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 examined in detail by molecular dynamics computations covering the range of temperature over which the cluster's orientationally disordered phase IV is stable. The aim was to characterize the molecular behavior in a bulklike region in order to interpret the kinetics of the spontaneous transformation to the ordered monoclinic phase occurring at colder temperatures. Of particular concern were the rotational motions about molecular 3-fold axes dictating molecular jump rates in the transition. The activation energy for rotational diffusion, mean librational frequencies, coefficients of rotational diffusion, and relaxation times were derived from the simulations by analyzing results in tens of a model based on a single rotational force constant and coefficient of friction. Prior experimental evidence about these quantities obtained by NMR, cold neutron diffraction, and dielectric techniques had been somewhat contradictory. A reasonable agreement was obtained between computational results and a subset of the experimental results.