Rotational Brownian motion in liquid and plastic crystalline CBr4 from far infrared induced absorptions

Abstract

The far infrared induced absorption bands of CBr4 in the liquid and plastic single crystalline states have been measured in order to study the changes in rotational dynamics brought about by the increased translational constraint and packing symmetry of the solid. Analysis of the band-shapes in terms of rotational Brownian type dynamics suggests that the barrier to such motion is slightly increased on going from the plastic crystal to the liquid a few degrees above the melting point, thus making the change of phase energetically desirable. The integrated intensity per molecule is greater in the liquid, although the macroscopic density is less, which suggests that the spatial disposition of the electrostatic (long range) part of the intermolecular potentials is important in determining the magnitude of the molecular induced dipole moment.The dipole cross-correlation function corresponding to these bands is compared with that of octopole-induced dipole absorption of a two molecule collision of spherical tops, and with the auto-correlation function for a Maxwellian ensemble of freely rotating molecules of this symmetry. It is found that the mean torque is greater initially in the condensed phases, thereafter, rotational motion is correlated.