Chemical Shift Changes and Line Narrowing in 13C

Apr 22, 2013 - Furthermore, the 13C NMR line widths suggest, because of the reorientation ... and phase equilibrium15−17 predictions of clathrate hy...
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Chemical Shift Changes and Line Narrowing in Hydrocarbon Clathrate Hydrates

13

C NMR Spectra of

Masato Kida,† Hirotoshi Sakagami,‡ Nobuo Takahashi,‡ and Jiro Nagao*,† †

Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohiraku, Sapporo, Hokkaido 062-8517, Japan ‡ Department of Materials Science and Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan ABSTRACT: The solid-state 13C NMR spectra of various guest hydrocarbons (methane, ethane, propane, adamantane) in clathrate hydrates were measured to elucidate the local structural environments around hydrocarbon molecules isolated in guest−host frameworks of clathrate hydrates. The results show that, depending on the cage environment, the trends in the 13C chemical shift and line width change as a function of temperature. Shielding around the carbons of the guest normal alkanes in looser cage environments tends to decrease with increasing temperature, whereas shielding in tighter cage environments tends to increase continuously with increasing temperature. Furthermore, the 13C NMR line widths suggest, because of the reorientation of the guest alkanes, that the local structures in structure II are more averaged than those in structure I. The differences between structures I and II tend to be very large in the lower temperature range examined in this study. The 13C NMR spectra of adamantane guest molecules in structure H hydrate show that the local structures around adamantane guests trapped in structure H hydrate cages are averaged at the same level as in the α phase of solid adamantane.

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INTRODUCTION Understanding the local structure and/or behavior of a molecule isolated in a nanoscale framework is an interdisciplinary challenge for technology using nanopores.1−3 For isolated macromolecules, recent reports have described direct visual observations of specific structures through imaging techniques.1−3 Although fundamental small molecules such as hydrogen4,5 and water6 can be isolated in small spaces, their local structural environments remain poorly understood. An understanding of the local structural environment of such fundamental molecules in small spaces is necessary to predict molecular function in material design. Small hydrocarbons are basic organic molecules that are also isolated in small cavities of the water/silicon oxide frameworks of clathrate compounds.7,8 Clathrate hydrates, which are representative clathrate compounds, consist of guest molecules and host polyhedral cages constructed from assembled water molecules. Clathrate hydrates have been studied extensively for applications as a potential energy resource and as energy storage materials for hydrogen or hydrocarbons in natural gas.4,5,7,9 The three known main clathrate hydrate frameworks are structure I (sI), structure II (sII), and structure H (sH). The conditions under which these frameworks are thermodynamically stable, which depend on the type of guest molecule, are controlled by pressure and temperature. A single molecule or multiple molecules are isolated in cages with average radii of 0.391− 0.571 nm in clathrate hydrate frameworks.7,10 Although the reorientation of guest molecules in guest−host systems is controlled mainly by the cage geometry and the electrostatic fields of the host water molecules,11 the molecular motion of guest molecules has persisted as a complex problem.12 Guest− © 2013 American Chemical Society

host interactions in clathrate hydrates can play a role in dictating the crystal structure type,13 which is extremely important from the perspectives of thermodynamic stability14 and phase equilibrium15−17 predictions of clathrate hydrates. Furthermore, the guest−host interactions in clathrate hydrates play a role in the expression of the unique phenomenon known as the self-preservation of clathrate hydrates.18 Self-preservation is a phenomenon by which some gas hydrates are anomalously preserved under conditions such that the gas hydrate is thermodynamically unstable and is a key for gas hydrate storage and transportation under milder conditions. 19−21 It is important to improve the understanding of guest−host interactions in clathrate hydrates from the perspectives not only of fundamental studies of clathrate compounds but also of applications to gas storage materials. Herein, we describe the local structural environments around guest hydrocarbons isolated in water frameworks in clathrate hydrates using solidstate 13C nuclear magnetic resonance (NMR) spectroscopy. This study examined clathrate hydrates synthesized from methane (CH4) and from mixtures of methane with ethane (CH4−C2H6), propane (CH4−C3H8), and adamantane (CH4− C10H16).

