Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems

There is interest in the role of ammonia on Saturn's moons Titan and Enceladus as the presence of water, methane, and ammonia under temperature and pressure conditions of the surface and interior make these moons rich environments for the study of phases formed by these materials. Ammonia is known to form solid hemi-, mono-, and dihydrate crystal phases under conditions consistent with the surface of Titan and Enceladus, but has also been assigned a role as water-ice antifreeze and methane hydrate inhibitor which is thought to contribute to the outgassing of methane clathrate hydrates into these moons' atmospheres. Here we show, through direct synthesis from solution and vapor deposition experiments under conditions consistent with extraterrestrial planetary atmospheres, that ammonia forms clathrate hydrates and participates synergistically in clathrate hydrate formation in the presence of methane gas at low temperatures. The binary structure II tetrahydrofuran + ammonia, structure I ammonia, and binary structure I ammonia + methane clathrate hydrate phases synthesized have been characterized by X-ray diffraction, molecular dynamics simulation, and Raman spectroscopy methods.     

Synthesis and characterization of a new structure of gas hydrate

Atoms and molecules <0.9 nm in diameter can be incorporated in the cages formed by hydrogen-bonded water molecules making up the crystalline solid clathrate hydrates. For these materials crystallographic structures generally fall into 3 categories, which are 2 cubic forms and a hexagonal form. A unique clathrate hydrate structure, previously known only hypothetically, has been synthesized at high pressure and recovered at 77 K and ambient pressure in these experiments. These samples contain Xe as a guest atom and the details of this previously unobserved structure are described here, most notably the host-guest ratio is similar to the cubic Xe clathrate starting material. After pressure quench recovery to 1 atmosphere the structure shows considerable metastability with increasing temperature (T <160 K) before reverting back to the cubic form. This evidence of structural complexity in compositionally similar clathrate compounds indicates that the reaction path may be an important determinant of the structure, and impacts upon the structures that might be encountered in nature.     

Crystal structure and encapsulation dynamics of ice II-structured neon hydrate

Neon hydrate was synthesized and studied by in situ neutron diffraction at 480 MPa and temperatures ranging from 260 to 70 K. For the first time to our knowledge, we demonstrate that neon atoms can be enclathrated in water molecules to form ice II-structured hydrates. The guest Ne atoms occupy the centers of D₂O channels and have substantial freedom of movement owing to the lack of direct bonding between guest molecules and host lattices. Molecular dynamics simulation confirms that the resolved structure where Ne dissolved in ice II is thermodynamically stable at 480 MPa and 260 K. The density distributions indicate that the vibration of Ne atoms is mainly in planes perpendicular to D₂O channels, whereas their distributions along the channels are further constrained by interactions between adjacent Ne atoms.Clathrate hydrates are a group of ice-like, crystalline inclusion compounds formed by water and “guest” gas molecules of suitable size, typically under low-temperature and high-pressure conditions (1). Within a clathrate lattice, water molecules form hydrogen-bonded frameworks and the guest molecules are situated in the framework cavities. Because of this storage capacity, clathrate hydrates have important applications in energy storage and production, gas separation and transportation, and carbon dioxide sequestration.Depending on the shape and size of the encaged gas molecules, most hydrate clathrates crystallize in one of three distinct crystal structures: cubic sI (Pm3¯ n), sII (Fd3¯ m), or hexagonal sH (P6/mmm) (1). Evidently, the size effect on the crystal structure of clathrate hydrate would be readily expected in systems with the simplest guests, noble gases, which makes research on noble gas hydrate very important for the fundamental thermodynamic understanding of gas hydrates. As expected, large noble gas atoms, such as Ar, Kr, and Xe, form classic clathrates of sI and sII structures at modest gas pressures (i.e., less than 1 GPa) (2–4). However, there was a long-standing controversy about whether smaller noble gas atoms such as He and Ne can form hydrates at all. This issue was not resolved until 1986, when Londono et al. (5) reported the first helium hydrate based on high-pressure neutron diffraction at 280–500 MPa. Unlike traditional clathrate hydrates, He hydrate possesses a filled-ice structure based on that of ice II. Ne is the second-smallest noble gas after He. From differential thermal analysis Dyadin et al. (6) reported indirect evidence of the formation of a classical sII clathrate hydrate between 200–300 MPa. This was later confirmed by Lokshin (7) by neutron diffraction, although no specific structural details were given. When the pressure is further increased to 350 MPa, Lokshin and Zhao (8) found that the sII-structured Ne hydrate breaks down into a pure ice-II structure with no Ne in the framework. Like Ar, the atom size of Ne under this pressure seems to be too large to fit into the free channels in ice-II structure along the c axis.In this work, we show for the first time to our knowledge, from in situ neutron diffraction experiments that Ne gas can be enclathrated in ice-II channels when the applied gas pressure reaches 480 MPa. To the best of our knowledge, this is the first unambiguous experimental demonstration that Ne hydrate can form an ice II structure. We have also carried out molecular dynamics (MD) simulations of Ne hydrate around 480 MPa to study the distribution of Ne atoms in the host ice-II lattice and to gain insight into mechanisms underlying the hydrate formation. These simulations are of particular importance because, unlike conventional hydrates such as CH₄ or CO₂ clathrates, the ice-II framework of Ne hydrate can be self-stabilized without the encapsulation of guest Ne atoms. This feature is very special in the clathrate hydrate family and thus extends our knowledge of hydrate science.     

