MOLECULAR MOTIONS IN PLASTICALLY CRYSTALLINE CLUSTERS OF TETRAGONAL TERT-BUTYL CHLORIDE - A MOLECULAR-DYNAMICS STUDY

The interior of a 188-molecule cluster of tert-butyl chloride was exam ined in detail by molecular dynamics computations covering the range o f temperature over which the cluster's orientationally disordered phas e IV is stable. The aim was to characterize the molecular behavior in a bulklike region in order to interpret the kinetics of the spontaneou s transformation to the ordered monoclinic phase occurring at colder t emperatures. Of particular concern were the rotational motions about m olecular 3-fold axes dictating molecular jump rates in the transition. The activation energy for rotational diffusion, mean librational freq uencies, coefficients of rotational diffusion, and relaxation times we re derived from the simulations by analyzing results in tens of a mode l based on a single rotational force constant and coefficient of frict ion. Prior experimental evidence about these quantities obtained by NM R, cold neutron diffraction, and dielectric techniques had been somewh at contradictory. A reasonable agreement was obtained between computat ional results and a subset of the experimental results.