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MATERIALS AND EXPERIMENTAL METHODS The simple CH4 hydrate sample was prepared by stirring 15.1 MPa of methane and 30 g of liquid water in a high-pressure reactor (volume ≈ 1.5 × 10−4 m3) at 275.5 K. The CH4−C2H6 Received: December 10, 2012 Revised: April 20, 2013 Published: April 22, 2013 4108

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Figure 1. Temperature dependence of CP-MAS 13C NMR spectra: (a) simple CH4 hydrate, (b) CH4−C2H6 hydrate formed from CH4−C2H6 mixtures containing 29.7% C2H6, (c) CH4−C2H6 hydrate formed from CH4−C2H6 mixtures containing 1.1% C2H6, (d) CH4−C3H8 hydrate, (e) CH4−C10H16 hydrate, and (f) solid C10H16.

adamantane reference. A methine carbon peak at 298 K was found at 29.47 ppm.24 The magnetic field drift of the magnetic field was within 4 Hz/h. The reported 13C NMR line widths (Δν1/2) are the full widths at half-maximum of a 13C NMR line fitted to a Lorentz−Gaussian function.

hydrate samples were prepared from 30 g of ice particles and two CH4−C2H6 mixtures containing 1.1% and 29.7% C2H6 using a milling-type high-pressure vessel22 with a volume of about 1 × 10−3 m3 at 263 K. The CH4−C3H8 hydrate sample was prepared from ice particles and 5% C3H8 using the same method as used for the CH4−C2H6 hydrates. The CH4−C10H16 hydrate was synthesized by a contact reaction in an ice−C10H16 powder mixture (11.9 g, 18.6 wt % C10H16 content) with 2 MPa of CH4 in a high-pressure reactor (volume ≈ 1.2 × 10−4 m3) at 269 K. All samples were recovered from the vessel using liquid nitrogen. The crystals in the CH4−C10H16 hydrate sample were identified using powder X-ray diffraction (PXRD). The PXRD profile was measured at a constant temperature of 143 K with flowing cooled dry N2 gas using an X-ray diffraction apparatus with Cu Kα radiation (40 kV, 249 mA, Rint-2500; Rigaku Corp.). A quartz glass capillary cell (diameter, 2.0 mm; thickness, 0.01 mm; length, 20 mm; Hilgenberg GmbH) was used for PXRD measurements. The PXRD profiles were acquired using a step width of 0.06° with a counting time of 1.2 s/step. The 13C NMR spectra of the clathrate hydrates were measured using an NMR spectrometer (JNM-AL400, 100 MHz; JEOL) at temperatures of 123−243 K with the crosspolarization magic-angle-spinning (CP-MAS) technique (1H radio-frequency field strength, 45 kHz; contact time, 10 ms; pulse delay time, 50 s; number of acquisitions, 16−80; spinning rate, 3.0−3.5 kHz). The precision of temperature control in the range of spinning rates used was within ±5 K, based on sample temperatures determined by the temperature sensitivity of the 207 Pb chemical shift of lead(II) nitrate.23 In addition, the 13C NMR spectra of solid C10H16 were obtained for the assignment of NMR lines from guest C10H16 in the CH4−C10H16 hydrate sample. The sample tube used for this study was made of zirconia (diameter, 6 mm; length, 22 mm; JEOL). The values of the 13C chemical shift were determined against an external

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RESULTS AND DISCUSSION Representative CP-MAS 13C NMR spectra of the CH4, CH4− C2H6, CH4−C3H8, and CH4−C10H16 hydrates are presented in Figure 1. The 13C NMR lines obtained from guest CH4, C2H6, and C3H8 molecules in each hydrate cage were assigned based on previously reported values.22,25−28 The lines from guest C10H16 molecules were assigned by comparison with those from solid C10H16 in the same temperature range. The CH4 hydrate showed two 13C NMR lines at −4.4 and −6.8 ppm at 123 K, which are assigned to CH4 molecules in 512 and 51262 cages, respectively, in sI (presented in Figure 1a). The sI unit cell comprises two small cages with 12 pentagonal faces (512) and six large cages with 12 pentagonal and 2 hexagonal faces (51262).7 The 13C NMR spectra of CH4 hydrate at temperatures higher than 183 K obtained in this study constitute information related directly to guest CH4 molecules in the remaining clathrate hydrate framework in decomposing or anomalously preserved crystals. Although CH4 hydrate is stable below approximately 190 K under methane at atmospheric pressure according to the phase equilibrium prediction by CSMHYD,29 CH4 hydrate under conditions of a lack of CH4 in the gas phase is thermodynamically unstable, even at temperatures lower than 193 K. An XRD study of the dissociation of clathrate hydrate lattices by Takeya and Ripmeester confirmed that the methane hydrate crystal framework begins to decompose at approximately 180 K,18 which is lower than the equilibrium temperature of methane hydrate under an atmospheric pressure of methane. Reportedly, most clathrate hydrates commence decomposition at temper4109

