Molecular Motions in Plastically Crystalline Clusters of Tetragonal tert-Butyl Chloride. A Molecular Dynamics Study

Abstract

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 terms 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.

Molecular Dynamics Study of Homogeneous Nucleation in Supercooled Clusters of tert-Butyl Chloride. Transition from Tetragonal to Monoclinic Phase

“…Application of the model of unbiased walks to the temperature dependent coefficient of rotational diffusion derived elsewherelo yielded a molecular jump frequency from the tetragonal to the monoclinic phase of v(T) = 7.4 x 10l2 exp(-Ea/RT) (9) with activation energy Ea of 5480 J/mol. Note that this activation energy is different from that cited for the diffusion coefficient in ref 10. The discrepancy is only apparent.…”

“…As outlined elsewhere,' I the tension in the stressed interfacial region is small at first but grows rapidly as the postcritical nucleus continues to grow. In images of the clusters plotted at various stages of transformation ( Figure 1) the outer tetragonal Eac, J mol-' 5480 10 phase even appears to become less crystalline as the monoclinic core grows. These considerations suggest that while orientational jump rates derived from rotational diffusion coefficients can reasonably characterize the early stages of the structural fluctuations leading to nucleation, they are totally inadequate to reproduce the observed growth rates of the postcritical nuclei.…”

“…For such a function, the mean time for diffusing into the new phase at 90 K is just 54% of the time required for an unbiased walk if u2 is taken to be just half the t = 0 intercept of ((4-40)~) shown in Figure 2 of ref 10. In view of the speculative nature of the modeling, the effect of the above bias can perhaps be regarded as an estimate of the uncertainty in the jump frequency derived from the coefficient of diffusion.…”

“…This proved to be correct. [9][10][11] The present paper describes the results of one of the first such sets of simulations, that of the transition of solid tort-butyl chloride from the tetragonal12•13 to a lowertemperature phase. It has already been reported how the unknown structure of the lower-temperature phase was correctly deduced by the MD computations simply by observing how the molecules spontaneously rearranged when the solid was cooled.…”

“…Therefore, the required information was extracted from the MD runs, as documented else-® Abstract published in Advance ACS Abstracts, July 15, 1995. 0022-3654/95/2099-12444$09.00/0 where. 10 In the following we present the results of our study of nucleation rates in solid tort-butyl chloride.…”