Tsun-Mei Chang

Molecular dynamics study of water clusters, liquid, and liquid–vapor interface of water with many-body potentials

The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structural properties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of v...

Molecular dynamics simulations of CCl4–H2O liquid–liquid interface with polarizable potential models

The results from molecular dynamics simulations of the equilibrium properties of the CCl4–H2O liquid–liquid interface at room temperature are presented. The interactions between H2O–H2O, H2O–CCl4, and CCl4–CCl4 are described using the polarizable potential models developed in our laboratory. To our knowledge, this work is the first molecular dynamics simulations of the liquid–liquid interfacial equilibrium properties that explicitly includes nonadditive polarization effects. Molecular dynamics results of a 300 ps simulation following an extensive equilibration process indicate that the liquid interface is very stable, the density profile of H2O is very smooth, while that of CCl4 exhibits some oscillations. It is found that locally there is a sharp transition from one liquid phase to the other, but the overall interface is broadened by thermal fluctuations as indicated by the liquid density profiles. Calculated radial distribution functions suggest that the local structures of CCl4 and H2O remain unchanged from the bulk liquid to the interface. However, the interface does induce orientational order of H2O and CCl4 molecules. To study the polarization effects on the liquid–liquid interfacial equilibrium properties, we have calculated the total and induced dipole moments of H2O and CCl4 molecules as a function of the distance normal to the interface. The calculated dipole moments of the water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to the bulk values. This behavior can be attributed to the changes of the hydrogen bonding patterns and the orientation of water molecules near the interface. The induced dipole moments of the CCl4 molecules near the interface, on the other hand, are significantly enhanced. This is due in part to the strong local field induced by the water molecules at the interface. The calculated electric potentials using the dipole moment approach help us to analyze the orientations of water and CCl4 molecules at the interface.

Molecular dynamics simulations of liquid, interface, and ionic solvation of polarizable carbon tetrachloride

In this study, we construct a nonadditive polarizable model potential to describe the intermolecular interactions between carbon tetrachloride, CCl4, based on classical molecular dynamics techniques. The potential parameters are refined to accurately describe the experimental thermodynamic and structural properties of liquid CCl4 at 298 K. We then carried out additional liquid CCl4 simulations at temperatures in the range of 250–323 K to examine the temperature dependence of the thermodynamic properties. The computed liquid densities and the enthalpies of vaporization are in excellent agreement with experimental values. The structures of liquid CCl4 can be analyzed by examining the radial distribution functions and angular distribution functions. It is found that the liquid CCl4 forms an interlocking structure and that a local orientational correlation is observed between neighboring CCl4 molecules. We also investigate the CCl4 liquid/vapor interface using this potential model. The density profile shows t...

Computational studies of structures and dynamics of 1,3-dimethylimidazolim salt liquids and their interfaces using polarizable potential models.

The structures, thermodynamics, and dynamical properties of bulk and air/liquid interfaces of three ionic liquids, 1,3-dimethylimidazolium [dmim](+) with Cl(-), Br(-), and I(-) were studied using molecular dynamics techniques and polarizable potential models. In bulk melts, the radial distribution functions reveal a significant long-range structural correlation in these ionic liquids. The single-ion dynamics are studied via mean-square-displacements, velocity and orientational correlation functions. We observe that anion size plays an important role in the dynamics of ionic liquids, with larger anions inducing faster cation and anion motion. The computed density profiles of the ionic liquid/vapor interface exhibit oscillatory behavior, indicative of surface layering at the interface. The computed surface tensions indicate small differences between these ionic liquids and decrease with the increasing anion size. The magnitudes of the computed potential drops of these ionic liquids are found to be small and negative and increase with the decreasing anion size. These results could imply that the cation dipoles on average orient more in the interfacial plane than perpendicular to it. Our results showed that anion type plays a major role in determining IL interfacial behavior.

Molecular Mechanism of CO2 and SO2 Molecules Binding to the Air/Liquid Interface of 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid: A Molecular Dynamics Study with Polarizable Potential Models

Molecular dynamics simulations with many-body interactions were carried out to understand the bulk and interfacial absorption of gases in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). A new polarizable molecular model was developed for BMIMBF4, which was found to give the correct liquid density but which also had good agreement with experiment for its surface tension and X-ray reflectivity. The potential of mean force of CO(2) and SO(2) was calculated across the air-BMIMBF4 interface, and the bulk free energies were calculated with the free-energy perturbation method. A new polarizable model was also developed for CO(2). The air-BMIMBF4 interface had enhanced BMIM density, which was mostly related to its butyl group, followed by enhanced BF4 density a few angstroms toward the liquid bulk. The density profiles were observed to exhibit oscillations between high BMIM and BF4 density indicating the presence of surface layering induced by the interface. The potential of mean force for CO(2) and SO(2) showed more negative free energies in regions of enhanced BF4 density, while more positive free energies were found in regions of high BMIM density. Moreover, these gases showed free-energy minimums at the interface, where the BMIM alkyl groups were found to be most prevalent. Our results show the importance of ionic liquid interfacial ordering for understanding gas solvation in them.