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atures of 180−200 K, irrespective of the guest molecule, although their stability conditions differ greatly.18 Moreover, in some cases, methane hydrate dissociation slows in association with ice formation even if the pressure−temperature conditions are outside the thermodynamically stable conditions on the phase diagram, which can lead to metastable methane hydrate.18,30,31 This phenomenon, called self-preservation, appears prominently in simple CH4 hydrate.18 Although selfpreservation was believed to be absent for simple C2H6 and C3H8 hydrates,18 recent reports have described the selfpreservation phenomena of simple C2H6 and CH4−C2H6 hydrates32 and natural gas hydrate containing C2H6 or C3H8.21 According to the 13C NMR technique, the single 13C NMR line from gaseous CH4 appears far upfield of the lines from CH4 in clathrate hydrate cages.33 Although the selected temperature range between 193 and 243 K under nonpressurized conditions are outside the thermodynamically stable region in the phase diagram of CH4 hydrate, the detection of two 13C NMR lines from guest CH4 molecules at temperatures above 193 K indicates that a certain amount of CH4 hydrate crystals remained during data acquisition. In this study, the 13C NMR line from gaseous CH4 released by clathrate framework dissociation did not appear because unsealed NMR sample tubes were used for all samples. For the CH4−C2H6 hydrates, the 13C NMR spectra reflect the difference in clathrate hydrate structure depending on the mixing ratio of CH4 and C2H6.22,26,27 As presented in Figure 1b, the clathrate hydrate formed from the CH4−C2H6 mixture containing 29.7% C2H6 shows three 13C NMR lines at 7.9, −4.3, and −6.7 ppm at 123 K, which are assigned to C2H6 in 51262 cages, CH4 in 512 cages, and CH4 in 51262 cages, respectively, in the sI hydrate framework. The 13C NMR spectra in Figure 1c show that the clathrate hydrate formed from the CH4−C2H6 mixture containing 1.1% C2H6 had an sII framework. The sII unit cell comprises 16 small cages with 12 pentagonal faces (512) and 8 large cages with 12 pentagonal and 4 hexagonal faces (51264).7 The three 13C NMR lines at 6.2, −4.4, and −8.3 ppm at 123 K are derived from C2H6 in 51264 cages, CH4 in 512 cages, and CH4 in 51264 cages, respectively, in the sII hydrate framework. The 13C NMR spectra of the CH4− C2H6 hydrates with sI and sII frameworks were observed at 123−203 and 123−183 K, respectively. For the CH4−C3H8 hydrate, comparisons between the chemical shifts obtained for guest CH4 and C3H8 and previously reported values25,28 showed the hydrate to have an sII framework. The CH4−C3H8 hydrate at 123 K yielded four 13 C NMR signals from CH3 groups of C3H8 in 51264 cages (17.6 ppm), CH2 group of C3H8 in 51264 cages (16.9 ppm), CH4 in 512 cages (−4.4 ppm), and CH4 in 51264 cages (−8.3 ppm) of sII. The line from CH4 in 51264 cages of sII disappears at 223 K, implying that the least energetically stable gas-filled cage has decomposed at that temperature. Adamantane forms sH hydrate in the presence of a help gas such as CH4 or xenon.34−36 The powder X-ray diffraction technique revealed direct evidence that the CH4−C10H16 sample contained sH hydrate (Figure 2). The diffraction peaks were assigned according to the reported data.36−38 The sH hydrate in the CH4−C10H16 sample had a hexagonal lattice with a = 12.31 Å and c = 9.91 Å, in good agreement with the results reported previously.36 The sH unit cell comprises three small cages with 12 pentagonal faces (512); two medium cages with 3 square, 6 pentagonal, and 3 hexagonal faces (435663); and a large cage with 12 pentagonal and 8 hexagonal faces

Figure 2. PXRD profile of CH4−C10H16 hydrate at 143 K. Unlabeled peaks are diffraction peaks from sH hydrate crystal. Open and solid triangle symbols denote positions of diffraction peaks from hexagonal ice and solid adamantane, respectively.

