Low temperature Raman spectra of hydrogen in simple and binary clathrate hydrates

Abstract

The Raman spectrum of hydrogen clathrate hydrates has been measured, as a function of temperature, down to 20 K. Rotational bands of H(2) and HD, trapped into the small cages of simple (H(2)O-H(2)) and binary (H(2)O-THF-H(2)) hydrates, have been analyzed and the fivefold degeneracy of the molecular J=2 rotational level has been discussed in the light of the available theoretical calculations. The vibrational frequencies of H(2) molecules encapsulated in the large cages of simple hydrates turn out to be well separated from those pertaining to the small cages. Comparison with the equivalent D(2) spectra allowed us to assign the large cavity vibrational frequencies to three couples of Q(1)(1)-Q(1)(0) H(2) vibrational modes. Populations of ortho and para species have been measured as a function of time from rotational spectra and the rate of ortho-para conversion has been estimated for both simple and binary hydrates. We suggest, using the H(2) vibrational spectra, a model to analyze the cage population in simple hydrates.

Molecular-Dynamics and First-Principles Calculations of Raman Spectra and Molecular and Electronic Structure of Hydrogen Clusters in Hydrogen Clathrate Hydrate

“…1,37 The low-frequency end around 4120-4125 cm -1 of the experimental spectra is assigned to H 2 in the S 1 cages and the high-frequency end around 4125-4150 cm -1 to H 2 in the L cage (Figure 2), consistent with recent Raman spectroscopic studies of hydrogen clathrate. 14,15 However, overlap between S 1 and L 1-4 peaks may occur as suggested by the calculations (Figure 2a). This assignment is also consistent with assigning Raman peaks between 4120 and 4125 cm -1 to the H 2 singly occupied small cages in THF hydrogen clathrate.…”

“…14,15 This kind of behavior can be attributed to very slow ortho-to para-H 2 conversion, which involves a change of H 2 nuclear spins at lower temperature of samples made at higher temperature.…”

“…Indeed, recent experiments using Raman spectroscopy support a mixture of different hydrogen occupancies in the large cages. 14,15 However, uncertainty regarding the optimal hydrogen occupancy in the small cages remains. Although most of the recent experiments support single occupancy in the small cages, 3,[14][15][16] it is not clear that hydrogen clathrate with hydrogen doubly occupied small cages is possible as previously reported in a high-pressure experiment.…”

“…14,15 However, uncertainty regarding the optimal hydrogen occupancy in the small cages remains. Although most of the recent experiments support single occupancy in the small cages, 3,[14][15][16] it is not clear that hydrogen clathrate with hydrogen doubly occupied small cages is possible as previously reported in a high-pressure experiment. 1 There is also some uncertainty about the Raman frequency range of hydrogen molecules in the doubly occupied small cages.…”

Dynamics of hydrogen guests in ice XVII nanopores

“…In hydrogen clathrate hydrates, which possess a similar chemical composition, although a different structure and stoichiometry, the hydrogen molecules confined in nearly spherical cages 4 perform an almost free rotation and a deeply non-harmonic center-of-mass (CoM) vibrational motion (rattling), both of which have been experimentally investigated by inelastic neutron scattering [5][6][7][8][9] and Raman scattering [10][11][12][13] . In this paper we discuss the results of a combined experimental and simulation study on the dynamics of the H 2 guests in D 2 O ice XVII and of the D 2 O host lattice.…”