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Two new model systems of methane hydrate, larger than the previous systems, are constructed. One consists of 63 small and large cages with a small cage at the centre. The other has 65 small and large cages with a large cage at the centre. Three different H-bonding network patterns between water are formed, and three random orientations of methane in each cage are chosen. Using the surface water fixed method, we obtained the energy minimum conformations, fitted to the X-ray crystallographic structure. With normal mode analysis, we calculated frequencies of 2915.1 cm(-1) for a small cage, and 2911.0 cm(-1) for a large cage. These frequencies are a little nearer to the Raman spectra than were previous model systems. Treating three force constants of anharmonic potential energy and the strength of H-bonding between methane and water as four parameters, we obtained frequencies of 2913.6 cm(-1) for a small cage, a little lower than the Raman, and 2906.6 cm(-1) for a large cage, a little higher than the Raman. The calculations thus almost reach the Raman spectra.
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PMID:Application of the independent molecule model to elucidate the dynamics of structure I methane hydrate: a third report. 1900 35

Increased interest in natural gas hydrate formation and decomposition, coupled with experimental difficulties in diffusion measurements, makes estimating transport properties in hydrates an important technological challenge. This research uses an equilibrium path sampling method for free energy calculations [Radhakrishnan, R.; Schlick, T. J. Chem. Phys. 2004, 121, 2436] with reactive flux and kinetic Monte Carlo simulations to estimate the methane diffusivity within a structure I gas hydrate crystal. The calculations support a water-vacancy assisted diffusion mechanism where methane hops from an occupied "donor" cage to an adjacent "acceptor" cage. For pathways between cages that are separated by five-membered water rings, the free energy landscape has a high barrier with a shallow well at the top. For pathways between cages that are separated by six-membered water rings, the free energy calculations show a lower barrier with no stable intermediate. Reactive flux simulations confirm that many reactive trajectories become trapped in the shallow intermediate at the top of the barrier leading to a small transmission coefficient for these paths. Stable intermediate configurations are identified as doubly occupied off-pathway cages and methane occupying the position of a water vacancy. Rate constants are computed and used to simulate self-diffusion with a kinetic Monte Carlo algorithm. Self-diffusion rates were much slower than the Einstein estimate because of lattice connectivity and methane's preference for large cages over small cages. Specifically, the fastest pathways for methane hopping are arranged in parallel (nonintersecting) channels, so methane must hop via a slow pathway to escape the channel. From a computational perspective, this paper demonstrates that equilibrium path sampling can compute free energies for a broader class of coordinates than umbrella sampling with molecular dynamics. From a technological perspective, this paper provides one estimate for an important transport property that has been difficult to measure. In a hydrate I crystal at 250 K with nearly all cages occupied by methane, we estimate D approximately 7 x 10(-15) X m(2)/s where X is the fraction of unoccupied cages.
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PMID:Path sampling calculation of methane diffusivity in natural gas hydrates from a water-vacancy assisted mechanism. 1905 89

High-pressure experiments of ethane hydrate and methane-ethane mixed hydrates with five compositions were performed using a diamond anvil cell in a pressure range of 0.1-2.8 GPa at room temperature. X-ray diffractometry and Raman spectroscopy showed structural changes as follows. The initial structure, structure I (sI), of ethane hydrate was retained up to 2.1 GPa without any structural change. For the mixed hydrates, sI was widely distributed throughout the region examined except for the methane-rich and lower pressure regions. For the ethane-rich and intermediate composition regions (73 mol % ethane sample and 53% sample), sI was maintained up to 2.1 GPa. With increasing methane component (34% and 30% samples), sI existed at pressures from 0.1 to about 1.0 GPa. Hexagonal structure (sH) appeared in addition to sI at 1.3 GPa for the 34% sample and at 1.1 GPa for the 30% sample. By further increasing the methane component (22% sample), structure II (sII) existed solely up to 0.3 GPa. From 0.3 to 0.6 GPa, sII and sI coexisted, and from 0.6 to 1.0 GPa only sI existed. At 1.2 GPa sH appeared, and sH and sI coexisted up to 2.1 GPa. Above 2.1 GPa, ethane hydrate and all of the mixed hydrates decomposed into ice VI and ethane fluid or methane-ethane fluid, respectively. The Raman study revealed that occupation of the small cages by ethane molecules occurred above 0.1 GPa in ethane hydrate and continued up to decomposition at 2.1 GPa, although it is thought that ethane molecules are contained only in the large cage.
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PMID:Structural changes and preferential cage occupancy of ethane hydrate and methane-ethane mixed gas hydrate under very high pressure. 1907 24