(51268).7 The PXRD pattern also revealed that the sample included a certain amount of unreacted solid C10H16 and hexagonal ice crystals. The CH4−C10H16 hydrate showed four 13 C NMR lines at 223 K. The most upfield line at −4.5 ppm can be assigned to the overlapping NMR lines of CH4 molecules in 512 and 435663 cages of sH in comparison with previous results.39,40 In sH hydrate systems containing CH4 as a help gas, the 13C NMR lines from CH4 in 512 and 435663 cages can be indistinguishable depending on the type of largemolecule guest and the temperature of the NMR measurements.40 Reportedly, the overlapping line is observed at −4.40 ppm at 193 K in 2,2-dimethylbutane and methylcyclohexane systems in the presence of CH4 as a help gas, which is shifted upfield by 0.30 ppm compared with the line from CH4 in 512 cages of pure CH4 hydrate.40 The chemical shift of the most upfield line of the CH4−C10H16 hydrate at 193 K was estimated as −4.4 ppm based on the temperature-dependent change in chemical shift in this study, which is in good agreement with the reported overlapping line at 193 K. Comparison with the 13 C NMR lines of solid C10H16 (spectrum at 223 K in Figure 1f) enabled the assignment of the other downfield NMR lines from C10H16 molecules. The lines at 28.5, 29.4, and 38.4 ppm are assigned to methine carbons (CH) of C10H16 in 51268 cages in sH at 223 K, CH carbons of unreacted solid C10H16, and indistinguishable methylene carbons (CH2) of C10H16 in 51268 cages in sH and unreacted solid C10H16, respectively. Lines from unreacted solid C10H16 in the sample were not detected at temperatures below 163 K, which is probably attributable to a combination of effects: the motional region of the system, long spin−lattice relaxation times, and a short spin−lattice relaxation time in the rotating frame of solid C10H16. The 13C chemical shifts of guest hydrocarbons as a function of temperature are presented in Figure 3. The chemical shifts changed as the temperature was increased depending on the type of guest molecule and hydrate cage, which suggests that the guest−host interactions changed with increasing temperature. For guest CH4 molecules in the sI, sII, and sH hydrate frameworks, all 13C NMR lines except for the overlapping line from CH4 in 512 and 435663 cages in the CH4−C10H16 hydrate with an sH framework shifted downfield with increasing temperature, indicating a decrease in shielding around the carbon (Figure 3a). The overlapping line from CH4 in the 512 and 435663 cages showed no change with increasing temperature, which masks the individual shielding changes in guest CH4 in the sH hydrate framework. For guest C2H6 in the sI and sII frameworks, the 13C chemical shifts showed a completely different response to increasing temperature. The 13C chemical shift of C2H6 in 51262 cages of sI tended to become smaller with increasing temperature, whereas that in 51264 cages of sII was constant in the temperature range examined. The shielding 4110

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Figure 3. 13C chemical shifts of guest hydrocarbons as a function of temperature: (a) CH4, (b) C2H6, (c) C3H8, (d) C10H16 (CH), and (e) C10H16 (CH2).

Table 1. Slopes of the Linear Fits of the Change in 13C Chemical Shifts of Guests with Increasing Temperature (dδ/dT) and the Guest-to-Cage Cavity Ratio13 guest molecule(s) CH4

C2H6 C3H8 C10H16 a

CH3 CH2 CH

cage

structure

hydrate

dδ/dT (ppm/K)

error (ppm/K)

guest-to-cage cavity ratioa

12

sI sI sI sI sII sII sII sII sI sII sII sII sH

CH4 CH4 CH4−C2H6 CH4−C2H6 CH4−C2H6 CH4−C2H6 CH4−C3H8 CH4−C3H8 CH4−C2H6 CH4−C2H6 CH4−C3H8 CH4−C3H8 CH4−C10H16

0.0031 0.0038 0.0007 0.0024 0.0017 0.0032 0.0022 0.0032 −0.0029 0.0000 −0.0003 −0.0023 −0.0017

0.0002 0.0003 0.0006 0.0004 0.0004 0.0004 0.0002 0.0004 0.0006 0.0000 0.0002 0.0003 0.0006

0.855 0.744 0.855 0.744 0.868 0.652 0.868 0.652 0.939 0.826 0.943 0.943 0.84

5 51262 512 51262 512 51264 512 51264 51262 51264 51264 51264 51268

Reference 13.

cages.13 In the sH hydrate system, C10H16, a spherical guest molecule with high symmetry, is isolated in the oblate 51268 cage. In the sI and sII hydrate systems, normal (n-) alkanes with small spherical or chainlike shapes are trapped in cavities with more spherical symmetry than the 51268 cavity of sH. The temperature-dependent 13C chemical shifts for various n-alkane guests enables the determination of the relationship between changes in the local structural environment around the guest n-alkanes and the tightness of fit of the guest molecules in the host cages. In general, examination of the fitness of the guest molecular size to the host cages provides valuable information about the guest occupancy and thermodynamic properties of clathrate hydrates.7 The slopes of linear fits of the changes in 13C chemical shift with increasing temperature (denoted as dδ/dT) and the guest-to-cage cavity ratio are presented in Table 1. The guest-to-cage cavity ratios are values from the literature, defined as the molecular diameter of the guest divided by the cavity diameter of the host cage,13 where the cavity diameter of the host cage is obtained from the

