6th International Conference on Gas Hydrates

RAMAN SPECTROSCOPIC STUDIES OF HYDROGEN CLATHRATE HYDRATES Strobel, Timothy A. 2008

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  RAMAN SPECTROSCOPIC STUDIES OF HYDROGEN CLATHRATE HYDRATES   Timothy A. Strobel, Carolyn A. Koh, E. Dendy Sloan    Center for Hydrate Research Department of Chemical Engineering Colorado School of Mines 1600 Illinois Street, Golden, CO, 80401 USA    ABSTRACT Raman spectroscopic measurements of various hydrogen bearing clathrate hydrates have been performed under high (< 1cm -1 ) and low resolution (>2 cm -1 ) conditions.  Raman bands for hydrogen in most common clathrate hydrate cavities have been assigned.  Unlike most clathrate hydrate guests, the general observation is no longer valid that the larger the clathrate cavity in which a guest resides, the lower the vibrational frequency.  This is rationalized by the multiple hydrogen occupancies in larger clathrate cavities.  Both the roton and vibron bands for hydrogen clathrates illuminate interesting quantum dynamics of the enclathrated hydrogen molecules.  At 77K, the progression from ortho to para H2 occurs over a relatively slow time period (days).  The para contribution to the roton region of the spectrum exhibits the triplet splitting also observed in solid para H2.  The complex vibron region of the Raman spectrum has been interpreted by observing the change in population of these bands with temperature and with isotopic substitution by deuterium.  Raman spectra from H2 and D2 hydrates suggest that the occupancy patterns between the two hydrates are analogous. The Raman measurements demonstrate that this is an effective and convenient method to determine the relative occupancy of hydrogen molecules in different clathrate cavities.  Keywords: hydrogen, deuterium, THF, multiple occupancy, Raman spectroscopy    Corresponding author: Phone: +1 303 273 3723 Fax +1 303 273 3720 E-mail: esloan@mines.edu NOMENCLATURE ν             Vibrational quantum number J              Rotational quantum number m            Component of J along z-axis I              Total nuclear spin gs            Statistical weight QΔν(J)     Q branch vibron bands S Δν(J)     S branch roton bands  INTRODUCTION Clathrate hydrates are molecular inclusion compounds that trap small molecules within polyhedral hydrogen-bonded water cavities [1]. Historically it was thought that the hydrogen molecule was too small to contribute to the stability of these compounds and H2 was thought to act as a diluent to the fugacity of other components in gas stream mixtures [2].  Over the past six years it has been established that hydrogen can act as a suitable hydrate guest molecule in both single and mixed hydrates [3, 4].  Pure hydrogen hydrate forms hydrate structure II (sII) and may contain up to four hydrogen molecules in the large cavities and one hydrogen molecule in the small cavities [5].  In binary hydrates with hydrogen and other large sII forming molecules like tetrahydrofuran (THF), hydrogen may Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. partially or completely occupy the small dodecahedral cavity [6, 7].  Additionally, binary structure I (sI) hydrates have been made from mixtures of methane+hydrogen and carbon dioxide+hydrogen [8, 9].  Binary structure H (sH) hydrates have been synthesized with hydrogen and appropriately sized sH forming molecules like methylcyclohexane (MCH) [10].  Also, hydrogen may be contained within the small cavities of semi-clathrate structures [11].  The nuclei of the H2 molecule are composed of two indistinguishable fermions constraining the overall wavefunction to be antisymmetric. Thus, H2 molecules with antiparallel nuclear spins (I = 0) can only exist with even rotational states J = 0 , 2, … and H2 molecules with parallel spins (I = 1) can only exist with odd rotational states J = 1 , 3, ... [12]. The nuclei of D2 are composed of two indistinguishable bosons constraining the overall wavefunction to be symmetric. D2 molecules with I = 0 or 2 can only exist with even rotational states and D2 molecules with I = 1 only exist with odd rotational states. These symmetry constraints yield two types of hydrogen: ortho and para. Ortho is designated the species with the greatest statistical weight, gs.  