[MIS21-P01] Raman spectroscopic analysis of isotopologue methane hydrates
Keywords:methane hydrate, Raman spectroscopy, isotopologues
13CH4 hydrate sample was synthesized in a small pressure cell (volume: 5 mL). Fine ice powder (1g) was put in the pressure cell, and introduced appropriate amount of 13CH4 (purity: 99.5%, Taiyo-Nissan). 13CH4 hydrate was formed by melting the fine ice powder at the temperature of 273.2 K under high pressure of 13CH4. We retrieved the hydrate sample at 77 K and its Raman spectra was obtained at 123 K in the range 2,500-3,300 cm-1 using a Raman spectrometer (RMP-210, JASCO Corporation). The Raman peaks were fitted in the range 2,800-3,000 cm-1 for the C-H stretching peaks of methane using a Voigt function to obtain the integrated intensities of the two peaks corresponding to methane encaged in the large and small cages of the cubic structure I.
Raman shifts for the C-H stretching and bending modes of 13CH4 was 0.8 cm-1 and 14 cm-1 smaller than those of 12CH4, respectively, suggesting that 13C-H bonds affect its vibrational frequency. Hydration number of 13CH4 was estimated as 6.00±0.02, almost the same as that of 12CH4 (6.02±0.02). Therefore, Cage occupancies of 13CH4 and 12CH4 hydrates showed no difference between them.
Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH3D and CD4. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203