Polyhedral Clathrate Hydrates. IX. Structure of Ethylene Oxide Hydrate

A detailed single-crystal structure analysis is reported for ethylene oxide hydrate. This confirms the polyhedral host lattice formed by 46 hydrogen-bonded water molecules in a cubic unit cell of 12.03 Å with space-group symmetry Pm3n. Disordered ethylene oxide molecules occupy the six equivalent tetrakaidecahedral cavities, and their electron-density distribution suggests that they are hindered axial rotors. Additional guest molecules, either ethylene oxide, oxygen, or nitrogen molecules occupy the two dodecahedral cavities with high disorder and low statistical weight. The compound is therefore nonstoichiometric having the limiting formula 2M·6C2H4O·46H2O, in which the percentage of M is variable. If M is assumed to be entirely ethylene oxide, as is likely from the method of preparation, the composition of the crystals obtained from an equilibrated aqueous solution of 8 mole % ethylene oxide at 9°C is 6.4C2H4O·46H2O.