Ortho H2 refers to molecules with I = 1, odd J and gs = 3, and para H2 refers to molecules with I = 0, even J and gs = 1. Ortho D2 refers to molecules with I = 0 or 2, even J and gs = 6, and para D2 refers to molecules with I = 1, odd J and gs = 1. At high temperatures (T>>Behc/kB) the ortho to para ratio reduces to the ratio of statistical weights – 3:1 for H2, and 2:1 for D2. These mixtures are termed normal [12].  The inclusion of H2 molecules within clathrate hydrate cavities presents a unique system to study the quantum dynamics of confined hydrogen and interactions with the water host lattice. Features such as multiple cavity occupation and H2-H2 separation distances smaller than that observed in solid hydrogen demonstrate unique characteristics warranting additional studies. Furthermore, a fundamental understanding of these compounds is required if they are to be realized as functional hydrogen storage materials. In this work we have investigated the molecular behavior of hydrogen molecules contained with various clathrate hydrate cavities via Raman spectroscopy.  EXPERIMENTAL Two types of experimental techniques were used in this study: (1) in situ capillary tube experiments and (2) ex situ experiments in which hydrate was formed in a high pressure stainless steel cell. For the in situ experiments, hydrates were formed in fused silica, square walled capillary tubes. Details of this procedure are given elsewhere [13].  For ex situ experiments, ~1 g of finely ground ice powder (<180 μm) was loaded into a 316ss high pressure cell and pressurized with either H2 of D2. Experiments were also conducted with THF hydrate and ice powder saturated with various sH forming molecules. Hydrate formation periods consisted of about 24 hours which was deemed sufficient for complete conversion, for all systems except some sH systems, by X-ray diffraction measurements. After the conversion period the cell was quenched in liquid nitrogen and the hydrate samples were removed and measured with Raman in a nitrogen cryostat.  Two Raman spectrometers were used in this study: a Renishaw MKIII and a Horiba Jobin Yvon LabRamHR. The Renishaw spectrometer utilized an Ar +  laser with 514.5 nm light as an excitation source. Scattered light was collected in back- scatter geometry and dispersed off of a 2400 gv/mm grating over a 300 mm focal length. The resolution of this system, estimated from the linear dispersion coefficient and entrance slit (50 μm) was about 2 cm -1 . The LabRamHR spectrometer used a 532 nm diode laser as an excitation source. Scattered light was collected in back-scatter geometry and dispersed off of an 1800 gv/mm or 2400 gv/mm grating over an 800 mm focal length. The entrance slit was set at 50 μm. Typical spectral resolution ranged from 0.5 to 0.9 cm -1 .  TRENDS IN CLATHRATE HYDRATES Raman spectra for the Q branch of hydrogen bearing clathrates for all three common structures are shown in Figure 1.  At 77 K and 1 MPa, gaseous hydrogen shows two bands separated by approximately 6 cm -1  with an intensity ratio of about 3:1.  These two bands originate from vibrational excitations of hydrogen molecules with vibrational quantum number v=0 to v=1 for molecules with rotational quantum numbers J=0 and J=1 (Q1(0) and Q1(1)).  The intensity ratio is inherent to the equilibrium ratio of ortho to para H2 and rotational population distribution factor. Compared with the gaseous phase, hydrogen contained within the various clathrate cavities vibrates at a lower frequency, and exhibits a broader peak shape. 4100 4120 4140 4160 4180 Raman Shift (cm -1 ) In te n s it y  ( A .U .) Gaseous H2 sII H2 Hydrate sI+sII CH4 / H2 Hydrate sII THF / H2 Hydrate sH MTBE / H2 Hydrate  Figure 1.  Raman spectra of various hydrogen bearing clathrates  For THF+H2 hydrate, THF almost completely occupies the large sII cavities, and hydrogen occupies the small cavities up to one hydrogen molecule [6, 7, 14]. Therefore, contributions in the vibron region of hydrogen in the Raman spectrum of THF+H2 hydrate solely reflect singly occupied small cavities of sII hydrate. sH hydrate contains three cavity types.  The large 5 12 6 8  cavity houses molecules like methyl-tert-butyl ether (MTBE) and the two smaller 5 12  and 4 3 5 6 6 3  cavities contain smaller stabilizing molecules.  