[MIS17-P07] Effect of temperature on the C–H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates
Keywords:gas hydrate, Raman spectroscopy, powder X-ray diffraction, molecular vibrations
Three typical crystal structures of clathrate hydrates have been identified on Earth: cubic structure I (sI), cubic structure II (sII), and hexagonal structure H (sH). Small guest molecules such as methane (CH4) or ethane form sI-type hydrates, whereas larger molecules like propane or 2-methylpropane (C4H10) form sII hydrates. sH hydrates are obtained from even larger molecules, e.g., 2-methylbutane (C5H12) or 2,2-dimethylbutane (C6H14), in the presence of a help gas as CH4. In this study, the Raman spectra of the C–H symmetric stretch region of CH4 enclathrated within various sH hydrates and structure I CH4 hydrates were analyzed in the temperature range of 83–183 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH4 in host water cages increased because of varying temperature, and the sizes of host water cages also increased; variation of CH4 in small cages was less than in larger cages. Comparing the variations of the C–H symmetric stretching frequencies of CH4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH4 molecule in water cages.
Sloan and Koh (2008) Clathrate Hydrates of Natural Gases, 3rd ed., CRC Press: Boca Raton, FL, USA