Abnormal Proton Positioning of Water Framework in the Presence of

The unique host–guest interactions of ionic clathrate hydrates, as distinct from those of other nonionic clathrate hydrates, need to be investigated...
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Abnormal Proton Positioning of Water Framework in the Presence of Paramagnetic Guest within Ion-Doped Clathrate Hydrate Host Kyuchul Shin,† Minjun Cha,‡ Wonhee Lee,‡ Yutaek Seo,† and Huen Lee*,‡ †

Division of Ocean Systems Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The unique host−guest interactions of ionic clathrate hydrates, as distinct from those of other nonionic clathrate hydrates, need to be investigated to understand their inherent physicochemical features, but direct observation of ionic host geometry has not yet been attempted. In this study, we first report the distortions of the water−water connection in the charged cages caused by orbital mixing between a paramagnetic guest and an ion-doped host, and by electrostatic repulsion between the cationic host and guest via the direct observation with using synchrotron high-resolution powder diffraction analysis. The present findings well explain the mechanisms of unique phenomena occurring in ionic clathrate hydrates with paramagnetic guests.

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we first report the direct observation of the ionic host geometry in the presence of a paramagnetic guest and identify the abnormal positioning of protons in the water framework. For this, we chose a tetramethylammonium hydroxide (Me4NOH)13 + O2 binary hydrate system, which was previously reported to form a superoxide ion by irradiation and to show extraordinary antiferromagnetic superexchangelike interactions between O2 guests.9,10 The unusual host geometry was identified from 1H NMR spectroscopy and synchrotron high-resolution powder diffraction (HRPD) analysis.

INTRODUCTION Clathrate hydrates are well-known host−guest compounds that are commonly stabilized by van der Waals interactions between the guest gases or hydrocarbon molecules and the hydrogenbonded water network, which acts as the host framework.1 A particular species in this category, ionic clathrate hydrates, which contains hydrophobic cationic or anionic guests in a counterion incorporated water framework,2 has recently gained attention because of its potential applicability as a solid electrolyte for supercapacitors3 or for gas sensors4 and as clathrate material for gas separation5 or storage.6 The unique host−guest interactions, as distinct from those of other nonionic clathrate hydrates, which often cause rapid proton conduction7 or unusual structural transformation with many vacant cages,8 thus need to be investigated further to understand their inherent physicochemical features; such knowledge can be useful in developing practical applications with novel icy materials. We previously reported superoxide ion formation9 and superexchange-like interaction10 in ionic clathrate hydrates with paramagnetic guest molecules such as O2. These unusual phenomena arise from the interaction between the ionic host and the paramagnetic guest. Although a few recent studies have focused on the magnetic properties of O2 molecules in clathrate hydrates,9−12 direct observation of host geometry, especially of the positions of the hydrogen atoms in the water framework, has not yet been attempted. A comprehensive understanding of the geometry of the host, which strongly interacts with the guest molecule, is urgent and essential for tuning the host framework; such tuning could help in the development of practical applications of ionic clathrate hydrates. In this work, © 2014 American Chemical Society

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EXPERIMENTAL DETAILS Sample Preparation. A well-mixed solution (1 Me4NOH:16 H2O) was frozen at 213 K for 1 day and was then finely powdered (∼200 μm) under liquid nitrogen temperature. The solid was put into a high-pressure cell and exposed to O2, N2, or H2 (120 bar) at 213 K for 1 week. The synthesized hydrate sample was stored in liquid nitrogen and finely ground (∼45 μm) again for the HRPD experiment. A part of the Me4NOH + O2 hydrate sample was irradiated at 60 kGy dose (15 kGy per 1 h) by a 60Co γ-ray source at KAERI in Jeongeup, Korea. The sample was being immersed in liquid nitrogen during the irradiation. NMR Spectroscopy. The 1H NMR spectra were collected on a Varian UnityINOVA600 600 MHz solid-state NMR instrument. The hydrate samples were packed in a 4 mm Received: May 12, 2014 Revised: June 23, 2014 Published: June 23, 2014 15193

