The molecular motion in crystalline, plastic, and liquid neopentane studied by cold neutron scattering experiments

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Abstract
Cold neutron scattering experiments have been performed on neopentane in the liquid state at 265 K and in the solid state at the temperatures 135, 148, 173, 213, 239, and 253 K. The measured data were interpreted by fitting computer-calculated cross sections, based on various physical models, to the experimentally obtained ones. In this procedure the translational motion was supposed to be known and the temperature dependence of the molecular rotations was particularly studied. The following was concluded: In the crystalline phase the rotations are hindered such that the molecule mainly performs small-angle librations around equilibrium orientations. In the plastic phase the rotational motions may be visualized as a rapid succession of alternating steps, described by librations and free rotations. The mean life times τ0 and τ1, of these alternating states is, however, short: between 3.5 and 0.25×10−12 s for the librations (τ0) and between 0.8 and 0.4×10−12 s for the free rotations (τ1). This implies that the free rotations involving orientational changes are strongly damped, whereas the librations are strongly damped near the melting point at 257 K and somewhat better defined near the plastic–crystalline phase transition at 140 K. Both motions are of such a short duration that the compounded motion may rather well be approximated by the Langevin model for rotation. In this model a friction coefficient is the only disposable parameter. In the liquid phase at 265 K the free rotations appear even more damped, τ1∼0.2×10−12 s, whereas the librations seem to persist slightly longer than just below the melting point τ0∼1 ×10−12 s. This implies that these molecules show a tendency of more free rotations in the solid than in the liquid for states close to the melting temperature.