Distribution of Butane in the Host Water Cage of Structure II Clathrate Hydrates

To understand host-guest interactions of hydrocarbon clathrate hydrates, we investigated the crystal structure of simple and binary clathrate hydrates including butane (n-C4H10 or iso-C4H10) as the guest. Powder X-ray diffraction (PXRD) analysis using the information on the conformation of C4H10 molecules obtained by molecular dynamics (MD) simulations was performed. It was shown that the guest n-C4H10 molecule tends to change to the gauche conformation within host water cages. Any distortion of the large 5(12)6(4) cage and empty 5(12) cage for the simple iso-C4H10 hydrate was not detected, and it was revealed that dynamic disorder of iso-C4H10 and gauche-nC(4)H(10) were spherically extended within the large 5(12)6(4) cages. It was indicated that structural isomers of hydrocarbon molecules with different van der Waals diameters are enclathrated within water cages in the same way owing to conformational change and dynamic disorder of the molecules. Furthermore, these results show that the method reported herein is applicable to structure analysis of other host-guest materials including guest molecules that could change molecular conformations.