surrounding the carbons of C2H6 in 51262 cages of sI increased continuously with increasing temperature, whereas shielding environment around C2H6 in 51264 cages of sII remained unchanged. For guest C3H8 in the CH4−C3H8 hydrate with sII, as the temperature increased, the two 13C NMR lines from C3H8 in 51264 cages of sII were shifted upfield in the same manner as described in an earlier report28 on pure C3H8 hydrate. However, whereas the 13C NMR lines from CH and CH2 carbons of guest C10H16 in CH4−C10H16 hydrate were also shifted upfield with increasing temperature, the line from CH2 carbons of guest C10H16 changed nonlinearly with temperature, exhibiting behavior that was different from that of other guests. The differences among guests in the changes with temperature are expected to result from differences in the van der Waals contacts between the guest molecules and the cage walls based on the shapes of the guest molecules and pentagonal cages. The 51268 cavity is the most oblate of the five principal hydrate cages. Therefore, the shape of the occupying molecule is a more important consideration than for other 4111

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average cavity radius minus the diameter of water.13 The relationship between the slope and the guest-to-cage cavity ratio13 is presented in Figure 4, which shows that the

The 13C NMR line widths (Δν1/2) of n-alkane guests in sI and sII hydrate frameworks as a function of temperature are presented in Figure 5. The 13C NMR lines from guest CH4,

Figure 4. Relationship between the slope of a linear fit to the change in 13C chemical shift of guests with increasing temperature (dδ/dT) and the guest-to-cage cavity ratio.13 Solid and open square symbols indicate dδ/dT values for n-alkane guests in sI and sII and C10H16 (CH carbons) guest in sH, respectively.

temperature-dependent 13C chemical shifts of n-alkane guests are related to the tightness of the fit of the guest within the hydrate cage cavities. The slope of the linear fit for CH carbons of guest C10H16 is included in Table 1 and Figure 4 for comparison with those for the family of guest n-alkanes. For the family of n-alkanes, negative slopes, which indicate increases in electron density and magnetic shielding, were observed for the tighter guest molecules with a high guest-to-cage cavity ratio, such as C2H6 in 51262 cages and C3H8 in 51264 cages. The other cases with looser fits showed positive slopes, which indicate decreased electron density and magnetic shielding. In the temperature range examined in this study, the crystal lattice of clathrate hydrate commonly expands with increasing temperature.41 Therefore, the results obtained in this study suggest that the electron density around tighter-fitting guests increases as a result of changes in guest−host interaction during lattice expansion of the hydrate framework. The difference in the change in electron density during the temperature increase between the more tightly and more loosely fitting guest nalkanes is expected to be attributable to the difference in the relative increase of the lattice parameter with guest molecular size or to the change in guest molecular motion. For C10H16 guests in sH, even though C10H16 in the 51268 cage is a relatively loose cage environment (guest-to-cage cavity ratio of 0.84),13 in the cases examined in this study, the plot of the guest C10H16 data (CH groups) seemed to be similar to those for the tighter cage environments. This result could be caused by the large variation in the estimation of the cavity diameter of 51268 cages and the highly symmetric molecular structure of C10H16. The size ratio of C10H16 to the cavity of the 51268 cage in sH is based on the average cage radius determined by the atomic distance for each cage oxygen to the center of the extremely oblate cage.13

Figure 5. 13C NMR line widths (Δν1/2) of n-alkane guests in sI and sII hydrate frameworks as a function of temperature.

C2H6, and C3H8 in sI and sII frameworks were found to narrow with increasing temperature, suggesting that the increase in temperature activated molecular motion of the guest alkanes trapped in the host cages. The temperature dependence of Δν1/2 shows a clear difference between the n-alkane guests trapped in sI (upper shaded area in Figure 5) and those trapped in sII (lower shaded area in Figure 5). The fact that the Δν1/2 values for the n-alkane guests in sII are much smaller than those in sI in the lower temperature range studied in this work suggests that the local structures around the n-alkane guests in sII are more locally averaged than those in sI. For the sII hydrates, the Δν1/2 values for C2H6 and C3H8 molecules were smaller than those of CH4 in the 512 cages of the sII gas hydrates in the temperature range studied, implying that the methyl-group reorientations in C2H6 and C3H8 guests give a sharp line. The rates of change in Δν1/2 for the n-alkane guests in the studied temperature range were greater in the sI hydrate framework than in the sII hydrate framework, suggesting that the temperature selectively affects the averaging around the CH4 and C2H6 molecules in the sI framework. In the temperature ranges studied, the unit cells of sI and sII reportedly expand with different thermal expansion coefficients with increasing temperature.41 The lattice expansion is apparently the most likely candidate for the cause of the difference in the averaging around guest molecules with increasing temperature. In addition, the difference in Δν1/2 between sI and sII hydrates can result from the relative increase of the lattice parameter with guest molecular size. However, changes in Δν1/2 for the CH4−C10H16 hydrate with the sH framework are compared with those for solid C10H16 in Figure 4112

