ORIENTATION-TRANSLATION AND ORIENTATION-ORIENTATION CORRELATIONS IN NEOPENTANE PLASTIC CRYSTALS - COMPUTER-SIMULATION

The dynamics of orientationally disordered crystals, such as neopentan e C(CH3)(4), couple translational and orientational variables in a com plex way. From the results of a molecular dynamics simulation of the o rientationally disordered phase of neopentane, simple geometrical meth ods are used to define functions that can be displayed on a two-dimens ional surface and yield practical information, and from these are obta ined the single-molecule orientation-translation coupling, the couplin g between the translation of a molecule and the orientation of one of its neighbours, and the orientation-orientation coupling of two neighb ouring molecules. The single-molecule orientation-translation term, al though weak, is consistent with experimental results. A distinct orien tation-orientation coupling is visible when a C-C bond of one molecule points at a neighbouring molecule, while a bond of that neighbour poi nts 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 c orrelation, 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 o nly inferred from partial experimental data. Predictions are made for the type of information that is likely to be obtained from neutron dif fuse scattering.