Anisotropic atomic displacement parameters and molecular motions in mesogenic crystals

In this paper we describe the results of vibrational analysis of molecules in the crystalline phase of three mesogenic compounds of the series n-alkyl cyclohexyl cyanophenyl pyrimidine from experimental atomic displacement pa rameters using the rigid body model of Cruickshank. The rigid body model ha s been applied so far for estimation of corrections in bond lengths arising from the librational modes of motion. We have undertaken this vibrational analysis to find out whether we can get any idea about the transition from crystalline to liquid crystalline phase. Two tensors T (translation) and L (libration) are used to describe the motion. These tensors have been diagon alized to obtain the principal axes and amplitudes of translational and lib rational motions.