Retracted Article: Effect of temperature and large guest molecules on the C–H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates

Abstract

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 83-183 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.

Structure H Clathrate Hydrates in Methane−Halogenic Large Molecule Substance–Water Systems

“…The thermal lattice expansions of the sH hydrates hosting CH 4 and halogenic cyclohexanes (ClCH, ICH, and BrCH) were observed by PXRD measurements as the temperature increased from 83 to 183 K. The lattice expansions during temperature ramping are plotted in Figure . The lattice constants linearly increased with temperature, as reported for other clathrate hydrates. , The lattice expansions of the sH hydrates were anisotropic along the a - and c -axes . Such anisotropic behavior originates from the size of the help guest .…”

“…Fuseya et al demonstrated a shift toward higher wavenumbers in the C–H peak of CH 4 in sI hydrate and the composite one C–H peak in sH hydrates during temperature ramping. Meanwhile, the peaks in the spectra of sH hydrates cannot be spectrally resolved.…”

“…Relationships between help guests/LMGS and the crystallographic properties of sH hydrates (such as lattice constants, guest–host interactions, and phase stabilities) have been extensively reported in the literature. ,− For example, as the LMGS volume increases, the unit cell volume of the sH hydrate increases, whereas the lattice constants along the c and a axes decrease and increase, respectively. , Conversely, Murayama et al suggested that as the size of the helper guests increases, the c lattice constant increases remarkably more rapidly than the a lattice constant . Which guest parameters determine the crystallographic properties in sH hydrates remains insufficiently understood.…”

Effects of Large Guest Molecular Structure on Thermal Expansion Behaviors in Binary (C4H8O + CH4) Clathrate Hydrates

“…The cooperative relationship between the O–H stretching mode and the O:H stretching phonon in ice or the THF hydrate system has been reported in previous studies. , During the annealing process, the intermolecular hydrogen bonding of the hydrate cage is lengthened and softened with lattice expansion, showing a red shift, and the intramolecular covalent bonding is shortened and stiffened, showing a blue shift. Also, Fuseya et al reported the effects of hydrate cage size on the shift of C–H stretching mode when the temperature increases . The slight blue shift of C–H stretching mode in Figure is similar to the results of the aforementioned study, but there was no significant difference depending on the type of guest molecules.…”