Munetaka Takeuchi

Free-energy and structural analysis of ion solvation and contact ion-pair formation of Li(+) with BF4(-) and PF6(-) in water and carbonate solvents.

Free energy of contact ion-pair (CIP) formation of lithium ion with BF(4)(-) and PF(6)(-) in water, propylene carbonate (PC), dimethyl carbonate (DMC) are quantitatively analyzed using MD simulations combined with the energy representation method. The relative stabilities of the mono-, bi-, and tridentate coordination structures are assessed with and without solvent, and water, PC, and DMC are found to favor the CIP-solvent contact. The monodentate structure is typically most stable in these solvents, whereas the configuration is multidentate in vacuum. The free energy of CIP formation is not simply governed by the solvent dielectric constant, and microscopic analyses of solute-solvent interaction at a molecular level are then performed from energetic and structural viewpoints. Vacant sites of Li(+) cation in CIP are solvated with three carbonyl oxygen atoms of PC and DMC solvent molecules, and the solvation is stronger for the monodentate CIP than for the multidentate. Energetically favorable solute-solvent configurations are shown to be spatially more restricted for the multidentate CIP, leading to the observation that the solvent favors the monodentate coordination structure.

Raman Spectroscopic Study, DFT Calculations and MD Simulations on the Conformational Isomerism of N-Alkyl-N-methylpyrrolidinium Bis-(trifluoromethanesulfonyl) Amide Ionic Liquids

The conformational behaviors of N-alkyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquids (alkyl; propyl and butyl, [P(1n)][TFSA]; n = 3 and 4) were studied by Raman spectroscopy in the frequency range of 200-1700 cm(-1) at different temperatures. Observed Raman spectra in the frequency range 870-960 cm(-1) for [P(13)][TFSA] and at 860-950 cm(-1) for [P(14)][TFSA] depend on the temperature, indicating that pseudo rotational isomerization of the pyrrolidinium ring exists in the ionic liquids. DFT calculations revealed that the pseudo rotational potential energy surfaces for P(13)(+) and P(14)(+) ions were similar to each other, i.e., the e6 isomer is the global minimum, whereas the three other isomers e1, e4, and e5 are ca. 3 kJ mol(-1) higher in energy. Optimized geometries with no imaginary frequency were successfully obtained for the e6, e1, and e4 isomers. For both cations, the theoretical Raman spectra of the e6 isomers reproduce well the observed data. To explain their observed Raman spectra in a reasonable way, it is necessary to consider one or more species as predicted by DFT calculations, i.e., the e4 isomer of P(13)(+) rather than the e1, or the e1 isomer of P(14)(+) rather than the e4. In addition, the torsion energy potentials of the alkyl chains of the cations were scanned by DFT calculations. It turns out that the alkyl chains of the cations prefer all trans conformations. It should be emphasized that the alkyl chains of the pyrrolidinium cations show remarkably different conformational behaviors comparing with those of the imidazolium. The isomerization enthalpies Delta(iso)H degrees from the e6 to the e4 isomer of P(13)(+) and to e1 of P(14)(+) were reasonably estimated from the temperature dependence of Raman spectra based on our proposed assignments to be 2.9 kJ mol(-1) for P(13)(+) and 4.2 kJ mol(-1) for P(14)(+), respectively. Thus evaluated experimental Delta(iso)H degrees values, which may contain some uncertainties, are in agreement with those predicted by DFT calculations and MD simulations suggesting that pseudo rotational isomerization equilibria are established in the examined N-alkyl-N-methylpyrrolidinium ionic liquids. The conformational behavior of TFSA(-) was also investigated. The Delta(iso)H degrees from the trans (trifluoromethyl groups on opposite sides of the S-N-S plane) to the cis isomer were evaluated to be 4.2 kJ mol(-1) for [P(13)][TFSA] and 3.5 kJ mol(-1) for [P(14)][TFSA], respectively, which are similar to that for the 1-ethyl-3methylimidazolium ionic liquid.