For the Raman spectrum of MTBE+H2 hydrate, the encapsulation of hydrogen within both of these cavities is consistent with the increase in peak breadth and decrease in shoulder definition when compared with that of THF+H2 hydrate.  It is expected that the vibron bands for the mid- and small-sized cavities of sH occur at similar frequency as the difference in radii of these cavities is about 0.1 Å [1].  By comparing the spectra of sII THF+H2 hydrate with pure sII H2 hydrate, it is apparent that the lowest frequency contribution to the Raman spectrum of pure H2 hydrate is due to singly occupied small cavities of sII hydrate.  Thus, the three highest frequency bands must be due to hydrogen molecules contained within the large cavities.  The spectrum for the sI+sII CH4+H2 hydrate shows the same trends as pure H2 hydrate, although the shoulder on the small cavity peak is larger, indicating possible contributions from the sI small cavities. Unlike most clathrate hydrate guests, the general observation is no longer valid that the larger the clathrate cavity in which a guest resides, the lower the vibrational frequency. For the large cavity of H2 hydrate, multiple occupancies create an effectively “tighter” environment for the H2 molecules. Thus, H2 molecules multiply occupied in large cavities vibrate at a higher frequency than singly occupied small cavities.  DETAILED PEAK ASSIGNMENTS In order to investigate the contributions from hydrogen to the Raman spectra of H2 bearing hydrates in greater detail, Raman measurements were performed at higher resolution (<1 cm -1 ). Figure 2 shows Raman spectra for gaseous H2 at 100 K and 100 MPa and THF+H2 hydrate formed at 150 MPa and 250 K, measured immediately after the formation period at 0.1 MPa and 77 K, and after six days in liquid nitrogen at the same conditions.  The spectrum for THF+H2 hydrate measured immediately after the formation period is very similar (though shifted to lower frequency) to that of gaseous H2: two bands separated by about 6 cm -1  with an intensity ratio of near 3:1.  These observations are consistent with both ortho and para H2 molecules occupying the small cavities of the sII hydrate.  Intuitively this result is consistent because the clathrate was formed from a normal mixture of ortho and para H2 at room temperature (the gas bottle was stored at ambient conditions) and the clathrate occupancy should reflect those compositions.  Therefore, it would appear that both ortho and para H2 molecules occupy the small cavities.  To test this hypothesis, the clathrate was left in liquid nitrogen for a period of six days.  After the six day period in liquid nitrogen the equilibrium distribution of ortho to para H2 should shift to reflect a slightly para rich mixture [15].  Figure 2 demonstrates that this conversion does occur and we can conclude that these contributions to the Raman spectrum are due to H2 molecules in the two lowest energy rotational quantum states.  0 0.2 0.4 0.6 0.8 1 4110 4120 4130 4140 4150 4160 4170 4180 Raman Shift (cm -1 ) R e la ti v e  I n te n s it y  ( A .U .)  Figure 2. Raman spectra of gaseous H2 at 100 MPa and 100K (dashed), THF+H2 hydrate at 0.1 MPa and 77 K (dotted), THF+H2 at 0.1 MPa and 77 K after six days at 77 K (solid).  Based upon these results and the Raman spectra in Figure 1, it is expected that two bands (Q1(0) and Q1(1)) comprise the total vibron contribution for any singly occupied hydrate cavity at 77 K.  At lower temperature a single para band will dominate.  At higher temperatures additional rotational states become populated and additional peaks are observed in the spectra.  We have clearly observed this feature for THF+H2 hydrate above 150 K [16].  As temperature increases the natural peak width of each vibrational band broadens in the vibron spectrum.  This results in a single broad feature for hydrogen in the clathrate at higher temperatures (~280 K) [4, 17].  Pure hydrogen hydrate was synthesized at 150 MPa and 250 K. After the formation period, the hydrate was measured under high resolution conditions at 77 K and 0.1 MPa (Figure 3, day 1). The hydrogen hydrate measured immediately after the formation period (day 1) showed six distinct contributions.  