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 (4³5⁶6³ and 5¹² cages, respectively), θ_M/θ_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 ∼ 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.