In this report, we present a detailed powder x-ray diffraction study of the structural properties and charge density topology of structure I Xe clathrate hydrate under high pressure and room temperature. The pressure dependence of the structural parameters was determined by applying a Rietveld analysis to the experimental data. The combined Rietveld/maximum entropy method was used to derive the most probable charge density distribution at each pressure. Our results show that the charge density distribution of the encaged Xe atoms differs depending on the type of host cage at all pressures. Spherical electron density distributions were observed for the Xe atoms in the small cages while the atoms in the large cages showed longitudinal elongated electronic distributions. Along with the observed cage deformations, the change in electronic density distribution represents a clear indication that the guest-host interaction differs significantly between the small and large cages at high pressures. A similar behavior has been previously reported in low-temperature studies of methane clathrate hydrate.
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PMID:Electron density topology of cubic structure I Xe clathrate hydrate at high pressure. 1912 17

The adsorption of several quadrupolar and nonpolar gases on the Metal Organic Framework Cu-BTC has been studied by combining experimental measurements and Monte Carlo simulations. Four main adsorption sites for this structure have been identified: site I close to the copper atoms, site I' in the bigger cavities, site II located in the small octahedral cages, and site III at the windows of the four open faces of the octahedral cage. Our simulations identify the octahedral cages (sites II and III) and the big cages (site I') as the preferred positions for adsorption, while site I, near the copper atoms, remains empty over the entire range of pressures analyzed due to its reduced accessibility. The occupation of the different sites for ethane and propane in Cu-BTC proceeds similarly as for methane, and shows small differences for O2 and N2 that can be attributed to the quadrupole moment of these molecules. Site II is filled predominantly for methane (the nonpolar molecule), whereas for N2, the occupation of II and I' can be considered almost equivalent. The molecular sitting for O2 shows an intermediate behavior between those observed for methane and for N2. The differences between simulated and experimental data at elevated temperatures for propane are tentatively attributed to a reversible change in the lattice parameters of Cu-BTC by dehydration and by temperature, blocking the accessibility to site III and reducing that to site I'. Adsorption parameters of the investigated molecules have been determined from the simulations.
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PMID:Identification of adsorption sites in Cu-BTC by experimentation and molecular simulation. 1912 60

Molecular dynamics simulations have been carried out to study decomposition of methane hydrate at different cage occupancies. The decomposition rate is found to depend sensitively on the hydration number. The rate of the destruction of the cages displays Arrhenius behavior, consistent with an activated mechanism. During the simulations, reversible formation of partial water cages around methane molecules in the liquid was observed at the interface at temperatures above the computed hydrate decomposition temperature.
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PMID:Molecular dynamics simulations of methane hydrate decomposition. 1912 2

An endohedral methane complex of a fullerene derivative is first synthesized by insertion of a methane molecule through the opening of an open-cage C(60) derivative. The trapped methane is confirmed by NMR spectroscopy and mass spectrometry. Both methane carbon and protons show remarkable upfield shifts in NMR, characteristic of a chemical species in a fullerene cage. CH(4) protons appear as one equivalent signal in the (1)H NMR spectrum, suggesting that even methane can rotate in a C(60) cage.
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PMID:Methane in an open-cage [60]fullerene. 1936 84

