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An isotopic effect arising from the substitution of superheavy water molecules for normal water molecules in ice (Ih) has been studied by the lattice dynamics method in a quasi-harmonic approximation using a rigid three-point potential modified to reproduce the superheavy water properties. It has been shown that the considerable variation of the vibrational state density upon substituting 12.5, 50, and 100% of water molecules takes place only in the range of libration. The temperature dependence of the superheavy ice density has been calculated, and the density maximum for this ice near 60 K has been predicted. A relationship between the melting point of (T2O + H2O)-ice Ih and the T2O molecule concentration in its structure has been constructed, and this relationship has been found to be linear. [ABSTRACT FROM AUTHOR]

The effect of silicon dioxide nanoparticles on the formation of hydrate phases in the presence of CH4/CO2 has been studied. The theoretical experiment has been carried out by molecular dynamics methods at initial pressures in the system of 2.4 and 1.2 MPa and a temperature of 271 K for methane and carbon dioxide systems. The results showed that in the presence of silicon dioxide nanoparticles, the induction time of the methane hydrate formation decreased by 79%, and the amount of methane trapped in the hydrate cavity increased by 55.8% at a pressure of 2.4 MPa. In the presence of silicon dioxide nanoparticles, the induction time for the formation of carbon dioxide hydrate decreased by 62%, and the amount of carbon dioxide trapped in the hydrate cavity increased by 27.8% at a pressure of 1.2 MPa. [ABSTRACT FROM AUTHOR]

The structure and vibrational properties of high- and low-density amorphous (HDA and LDA, respectively) ices have been determined using reverse Monte Carlo, molecular dynamics, and lattice dynamics simulations. This combined approach leads to a more accurate and detailed structural description of HDA and LDA ices when compared to experiment than was previously possible. The water molecules in these ices form well connected hydrogen-bond networks that exhibit modes of vibration that extend throughout the solid and can involve up to 70% of all molecules. However, the networks display significant differences in their dynamical behavior. In HDA, the extended low-frequency vibrational modes occur in dense parallel two dimensional layers of water that are approximately 10 nm thick. In contrast, the extended modes in LDA resemble a holey structure that encapsulates many small pockets of nonparticipating water molecules. [ABSTRACT FROM AUTHOR]

Dynamic properties of β-hydroquinone (Qβ) clathrate with various guest molecules have been calculated by the method of lattice dynamics within the atom–atom potential approximation; van der Waals interactions, H-bond interactions, and electrostatic interactions for charges localized on the atoms are included. External vibration spectra have been calculated for the empty framework of Qβ and with different guest molecules (HCN, CO2, SO2, C2N2, C2H6, CH3OH, O2CO). Vibrational frequencies of the guest molecules in cavities are determined. The phonon density of states for these compounds has been calculated. The influence of the encaged guest molecules on the vibrational spectrum of the host lattice has been investigated. The most visible changes are observed upon the insertion of guest molecules with large dipole moment or those positioned asymmetrically inside the Qβ clathrate cages. Peculiarities of different interactions have been revealed. Analysis of the calculated and experimental results have been carried out. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The sorption of CO, CO2 and C2H2 by two urotropine-containing porous metal–organic framework materials [Zn4(dmf)(ur)2(ndc)4] (H2ndc = 2,6-naphthalenedicarboxylic acid; ur = urotropine; dmf = dimethylformamide) and [Zn11(H2O)2(ur)4(bpdc)11] (H4bpdc = 4,4′-biphenyldicarboxylic acid) incorporating free N-donors has been investigated. These materials show pronounced affinity for CO2 and C2H2, and these observations are supported by interaction energy and ab initio DFT calculations. [ABSTRACT FROM AUTHOR]

