Raman spectra of vibrational and librational modes in methane clathrate hydrates using density functional theory

摘要

The sI type methane clathrate hydrate lattice is formed during the process of nucleation where methane gas molecules are encapsulated in the form of dodecahedron (5 ~(12)CH _4) and tetrakaidecahedron (5 ~(12)6 ~2CH _4) water cages. The characterization of change in the vibrational modes which occur on the encapsulation of CH _4 in these cages plays a key role in understanding the formation of these cages and subsequent growth to form the hydrate lattice. In this present work, we have chosen the density functional theory (DFT) using the dispersion corrected B97-D functional to characterize the Raman frequency vibrational modes of CH _4 and surrounding water molecules in these cages. The symmetric and asymmetric C-H stretch in the 5 ~(12)CH _4 cage is found to shift to higher frequency due to dispersion interaction of the encapsulated CH _4 molecule with the water molecules of the cages. However, the symmetric and asymmetric O-H stretch of water molecules in 5 ~(12)CH _4 and 5 ~(12)6 ~2CH _4 cages are shifted towards lower frequency due to hydrogen bonding, and interactions with the encapsulated CH _4 molecules. The CH _4 bending modes in the 5 ~(12)CH _4 and 5 ~(12)6 ~2CH _4 cages are blueshifted, though the magnitude of the shifts is lower compared to modes in the high frequency region which suggests bending modes are less affected on encapsulation of CH _4. The low frequency librational modes which are collective motion of the water molecules and CH _4 in these cages show a broad range of frequencies which suggests that these modes largely contribute to the formation of the hydrate lattice.