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zirconia rotor at liquid nitrogen temperature, and the rotor was loaded into a variable-temperature probe at 183 K. The 1H NMR spectra of the samples were recorded at 183 or 203 K with magic-angle spinning at 7 kHz; the proton resonance peak of tetramethyl-silane, assigned a chemical shift of 0 ppm at 298 K, was used as an external chemical shift reference. Structural Characterization by Synchrotron HighResolution Powder Diffraction. The HRPD pattern was recorded at 5 K using the High-Resolution Powder Diffraction beamline (9B) at Pohang Accelerator Laboratory (PAL) in Korea. The synchrotron beam vertically collimated by a mirror was monochromatized (λ = 1.54900 Å) by a double-crystal Si (1 1 1) monochromator. Seven detector sets composed of Soller slits, flat Ge (1 1 1) crystal analyzers, antiscatter baffles, and scintillation detectors were installed to collect the diffracted beam for each separated 20° range. The γ-irradiated Me4NOH + O2 hydrate powder stored in liquid nitrogen was quickly transferred to the sample stage cooled down to 80 K in air, and the diffraction pattern was measured after the temperature was decreased to 5 K using liquid helium. The experiment was carried out in the step mode with a fixed time of 2 s and a step size of 0.0025° for 2θ = 7 to 127.5° with a 0.5° overlap to the next detector bank up. The whole HRPD measurement was conducted for about 6 h. The obtained pattern was refined using the Rietveld method with four phases, two Fd3̅m hydrate phases, hexagonal ice, and copper that originated from the sample stage. The rigid bodies of the guest molecules, Me4N+, O2, and O2−, were inserted into the centers of cages, and the rotations and site occupancies of the rigid bodies (bond lengths for O2: 1.20 Å and O2−: 1.34 Å)14 were refined. For two hydrate phases (phase 1 and phase 2; see below), soft distance constraints for the water molecules (O−H covalent bond length: 0.98 Å and O···H hydrogen bond length: 1.74 Å) were applied and the isotropic temperature factor (B value) of a hydrogen atom of a water molecule or a methyl group was assumed to be 1.5 times the B value of the atom to which the hydrogen had bonded. The B values for the carbon atoms of Me4N+ were constrained to be identical.

Figure 1. 1H NMR spectra of unirradiated Me4NOH binary hydrates.

paramagnetic O2 guest and the charged host water molecules. Here, we note that the spectrum of the O2 hydrate in Figure 1 was measured at 183 K, while the others were measured at 203 K, because peaks of Me4NOH + O2 hydrate were too broad at 203 K (see Figure S1 in the Supporting Information). The intensities of host signals in 1H NMR spectra decrease accordingly as the temperature decreases. Additionally, the reversed peak intensities between at 8.0 ppm and at 7.2 ppm as the temperature decreases mean faster water reorientation at 7.2 ppm, implying higher probability of OH− there at lower temperature. The next step was to identify the geometry of the water cages, which gave rise to a different NMR result of the O2 guest from those of others. For this purpose, an HRPD pattern of the Me4NOH + O2 hydrate was obtained using a synchrotron beamline at 5 K and refined using the Rietveld method and the Fullprof program.16 However, there were some difficulties in the diffraction analysis for this system. We previously reported that O2 guests in the Me4NOH hydrate were partly converted to superoxide ion by γ-irradiation;9 thus, X-ray irradiation during the diffraction experiments can cause a nonuniform phase transition in the powder sample. Although the neutron powder diffraction technique can prevent the formation of a superoxide during the diffraction experiment, the deuteration of Me4NOH, performed to guarantee a low-intensity incoherent scattering background, creates new difficulties.17 Additionally, laboratory-scale powder X-ray diffraction (PXRD) equipment is not appropriate for the present study because the atomic scattering factor of hydrogen is too small to use for the refinement of low-resolution PXRD patterns; however, its contribution on the patterns is not negligible. A method based on the virtual species of H2O, which has the sum of atomic scattering factors of two hydrogens and one oxygen, is good for refining laboratory PXRD patterns of icy crystals,18 but the geometry of the hydrogen atom would be omitted in the results. For these reasons, a synchrotron HRPD experiment was used to carry out the identification of the host geometry. As a pretreatment to convert most of the O2 to superoxide, the Me4NOH + O2 hydrate sample was γ-irradiated before the experiment, thus preventing additional phase transitions during synchrotron beam radiation.

