Journal
JOURNAL OF CHEMICAL PHYSICS
Volume 130, Issue 1, Pages -Publisher
AMER INST PHYSICS
DOI: 10.1063/1.3046678
Keywords
deuterium; hydrogen neutral molecules; isotope effects; organic compounds; Raman spectra; red shift; rotational states; rotons; vibrational states
Funding
- DOE [DE-FG02-05ER46242]
- U.S. Department of Energy (DOE) [DE-FG02-05ER46242] Funding Source: U.S. Department of Energy (DOE)
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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.
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