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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), θM/θS, 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.

[1] We report the field observation of hydrate deposits of different crystal structures in the same cores of a mud volcano in the Kukuy Canyon. We link those deposits to chemical fractionation during gas hydrate crystallization. Gas composition and crystallographic analyses of hydrate samples reveal involvement of two distinct gas source types in gas hydrate formation at present or in the past: microbial (methane) and thermogenic (methane and ethane) gas types. The clathrate structure II, observed for the first time in fresh water sediments, is believed to be formed by higher mixing of thermogenic gas. Citation: Kida, M., et al. (2006), Coexistence of structure I and II gas hydrates in Lake Baikal suggesting gas sources from microbial and thermogenic origin,

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Movers and shakers: Vibrational states of CH4 molecules encaged in three clathrate hydrate structures are studied (see picture). Guest methane distribution in the structure‐H 512 and 435663 host cavities is revealed for the first time. Raman profiles of the CH4 vibration are dependent not only on types of water cages, but also on clathrate structures (guest compositions), suggesting distinctive differences in molecular interactions between the three hydrate systems.

Isotopic fractionation of carbon and hydrogen in methane and ethane during the formation of gas hydrates was investigated. The gas hydrate samples were experimentally prepared in a pressure cell and isotopic compositions of both residual and hydrate‐bound gases were measured. δD of hydrate‐bound molecules of methane and ethane hydrates was several per mil lower than that of residual gas molecules in the formation processes, while there was no difference in the case of δ13C. These isotopic differences in δD are enough small for discussing the source types of hydrate‐bound gases using the δ13C‐δD diagram of Whiticar et al. [1986]. These results may provide useful insight into the formation process of gas hydrates.