X-ray diffraction and optical studies of three mesogens of a homologous series of alkyl cyclohexyl cyanophenyl pyrimidine

Abstract

The crystal and molecular structures of the three compounds belonging to the homologous series of alkyl cyclohexyl cyanophenyl pyrimidine had been determined by us earlier. In this paper we analyse our X-ray and optical data of the same compounds over their entire mesomorphic range. We have measured the refractive indices and densities at different temperatures. The orientational order parameter values <P₂> have been calculated from molecular polarizabilities. Orientational order parameters <P₂> and <P₄> have also been determined from small angle X-ray diffraction photographs of magnetically aligned samples. Molecular parameters have also been determined from this X-ray study. Experimental order parameters are compared with both Maier–Saupe and Humphries–James–Luckhurst theories. The experimental <P₂> values for one of the compounds (heptyl) possessing Smectic A phase have been compared with McMillan's model.

Key Words:

• Order parameters
• polarizabilities
• X-ray diffraction