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RESULTS AND DISCUSSION As a first step, the Me4NOH + O2 binary hydrate sample was prepared by exposing a finely powdered solid mixture of Me4NOH and H2O (1:16) to 120 bar O2 gas at 213 K for a week. Then, the 1H solid-state NMR spectrum of the prepared sample was obtained and compared with those of other binary Me4NOH hydrates (Figure 1). In our previous report, hydroxide-doped host frameworks showed characteristic sharp peaks in their 1H NMR spectra owing to the fast reorientation of water molecules around the hydroxide ions.15 This makes 1H NMR spectroscopy a useful method for predicting the state of the host framework in ionic clathrate hydrates. As can be seen in Figure 1, Me4NOH binary hydrates with N2 or H2, as expected, show one sharp signal at 7.0 ppm, indicating an identical environment for proton reorientation in the whole host framework. On the other hand, in the case of the O2 guest, two broad, but clearly distinguishable, peaks were observed at 7.2 and 8.0 ppm. The broadness of the peaks was a result of the paramagnetic nature of O2. This implies that at least two chemically different states of hydrogen atoms exist in the Me4NOH + O2 hydrate framework. Because all three Me4NOH hydrates of O2, N2, and H2 have the same crystal structure (cubic, Fd3̅m),12 the difference seen in the 1H NMR spectra can be said to arise from the interaction between the 15194

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Figure 2 shows the refined pattern of the γ-irradiated Me4NOH + O2 hydrate sample with reliability factors of χ2 =

Figure 3. Vertical (top) and horizontal (bottom) views of hexagonal faces of (a) phase 1, (b) phase 2, and (c) Me4NOH + H2 hydrate.

refinement are given in the Supporting Information, and Figure S4 provides the reason that the HRPD pattern of Me4NOH + H2 hydrate was chosen instead of Me4NOH + N2 hydrate for the diamagnetic case). A hydrate cage is composed of hydrogen-bonded water molecules; protons are commonly positioned near the axis of the oxygen−oxygen connection with half site occupancies, as shown in Figure 3c. However, for phase 1 and phase 2, the protons are definitely bent out of the O−O edges of the hexagonal rings, as shown in Figure 3a,b (We note that the protons represented in Figure 3 are disordered ones in the tetrahedrally hydrogen-bonded host framework. The figures do not mean that the intramolecular water angles are severely distorted). The Og−Hgg(h)···Og angles are only 128.8° for phase 1 and 138.1° for phase 2, whereas that of the Me4NOH + H2 hydrate is 168.8°. Considering phase 1, it appears that the bent hydrogen atoms, shown in Figure 3a, have proton conductivity different from the other protons, which are normally hydrogen-bonded along the O−O edges of the host framework. Thus, the two distinguishable peaks (at 8.0 and 7.2 ppm) in the 1H NMR spectrum (Figure 1) for the Me4NOH + O2 hydrate must represent the abnormally positioned protons on the hexagonal rings and in the rest of the host lattice, respectively. Whole images of the 51264 cages for three phases are shown in Figure 4 and of the unit cells in Figure 5. At this stage, the reason that abnormal proton ordering only occurs on the hexagonal faces should be considered. Water molecules in clathrate hydrates are tetrahedrally connected with each other, and the ideal tetrahedral angle is 109.5°. Therefore, hydrogen bonds in the hexagonal faces with an O−O−O angle of ∼120° might be less stable than those in the pentagonal faces with an angle of ∼108°.21−23 As previously reported,9,10 the extraordinary host−guest interactions between the paramagnetic O2 and the ionic host are obviously involved in the charge transfer from the OH− of the framework. This strong interaction may be accompanied by a distortion of the weakest hydrogen bond lying on the hexagonal rings. Distances of O− H, O···H, and O−O, calculated from the refinement results, are tabulated in Tables 1−3. The Og−Hgg(h) and Og···Hgg(h) distances, comprising the hexagonal faces, are 1.10 and 1.91 Å for phase 1 and 1.00 and 1.77 Å for the Me4NOH + H2 hydrate; however, the Og−Og distances are 2.74 and 2.75 Å, respectively. Complete distortion of the hydrogen bonds, despite the common O−O distance (2.74 Å) of the hexagonal faces in phase 1, might imply that the OH− of the host framework mainly exists within this area, at least at 5 K. It should be noted that the soft distance constraints for an O−H covalent bond length, 0.98 Å, and O···H hydrogen bond length, 1.74 Å, were applied in this refinement to avoid any refining

