Low-temperature Thermodynamic Study of the Empty Clathrate Hydrates

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Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate.

Methane trapped in the two distinct dodecahedral cages of the ionic clathrate hydrate of TBAB was studied by single crystal XRD and MD simulation

Thermal Performance Evaluation of TIM Combined with Residential Windows in Different Climatic Regions in Iran

Windows play a significant role in the increase and loss of heat from the building envelope and determine the quantity, quality, and distribution of daylight. A strategy that involves incorporating transparent insulating materials into a double-glazed window offers the potential to provide combined improvements in thermal and daylighting performance. The thermal properties of transparent insulation materials in windows depend on various factors, such as the type of insulation material, thickness, geometry and insulation structure, location, and orientation of the window, among others. The aim of this research is to optimize three criteria: "thickness," "location of transparent insulation relative to window layers," and "direction of the wall with transparent insulation of the building window." The goal is to minimize thermal loads and reduce energy consumption in residential buildings. To achieve this, a real model was selected, and Design Builder software was used to measure the "heating load," "cooling load," and the sum of these two loads as the "total thermal load" for all three criteria in three cities of Iran with different climates: Tehran (moderate climate), Ahvaz (warm climate), and Tabriz (cold climate). The results of the research showed that for the city of Tehran, 3-inch insulation in the middle of the double-glazed window and the south front is optimal. For the city of Tabriz, 5-inch insulation on the inner surface of the window and the western front is optimal. And for the city of Ahvaz, 3-inch insulation on the outer surface of the window and the eastern front is optimal. It is worth noting that the annual heating load and total annual heating load for all three criteria have the highest values in Tabriz city. Therefore, it is recommended to use HSNPS insulation in transparent windows to reduce energy consumption in Tabriz (cold climate)

Simple Ethers as Models of Sugar Molecules in Calculations of Vertical Excitation Energies of DNA and RNA Nucleosides

The ribose and deoxyribose molecules of RNA and DNA nucleosides are substituted with simple model compounds 1-methoxy-2-ethanol and 1-methoxypropane to mimic the effect of binding to sugars on the vertical excitation energies of purine and pyrimidine bases. The (R)-1-methoxy-2-ethanol, CH3OC*HCH2OH, for model ribose nucleosides and (R)-1-methoxypropane, CH3OC*HC2H5, for model deoxyribose nucleosides have minimal structural characteristics of ribose and deoxyribose molecules when attached to nucleic acid purine and pyrimidine bases. The bases are attached to the C1 carbon atom designated by the asterisk. The vertical excitation energies of these model nucleosides are calculated with the time-dependent density functional theory method at the B3LYP level with 6-311++G(d,p) and aug-cc-pVDZ basis sets. The attachment of the ether molecules qualitatively and quantitatively modifies the excited state energy levels of the model nucleosides when compared to the free bases. These changes can affect the deexcitation mechanisms for photoexcited nucleosides.Peer reviewed: YesNRC publication: Ye

Application of the linear isotherm regularity to selected fluid systems

Dense hard-sphere and Lennard-Jones fluids and also liquid mercury and water are studied to see if they obey the linear isotherm regularity suggested by Parsafar and Mason. For dense hard-sphere fluids a behavior consistent with the regularity is observed. Data from simulations of the Lennard-Jones fluid were observed to follow the trends proposed by the regularity. For mercury, agreement between the experimental data and the predictions of the regularity is obtained. This suggests that the scope of the regularity can be extended to include liquid metals. In the case of water, for pressure ranges that are not too large. quantitative agreement with the predictions of the regularity can be obtained. Over larger ranges of pressure, systematic deviations appear, but the agreement is still satisfactory. Based on the model previously proposed for the regularity, a discussion of some aspects of the parameters in the equation is given.NRC publication: N

