Temperature-Dependent Distortions of the Host Structure of Propylene Oxide Clathrate Hydrate

A detailed study of the water host of a structure II clathrate hydrate with propylene oxide as the guest has been performed on high-resolution, constant-wave length neutron powder diffraction data. The structure is cubic in space group Fd (3) over barm, with lattice parameters falling in the range (1.7086-1.72182) nm in the temperature range (5-250) K. Static disorder of water molecules and dynamic disorder of deuterium atoms produces a hydrogen-bond geometry in the host water structure, which is significantly different From that of (2)H(2)O ice I(h), but displays similarities to high-pressure ices and to crystalline hydrates of inorganic salts. The structural description of the water host can he divided into three regimes of temperature, where the bond distances and angles display greater or lesser similarity to those of (2)H(2)O ice I(h). These temperature-dependent variations in structure are related to size, shape, and dynamics of the guest molecules, as well as the dynamics of the host itself. As in other hydrates experiencing Strain from the enclathration of large guest, isotropic expansion of the local structure in the cubic hydrate of PO does not lead to the minimum-energy configuration. The variation in the hydrogen bond angles and distance provide a quantitative indicator of the strength of guest-host interactions and the deviation of the thermodynamic properties of hydrate hosts from those of ice I(h).