As in the THF+H2 hydrate at 77 K, each hydrogen environment is actually represented by two bands (ortho and para H2).  These ortho and para contributions are separated by ~6 cm -1  and initially have an intensity ratio of about 3:1. 4110 4120 4130 4140 4150 4160 4170 Raman Shift (cm -1 ) In te n s it y  ( A .U .) O P P P P P P O O O O O Day 1 Day 6  Figure 3. Vibron spectra for hydrogen hydrate measured at 77 K and 0.1 MPa at 1 day, and the same sample measure after six days in liquid nitrogen. Dashed lines connect ortho Q1(1) and para Q1(0) partners separated by ~6 cm -1 .  Therefore, the six bands shown in Figure 3 (day 1) actually represent three separate hydrogen environments (a fourth environment around 4133 cm -1  is also present, see discussion below).  The ortho-para pairs separated by 6 cm -1  with a 3:1: intensity ratios have been connected with dashed lines.  Additionally, after 6 days in liquid nitrogen, the ortho:para distribution changes to reflect a slightly para rich composition (Figure 3, day 6).  It is known that at ambient pressure, the occupancy of hydrogen in the large sII cavity can vary between two and four hydrogen molecules. Figure 4 shows neutron scattering results on D2 hydrate from Lokshin et al.[5] As the temperature was increased from about 70 K, the occupancy in the large cavity began to diminish from four to two hydrogen molecules.  Because each individual cavity must contain integer values of hydrogen molecules, average non-integer occupancy values must be caused by some distribution of multiply occupied cavities, each containing two, three, or four H2 molecules.  The small cavity occupancy remains unity until the thermodynamic melting temperature (~160 K) is achieved.  Therefore, if H2 hydrate is heated from 77 K, the change in the large cavity occupancy distribution should be evident in the Raman spectra, and the individual occupancy contributions can be assigned.  0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 60 80 100 120 140 160 180 Temperature /K D 2  O c c u p a n c y Small Cavity Large Cavity  Figure 4. Occupancy of D2 hydrate as a function of temperature at 0.1 MPa modified from Lokshin et al.[5]  Figure 5a shows H2 hydrate formed at 200 MPa and 250 K, measured at 77 K and 0.1 MPa.  This hydrate sample was then heated to 150 K and requenched in liquid nitrogen (Figure 5b) and heating/quenching was repeated two additional times (Figure 5c-d).  The hydrate was heated in order to decrease H2 occupancy, and quenched to stop this process as well as increase resolution by narrowing the band widths.  By examining the evolution and regression of bands over these heating and quenching cycles, the individual contributions from quadruple, triple and double occupancy are easily assigned.  The unperturbed sample (5a) shows the ortho and para small cage contribution as well as a high degree of quadruple large cage occupancy and a small amount of triple large cage occupancy. After the first heat / quench cycle (b), the triple occupancy bands grew and the quadruple occupancy bands shrank, relative to the small cage bands, consistent with the neutron scattering results [5].  Additionally, two new bands appeared representing double occupancy of the large cavities.  After the second heat / quench cycle, the quadruple occupancy contributions were almost completely gone, and the triple and double occupancy bands grew relative to the small cage contributions.  After the third heat / quench cycle, no quadruple occupancy was detectable, and the double occupancy bands were the most intense large cavity contribution.  A fourth heat / quench cycle resulted in decomposition of the hydrate.  It is worth noting that a hydrogen hydrate with only double occupancy in the large cavity and single occupancy in the small cavity was never observed in this study; a small amount of large cavity triple occupancy was always required for stability at these conditions.  4100 4120 4140 4160 Raman Shift (cm -1 ) In te n s it y  ( A .U .) (a) (b) (c) (d) 1@s 2@L 3@L 4@L  Figure 5. (a) vibron Raman spectrum of unperturbed hydrogen hydrate formed 200 MPa and 250 K, measured at 77 K and 0.1 MPa. (b)-(d) heat (150K) / quench (77 K) cycles (see text). Vertical lines indicate ortho and para cavity contributions: 1 H2 / small cage (1@s), 2H2/large cage (2@L), 3H2/large cage (3@L), 4 H2/large cage (4@L).  