The molecular motion in the solid phases of t-butyl chloride by cold neutron scattering

Abstract

Cold neutron scattering experiments have been performed on tertiary butyl chloride at the temperature 173 °K in phase III, 205 °K in phase II, and 235 °K in phase I. The crystal is plastic in phases I and II and ordinary crystalline in phase III. The experimental data are interpreted by fitting of computer calculated cross sections, based on various physical models. The main aim was to study the rotational motion. By use of the data from the phase III a reasonalbe Debye form for the translational spectrum is obtained and used also in phases II and I. Similarly the rather well separated torsional spectrum of the methyl groups is simply approximated. The rotational spectra dominate the elastic and near elastic scattering. Models ranging from free rotation to undamped libration, including isotropic Langevin rotational diffusion as an intermediate case when the rotational relaxation times are short, are tested. It is found, that no extreme model corresponding either to almost free rotation, or to almost undamped libration, or to uniaxial rotational diffusion can describe the data. On the other hand, either isotropic Langevin rotational diffusion or a stochastic two-step rotational diffusion model involving consecutive free rotations and librations fit the data well. It is found that the time a molecule needs for moving from one potential well to another is about 30% of the time it stays in a well. It is also found that both times are short, of order 0.2<τ<2.10−12 sec. It is therefore concluded that the Frenkel model type is not applicable to the rotational motion of t-butyl chloride in plastic phases I and II.