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 parameters using the rigid body model of Cruickshank. The rigid body model has 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 diagonalized to obtain the principal axes and amplitudes of translational and librational motions.