Reorientation dynamics in condensed phases

• Main Author: Markwick, Phineus R. L.
• Published: University of Edinburgh 2000
• Subjects: 541.2
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.657397

Using Raman lineshape analysis and dielectric relaxation techniques, the reorientation dynamics of single molecules in partially disordered phases is studied. Reorientation correlation functions for a prototypical isotropic liquid <i>CS</i>₂ are presented over a broad temperature range. Calculated values of the integrated correlation time, rms intermolecular torque and collision frequency suggest the onset of substancial local structuring in the liquid phase on cooling to below 220<i>K</i>. An <i>ab initio</i> DFT computer simulation shows that the intermolecular torque arises from a strong dipole moment interaction created by the bending mode vibration of <i>CS</i>₂ and by a weaker dipole moment interaction which oscillates on a much faster time-scale due to the anti-symmetric stretch vibrational mode. A solvation dynamics study of <i>CS</i>₂ in cyclohexane provides the first observation of the free-volume effect occurring at the isotropic-plastic crystal phase transition. Reorientation correlation functions are presented for various members of the nCB [4-n-Alkyl-4'-Cyanobiphenyl] homologous series in the isotropic phase. Two distinct relaxation processes are observed; A very fast intra-molecular relaxation mechanism assigned to the flexibility of the anisotropic molecules, and a slow exponentially-decaying rotational-translational relaxation process with a complex temperature dependence. The results of dielectric relaxation studies under high pressure of two liquid crystals (6PCH [4-trans-4-n-Hexyl-Cyclohexyl-Benzonitrile] and 8CB) are presented. The extraction of reorientational activation parameters in the isotropic, nematic and smectic A phase allows the formulation of a reorientational model. Steric effects determine the predominant relaxation process. Increasing the molecular alignment, reduces the steric interaction and facilitates molecular reorientation about the short-molecular axes, though molecular flexibility and dipole-dipole interactions also play a role. The order parameter in the nematic phase is seen to be strongly dependent on the rigidity of the constituent molecules.