Neutron diffraction study of structure I and structure II trimethylene oxide clathrate deuterate

Trimethylene oxide (TMO) forms structure I or II hydrates depending on stoichiometry and affords the opportunity to compare and contrast the temperature dependence of the crystal structure parameters for the two major hydrate structure types occluding a common guest molecule. Structures I and II TMO deuterates were synthesized from D₂O and hydrogenated TMO at the ratio of 6.25:1 D₂O:TMO (mole) for structure I and 17:1 D₂O:TMO for structure II. The samples were studied with powder neutron diffraction using the high-resolution powder diffractometer at the Japan Atomic Energy Research Institute, research reactor JRR-3M. The guest-host interactions of TMO in the 5¹²6² and 5¹²6⁴ cavities were modeled using rigid-body constraints. Data were collected at 10 (12 K for structure I), 40, 70, 100, 130, 160, 190, 220, and 250 K for both structure types. The volumes of the 5¹² and 5¹²6² or 5¹²6⁴ cages of both structure types were computed at each temperature. Cage volume calculations, rigid-body orientations, and rigid-body mean-squared displacement parameters revealed perturbations in the host lattice due to guest molecule motion/orientation for the structure I deuterate but not for the structure II deuterate. The guest molecule motion approximates that in the gas phase with some restrictions imposed by the deuterate cage. Below 105 K, the guest molecule tends to orient the ether oxygen toward the hexagonal face of the structure I 5¹²6² cage. Above 105 K the ether oxygen moves away from the hexagonal face of the 5¹²6² cage to an angle of about 50°, corresponding to an increase in volume for the 5¹²6² cage and a decrease in volume for the 5¹² cage. The most rapid changes with respect to temperature occur between 160 and 190 K. Similar behavior was not noted for the structure II deuterate.