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6. For solid C10H16, the Δν1/2 values for CH and CH2 carbons in the β phase of solid C10H16 (T ≤ 208.62 K) were found to be

cages and the highly symmetric molecular structure of C10H16. Our future work will specifically address a more detailed interpretation of the difference in the trends between the guest n-alkanes in sI and sII and the diamondoid hydrocarbon in sH. However, the changes in 13C NMR line width of guest nalkanes with increasing temperature were found to depend on the clathrate hydrate structure, suggesting that the local structure around the guest alkanes in sII are more averaged than those in sI. The local structure around adamantane guests trapped in structure H hydrate cages was found to be averaged at the same level as that in the α phase of solid adamantane.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. Y. Jin (AIST), H. Ohno (National Institute of Polar Research), and T. Uchida (Hokkaido University) for their fruitful discussions.

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REFERENCES

(1) Koshino, M.; Tanaka, T.; Solin, N.; Suenaga, K.; Isobe, H.; Nakamura, E. Imaging of Single Organic Molecules in Motion. Science 2007, 316, 853. (2) Koshino, M.; Solin, N.; Tanaka, T.; Isobe, H.; Nakamura, E. Imaging the Passage of a Single Hydrocarbon Chain through a Nanopore. Nat. Nanotechnol. 2008, 3, 595−597. (3) Kühne, D.; Klappenberger, F.; Krenner, W.; Klyatskaya, S.; Ruben, M.; Barth, J. V. Rotational and Constitutional Dynamics of Caged Supramolecules. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 21332− 21336. (4) Mao, W. L.; Mao, H. K.; Goncharov, A. F.; Struzhkin, V. V.; Guo, Q.; Hu, J.; Shu, J.; Hemley, R. J.; Somayazulu, M.; Zhao, Y. Hydrogen Clusters in Clathrate Hydrate. Science 2002, 297, 2247−2249. (5) Lee, H.; Lee, J.-W.; Kim, D. Y.; Park, J.; Seo, Y.-T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Tuning Clathrate Hydrates for Hydrogen Storage. Nature 2005, 434, 743−746. (6) Kurotobi, K.; Murata, Y. A Single Molecule of Water Encapsulated in Fullerene C60. Science 2011, 333, 613−616. (7) Sloan, E. D., Jr. Nature 2003, 426, 353−359. (8) Momma, K.; Ikeda, T.; Nishikubo, K.; Takahashi, N.; Honma, C.; Takada, M.; Furukawa, Y.; Nagase, T.; Kudoh, Y. New Silica Clathrate Minerals That Are Isostructural with Natural Gas Hydrates. Nat. Commun. 2011, 2, 196−197. (9) Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H.; Hemley, R. J. Hydrogen Storage in Molecular Clathrates. Chem. Rev. 2007, 107, 4133−4151. (10) Kuhs, W. F.; Chazallon, B. Cage Occupancy and Compressibility of Deuterated N2-Clathrate Hydrate by Neutron Diffraction. J. Inclusion Phenom. Mol. Recognit. Chem. 1997, 29, 65−77. (11) Davidson, D. W. The Motion of Guest Molecules in Clathrate Hydrates. Can. J. Chem. 1971, 49, 1224−1242. (12) Davidson, D. W.; Ratcliffe, C. I.; Ripmeester, J. A. 2H and 13C NMR Study of Guest Molecule Orientation in Clathrate Hydrates. J. Inclusion Phenom. Macrocyclic Chem. 1984, 2, 239−247. (13) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (14) Tanaka, H.; Kiyohara, K. The Thermodynamic Stability of Clathrate Hydrate. II. Simultaneous Occupation of Larger and Smaller Cages. J. Chem. Phys. 1993, 98, 8110−8118. (15) John, V. T.; Holder, G. D. Choice of Cell Size in the Cell Theory of Hydrate Phase Gas−Water Interaction. J. Phys. Chem. 1981, 85, 1811−1814.