Thermodynamic stability of hydrogen clathrates

The stability of the recently characterized type II hydrogen clathrate [Mao, W. L., Mao, H.-K., Goncharov, A. F., Struzhkin, V. V., Guo, Q., et al. (2002) Science 297, 2247-2249] with respect to hydrogen occupancy is examined with a statistical mechanical model in conjunction with first-principles quantum chemistry calculations. It is found that the stability of the clathrate is mainly caused by dispersive interactions between H2 molecules and the water forming the cage walls. Theoretical analysis shows that both individual hydrogen molecules and nH2 guest clusters undergo essentially free rotations inside the clathrate cages. Calculations at the experimental conditions--2,000 bar (1 bar = 100 kPa) and 250 K confirm multiple occupancy of the clathrate cages with average occupations of 2.00 and 3.96 H2 molecules per D-5(12) (small) and H-5(12)6(4) (large) cage, respectively. The H2-H2O interactions also are responsible for the experimentally observed softening of the H[bond]H stretching modes. The clathrate is found to be thermodynamically stable at 25 bar and 150 K.     

High-pressure transformations in xenon hydrates

A high-pressure investigation of the Xe*H(2)O chemical system was conducted by using diamond-anvil cell techniques combined with in situ Raman spectroscopy, synchrotron x-ray diffraction, and laser heating. Structure I xenon clathrate was observed to be stable up to 1.8 GPa, at which pressure it transforms to a new Xe clathrate phase stable up to 2.5 GPa before breaking down to ice VII plus solid xenon. The bulk modulus and structure of both phases were determined: 9 +/- 1 GPa for Xe clathrate A with structure I (cubic, a = 11.595 +/- 0.003 A, V = 1,558.9 +/- 1.2 A(3) at 1.1 GPa) and 45 +/- 5 GPa for Xe clathrate B (tetragonal, a = 8.320 +/- 0.004 A, c = 10.287 +/- 0.007 A, V = 712.1 +/- 1.2 A(3) at 2.2 GPa). The extended pressure stability field of Xe clathrate structure I (A) and the discovery of a second Xe clathrate (B) above 1.8 GPa have implications for xenon in terrestrial and planetary interiors.     

Transformations in methane hydrates

Detailed study of pure methane hydrate in a diamond cell with in situ optical, Raman, and x-ray microprobe techniques reveals two previously unknown structures, structure II and structure H, at high pressures. The structure II methane hydrate at 250 MPa has a cubic unit cell of a = 17.158(2) A and volume V = 5051.3(13) A(3); structure H at 600 MPa has a hexagonal unit cell of a = 11.980(2) A, c = 9.992(3) A, and V = 1241.9(5) A(3). The compositions of these two investigated phases are still not known. With the effects of pressure and the presence of other gases in the structure, the structure II phase is likely to dominate over the known structure I methane hydrate within deep hydrate-bearing sediments underlying continental margins.     

