Ozone storage capacity in clathrate hydrates formed by O3 + O2 + N2 + CO2 gas mixtures.

Ozone storage capacity in clathrate hydrates formed from gas mixtures of O3 + O2 + N2 + CO2 was studied. It was found that in such system the amount of ozone included in the hydrate phase can be at least several times higher than for the experimentally described O3 + O2 + CO2 gas hydrates. The most promising thermobaric conditions and gas phase compositions for the formation of ozone containing hydrates from gas mixtures which include nitrogen are suggested on the basis of the obtained results.

Influence of N2 on Formation Conditions and Guest Distribution of Mixed CO2 + CH4 Gas Hydrates.

In this contribution, a method based on a solid solution theory of clathrate hydrate for multiple cage occupancy, host lattice relaxation, and guest-guest interactions is presented to estimate hydrate formation conditions of binary and ternary gas mixtures. We performed molecular modeling of the structure, guest distribution, and hydrate formation conditions for the CO2 + CH4 and CO2 + CH4 + N2 gas hydrates. In all considered systems with and without N2, at high and medium content of CO2 in the gas phase, we found that CO2 was more favorable in occupying clathrate hydrate cavities than CH4 or N2. The addition of N2 to the gas phase increased the ratio concentration of CO2 in comparison with the concentration of CH4 in clathrate hydrates and made gas replacement more effective. The mole fraction of CO2 in the CO2 + CH4 + N2 gas hydrate rapidly increased with the growth of its content in the gas phase, and the formation pressure of the CO2 + CH4 + N2 gas hydrate rose in comparison to the formation pressure of the CO2 + CH4 gas hydrate. The obtained results agreed with the known experimental data for simple CH4 and CO2 gas hydrates and the mixed CO2 + CH4 gas hydrate.

Modular, homochiral, porous coordination polymers: rational design, enantioselective guest exchange sorption and ab initio calculations of host-guest interactions.

Two new, homochiral, porous metal-organic coordination polymers [Zn(2)(ndc){(R)-man}(dmf)]⋅3DMF and [Zn(2)(bpdc){(R)-man}(dmf)]⋅2DMF (ndc=2,6-naphthalenedicarboxylate; bpdc=4,4'-biphenyldicarboxylate; man=mandelate; dmf=N,N'-dimethylformamide) have been synthesized by heating Zn(II) nitrate, H(2)ndc or H(2)bpdc and chiral (R)-mandelic acid (H(2)man) in DMF. The colorless crystals were obtained and their structures were established by single-crystal X-ray diffraction. These isoreticular structures share the same topological features as the previously reported zinc(II) terephthalate lactate [Zn(2)(bdc){(S)-lac}(dmf)]⋅DMF framework, but have larger pores and opposite absolute configuration of the chiral centers. The enhanced pores size results in differing stereoselective sorption properties: the new metal-organic frameworks effectively and stereoselectively (ee up to 62 %) accommodate bulkier guest molecules (alkyl aryl sulfoxides) than the parent [Zn(2)(bdc){(S)-lac}(dmf)]⋅DMF, while the latter demonstrates decent enantioselectivity toward precursor of chiral anticancer drug sulforaphane, CH(3)SO(CH(2))(4)OH. The new homochiral porous metal-organic coordination polymers are capable of catalyzing a highly selective oxidation of bulkier sulfides (2-NaphSMe (2-C(10)H(7)SMe) and PhSCH(2)Ph) that could not be achieved by the smaller-pore [Zn(2)(bdc){(S)-lac}(dmf)]⋅DMF. The sorption of different guest molecules (both R and S isomers) into the chiral pores of [Zn(2)(bdc){(S)-lac}(dmf)]⋅DMF was modeled by using ab initio calculations that provided a qualitative explanation for the observed sorption enantioselectivity. The high stereo-preference is accounted for by the presence of coordinated inner-pore DMF molecule that forms a weak C-H...O bond between the DMF methyl group and the (S)-PhSOCH(3) sulfinyl group.

Accurate description of phase diagram of clathrate hydrates at the molecular level.

In order to accurately estimate the thermodynamic properties of hydrogen clathrate hydrates, we developed a method based on the solid solution theory of van der Waals and Platteeuw. This model allows one to take into account the influence of guest molecules on the host lattice and guest-guest interactions--especially when more than one guest molecule occupies a cage. The free energies, equations of state, and chemical potentials of hydrogen and mixed propane-hydrogen clathrate hydrates of cubic structure II with different cage fillings have been estimated using this approach. Moreover, the proposed theory has been used for construction p-T phase diagrams of hydrogen hydrate and mixed hydrogen-propane hydrates in a wide range of pressures and temperatures. For the systems with well defined interactions the calculated curves of "guest gas-hydrate-ice I(h)" equilibrium agree with the available experimental data. We also believe that the present model allows one not only to calculate the hydrogen storage ability of known hydrogen hydrate but also predict this value for structures that have not yet been realized by experiment.

Theoretical study of phase transitions in Kr and Ar clathrate hydrates from structure II to structure I under pressure.

The theory developed in our earlier papers is extended to predict dynamical and thermodynamic properties of clathrate structures by accounting for the possibility of multiple filling of cavities by guest molecules. The method is applied to the thermodynamic properties of argon and krypton hydrates, considering both structures I (sI) and II (sII), in which the small cages can be singly occupied and large cages of sII can be singly or doubly occupied. It was confirmed that the structure of the clathrate hydrate is determined by two main factors: intermolecular interaction between guest and host molecules and the configurational entropy. It is shown that for guests weakly interacting with water molecules, such as argon or krypton, the free energy of host lattices without the contribution of entropy is the main structure-determining factor for clathrate hydrates, and it is a cause of hydrate sII formation at low pressure with these guests. Explicit account of the entropy contribution in the Gibbs free energy allows one to determine the stability of hydrate phases and to estimate the line of structural transition from sII to sI in P-T plane. The structural transition between sII and sI in argon and krypton hydrates at high pressure is shown to be the consequence of increasing intermolecular interaction and the degree of occupancy of the large cavities.

Solvation Mechanism of Task-Specific Ionic Liquids in Water: A Combined Investigation Using Classical Molecular Dynamics and Density Functional Theory.

The solvation behavior of task-specific ionic liquids (TSILs) containing a common, L-histidine derived imidazolium cation [C20H28N3O3](+) and different anions, bromide-[Br](-) and bis(trifluoromethylsulfonyl)amide-[NTF2](-), in water is examined, computationally. These amino acid functionalized ionic liquids (ILs) are taken into account because of their ability to react with rare earth metal salts. It has been noted that the TSIL with [Br](-) is more soluble than its counterpart TSIL with [NTF2](-), experimentally. In this theoretical work, the combined classical molecular dynamics (CMD) and density functional theory (DFT) calculations are performed to study the behavior of the bulk phase of these two TSILs in the vicinity of water (H2O) molecules with different concentrations. Initially, all the constructed systems are equilibrated using the CMD method. The final structures of the equilibrated systems are extracted for DFT calculations. Under CMD operation, the radial distribution function (RDF) plots and viscosity of TSILs are analyzed to understand the effect of water on TSILs. In the DFT regime, binding energy per H2O, charge transfer, charge density mapping, and electronic density of states (EDOS) analyses are done. The CMD results along with the DFT results are consolidated to support the hydrophilic and hydrophobic nature of the TSILs. Interestingly, we have found a strong correlation between the viscosity and the EDOS results that leads to an understanding of the hydration properties of the TSILs.