Figure 6. Changes in Δν1/2 for CH4−C10H16 hydrate and solid C10H16.

much larger than those in the α phase of solid C10H16 (T ≥ 208.62 K), which shows good agreement with the values reported previously by Rothwell and Waugh.42 This fact suggests that the reorientation rate of C10H16 molecules in the β phase is lower than that in the α phase.42 The 13C NMR lines from guest C10H16 in 51268 cages of sH maintained sharpness even at temperatures below the transition point43 between the α and β phases of solid C10H16. This result provides direct evidence that the reorientation rate of C10H16 molecules isolated in 51268 cages is the same as that in the α phase of solid C10H16 at around 210 K.

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CONCLUSIONS The solid-state 13C NMR spectra of various clathrate hydrates with sI, sII, and sH frameworks were measured to elucidate the local structural environments around hydrocarbon molecules trapped in the guest−host frameworks of clathrate hydrates. The results showed that the changes in the 13C chemical shifts of the guest n-alkanes with increasing temperature depend on the cage environment. The shielding around the carbons of guest CH4 in sI and sII decreased continuously with increasing temperature, whereas those of guest C2H6 and C3H8 in sI and sII showed no change or increased with increasing temperature. This finding suggests that the chemical shifts of guest n-alkanes in looser cage environments tend to decrease with increasing temperature, whereas those in tighter cage environments tend to increase with increasing temperature. Although C10H16 in 51268 cages in sH represents relatively loose cage environment for the cases examined in this study, the changes in shielding around the carbons of guest C10H16 with increasing temperature seem to be similar to those in the tighter cage environments. This would result from the large variation in the estimation of the cavity diameter of the oblate-shaped 51268 4113

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(16) Sparks, K. A.; Tester, J. W. Intermolecular Potential Energy of Water Clathrates: The Inadequacy of the Nearest-Neighbor Approximation. J. Phys. Chem. 1992, 96, 11022−11029. (17) Pimpalgaonkar, H.; Veesam, S. K.; Punnathanam, S. N. Theory of Gas Hydrates: Effect of the Approximation of Rigid Water Lattice. J. Phys. Chem. B 2011, 115, 10018−10026. (18) Takeya, S.; Ripmeester, J. A. Dissociation Behavior of Clathrate Hydrates to Ice and Dependence on Guest Molecules. Angew. Chem., Int. Ed. 2008, 47, 1276−1279. (19) Yakushev, V. S.; Istomin, V. A. Gas-Hydrates Self-Preservation Effect. In Physics and Chemistry of Ice; Maeno, N., Hondoh, T., Eds.; Hokkaido University Press: Sapporo, Japan, 1992; pp 136−140. (20) Gudmundson, J.; Borrehaug, A. Frozen Hydrate for Transport of Natural Gas. In Proceedings of the Second International Conference on Natural Gas Hydrates; Monfort, J. P., Ed.; 1996; pp 415−422. (21) Takeya, S.; Yoneyama, A.; Ueda, K.; Mimachi, H.; Takahashi, M.; Sano, K.; Hyodo, K.; Takeda, T.; Gotoh, Y. Anomalously Preserved Clathrate Hydrate of Natural Gas in Pellet Form at 253 K. J. Phys. Chem. C 2012, 116, 13842−13848. (22) Kida, M.; Sakagami, H.; Takahashi, N.; Hachikubo, A.; Shoji, H.; Kamata, Y.; Ebinuma, T.; Narita, H.; Takeya, S. Estimation of Gas Composition and Cage Occupancies in CH4−C2H6 Hydrates by CPMAS 13C NMR Technique. J. Jpn. Pet. Inst. 2007, 50, 132−138. (23) Bielecki, A.; Burum, D. P. Temperature Dependence of 207Pb MAS Spectra of Solid Lead Nitrate. An Accurate, Sensitive Thermometer for Variable-Temperature MAS. J. Magn. Reson. A 1995, 116, 215−220. (24) Hayashi, S.; Hayamizu, K. Chemical Shift Standards in HighResolution Solid-State NMR (1) 13C, 29Si, and 1H Nuclei. Bull. Chem. Soc. Jpn. 1991, 64, 685−687. (25) Ripmeester, J. A.; Ratcliffe, C. I. Low-Temperature CrossPolarization/Magic Angle Spinning 13C NMR of Solid Methane Hydrates: Structure, Cage Occupancy, and Hydration Number. J. Phys. Chem. 1988, 92, 337−339. (26) Subramanian, S.; Ballard, A. L.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr. Structural Transitions in Methane + Ethane Gas Hydrates Part I: Upper Transition Point and Applications. Chem. Eng. Sci. 2000, 55, 5763−5771. (27) Kida, M.; Khlystov, O.; Zemskaya, T.; Takahashi, N.; Minami, H.; Sakagami, H.; Krylov, A.; Hachikubo, A.; Yamashita, S.; Shoji, H.; Poort, J.; Naudts, L. Coexistence of Structure I and II Gas Hydrates in Lake Baikal Suggesting Gas Sources from Microbial and Thermogenic Origin. Geophys. Res. Lett. 2006, 33, L24603. (28) Kida, M.; Hori, A.; Sakagami, H.; Takeya, S.; Kamata, Y.; Takahashi, N.; Ebinuma, T.; Narita, H. 13C Chemical Shifts of Propane Molecules Encaged in Structure II Clathrate Hydrate. J. Phys. Chem. A 2011, 115, 643−647. (29) CSMHYD is a phase-equilibrium calculation program package developed at Colorado School of Mines. See: Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (30) Takeya, S.; Shimada, W.; Kamata, Y.; Ebinuma, T.; Uchida, T.; Nagao, J.; Narita, H. In Situ X-ray Diffraction Measurements of the Self-Preservation Effect of CH4 Hydrate. J. Phys. Chem. A 2001, 105, 9756−9759. (31) Stern, L. A.; Circone, S.; Kirby, S. H. Anomalous Preservation of Pure Methane Hydrate at 1 atm. J. Phys. Chem. B 2001, 105, 1756− 1762. (32) Uchida, T.; Kida, M.; Nagao, J. Dissociation Termination of Methane−Ethane Hydrates in Temperature-Ramping Tests at Atmospheric Pressure below the Melting Point of Ice. ChemPhysChem 2011, 12, 1652−1656. (33) Dec, S. F.; Bowler, K. E.; Stadterman, L. L.; Koh, C. A.; Sloan, E. D., Jr. Direct Measure of the Hydration Number of Aqueous Methane. J. Am. Chem. Soc. 2006, 128, 414−415. (34) Ripmeester, J. A.; Ratcliffe, C. I. 129Xe NMR Studies of Clathrate Hydrates: New Guests for Structure II and Structure H. J. Phys. Chem. 1990, 94, 8773−8776.