Hydrogen storage in molecular compounds

At low temperature (T) and high pressure (P), gas molecules can be held in ice cages to form crystalline molecular compounds that may have application for energy storage. We synthesized a hydrogen clathrate hydrate, H(2)(H(2)O)(2), that holds 50 g/liter hydrogen by volume or 5.3 wt %. The clathrate, synthesized at 200-300 MPa and 240-249 K, can be preserved to ambient P at 77 K. The stored hydrogen is released when the clathrate is warmed to 140 K at ambient P. Low T also stabilizes other molecular compounds containing large amounts of molecular hydrogen, although not to ambient P, e.g., the stability field for H(2)(H(2)O) filled ice (11.2 wt % molecular hydrogen) is extended from 2,300 MPa at 300 K to 600 MPa at 190 K, and that for (H(2))(4)CH(4) (33.4 wt % molecular hydrogen) is extended from 5,000 MPa at 300 K to 200 MPa at 77 K. These unique characteristics show the potential of developing low-T molecular crystalline compounds as a new means for hydrogen storage.     

High-pressure/low-temperature neutron scattering of gas inclusion compounds: progress and prospects

Alternative energy resources such as hydrogen and methane gases are becoming increasingly important for the future economy. A major challenge for using hydrogen is to develop suitable materials to store it under a variety of conditions, which requires systematic studies of the structures, stability, and kinetics of various hydrogen-storing compounds. Neutron scattering is particularly useful for these studies. We have developed high-pressure/low-temperature gas/fluid cells in conjunction with neutron diffraction and inelastic neutron scattering instruments allowing in situ and real-time examination of gas uptake/release processes. We studied the formation of methane and hydrogen clathrates, a group of inclusion compounds consisting of frameworks of hydrogen-bonded H(2)O molecules with gas molecules trapped inside the cages. Our results reveal that clathrate can store up to four hydrogen molecules in each of its large cages with an intermolecular H(2)-H(2) distance of only 2.93 A. This distance is much shorter than that in the solid/metallic hydrogen (3.78 A), suggesting a strong densification effect of the clathrate framework on the enclosed hydrogen molecules. The framework-pressurizing effect is striking and may exist in other inclusion compounds such as metal-organic frameworks (MOFs). Owing to the enormous variety and flexibility of their frameworks, inclusion compounds may offer superior properties for storage of hydrogen and/or hydrogen-rich molecules, relative to other types of compounds. We have investigated the hydrogen storage properties of two MOFs, Cu(3)[Co(CN)(6)](2) and Cu(3)(BTC)(2) (BTC = benzenetricarboxylate), and our preliminary results demonstrate that the developed neutron-scattering techniques are equally well suited for studying MOFs and other inclusion compounds.     

Nonequilibrium air clathrate hydrates in Antarctic ice: a paleopiezomdter for polar ice caps

"Craigite," the mixed-air clathrate hydrate found in polar ice caps below the depth of air-bubble stability, is a clathrate mixed crystal of approximate composition (N2O2).6H2O. Recent observations on the Byrd Station Antarctic core show that the air hydrate is present at a depth of 727 m, well above the predicted depth for the onset of hydrate stability. We propose that the air hydrate occurs some 100 m above the equilibrium phase boundary at Byrd Station because of "piezometry"--i.e., that the anomalous depth of hydrate occurrence is a relic of a previous greater equilibrium depth along the flow trajectory, followed by vertical advection of ice through the local phase-boundary depth. Flowline trajectories in the ice based on numerical models show that the required vertical displacement does indeed occur just upstream of Byrd Station. Air-hydrate piezometry can thus be used as a general parameter to study the details of ice flow in polar ice caps and the metastable persistence of the clathrate phase in regions of upwelling blue ice.     

