Jean-Christophe Soetens

Molecular dynamics simulation and X-ray diffraction studies of ethylene carbonate, propylene carbonate and dimethyl carbonate in liquid phase

Abstract Molecular Dynamics simulations of pure ethylene carbonate, propylene carbonate and dimethyl carbonate have been carried out at different temperatures in the liquid phase. The all atom OPLS parameters are used to describe van der Waals interactions and the atomic charges are obtained from ab initio 6-31G** SCF calculations. For the flexible models, various force fields have been tested and Cff91 has been chosen to describe intramolecular interactions. Thermodynamical and dynamical properties are reported and a reasonable agreement with the available experimental data is observed. Simulated structure of ethylene carbonate and propylene carbonate at 323 and 473 K are compared with the experimental one determined at the same temperatures using X—ray diffraction experiment.

Effect of distributing multipoles and polarizabilities on molecular dynamics simulations of water

Abstract A series of molecular dynamics simulations of liquid water has been carried out using different electrostatic models. The models tested include the distributed point charges and dipoles and the corresponding polarizabilities obtained from Stones partitioning (DMA and DPA) and those obtained from an extension of Baders topological theory of atoms in a molecule. It is shown that (i) distributing the polarizability on atomic sites has an important effect on liquid properties, (ii) liquid properties are sensitive to the values of the distributed charges and dipole moments and less dependent on the choice of the polarizability partitioning.

Hydrogen bonding in supercritical tert-butanol assessed by vibrational spectroscopies and molecular-dynamics simulations.

We have investigated the state of aggregation in supercritical tert-butanol (T = 523 K,0.05 < rho < 0.4 g cm(-3)) by means of vibrational spectroscopies (infrared and Raman) and molecular-dynamics (MD) simulations. A quantitative band shape analysis of the spectra associated with the OH stretching mode of tert-butanol has been done using activities computed by ab initio calculations on small clusters. This allows us to determine the degree of hydrogen bonding and populations of oligomers. These latter quantities have been derived from MD simulations and very consistent results are found with experiments. These results show that hydrogen bond still exist in supercritical tert-butanol and that the fluid mainly consists of oligomers smaller than tetramers.

The quest for ion pairing in supercritical aqueous electrolytes

We have investigated the local structure around the bromine ion in three different alkali-brominated electrolytes in both ambient and supercritical conditions by means of the Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. Some defects of the ion-ion contributions obtained in the simulations were observed, although further measurements would be necessary to correct the potential parameters. The phase contrast method presented in this paper, based on a substitution of the ion which acts as an X-ray scatterer in the ion absorber environment, offers new opportunities for the exploration of ion-pairing in other systems or thermodynamic conditions.

Combining extended x-ray absorption fine structure with numerical simulations for disordered systems

Over the past two decades x-ray absorption spectroscopy has proven to be a valuable tool for the study of the short-range order in a wide variety of materials, including disordered systems such as superionic conductors as well as glasses, amorphous and liquid systems in general. A number of methods have been proposed to analyse EXAFS data. However, in the case of disordered systems, only the ones taking the distribution of atomic environments into account should be retained. Molecular dynamics (MD) simulations are a valuable tool in this respect, as will be shown from results obtained in a supercritical aqueous solution.

Investigation of the Local Structure in Sub and Supercritical Ammonia Using the Nearest Neighbor Approach: A Molecular Dynamics Analysis