Figure 2. (a) HRPD pattern of γ-irradiated Me4NOH + O2 hydrate powder (red open circle: observed; black line: calculated; blue line: the difference between observed and calculated). Tick marks indicate the reflections of Me4NOH + O2 hydrate (phase 1, first row), Me4NOH + O2− hydrate (phase 2, second row), hexagonal ice (third row), and solid copper from the sample stage (fourth row). (b) Magnification of the pattern in the range of 29.0−32.0° of 2θ (green arrows: phase 1; blue arrows: phase 2).

2.38 and Rwp = 16.5% (background subtracted). Two hydrate phases exist (Figure 2b), the Me4NOH + O2 hydrate (phase 1) with a cubic Fd3m ̅ structure (Z = 8) and lattice parameter of 17.1567(1) Å (V = 5050.14(7) Å3; unit cell formula: 7.69Me4NOH·8.99O2 ·128H2O) and the Me4NOH + O2− hydrate (phase 2),19 of the same space group, with a lattice parameter of 17.1123(2) Å (V = 5011.00(9) Å3; unit cell formula: 8Me4 NOH·13.79O 2 −·128H 2O). The calculated weight fraction of phase 1 is 37% of the total hydrate phase; the other 63% was converted to superoxide hydrate by γirradiation. The atomic coordinates, isotropic temperature factors, and site occupancies of phase 1 and phase 2 were also obtained from the refinement and are given in the Supporting Information. A remarkable feature of the refinement results is the unexpected position of protons, which totally deviate from the hexagonal rings of the 51264 cages. Figure 3a,b provides the vertical and horizontal views of the six water molecules that form the hexagonal faces of phase 1 and phase 2.20 The Me4NOH + H2 hydrate, with a diamagnetic guest for comparison, is shown in Figure 3c (The HRPD pattern reported in ref 12 was refined for Figure 3c. The details of the 15195

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Figure 4. Whole images of the 51264 cages for (a) phase 1, (b) phase 2, and (c) Me4NOH + H2 hydrate.

Table 1. Distances of O−H, H···O, and O−O in Phase 1a Oe−Hea···Oa Oa−Hae···Oe Oe−Heg···Og Og−Hge···Oe Og−Hgg(p) ···Og Og−Hgg(h) ···Og

O−H (Å)

H···O (Å)

O−O (Å)

0.99 0.94 0.99 1.04 1.05 1.10

1.70 1.75 1.80 1.80 1.83 1.91

2.69 2.76 2.88 2.74

a HXY: OX−HXY···OY, and Oa: oxygen in 8a (site multiplicity and Wyckoff symbol) site. See Tables S1−S3 in the Supporting Information for details. Hgg(p): hydrogen of Og in pentagonal ring, and Hgg(h): in hexagonal ring.

Table 2. Distances of O−H, H···O, and O−O in Phase 2a Oe−Hea···Oa Oa−Hae···Oe Oe−Heg···Og Og−Hge···Oe Og−Hgg(p) ···Og Og−Hgg(h) ···Og

O−H (Å)

H···O (Å)

O−O (Å)

0.98 0.94 1.01 1.02 1.00 1.06

1.69 1.74 1.79 1.83 1.83 1.83

2.68 2.80 2.83 2.71

HXY: OX−HXY···OY, and Oa: oxygen in 8a (site multiplicity and Wyckoff symbol) site. See Tables S1−S3 in the Supporting Information for details. Hgg(p): hydrogen of Og in pentagonal ring, and Hgg(h): in hexagonal ring. a

Table 3. Distances of O−H, H···O, and O−O in Me4NOH + H2 Hydratea Oe−Hea···Oa Oa−Hae···Oe Oe−Heg···Og Og−Hge···Oe Og−Hgg(p) ···Og Og−Hgg(h) ···Og

O−H (Å)

H···O (Å)

O−O (Å)

