Orientational Ordering, Locking-in, and Distortion of CH4 Molecules in Methane Hydrate III under High Pressure

• Published in: Journal of physical chemistry. C Vol. 122; no. 20; pp. 11159 - 11166
• Main Authors: Schaack, Sofiane, Ranieri, Umbertoluca, Depondt, Philippe, Gaal, Richard, Kuhs, Werner F., Falenty, Andrzej, Gillet, Philippe, Finocchi, Fabio, Bove, Livia E.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 24.05.2018
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b02783

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in methane hydrate IIIthe stable form of methane for pressures above 2 GPa at room temperatureby combining high-pressure Raman spectroscopy with ab initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH4:D2O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20 GPa. This happens along with a progressive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa, the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.