Ocean methane hydrates as a slow tipping point in the global carbon cycle

We present a model of the global methane inventory as hydrate and bubbles below the sea floor. The model predicts the inventory of CH₄ in the ocean today to be ≈1600–2,000 Pg of C. Most of the hydrate in the model is in the Pacific, in large part because lower oxygen levels enhance the preservation of organic carbon. Because the oxygen concentration today may be different from the long-term average, the sensitivity of the model to O₂ is a source of uncertainty in predicting hydrate inventories. Cold water column temperatures in the high latitudes lead to buildup of hydrates in the Arctic and Antarctic at shallower depths than is possible in low latitudes. A critical bubble volume fraction threshold has been proposed as a critical threshold at which gas migrates all through the sediment column. Our model lacks many factors that lead to heterogeneity in the real hydrate reservoir in the ocean, such as preferential hydrate formation in sandy sediments and subsurface gas migration, and is therefore conservative in its prediction of releasable methane, finding only 35 Pg of C released after 3 °C of uniform warming by using a 10% critical bubble volume. If 2.5% bubble volume is taken as critical, then 940 Pg of C might escape in response to 3 °C warming. This hydrate model embedded into a global climate model predicts ≈0.4–0.5 °C additional warming from the hydrate response to fossil fuel CO₂ release, initially because of methane, but persisting through the 10-kyr duration of the simulations because of the CO₂ oxidation product of methane.     

Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates

Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. We explore here the swapping phenomenon occurring in structure I (sI) and structure II (sII) CH4 hydrate deposits through spectroscopic analyses and its potential application to CO2 sequestration at the preliminary phase. The present 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2+CO2 guests is surprising when compared with the rate of 64% for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2+CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII CH4 hydrates, we observe a spontaneous structure transition of sII to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension caused by structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest-cage conjugates possesses important implications for the diverse hydrate-based inclusion phenomena as illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.     

Interactions and Diffusion of Methane and Hydrogen in Microporous Structures: Nuclear Magnetic Resonance (NMR) Studies

Measurements of nuclear spin relaxation times over a wide temperature range have been used to determine the interaction energies and molecular dynamics of light molecular gases trapped in the cages of microporous structures. The experiments are designed so that, in the cases explored, the local excitations and the corresponding heat capacities determine the observed nuclear spin-lattice relaxation times. The results indicate well-defined excitation energies for low densities of methane and hydrogen deuteride in zeolite structures. The values obtained for methane are consistent with Monte Carlo calculations of A.V. Kumar et al. The results also confirm the high mobility and diffusivity of hydrogen deuteride in zeolite structures at low temperatures as observed by neutron scattering.     

Sol-Gel Synthesis and Antioxidant Properties of Yttrium Oxide Nanocrystallites Incorporating P-123

Yttrium oxide (Y₂O₃) nanocrystallites were synthesized by mean of a sol-gel method using two different precursors. Raw materials used were yttrium nitrate and yttrium chloride, in methanol. In order to promote oxygen vacancies, P-123 poloxamer was incorporated. Synthesized systems were heat-treated at temperatures from 700 °C to 900 °C. Systems at 900 °C were prepared in the presence and absence of P-123 using different molar ratios (P-123:Y = 1:1 and 2:1). Fourier transform infrared spectroscopy (FTIR) results revealed a characteristic absorption band of Y–O vibrations typical of Y₂O₃ matrix. The structural phase was analyzed by X-ray diffraction (XRD), showing the characteristic cubic phase in all systems. The diffraction peak that presented the major intensity corresponded to the sample prepared from yttrium chloride incorporating P-123 in a molar ratio of P-123:Y = 2:1 at 900 °C. Crystallites sizes were determined by Scherrer equation as between 21 nm and 32 nm. Antioxidant properties were estimated by 2,2-diphenyl-1-picrylhydrazyl (DPPH•) assays; the results are discussed.     

Structure of the soluble methane monooxygenase regulatory protein B.

The soluble methane monooxygenase (sMMO; EC 1.14.13.25) from the pseudothermophile Methylococcus capsulatus (Bath) is a three-component enzyme system that catalyzes the selective oxidation of methane to methanol. We have used NMR spectroscopy to produce a highly refined structure of MMOB, the 16-kDa regulatory protein of this system. This structure has a unique and intricate fold containing seven beta-strands forming two beta-sheets oriented perpendicular to each other and bridged by three alpha-helices. The rate and efficiency of the methane hydroxylation by sMMO depend on dynamic binding interactions of the hydroxylase with the reductase and regulatory protein components during catalysis. We have monitored by NMR the binding of MMOB to the hydroxylase in the presence and absence of the reductase. The results of these studies provide structural insight into how the regulatory protein interacts with the hydroxylase.     