0.97 0.95 1.00 1.01 1.01 1.00

1.71 1.73 1.77 1.78 1.78 1.76

2.68 2.77 2.79 2.75

HXY: OX−HXY···OY, and Oa: oxygen in 8a (site multiplicity and Wyckoff symbol) site. See Tables S1−S3 in the Supporting Information for details. Hgg(p): hydrogen of Og in pentagonal ring, and Hgg(h): in hexagonal ring. a

divergence, because the hydrogen atom position is too sensitive to its small scattering factor. Figure 6 shows the small 512 cages of the three hydrates, calculated and visualized from the refinement results. For phase 1 (Figure 6a), the protons shared by the hexagonal faces (green circles) bend out of the cage. The negative charge of the host framework should be inward because of the high molecular

Figure 5. Images of the unit cells of (a) phase 1, (b) phase 2, and (c) Me4NOH + H2 hydrate (red: oxygen; light blue: nitrogen; green: Hgg(h); white: rest of the hydrogens). Hexagonal rings are represented in the center of each image. The atoms of large guests are omitted except for centered nitrogen. 15196

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Figure 7. (a) Two neighboring O2 guests encaged in the 512 cages of phase 1 (b) and side view of (a).

O 2 − , which overrides the common ionic host−guest interactions. Our results show a 0.78% volume contraction (0.26% contraction for the lattice parameter) when phase 1 is converted to phase 2 due to γ-irradiation, even though the bond length of O2− (1.34 Å) is longer than that of O2 (1.20 Å).14 An interesting feature of phase 2 is that the protons in the hexagonal faces are positioned in a direction opposite to that of phase 1 (Figures 3b and 4b). They are bent inward from the 512 cage (Figure 6b). Considering that no significant signal for the hydroxyl radical was detected from the ESR spectrum of γirradiated Me4NOH + O2 hydrate,9,12 most of the OH groups existing in the host of phase 2 must be hydroxyl cations (1 OH: 1.72 O2− from Table S2, Supporting Information). Of course, the cations would tend toward the anionic superoxide in the 512 cage, rather than cationic Me4N+ in the 51264 cage, as shown in Figures 3b, 4b, and 6b. Another remarkable feature of phase 2 is the large isotropic temperature factor of the superoxide, 8.4(2) Å2, compared to that of O2 in phase 1 (0.26(21) Å2). This implies largely off-centered O2− guests in the 512 cages, which are presented because of their electrostatic attraction to Me4N+, accompanied by a contraction in the lattice parameter. The quite small isotropic temperature factor of O2 in phase 1, meaning small thermal motion of guest, is consistent with our previous study10 describing the overlap of π* orbitals between neighboring O2 guests via the p-orbital of OH−. Additionally, all the framework atoms in phase 2 have larger isotropic temperature factors than in phase 1. This could reflect poorer structural integrity due to the damage from γ-irradiation.

Figure 6. 512 cages of (a) phase 1, (b) phase 2, and (c) Me4NOH + H2 hydrate.

electron affinity of O2,12 and the abnormal proton ordering might be a reflection of the strong interaction between O2 and OH−. In our previous study,10 we calculated the mixing of π* orbitals, induced by the OH− of the host lattice, for two neighboring O2’s. The overlap of π* orbitals via the p-orbital of OH− was maximized when the negative charge of OH− was out of the pentagonal plane between two perpendicular O2’s (inplane position for proton in ref 10). The geometry of phase 1 obtained in this study satisfies the orientation condition calculated to maximize orbital mixing, as shown in Figure 7. Therefore, previously reported superexchange-like antiferromagnetic couplings between O2 guests10 are, as predicted, caused by the preferred host geometry of phase 1 at low temperature. The geometry of phase 2 was also obtained from the refined HRPD pattern. In previous studies,9,10,12 it was reported that γirradiation converted dioxygen to superoxide ion, accompanied by a contraction in the lattice framework. This contraction may be caused by the electrostatic attraction between Me4N+ and

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CONCLUSIONS With clathrate hydrate research continuously expanding, conventional models for the hydrophobic guest−host inter15197