In order for the large cavity hydrogen occupancy to change from four to three to two molecules without decomposition of the crystal structure, some sort of diffusive process must take place.  By examining the connectivity of cavities in sII clathrate (Figure 6), it is apparent that the large cages are connected in series by the sharing of hexagonal faces.  We propose that this hexagonal face sharing connectivity provides a migration pathway for hydrogen molecules to diffuse through the crystal structure, without affecting the small cavity occupancy.  Additionally, the energy barrier for hydrogen migration through a hexagonal face has been calculated to be much lower than that of a pentagonal face [18], thus this diffusive pathway appears to be that of the least resistance.   Figure 6. sII unit cell showing hexagonal face sharing connectivity between large cavities.  ROTON SPECTRA FOR H2 HYDRATES The rotational Raman spectra for gaseous hydrogen at 100 MPa and 100 K and for H2 hydrate formed at 150 MPa and 250 K, measured at 0.1 MPa and 77 K are shown in Figure 7.  The two bands in the gaseous phase spectrum arise from purely rotational transitions: S0(0) 354 cm -1  (J=0 → J=2, para) and S0(1) 587 cm -1  (J=1 → J=3, ortho).  In the clathrate phase, these bands are significantly broadened, and the near coincidence in frequency with the gaseous peaks indicates that the enclathrated hydrogen molecules undergo nearly free rotations.  The intensity ratio of ortho to para in the hydrate phase is much less than 3:1 as this sample was stored in liquid nitrogen for a 12 hour period.  It is of interest that the S0(0) band in the clathrate phase is split into three distinguishable peaks separated by about 4 cm -1 . This splitting can be understood in terms of the anisotropy of the hydrate cavities with respect to the orientation of the H2 molecule.  For the free hydrogen molecule, the rotational energy levels are degenerate by a value of 2J+1. These degenerate sub-levels of J, m, may take on values of J, (J-1), (J-2), … , -J.  When J is equal to zero, m can only have a value of zero and the spherical harmonic probability density is given by a sphere with no orientational dependence.  When J is equal to one, m may take on values of 0 or ±1 and the spherical harmonic probability densities are given by elongated or flattened spheroids.  For the free molecule, these m values are degenerate so that their energy levels are equal and only affect the rotational population factor reflected in the Raman peak intensity. 310 360 410 460 510 560 610 Raman Shift (cm -1 ) In te n s it y  ( A .U .) 330 340 350 360 370 Raman Shift (cm -1 ) In te n s it y  ( A .U .)  Figure 7. Rotational Raman spectra for gaseous H2 (bottom) and H2 hydrate (top).  It is well known that anisotropic crystal fields may lift the degeneracy of a freely rotating molecule [19].  If the degeneracy was completely lifted, the transition of J=0 to J=2 could occur between the J=0, m=0 level to any one of the five (slightly different) m levels for J=2.  However, this would mean that five transitions should be observed for the para H2 band in Figure 7 and in these data only three peaks are resolvable.  Recently, five dimensional quantum calculations have been performed for a single hydrogen molecule in a small sII cavity [20].  The results demonstrate this splitting effect with five separate energy levels for the J=2 state at 337.87 cm -1 , 339.29 cm -1 , 356.99 cm -1 , 378.66 cm -1  and 382.37 cm -1 .  The differences between the first two and last two values are very small (1.42 cm -1  and 3.71 cm -1 ), while the differences between the middle value and first and last values are about 20 cm -1 .  This result suggests that although the five m are split into separate energies, only three of these transitions may be resolvable as differences in energy between the first and last two is very small. Therefore we suggest that the three split bands in Figure 7 actually contain all five of these transitions with two of them convoluted in the lowest and highest frequency peaks (assuming that all of these transitions are Raman active).  This result is similar to the triplet splitting of the para- band in solid hydrogen induced by the hcp crystal field [19].  