater mobility of the chain and relatively weaker intermolecular interaction within the layer, at higher temperatures give rise to usual smectic B structure. This is in conformity with our observations of density measurements. The intensity of the CH3—O stretching vibrations has been found to be maximum in the smectic A phase of MBMBAC, indicating that the component of the dipole moment acting along the long molecular axis may also be responsible for the smectic-cholesteric transition in this compound.