Protein-ice interaction of an antifreeze protein observed with solid-state NMR

NMR on frozen solutions is an ideal method to study fundamental questions of macromolecular hydration, because the hydration shell of many biomolecules does not freeze together with bulk solvent. In the present study, we present previously undescribed NMR methods to study the interactions of proteins with their hydration shell and the ice lattice in frozen solution. We applied these methods to compare solvent interaction of an ice-binding type III antifreeze protein (AFP III) and ubiquitin a non-ice-binding protein in frozen solution. We measured ¹H-¹H cross-saturation and cross-relaxation to provide evidence for a molecular contact surface between ice and AFP III at moderate freezing temperatures of -35 °C. This phenomenon is potentially unique for AFPs because ubiquitin shows no such cross relaxation or cross saturation with ice. On the other hand, we detected liquid hydration water and strong water–AFP III and water–ubiquitin cross peaks in frozen solution using relaxation filtered ²H and HETCOR spectra with additional ¹H-¹H mixing. These results are consistent with the idea that ubiquitin is surrounded by a hydration shell, which separates it from the bulk ice. For AFP III, the water cross peaks indicate that only a portion of its hydration shell (i.e., at the ice-binding surface) is in contact with the ice lattice. The rest of AFP III’s hydration shell behaves similarly to the hydration shell of non-ice-interacting proteins such as ubiquitin and does not freeze together with the bulk water.     

Potential effects of gas hydrate on human welfare

For almost 30 years. serious interest has been directed toward natural gas hydrate, a crystalline solid composed of water and methane, as a potential (i) energy resource, (ii) factor in global climate change, and (iii) submarine geohazard. Although each of these issues can affect human welfare, only (iii) is considered to be of immediate importance. Assessments of gas hydrate as an energy resource have often been overly optimistic, based in part on its very high methane content and on its worldwide occurrence in continental margins. Although these attributes are attractive, geologic settings, reservoir properties, and phase-equilibria considerations diminish the energy resource potential of natural gas hydrate. The possible role of gas hydrate in global climate change has been often overstated. Although methane is a "greenhouse" gas in the atmosphere, much methane from dissociated gas hydrate may never reach the atmosphere, but rather may be converted to carbon dioxide and sequestered by the hydrosphere/biosphere before reaching the atmosphere. Thus, methane from gas hydrate may have little opportunity to affect global climate change. However, submarine geohazards (such as sediment instabilities and slope failures on local and regional scales, leading to debris flows, slumps, slides, and possible tsunamis) caused by gas-hydrate dissociation are of immediate and increasing importance as humankind moves to exploit seabed resources in ever-deepening waters of coastal oceans. The vulnerability of gas hydrate to temperature and sea level changes enhances the instability of deep-water oceanic sediments, and thus human activities and installations in this setting can be affected.     

Evidence of hollow golden cages

The fullerenes are the first "free-standing" elemental hollow cages identified by spectroscopy experiments and synthesized in the bulk. Here, we report experimental and theoretical evidence of hollow cages consisting of pure metal atoms, Au(n)(-) (n = 16-18); to our knowledge, free-standing metal hollow cages have not been previously detected in the laboratory. These hollow golden cages ("bucky gold") have an average diameter >5.5 A, which can easily accommodate one guest atom inside.     

Microstructures of FeCoNiMo and CrFeCoNiMo Alloys,and the Corrosion Properties in 1 M Nitric Acid and 1 M Sodium Chloride Solutions

FeCoNiMo and CrFeCoNiMo equimolar alloys were prepared by arc-melting. The microstructures of the as-cast alloys were examined by SEM, HREM and XRD; and a potentiodynamic polarization test of the as-cast alloys was undertaken to evaluate the corrosion resistance in the solutions. Results showed that both of FeCoNiMo and CrFeCoNiMo equimolar alloys had a dendritic structure. The dendrites of these two alloys were a single phase which was a simple cubic (SC) structure with large lattice constant; and the interdendrities of these two alloys had a dual-phased eutectic structure in which the phases were face-centered cubic (FCC) and simple cubic (SC). The hardness of CrFeCoNiMo alloy was higher than that of FeCoNiMo alloy. Additionally, the potentiodynamic polarization test showed that CrFeCoNiMo alloy was better than FeCoNiMo alloy in 1 M nitric acid and 1 M sodium chloride solutions. Adding chromium into FeCoNiMo alloy could increase corrosion resistance in these two solutions. All of the results indicated that the CrFeCoNiMo alloy surpassed FeCoNiMo alloy.     