(35) Lederhos, J. P.; Metha, P.; Nyberg, G. B.; Warn, K. J.; Sloan, E. D. Structure H Clathrate Hydrate Equilibria of Methane and Adamantane. AIChE J. 1992, 38, 1045−1048. (36) Kirchner, M. T.; Boese, R.; Billups, W. E.; Norman, L. R. Gas Hydrate Single-Crystal Structure Analyses. J. Am. Chem. Soc. 2004, 126, 9407−9412. (37) Ito, T. Pressure-Induced Phase Transition in Adamantane. Acta Crystallogr. B 1973, 29, 364−365. (38) Kozaki, T.; Taguchi, T.; Takeya, S.; Ohmura, R. Phase Equilibrium for Structure H Hydrates Formed with Methane plus Cycloheptane, Cycloheptanone, or Oxacycloheptane. J. Chem. Eng. Data 2010, 55, 3059−3062. (39) Shin, W.; Park, S.; Lee, J.-W.; Seo, Y.; Koh, D.-Y.; Seol, J.; Lee, H. Structure Transition from Semi- to True Clathrate Hydrates Induced by CH4 Enclathration. J. Phys. Chem. C 2012, 116, 16352− 16357. (40) Susilo, R.; Ripmeester, J. A.; Englezos, P. Characterization of Gas Hydrates with PXRD, DSC, NMR, and Raman Spectroscopy. Chem. Eng. Sci. 2007, 62, 3930−3939. (41) Hester, K. C.; Huo, Z.; Ballard, A. L.; Koh, C. A.; Miller, K. T.; Sloan, E. D. Thermal Expansivity for sI and sII Clathrate Hydrates. J. Phys. Chem. B 2007, 111, 8830−8835. (42) Rothwell, W. P.; Waugh, J. S. Transverse Relaxation of Dipolar Coupled Spin Systems under rf Irradiation: Detecting Motions in Solids. J. Chem. Phys. 1981, 74, 2721−2732. (43) Chang, S. S.; Westrum, E. F., Jr. Heat Capacities and Thermodynamic Properties of Globular Molecules. I. Adamantane and Hexamethylenetetramine. J. Phys. Chem. 1960, 64, 1547−1551.

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