Rotational motion of cyclic ether molecules in clathrate hydrates studied by neutron scattering. I. Quasielastic scattering by ethylene oxide

Clathrate hydrates are inclusion compounds in which guest molecules are encaged by a H₂O host lattice. The guests generally show a high rotational mobility. Ethylene oxide (EO) molecules occupy mainly H₂O cages of symmetry 42m. We have measured neutron scattering spectra at temperatures between 30 and 271 K using deuterated host lattice samples. Since the question of whether the EO molecules have preferred orientations within the cages is still open, we have compared the quasielastic region with predictions of models involving instantaneous random EO reorientations either around the cage axes or around randomly changing axes. From the Q-behaviour of the quasielastic intensities and from line-shape fits we have found that, in the time and temperature region covered in our measurements, a model including only 180° jumps around the molecular polar axis is able to describe adequately our results. For this model we have derived residence times, Debye-Waller factors, and the activation energy.