Vibrational Raman spectra of hydrogen clathrate hydrates from density functional theory

Abstract

Hydrogen clathrate hydrates are promising sources of clean energy and are known to exist in a sII hydrate lattice, which consists of H2 molecules in dodecahedron (5(12)) and hexakaidecahedron (5(12)6(4)) water cages. The formation of these hydrates which occur in extreme thermodynamic conditions is known to be considerably reduced by an inclusion of tetrahydrofuran (THF) in cages of these hydrate lattice. In this present work, we employ the density functional theory with a dispersion corrected (B97-D) functional to characterize vibrational Raman modes in the cages of pure and THF doped hydrogen clathrate hydrates. Our calculations show that the symmetric stretch of the H2 molecule in the 5(12)6(4)H2·THF cage is blueshifted compared to the 5(12)6(4)H2 cage. However, all vibrational modes of water molecules are redshifted which suggest reduced interaction between the H2 molecule and water molecules in the 5(12)6(4)H2·THF cage. The symmetric and asymmetric O-H stretch of water molecules in 5(12)H2, 5(12)6(4)H2, and 5(12)6(4)H2·THF cages are redshifted compared with the corresponding guest free cages due to interactions between encapsulated H2 molecules and water molecules of the cages. The low frequency modes contain contributions from contraction and expansion of water cages and vibration of water molecules due to hydrogen bonding and these modes could possibly play an important role in the formation of the hydrate lattice.

Clathrate hydrates: recent advances on CH4 and CO2 hydrates, and possible new frontiers

“…Futera et al 164 combined experimental Raman spectroscopy and MD to determine the vibrational modes of hydrogen molecules confined in structure II hydrates. 165,166 The simulations agreed qualitatively with the experimental Raman spectra. The ability of reproducing vibrational spectra is key for predicting thermal conductivities in systems that show many interfaces.…”

H2, HD, and D2 in the small cage of structure II clathrate hydrate: Vibrational frequency shifts from fully coupled quantum six-dimensional calculations of the vibration-translation-rotation eigenstates

“…In some of them, the H 2 molecules encapsulated in the isolated small or large hydrate cages were taken to be frozen in the geometry corresponding to the minimum energy of the system. [30][31][32] As a result, nuclear quantum effects are left out, in particular the averaging over the large-amplitude intermolecular vibrations of the guest H 2 molecules. In addition, since only isolated clathrate cages are considered, the effects of the condensed-matter environment are unaccounted for.…”

Intramolecular stretching vibrational states and frequency shifts of (H2)2 confined inside the large cage of clathrate hydrate from an eight-dimensional quantum treatment using small basis sets

“…In some of them, the H 2 molecules encapsulated in the isolated small or large hydrate cages were treated as static, frozen in the geometry corresponding to the minimum energy of the system. [33][34][35] This leaves out nuclear quantum effects, especially the averaging over the large-amplitude intermolecular vibrations of the guest H 2 molecules. This problem was also approached by combining classical molecular dynamics (MD) and PIMD simulations with electronic structure calculations at the DFT (B3LYP) and MP2 levels.…”