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Behavior Induced by Selective Injection of Guest Molecules into Clathrate Hydrates. J. Am. Chem. Soc. 2009, 131, 5736−5737. (12) Shin, K.; Cha, M.; Kim, H.; Jung, Y.; Kang, Y. S.; Lee, H. Direct Observation of Atomic Hydrogen Generated from the Water Framework of Clathrate Hydrates. Chem. Commun. 2011, 47, 674− 676. (13) Mootz, D.; Seidel, R. Polyhedral Clathrate Hydrates of a Strong Base: Phase Relations and Crystal Structures in the System Tetramethyl-ammonium Hydroxide-Water. J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 8, 139−157. (14) Bond lengths were calculated from Gaussian 03: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (15) Choi, S.; Shin, K.; Lee, H. Structure Transition and Tuning Pattern in the Double (Tetramethylammonium Hydroxide + Gaseous Guests) Clathrate Hydrates. J. Phys. Chem. B 2007, 111, 10224− 10230. (16) Rodriguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Physica B 1993, 192, 55−69. (17) Shin, K.; Lee, W.; Cha, M.; Koh, D. Y.; Choi, Y. N.; Lee, H.; Son, B. S.; Lee, S.; Lee, H. Thermal Expansivity of Ionic Clathrate Hydrates Including Gaseous Guest Molecules. J. Phys. Chem. B 2011, 115, 958−963. (18) Takeya, S.; Udachin, K. A.; Moudrakovski, I. L.; Susilo, R.; Ripmeester, J. A. Direct Space Methods for Powder X-ray Diffraction for Guest−Host Materials: Applications to Cage Occupancies and Guest Distributions in Clathrate Hydrates. J. Am. Chem. Soc. 2010, 132, 524−531. (19) In the refinement of this study, all guests in the 512 cages of phase 2 were assumed to be O2− because our previous study in ref 9 shows many indirect evidence that most of O2− generated by irradiation should be in phase 2. This does not mean that phase 2 should be purely O2− ionic hydrate. But, we note that the change of O−O distance (O2: 1.20 Å; O2−: 1.34 Å) practically did not affect the refinement results except for the isotropic temperature factors of O2 or O2−. (20) All visualizations of this work were performed by the VESTA program: Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (21) Jones, C. Y.; Nevers, T. J. Temperature-Dependent Distortions of the Host Structure of Propylene Oxide Clathrate Hydrate. J. Phys. Chem. C 2010, 114, 4194−4199. (22) Shin, K.; Kumar, R.; Udachin, K. A.; Alavi, S.; Ripmeester, J. A. Ammonia Clathrate Hydrates as New Solid Phases for Titan, Enceladus, and Other Planetary Systems. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 14785−14790. (23) Shin, K.; Udachin, K. A.; Moudrakovski, I. L.; Leek, D. M.; Alavi, S.; Ratcliffe, C. I.; Ripmeester, J. A. Methanol Incorporation in Clathrate Hydrates and the Implications for Oil and Gas Pipeline Flow Assurance and Icy Planetary Bodies. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 8437−8442. (24) Ripmeester, J. A. Hydrate Guest-Host Interactions, Structure, and Processes: A Molecular View. In Proceedings of the 7th International Conference on Gas Hydrates (ICGH), Edinburgh, U.K., July 17−21, 2011; p 751. (25) Alavi, S.; Udachin, K.; Ripmeester, J. A. Effect of Guest-Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates. Chem.Eur. J. 2010, 16, 1017−1025. (26) Alavi, S.; Takeya, S.; Ohmura, R.; Woo, T. K.; Ripmeester, J. A. Hydrogen-Bonding Alcohol-Water Interactions in Binary Ethanol, 1Propanol, and 2-Propanol+Methane Structure II Clathrate Hydrates. J. Chem. Phys. 2010, 133, 074505. (27) Udachin, K.; Alavi, S.; Ripmeester, J. A. Single Crystal X-ray Diffraction Observation of Hydrogen Bonding between 1-Propanol

actions of common clathrate hydrates now must deal with unusual interactions, such as guest−host hydrogen bonding or ionic interactions.24−28 The experiments reported here demonstrate distortion of the host geometry caused by orbital mixing between the nonionic guest and the ionic host or by electrostatic repulsion between the cationic host and guest. The present work might provide an important step toward understanding the nature of host−guest interactions in clathrate hydrates and toward the development of practical applications of ionic clathrate hydrates.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectroscopy and HRPD refinement details. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-42-350-3917. Fax: +82-42-350-3910. E-mail: h_ [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Research Foundation of Korea (No. 2010-0029176). HRPD experiments at PLS (Beamline 9B) were supported by POSTECH. The authors thank KAERI in Jeongup for γ-ray irradiation.

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REFERENCES

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