The properties of binary H2+ CH4 clathrate hydrates have been estimated using the extended van der Waals and Platteeuw statistical thermodynamic model that takes into account the lattice relaxation, host-guest, and guest-guest interactions as well as the quantum nature of guest behavior in the clathrate cavities. It has been found that at a small methane concentration in the gas phase the stable hydrate phase has cubic structure II (CS-II) and at a methane concentration of 6% stabilizes cubic structure I, which is metastable in the case of the pure hydrogen hydrate. This is in agreement with recent experimental data. The amount of hydrogen storage depends on the methane concentration in the gas phase as well as the thermodynamic conditions of hydrate formation. Hydrogen storage up to 2.6 wt. % can be achieved in the binary H2+ CH4 CS-II hydrate at T = 250K [ABSTRACT FROM AUTHOR]

Thephysics of gas hydrates are rich in interesting phenomena suchas anomalies for thermal conductivity, self-preservation effects fordecomposition, and others. Some of these phenomena are presumablyattributed to the resonance interaction of the rattling motions ofguest molecules or atoms with the lattice modes. This can be expectedto induce some specific features in the low-frequency (THz) vibrationalresponse. Here we present results for low-frequency Raman scatteringin a Xe hydrate, supported by numerical calculations of vibrationaldensity of states. A number of narrow lines, located in the rangefrom 18 to 90 cm–1, were found in the Raman spectrum.Numerical calculations confirm that these lines correspond to resonancemodes of the Xe hydrate. Also, low-frequency Raman scattering wasstudied during gas hydrate decomposition, and two scenarios were observed.The first one is the direct decomposition of the Xe hydrate to waterand gas. The second one is the hydrate decomposition to ice and gaswith subsequent melting of ice. In the latter case, a transient low-frequencyRaman band is observed, which is associated with low-frequency bands(e.g., boson peak) of disordered solids. [ABSTRACT FROM AUTHOR]

An original approach accounting for multiple cavity occupancy, host lattice relaxation and the description of the quantum nature of guest behaviour has been used for the estimation of the thermodynamic properties of pure hydrogen and binary C2H6 + H2 hydrates with the possibility of multiple filling of cavities by guest molecules. It has been found that the pure hydrogen cubic structure II (CS-II) hydrate is more thermodynamically stable than the cubic structure I (CS-I) hydrate in a wide range of p–T regions. However, at low pressure, the stabilisation of the CS-I hydrate can be realised for H2–C2H6–H2O systems even with small concentrations of ethane in the gas phase. However, in this case, the amount of stored hydrogen strongly depends on the ethane concentrations in the gas phase. At low concentration of ethane, the amount of hydrogen stored, 2.5 wt%, in CS-I hydrate can be achieved at T = 250 K. We believe that the present approach can be useful for understanding the thermodynamic properties of the binary hydrate and it can support the experimental exploration of novel hydrogen storage materials based on clathrate hydrates. [ABSTRACT FROM PUBLISHER]

Storage of hydrogen as hydrogen hydrate is a promising alternative technology to liquefied hydrogen at cryogenic temperatures or compressed hydrogen at high pressures. In this paper, computer simulation is performed based on the solid solution theory of clathrates of van der Waals and Platteeuw with some modifications that include in particular the account of multiple cage occupancies and host relaxation. The quasiharmonic lattice dynamics method employed here gives the free energy of clathrate hydrate to first order in the anharmonicity of intermolecular potential and permits to take into account quantum zero-point vibration of host lattice and hydrogen in the cages. It is employed to study the thermodynamic functions of binary (mixed) H2–CH4 hydrates of cubic structure II (sII) and hexagonal structure H (sH). It is shown that at divariant equilibrium "gas phase–gas hydrate" with increasing pressure the filling of large cavities by hydrogen proceeds gradually from single filling to the maximal number of hydrogen molecules in clusters included in large cages (four in sII and five in sH) preserving stability of the hydrogen–methane hydrates sII and sH. The results show that mass fraction of hydrogen in the mixed sH hydrate is significantly lower than in the mixed sII hydrate. Pressure of monovariant equilibrium "IceIh–gas phase–mixed sII hydrate" with increasing methane concentration in the gas phase lowers in comparison with the pressure of pure hydrogen hydrate formation. For the mixed hydrogen + methane sH hydrates, it was demonstrated that thermodynamic stability depends on the filling degree of small cavities by methane molecules and stability area shifts to lower pressure with increasing filling. [ABSTRACT FROM AUTHOR]