It is noted that the splitting of ~20 cm -1  from quantum calculations is much larger than the 4 cm -1  observed in this experiment.  The calculated splitting was also over estimated for the J=0 to J=1 transition from inelastic neutron scattering experiments on THF+H2 hydrate [21].  For the ortho H2 band in the clathrate phase, no direct splitting was detected.  Considering that the J=1 level will be split into three sub-levels, and that the J=3 level will be split into seven sub- levels, the possibility of numerous rotational transitions separated by small energies provides that it is likely that this may appear as one broad band in the spectrum.  TRENDS IN D2 HYDRATES In order to confirm the peak assignments presented in the previous section, experiments were performed on D2 bearing hydrates. In the case of D2, the differences in spin statistics and relative frequency shifts allow for the unambiguous assignment of observed Raman bands. Figure 8 shows Raman spectra for THF+D2 hydrate at 77 K and 0.1 MPa, formed at 150 MPa and 265 K, measured directly after formation and after a 10 day period in liquid nitrogen. At 77 K, the three lowest rotational states of D2 are populated as seen in the three bands (Q1(0), Q1(1) and Q1(2)) in Figure 8. This is in contrast with H2 which only has two rotational states populated at 77 K. In normal D2, the ortho (J=0 and 2) to para (J=1) ratio is 2:1 (66.67% ortho). However, the rotational constant for D2 (Be~30 cm -1 ) is about half that of H2  (Be~60 cm -1 ) [22]. As a result, the equilibrium ortho D2 concentration at 77 K (70%) is only about 5% different from the normal value. In the case of ortho H2 the equilibrium concentration at 77 K (51%) changes by over a factor of two from the normal value. Thus, no significant change was observed for the D2+THF hydrate after storage at 77 K, whereas the intensity of the ortho and para H2 bands of THF+H2 hydrate reversed after storage at 77 K. These results are in complete agreement with the previous assignments for H2.  For the free D2 molecule, the Q branch transitions Q1(0), Q1(1) and Q1(2) occur at 2993.5 cm -1 , 2991.4 cm -1  and 2987.2 cm -1  respectively [22]. In the case of H2 hydrates, the separation between the free gas Q1(0) and Q1(1) bands (6 cm -1 ) was in agreement with that of H2 contained within any of the various cavity environments. For the THF+D2 hydrate spectra (Figure 8), the frequencies Q1(0)- Q1(1)=2 cm -1  and Q1(1)-Q1(2)=4 cm -1  also agree with the gas phase separations and confirm the assignments presented.  Q1(0) Q1(1) Q1(2) 2960 2965 2970 2975 2980 2985 2990 Raman Shift (cm -1 ) In te n s it y  ( A .U .)  Figure 8. Raman spectra for THF+D2 hydrate formed at 100 MPa and 265 K. Solid line – Raman measurements directly after formation, dashed line – Raman measurement after 10 days in liquid nitrogen. The contribution at higher frequency (~2990cm -1 ) is from THF.  Pure D2 hydrate was formed at 192 MPa and 260 K. Figure 9 represents the same procedure for D2 hydrate as was performed for H2 hydrate for figure 5. Figure 9a shows the unperturbed D2 hydrate sample with predominantly quadruple occupancy of the large cavity and single occupancy of the small cavity, although as with the H2 hydrate at similar formation conditions, triple large cavity occupancy was also present. After the first heat / quench cycle (b), quadruple occupancy of the large cage was reduced, and double and triple occupancy increased relative to the small cage bands. The second heat quench cycle (c) showed primarily triple large cage occupancy with some double and quadruple. After the fourth cycle, the large cavities were mainly doubly occupied by D2 with small amount of triple large cavity occupancy. These results are directly applicable to the H2 data and unambiguously confirm the Raman peak assignments.  Additionally, the spectra presented for D2 suggest that H2 follows the same occupancy patterns under similar formation conditions and closes doubts as to whether D2 is a representative substitute for H2. As with the THF+D2 hydrate, after 10 days in liquid nitrogen, the simple D2 hydrate showed nearly identical Raman bands when compared with the measurement directly after formation. However, it is noted here that after the 10 day period, some amount of quadruple large cavity occupancy was lost, and triple and double occupancy increased. This indicates that although the hydrate is stable at these conditions by pressure and temperature, it is not stable by chemical potential. Thus, after significant storage in liquid nitrogen, some hydrogen is released.  