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Calculations of NMR properties for sI and sII clathrate hydrates of methane, ethane and propane.

ABSTRACT: Calculations of NMR parameters (the absolute shielding constants and the spin-spin coupling constants) for 5(12), 5(12)6(2) and 5(12)6(4) cages enclathrating CH4, C2H6 and C3H8 molecules are presented. The DFT/B3LYP/HuzIII-su3 level of theory was employed. The (13)C shielding constants of guest molecules are close to available experimental data. In two cases (the ethane in 5(12) and the propane in 5(12)6(2) cages) the (13)C shielding constants are reported for the first time. Inversion of the methyl/methylene (13)C and (1)H shielding constants order is found for propane in the 5(12)6(2) cage. Topological criteria are used to interpret the changes of values of NMR parameters of water molecules and they establish a connection between single cages and bulk crystal.

SUBMITTER: Siuda P

PROVIDER: S-EPMC4236610 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Clathrate hydrates in interstellar environment.

Project description:Clathrate hydrates (CHs) are ubiquitous in earth under high-pressure conditions, but their existence in the interstellar medium (ISM) remains unknown. Here, we report experimental observations of the formation of methane and carbon dioxide hydrates in an environment analogous to ISM. Thermal treatment of solid methane and carbon dioxide-water mixture in ultrahigh vacuum of the order of 10-10 mbar for extended periods led to the formation of CHs at 30 and 10 K, respectively. High molecular mobility and H bonding play important roles in the entrapment of gases in the in situ formed 512 CH cages. This finding implies that CHs can exist in extreme low-pressure environments present in the ISM. These hydrates in ISM, subjected to various chemical processes, may act as sources for relevant prebiotic molecules.

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Effect of temperature and large guest molecules on the C-H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates.

Project description:Large molecules such as 2-methylbutane (C5H12) or 2,2-dimethylbutane (C6H14) form structure H (sH) hydrates with methane (CH4) as a help gas. In this study, the Raman spectra of the C-H symmetric stretch region of CH4 enclathrated within various sH hydrates and structure I CH4 hydrates were analyzed in the temperature range 137.7-205.4 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH4 in host-water cages increased because of varying temperature, and the sizes of the host-water cages also increased; variation of CH4 in small cages was less than in larger cages. Comparing the variations of the C-H symmetric stretching frequencies of CH4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH4 molecule in water cages.

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Project description:The amount of energy in natural gas hydrates is thought to be equivalent to twice that of all other fossil fuels combined. However, economic and safe energy recovery has remained a challenge till now. To develop a novel method of breaking the hydrogen bonds (HBs) surrounding the trapped gas molecules, we investigated the vibrational spectra of the HBs of gas hydrates with structure types II and H. Two models of 576-atom propane-methane sII hydrate and 294-atom neohexane-methane sH hydrate were built. A first-principles density functional theory (DFT) method was employed using the CASTEP package. The simulated spectra were in good agreement with the experimental data. Compared with the partial phonon density of states of guest molecules, we confirmed that the experimental infrared absorption peak in the terahertz region mainly arose from HB vibrations. By removing the components of guest molecules, we found that the theory of two kinds of hydrogen bond vibrational modes applies. The use of a terahertz laser to enable resonance absorption of HBs (at about 6 THz, to be tested) may therefore lead to the rapid melting of clathrate ice and release of guest molecules.

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Hydrogen and Deuterium Molecular Escape from Clathrate Hydrates: "Leaky" Microsecond-Molecular-Dynamics Predictions.

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| S-EPMC8279647 | biostudies-literature

Retracted Article: Effect of temperature and large guest molecules on the C-H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates.

Project description:Large molecules such as 2-methylbutane (C5H12) or 2,2-dimethylbutane (C6H14) form structure H (sH) hydrates with methane (CH4) as a help gas. In this study, the Raman spectra of the C-H symmetric stretch region of CH4 enclathrated within various sH hydrates and structure I CH4 hydrates were analyzed in the temperature range 83-183 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH4 in host water cages increased because of varying temperature, and the sizes of the host water cages also increased; variation of CH4 in small cages was less than in larger cages. Comparing the variations of the C-H symmetric stretching frequencies of CH4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH4 molecule in water cages.

| S-EPMC9077694 | biostudies-literature

Mechanical instability of monocrystalline and polycrystalline methane hydrates.

Project description:Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments.

| S-EPMC4659946 | biostudies-literature

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