Orientation-translation and orientation-orientation correlations in neopentane plastic crystals. Computer simulation

Abstract

The dynamics of orientationally disordered crystals, such as neopentane C(CH₃)₄, couple translational and orientational variables in a complex way. From the results of a molecular dynamics simulation of the orientationally disordered phase of neopentane, simple geometrical methods are used to define functions that can be displayed on a two-dimensional surface and yield practical information, and from these are obtained the single-molecule orientation-translation coupling, the coupling between the translation of a molecule and the orientation of one of its neighbours, and the orientation-orientation coupling of two neighbouring molecules. The single-molecule orientation-translation term, although weak, is consistent with experimental results. A distinct orientation-orientation coupling is visible when a C-C bond of one molecule points at a neighbouring molecule, while a bond of that neighbour points away from the former. Isotropy around the connecting vector tends to disqualify pseudospin-type models for neopentane. The most salient feature, however, is the strong two-molecule orientation-translation correlation, which confirms the motion of one molecule when a C-C bond of one of its neighbours points in its direction; up to now this was only inferred from partial experimental data. Predictions are made for the type of information that is likely to be obtained from neutron diffuse scattering.