Interpreting Vibrational Sum-frequency Spectra of Sulfur Dioxide at the Air/Water Interface: A Comprehensive Molecular Dynamics Study

We investigated the solvation and spectroscopic properties of SO(2) at the air/water interface using molecular simulation. Molecular interactions from both Kohn-Sham (KS) density functional theory (DFT) and classical polarizable models were used to understand the properties of SO(2):(H(2)O)(x) complexes in the vicinity of the air/water interface. The KS-DFT was included to allow comparisons with vibrational sum-frequency spectroscopy through the identification of surface SO(2):(H(2)O)(x) complexes. Using our simulation results, we were able to develop a much more detailed picture of the surface structure of SO(2) consistent with spectroscopic data obtained by Richmond and co-workers (J. Am. Chem. Soc. 2005, 127, 16806). We also found many similarities and differences between the two interaction potentials, including a noticeable weakness of the classical potential model in reproducing the asymmetric hydrogen bonding of water with SO(2) due to its inability to account for SO(2) resonance structures.

Transfer of CH4 across the H2OCCl4 liquid-liquid interface with polarizable potential models

Abstract Molecular dynamics simulations are carried out to investigate the free energy profile of transferring a methane molecule across the H 2 OCCl 4 liquid-liquid interface. The computed free energy curve decreases monotonically from bulk H 2 O to bulk CCl 4 . Examination of the solvation structures suggests that the transfer involves a smooth change of the composition of the solvation shell around CH 4 . The computed free energy of transfer estimated from the potential of mean force is 3.8 ± 0.7 kcal/mol. This value is in good agreement with the value of 1.9 kcal/mol, supporting our approach.

Experimental and Theoretical Study of Molecular Response of Amine Bases in Organic Solvents

Reorientational correlation times of various amine bases (namely, pyridine, 2,6-lutidine, 2,2,6,6-tetramethylpiperidine) and organic solvents (dichloromethane, toluene) were determined by solution-state NMR relaxation time measurements and compared with predictions from molecular dynamics (MD) simulations. The amine bases are reagents in complex reactions catalyzed by frustrated Lewis pairs (FLP), which display remarkable activity in metal-free H2 scission. The comparison of measured and simulated correlation times is a key test of the ability of recent MD and quantum electronic structure calculations to elucidate the mechanism of FLP activity. Correlation times were found to be in the range of 1.4-3.4 (NMR) and 1.23-5.28 ps (MD) for the amines and 0.9-2.3 (NMR) and 0.2-1.7 ps (MD) for the solvent molecules.

Computational studies of liquid water and diluted water in carbon tetrachloride.

Molecular dynamics simulations were carried out to study solvent effects on the energetic and dynamical properties of water molecules in liquid water and in carbon tetrachloride (CCl4). In these studies, the free-energy profiles or potentials of mean force (PMF) for water dimers in both solvents were computed. The computed PMF results showed a stable minimum near 3 A for the O-O separation, with a minimum free energy of about -2.8 kcal/mol in CCl4, as compared to a value of -0.5 kcal/mol in liquid water. The difference in free energy in water as compared to that in CCl4 was expected and is the result of competition from surrounding water molecules that are capable of forming hydrogen bonds in the liquid water. This capability is absent in the diluted water found in CCl4. We found that the rotational motions of H2O/D2O were nonisotropic, with the out-of-plane vector correlation times in H2O/D2O varying from 5.6/5.8 ps at 250 K to 0.57/0.56 ps at 350 K and the corresponding OH/OD bond vectors varying from 6.5/7.7 ps to 0.75/0.75 ps. The results compare reasonably well to the available NMR experimental and computer simulation data on the same system (Farrar; Skinner; et al. J. Am. Chem. Soc. 2001, 123, 8047). For diluted water in CCl4, we found the computed rotational correlation times also were nonisotropic and much longer than the corresponding NMR experimental values at the same concentration (Farrar; et al. J. Phys. Chem. A 2007, 111, 6146). Upon analyzing the water hydrogen-bonding patterns as a function of water concentration, we conclude that the differences in the rotational correlation times mainly result from the formation of water hydrogen-bonding networks as the water concentration is increased in liquid CCl4. In addition, we found the rotational correlation times to be substantially faster in liquid CCl4 than in liquid water.

Transfer of chloroform across the water–carbon tetrachloride liquid–liquid interface

Using the constrained molecular dynamics technique, the mass transport of a chloroform molecule across the CC14/H2O interface is investigated. As expected, the transfer free energy is found to decrease monotonically from the aqueous phase into the nonpolar carbon tetrachloride liquid. The presence of the solute exerts essentially no perturbation to the interface, and to the peak positions of the solute–solvent atomic radial distribution functions. These observations suggest that the transport of the chloroform molecule involves a smooth change in the solvent composition of the solvation shells around the solute molecule.