Solvation of the Amphiphilic Diol Molecule in Aliphatic Alcohol-Water and Fluorinated Alcohol-Water Solutions

We investigated the solvation properties of aqueous solutions of aliphatic alcohols and fluorinated alcohols. These included ethanol (EtOH), 2-propanol (2-PrOH), 2,2,2-trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The amphiphilic diol, 1,4-pentanediol (1,4-PD), was used as the solute to probe solvation properties at the molecular level. Small-angle neutron scattering (SANS) experiments revealed that the inherent microheterogeneity of HFIP-water binary solutions was significantly enhanced by addition of 1,4-PD. In contrast, the addition of 1,4-PD to EtOH-, 2-PrOH-, and TFE-water solutions hardly changed the mixing state. Molecular dynamics simulations were used to obtain the spatial distribution functions for the oxygen atom of water molecules and the carbon and fluorine atoms of alcohol molecules around 1,4-PD. Of the alcohols studied, these spatial distributions illustrated that HFIP molecules formed the strongest hydrophobic solvation shell around the hydrocarbons of 1,4-PD. This preferential solvation of 1,4-PD by HFIP leads to enhancement of HFIP clusters in the solutions. (13)C NMR and infrared spectroscopic measurements on 1,4-PD in the different alcohol-water solutions suggested that the number of water molecules around the hydrocarbons of 1,4-PD decreased in aliphatic alcohol-water solutions. Additionally, HFIP molecules are thought to strongly interact with the hydrocarbons of 1,4-PD in HFIP-water solutions.

Perturbed Molecular Dynamics for Calculating Thermal Conductivity of Zirconia

A perturbation method is modified to calculate the thermal conductivity of ionic crystals by molecular dynamics simulation. The energy flux and perturbation tensor are formulated so as to contain only phase space variables and a single convergence parameter. The characteristics of the perturbation method are studied using ZrO2 as a model ionic crystal. The energy flux due to the perturbation is found to exhibit long time-scale oscillations due to the inclusion of long-range Coulombic interactions. In the linear response regime, the calculated thermal conductivity is independent of the external force field parameter, which is the only arbitrary parameter in the method. However, it is found that the external force field parameter plays an important role in minimizing thermal noise in the energy flux. An exponential relationship between thermal conductivity and the maximum external force field parameter is found, by which one can select an external force field parameter suitable for performing thermal conductivity calculations on different ionic materials without needing to carry out numerous preliminary simulations.

Thermal Conductivity of Zirconia for Thermal Barrier Coatings: A Perturbed Molecular Dynamics Study

Thermal conductivity of pure and Y2O3-doped ZrO2 was calculated using a perturbed molecular dynamics method in order to analyze phonon scattering mechanism which is responsible for the reduction of thermal conductivity. Although absolute values of thermal conductivity were overestimated due to a simple model used in this study, relative values were in good agreement with experiment, which indicates that phonon scattering due to Y2O3 addition is reproduced well. It is found from quantitative analysis of the phonon scattering using the mean field theory that decrease of the thermal conductivity upon Y2O3 addition is attributed not only to the introduction of O2- vacancies but also to substitution of Y3+ ions for Zr4+ ions.

MOLECULAR DYNAMICS SIMULATION OF AN ANTIFERROELECTRIC LIQUID CRYSTALLINE MOLECULE MHPOBC: CONFORMATIONAL TRANSITION IN SMECTIC PHASES

Molecular dynamics (MD) simulation was carried out for an antiferroelectric liquid crystalline molecule, (S)-4-[(1-methylheptyloxycarbonyl]phenyl 4′-octyloxybiphenyl-4′-carboxylate (MHPOBC), to analyze its conformational property in smectic liquid crystalline phases. The MD simulation reproduces a “solid–SmCx–SmA–isotropic” phase transition. The MHPOBC molecules in the SmCx phase are packed in a tilted layered structure, while in the SmA phase the molecular tilt disappears. The chiral alkyl chain in the crystalline phase has a larger gauche population at the C*–C bond, which gives rise to a “highly bent structure”. In smectic phases, the conformation of both chiral and achiral alkyl chains is almost fully liberated, and the average molecular shape changes into a “moderately bent structure”. Comparison of the results of MD simulation and molecular orbital calculation indicates that the smectic phases of MHPOBC can be distinguished from the solid phase in terms of the alkyl chain conformation, and the SmCx and SmA phases are distinguished from each other in terms of molecular packing structure.