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH(3)) in a solid krypton matrix at 17-31 K temperature range. The radicals were produced by dissociating methane by plasma bursts generated by a focused 193 nm excimer laser radiation during the krypton gas condensation on the substrate. The ESR spectrum exhibits only isotropic features at the temperature range examined, and the intensity ratio between the symmetric (A) and antisymmetric (E) spin state lines exhibits weaker temperature dependence than in a solid argon matrix. However, the general appearance of the methyl radical spectrum depends strongly on temperature due to the pronounced temperature dependency of the E state linewidths. The rotational energy level populations are analyzed based on the static crystal field model, pseudorotating cage model, and quantum chemical calculations for an axially symmetric, planar rotor. Crystal field strength parameter values of -140 cm(-1) in Ar and -240 cm(-1) in Kr match most closely the experimentally observed rotational energy level shifts from the gas phase value. In the alternative model, considering the lattice atom movement in a pseudorotating cage, the effective lowering of the rotational constants B and C to 80%-90% leads to similar effects.
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PMID:Rotation of methyl radicals in a solid krypton matrix. 1940 91

Gas hydrates represent an attractive way of storing large quantities of gas such as methane and carbon dioxide, although to date there has been little effort to optimize the storage capacity and to understand the trade-offs between storage conditions and storage capacity. In this work, we present estimates for gas storage based on the ideal structures, and show how these must be modified given the little data available on hydrate composition. We then examine the hypothesis based on solid-solution theory for clathrate hydrates as to how storage capacity may be improved for structure II hydrates, and test the hypothesis for a structure II hydrate of THF and methane, paying special attention to the synthetic approach used. Phase equilibrium data are used to map the region of stability of the double hydrate in P-T space as a function of the concentration of THF. In situ high-pressure NMR experiments were used to measure the kinetics of reaction between frozen THF solutions and methane gas, and (13)C MAS NMR experiments were used to measure the distribution of the guests over the cage sites. As known from previous work, at high concentrations of THF, methane only occupies the small cages in structure II hydrate, and in accordance with the hypothesis posed, we confirm that methane can be introduced into the large cage of structure II hydrate by lowering the concentration of THF to below 1.0 mol %. We note that in some preparations the cage occupancies appear to fluctuate with time and are not necessarily homogeneous over the sample. Although the tuning mechanism is generally valid, the composition and homogeneity of the product vary with the details of the synthetic procedure. The best results, those obtained from the gas-liquid reaction, are in good agreement with thermodynamic predictions; those obtained for the gas-solid reaction do not agree nearly as well.
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PMID:Tuning the composition of guest molecules in clathrate hydrates: NMR identification and its significance to gas storage. 1959 1

By performing constrained molecular dynamics simulations in the methane-water system, we successfully calculated the potential of mean force (PMF) between a dodecahedral water cage (DWC) and dissolved methane for the first time. As a function of the distance between DWC and methane, this is characterized by a deep well at approximately 6.2 A and a shallow well at approximately 10.2 A, separated by a potential barrier at approximately 8.8 A. We investigated how the guest molecule, cage rigidity and the cage orientation affected the PMF. The most important finding is that the DWC itself strongly adsorbs methane and the adsorption interaction is independent of the guests. Moreover, the activation energy of the DWC adsorbing methane is comparable to that of hydrogen bonds, despite differing by a factor of approximately 10% when considering different water-methane interaction potentials. We explain that the cage-methane adsorption interaction is a special case of the hydrophobic interaction between methane molecules. The strong net attraction in the DWC shell with radii between 6.2 and 8.8 A may act as the inherent driving force that controls hydrate formation. A cage adsorption hypothesis for hydrate nucleation is thus proposed and discussed.
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PMID:Why can water cages adsorb aqueous methane? A potential of mean force calculation on hydrate nucleation mechanisms. 1989 May 29


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