Methanol Adsorption and Reaction on Samaria Thin Films on Pt(111)

We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmO_x) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmO_x films, eventually leading to the desorption of CO and H₂ which desorbs at temperatures in the range 400–600 K. Small quantities of CO₂ are also detected mainly on as-prepared Sm₂O₃ thin films, but the production of CO₂ is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmO_x structures influence the chemical behavior. Over repeated TPD experiments, a SmO_x structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmO_x films or as OH groups on the SmO_x surfaces.     

High-temperature in situ crystallographic observation of reversible gas sorption in impermeable organic cages

Crystallographic observation of adsorbed gas molecules is a highly difficult task due to their rapid motion. Here, we report the in situ single-crystal and synchrotron powder X-ray observations of reversible CO₂ sorption processes in an apparently nonporous organic crystal under varying pressures at high temperatures. The host material is formed by hydrogen bond network between 1,3,5-tris-(4-carboxyphenyl)benzene (H₃BTB) and N,N-dimethylformamide (DMF) and by π–π stacking between the H₃BTB moieties. The material can be viewed as a well-ordered array of cages, which are tight packed with each other so that the cages are inaccessible from outside. Thus, the host is practically nonporous. Despite the absence of permanent pathways connecting the empty cages, they are permeable to CO₂ at high temperatures due to thermally activated molecular gating, and the weakly confined CO₂ molecules in the cages allow direct detection by in situ single-crystal X-ray diffraction at 323 K. Variable-temperature in situ synchrotron powder X-ray diffraction studies also show that the CO₂ sorption is reversible and driven by temperature increase. Solid-state magic angle spinning NMR defines the interactions of CO₂ with the organic framework and dynamic motion of CO₂ in cages. The reversible sorption is attributed to the dynamic motion of the DMF molecules combined with the axial motions/angular fluctuations of CO₂ (a series of transient opening/closing of compartments enabling CO₂ molecule passage), as revealed from NMR and simulations. This temperature-driven transient molecular gating can store gaseous molecules in ordered arrays toward unique collective properties and release them for ready use.Guest capture, storage, and removal in porous materials have been interesting topics in chemistry (1–7). The porosity of solids is one of the important factors determining their potential applications in guest storage. Because the storage generally uses void spaces interconnected through large open channels between the voids inside the crystal, nonporous or seemingly nonporous materials have received less attention. Over the past decades, nevertheless, due to the potential for selective guest capture and release in a controlled manner there have been attempts to study such materials which include calixarenes (8–17), 4-phenoxyphenol (18), biconcave molecules (19), tris(5-acetyl-3-thienyl)methane (20), metallocyclic complex (21), clarithromycin (22), and metal-organic frameworks (23, 24).Single-crystal X-ray diffraction can provide the crucial information on the binding interactions or structural changes of guest molecules within pores (25–27). However, the crystallographic observation of adsorbed gas adsorbents generally has not been possible due to the poor crystalline order of adsorbents upon removing residual solvent/guest molecules and the high mobility of gases even at low temperatures. Only a limited number of gas-adsorbed single-crystal structures have been determined at low temperatures (16, 28–36).If the permanent channels large enough for the guest diffusion between voids are not present, such a material is nonporous and impermeable to the guest molecules even if there are cages available for the guest storage. If transient pathways between voids can be made with molecular gates in special conditions, the cages can be used for storing special molecules in well-ordered arrays. Furthermore, if they confine the gas molecules in cages, it would be possible to observe the gas molecule using single-crystal X-ray crystallography at high temperatures, even for the relatively weak interactions between the frameworks and gas molecules. Based on this assumption, we studied the reversible CO₂ sorption in a seemingly nonporous organic crystalline cage material composed of 1,3,5-tris-(4-carboxyphenyl)benzene (H₃BTB) and N,N-dimethylformamide (DMF) using in situ single-crystal and synchrotron powder X-ray diffraction at 323 K. The dynamical motion of CO₂ is investigated using solid-state NMR as well as density functional calculations of transition pathways and molecular dynamics (MD) simulations.