Effect of temperature and large guest molecules on the C-H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates.

Large molecules such as 2-methylbutane (C 5 H 12 ) or 2,2-dimethylbutane (C 6 H 14 ) form structure H (sH) hydrates with methane (CH 4 ) as a help gas. In this study, the Raman spectra of the C-H symmetric stretch region of CH 4 enclathrated within various sH hydrates and structure I CH 4 hydrates were analyzed in the temperature range 137.7-205.4 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH 4 in host-water cages increased because of varying temperature, and the sizes of the host-water cages also increased; variation of CH 4 in small cages was less than in larger cages. Comparing the variations of the C-H symmetric stretching frequencies of CH 4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH 4 molecule in water cages.