Correlations in the plastic crystal phase of n-butane

Abstract

The molecular dynamics simulations of plastic crystalline phase I of n-butane have been analysed for the intermolecular correlations associated with the transition to the metastable phase II on cooling. The long-term objective is to understand these correlated motions, to suggest a mechanism for the transition, and to propose relevant supportive experiments. The simulation was done on the DAP, a SIMD computer with 4096 processing elements, and the mapping strategy used is described. The MD sample contained 2048 molecules, bounded cyclically following the constant pressure algorithm of Parrinello and Rahman. Phase I is a 1-d rotator structure, there being orientational disorder about a unique molecular axis giving rise to a characteristic ODF (orientational distribution function). The simulations have been done in close conjunction with experiment, and the ODF has been confirmed by experiment. The present analysis of the ODF and the molecular motion trajectories points to an ordering primarily in the crystallographic bc-plane; the possibility of confirmatory experiments is discussed.