2955 2965 2975 2985 2995 3005 3015 Raman Shift (cm -1 ) In te n s it y  ( A .U .) 1@s 2@L 3@L 4@L (a) (b) (c) (d)  Figure 9. (a) Vibron Raman spectrum of unperturbed D2 hydrate formed 192 MPa and 260 K, measured at 77 K and 0.1 MPa. (b)-(d) heat (150K) / quench (77 K) cycles (see text). Vertical lines indicate ortho Q1(0) and para Q1(1) cavity contributions: 1 H2 / small cage (1@s), 2H2/large cage (2@L), 3H2/large cage (3@L), 4 H2/large cage (4@L). Q1(2) positions are not labeled for clarity.  For the rotational spectrum of D2 hydrate, the same general trends as with H2 hydrate were observed. However, in the case of D2, three rotational bands were present rather than two. The S0(0) ortho D2 band was split into three resolvable peaks separated by about 4 cm -1 . The S0(1) and S0(2) bands were not resolvable into separate components. These results suggest that the anisotropy of the environments for both H2 and D2 are very similar.  ABSOLUTE HYDROGEN OCCUPANCY The results of the present study suggest that Raman spectroscopy is a powerful tool for determining the relative cage occupancies of hydrogen hydrates. As opposed to neutron scattering experiments, the distributions of different hydrogen environments are directly observable via Raman. Additionally, the relative intensities of the various hydrogen environments are related to the concentrations of those environments in the sample. Therefore, with knowledge of the relative polarizability change between multiply occupied hydrate cavities, this technique should allow for an absolute measurement of the hydrogen content with the small cavity occupancy as an internal standard. This method should also be applicable to measuring the hydrogen content of other types of hydrogen storage materials.  CONCLUSIONS Raman spectroscopic measurements of various hydrogen bearing clathrate hydrates have been performed under high (< 1cm -1 ) and low resolution (>2 cm -1 ) conditions.  Raman bands for hydrogen in most common clathrate hydrate cavities have been assigned.  Unlike most clathrate hydrate guests, the general observation is no longer valid that the larger the clathrate cavity in which a guest resides, the lower the vibrational frequency.  This is rationalized by the multiple hydrogen occupancies in larger clathrate cavities.  Both the roton and vibron bands for hydrogen clathrates illuminate interesting quantum dynamics of the enclathrated hydrogen molecules.  At 77K, the progression from ortho to para H2 occurs over a relatively slow time period (days).  The para contribution to the roton region of the spectrum exhibits the triplet splitting also observed in solid para H2.  The complex vibron region of the Raman spectrum has been interpreted by observing the change in population of these bands with temperature and with isotopic substitution by deuterium. Raman spectra from H2 and D2 hydrates suggest that the occupancy patterns between the two hydrates are analogous.  The Raman measurements demonstrate that this is an effective and convenient method to determine the relative occupancy of hydrogen molecules in different clathrate cavities.    REFERENCES [1] Sloan, E. D. and Koh, C. A. Clathrate Hydrates of Natural Gases, Third Edition. Boca Raton: Taylor & Francis - CRC Press, 2008. [2] Holder, G. D., Stephenson, J. L., Joyce, J. J., John, V. T., Kamath, V. A. and Malekar, S. Formation of Clathrate Hydrates in Hydrogen- Rich Gases. Ind. Eng. Chem. Proc. Des. Dev. 1983;22:170. [3] Mao, W. L., Mao, H. K., Goncharov, A. F., Struzhkin, V. V., Guo, Q. Z., Hu, J. Z., Shu, J. F., Hemley, R. J., Somayazulu, M. and Zhao, Y. S. Hydrogen clusters in clathrate hydrate. Science 2002;297:2247. [4] Florusse, L. J., Peters, C. J., Schoonman, J., Hester, K. C., Koh, C. A., Dec, S. F., Marsh, K. N. and Sloan, E. D. Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate. Science 2004;306:469. [5] Lokshin, K. A., Zhao, Y., He, D., Mao, W. 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