Raman spectroscopic studies of hydrogen clathrate hydrates

Raman spectroscopic measurements of simple hydrogen and tetrahydrofuran+hydrogen sII clathrate hydrates have been performed. Both the roton and vibron bands illuminate interesting quantum dynamics of enclathrated H-2 molecules. The complex vibron region of the Raman spectrum has been interpreted by observing the change in population of these bands with temperature, measuring the absolute H-2 content as a function of pressure, and with D-2 isotopic substitution. Quadruple occupancy of the large sII clathrate cavity shows the highest H-2 vibrational frequency, followed by triple and double occupancies. Singly occupied small cavities display the lowest vibrational frequency. The vibrational frequencies of H-2 within all cavity environments are redshifted from the free gas phase value. At 76 K, the progression from ortho- to para-H-2 occurs over a relatively slow time period (days). The rotational degeneracy of H-2 molecules within the clathrate cavities is lifted, observed directly in splitting of the para-H-2 roton band. Raman spectra from H-2 and D-2 hydrates suggest that the occupancy patterns between the two hydrates are analogous, increasing confidence that D-2 is a suitable substitute for H-2. The measurements suggest that Raman is an effective and convenient method to determine the relative occupancy of hydrogen molecules in different clathrate cavities.