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 (C5H12) or 2,2-dimethylbutane (C6H14) form structure H (sH) hydrates with methane (CH4) as a help gas. 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 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 CH4 in host-water cages increased because of varying temperature, and the sizes of the 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.