A molecular dynamics simulation of the plastic phase of hexachloroethane

Abstract

A molecular dynamics simulation of the plastic phase of the pseudo-octahedral molecule C₂Cl₆ has been carried out using a 6-exp potential model taken from the literature. A plastic phase has been found for a temperature range which is in good agreement with experiment. The calculated thermal averages of the centre of mass displacements and the orientational cubic harmonics are also in good agreement with the experimental values. The calculated atomic orientational probability distribution shows maxima along the [100] and [111] directions for the Cl and C atoms, respectively, and the distribution is isotropic over a wide angular range about the maxima. An analysis of the instantaneous molecular positions shows that the molecules have a larger probability of rotating, and perform sudden reorientations around the [100] crystal directions. It has been found that the molecular C-C axis plays no important role in the molecular dynamics, which is identical to what is found for the plastic phase of the octahedral molecule SF₆. A search has been made for the formation of linear clusters of molecules as suggested in the literature but these do not appear in the simulation. A correlated repulsion between molecules and their next-nearest neighbours has been found so that the molecules avoid close contacts of the chlorine atoms along the [100] directions by performing rotations about the [110] crystal directions. The single-molecule rotational potential is calculated and compared with the experimental one, showing that the potential energy barrier for molecular rotations about the [100] directions is considerably lower than for the [110] and [111] rotations. The single-molecule dynamics are also studied and the translational power spectrum reveals a strong translational-rotational coupling whereas the rotational spectrum shows an isotropic rotational diffusion behaviour. The molecules are found to librate around ideal positions for an average residence time of 5·4 ps between consecutive reorientational jumps.