Structure H (sH) Clathrate Hydrate with New Large Molecule Guest Substances

This study characterized new structure H (sH) clathrate hydrates with bromide large-molecule guest substances (LMGSs) bromocyclopentane (BrCP) and bromocyclohexane (BrCH), using powder X-ray diffraction (PXRD) and Raman spectroscopy. The lattice parameters of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined from their PXRD profiles. On the basis of their Raman spectra, the M-cage to S-cage occupancy ratio (4(3)5(6)6(3) and 5(12) cages, respectively), theta(M)/theta(S), was estimated to be approximately 1.3, and the Raman shift of the symmetric C H vibrational modes of CH4 in S- and M-cages was 2911.1 and 2909.1 cm(-1), respectively. The phase-equilibrium conditions of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined by an isochoric method. A comparison between the equilibria of sH hydrates with BrCP and BrCH and those with other typical nonpolar and polar LMGSs (methylcyclopentane, MCP; methylcyclohexane, MCH; neohexane, NH; and tert-butyl methyl ether, TBME) at the same temperature revealed that the equilibrium pressure increased in the order NH < MCH < BrCH < TBME similar to MCP < BrCP. The phase stabilities of sH hydrates can be determined by not only molecular geometry but also their polar properties, which affect guest-host interactions.