Density corrections to transport coefficients from time correlation functions

A new method for deriving first order density corrections to transport coefficients using projection operators in the time correlation function formalism is developed. Low and moderately dense gas transport coefficients are standardly calculated from a form of the generalized Boltzmann equation. This equation being solved to first order density corrections for repulsive potentials at the binary collision level by Snider and Curtiss and later extended to include the effects associated with the static presence of a third particle on a binary collision by Hoffman and Curtiss. Rainwater and Friend added extra contributions for the presence of bound pairs when the molecules have an attractive potential. They utilized the Stogryn - Hirschfelder theory for the bound pairs and performed detailed numerical calculations of the resultant formulas. While the numerical calculations give good agreement with experiment, questions remain as to the nature and rigor of the assumptions made in obtaining the final formulas, especially the ad hoc addition of bound pair contributions to the density corrections of systems with repulsive potentials, and the extent that these approximations affect the final numerical results. To study these questions, the time correlation function formulas for the transport coefficients were chosen as an alternative route to determine first order density corrections. The time correlation formulas are formally exact and so the density corrections can be usefully compared to those of the generalized Boltzmann equation. Kawasaki and Oppenheim had previously derived formal expressions for first order density corrections to the shear viscosity for a gas of molecules with a repulsive potential, but their results had not been reduced to a form that could be directly compared to those of Snider and Curtiss. As a first step in the study of the time correlation function formalism, the density corrections of Kawasaki and Oppenheim are shown to be equivalent to those of Snider and Curtiss along with an additional correction for three-body collisions. The projection operator method developed in this thesis does not have the infinite series resummation procedure used by Kawasaki and Oppenheim and is an alternative route to obtaining density corrections from the time correlation functions. At low pressures, projection operators are defined which only consider kinetic contributions to the flux function and expressions for the lowest order transport coefficients along with their higher moment corrections are derived. These expressions are consistent with the solution of the Boltzmann equation. The first order density correction from bound pairs on the transport coefficients are approximated by treating the system as a binary gas mixture consisting of free molecules and bound pairs. The results of viewing the system from the molecular picture and the atomic picture with appropriate projection operators are shown to be consistent with one another and also with the Boltzmann equation for binary mixtures. Density corrections in moderately dense gases also arise from potential contributions to the flux. Projection operators which account for both the kinetic and potential flux contributions are required in order to derive explicit expressions for the first order density corrections to the viscosity and thermal conductivity. It is observed that these corrections are consistent with those of Snider and Curtiss with the added Hoffman and Curtiss correction and a term which takes explicit account of three-particle collisions. In the treatment of mixtures and potential interaction effects, the calculation of a transport coefficient is reduced to an equivalent matrix inversion problem. The binary collision expansion of the respective resolvent in the matrix elements in these formulas allows the transport coefficient to be expressed in terms of integrals over functions of the intermolecular potential. The projection operator for each system is determined in a straightforward manner with reference to the particular flux tensor in the time correlation formula. Reduction of the general formula to relations suitable for numerical calculation involves the resolvent expansion onto the appropriate projected subspace, and the subsequent binary collision expansion to reduce the iV-particle resolvent to a tractable form.Science, Faculty ofChemistry, Department ofGraduat

Non-equilibrium adiabatic molecular dynamics simulations of methane clathrate hydrate decomposition

Nonequilibrium, constant energy, constant volume (NVE) molecular dynamics simulations are used to study the decomposition of methane clathrate hydrate in contact with water. Under adiabatic conditions, the rate of methane clathrate decomposition is affected by heat and mass transfer arising from the breakup of the clathrate hydrate framework and release of the methane gas at the solid-liquid interface and diffusion of methane through water.We observe that temperature gradients are established between the clathrate and solution phases as a result of the endothermic clathrate decomposition process and this factor must be considered when modeling the decomposition process. Additionally we observe that clathrate decomposition does not occur gradually with breakup of individual cages, but rather in a concerted fashion with rows of structure I cages parallel to the interface decomposing simultaneously. Due to the concerted breakup of layers of the hydrate, large amounts of methane gas are released near the surface which can form bubbles that will greatly affect the rate of mass transfer near the surface of the clathrate phase. The effects of these phenomena on the rate of methane hydrate decomposition are determined and implications on hydrate dissociation in natural methane hydrate reservoirs are discussed.Peer reviewed: YesNRC publication: Ye

Molecular dynamics studies of melting and some liquid-state properties of 1-ethyl-3-methylimidazolium hexafluorophosphate [emim][PF6]

Molecular dynamics simulations are used to study the liquid-state properties and melting of 1-ethyl-3-methylimidazolium hexafluorosphosphate [emim][PF6] using the force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] and geometric constants from crystallographic data. The structures of the solid and liquid states are characterized by carbon-carbon, carbon-phosphorous, and phosphorous-phosphorous radial distribution functions. Spatial correlations among the ions are strong in the liquid state. The cohesive energy density and the temperature dependences of the molar volume and density of the liquid have been computed. The melting point is determined by equilibrating the solid-state supercells in which void defects have been introduced to eliminate the free-energy barrier for the formation of a solid-liquid interface. The computed melting point is 375+/-10 K, which is approximately 10% higher than the experimental value of 333 K.Peer reviewed: YesNRC publication: Ye

Vertical excitation energies for ribose and deoxyribose nucleosides

Vertical excitation energies for DNA and RNA nucleosides are determined with electron structure calculations using the time-dependent density functional theory (TDDFT) method at the B3LYP/6-311++G(d,p) level for nucleoside structures optimized at the same level of theory. The excitation energies and state assignments are verified using B3LYP/aug-cc-pVDZ level calculations. The nature of the first four excited states of the nucleosides are studied and compared with those of isolated bases. The lowest npi* and pipi* transitions in the nucleoside remain localized on the aromatic rings of the base moiety. New low-energy npi* and pisigma* transitions are introduced in the nucleosides as a result of bonding to the ribose and deoxyribose molecules. The effect on the low-lying excited state transitions of the binding to phosphate groups at the 5prime- and 3prime,5prime-hydroxyl sites of the uracil ribose nucleoside are also studied. Some implications of these calculations on the de-excitation dynamics of nucleic acids are discussed. ? 2007 Wiley Periodicals, Inc. J Comput Chem, 2007.NRC publication: Ye