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

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 (H2O-H-2) and binary (H2O-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) (O) 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. (C) 2008 American Institute of Physics.