Molecular dynamics simulations of ammonia were performed in the (N,P,T) ensemble along the isobar 135 bar and in the temperature range between 250 and 500 K that encompasses the sub and supercritical states of ammonia. Six simple interaction potential models (Lennard-Jones pair potential between the atomic sites, plus a Coulomb interaction between atomic partial charges) of ammonia reported in the literature were analyzed. Liquid-gas coexistence curve, critical temperature, and structural data (radial distribution functions) have been calculated for all models and compared with the corresponding experimental data. After choosing the appropriate potential model, we have investigated the local structure by analyzing the nearest neighbor radial, mutual orientation, and interaction energy distributions. The change in the local structure was traced back to the change of the nonlinear behavior (which is more pronounced at low temperatures) of the average distance between a reference ammonia molecule and its subsequent nearest neighbor. Our results suggest to use the position of the maximum in the fluctuation of the average distance to define the border of the first solvation shell (particularly at high temperature when the minimum of the radial distribution is not well-defined). Indeed, the effect of the temperature on the position of this maximum shows clearly that the spatial extent of the solvation shell increases with a concomitant decrease of the involved number of ammonia molecules. Furthermore, our results show that the signature of the hydrogen bonding is mainly observed for temperature below 300 K. This signature is quantified by a short distance contribution to the closest radial nearest neighbor distribution, by a strong mutual orientation (defined by the angles between the axis joining the nitrogen atoms and the molecular axes) and by a strong attractive character of the total interaction energy.

Molecular dynamics simulation study of cyclohexane guest molecules in the cyclohexane/thiourea inclusion compound

Molecular dynamics simulations of the cyclohexane/thiourea inclusion compound within the high temperature rhombohedral phase (at 173 and 273 K) are presented. The simulated systems consist of 96/288 cyclohexane/thiourea molecules, corresponding to 12 contiguous host thiourea channels of approximately 50 A length. The atomic trajectories were obtained over 600 ps assuming rigid molecules. The orientational distribution of cyclohexane guest molecules is described through the polar angles between the guest molecular C m symmetry axis and the thiourea host reference frame. From the molecular dynamics simulations we obtain the three-dimensional orientational distribution functions POh; uU. It is shown that the cyclohexane guest molecules reorient about the C c and the C2 symmetry axes of the crystallographic 32 site. The incoherent intermediate scattering functions have been computed from molecular dynamics trajectories, allowing the development of theoretical models that could be used to interpret the dynamics of cyclohexane guest molecules from experiments. ” 2000 Elsevier Science B.V. All rights reserved.

Water–Methanol Mixtures: Simulations of Mixing Properties over the Entire Range of Mole Fractions

Numerous experimental and theoretical investigations have been devoted to the hydrogen bond in pure liquids and mixtures. Among the different theoretical approaches, molecular dynamics (MD) simulations are predominant in obtaining detailed information, on the molecular level, simultaneously on the structure and the dynamics. Water and methanol are the two most prominent hydrogen-bonded liquids, and they and their mixtures have consequently been the subject of many studies; we revisit here the problem of the mixtures. An important first step is to check whether a classical potential model, the components of which are deemed to be satisfactory for the pure liquids, is able to reproduce the known thermodynamic excess properties of the mixtures sufficiently well. We have used the available BJH (water) and PHH (methanol) flexible models because they are by construction mutually compatible and also well suited to study, in a second step, some dynamic property characteristic of hydrogen-bonded liquids. In this article we show that these models, after a slight reparametrization for use in NpT simulations, reproduce the essential features of the excess mixing and molar properties of water-methanol mixtures. Furthermore, in the pure liquids, the agreement of the radial distribution functions with experiment remains as satisfactory as before. Similarly, the translation self-diffusion coefficients D are modified by less than 10%. In the mixtures, they evolve nonmonotonously as a function of mole fraction.

Static Dielectric Constant of the Polarizable Stockmayer Fluid. Comparison of the Lattice Summation and Reaction Field Methods

Abstract The static dielectric constant of the polarizable Stockmayer fluid is computed through Molecular Dynamics simulations where long range electrostatic interactions are computed either by reaction field or by lattice summation method (Ladd expansion). The effects on the simulated dielectric constant of the truncation of the Ladd expansion and of the cutoff distance in reaction field geometry have been explored for different values of the polarizability.

Supercritical ammonia: A molecular dynamics simulation and vibrational spectroscopic investigation

Combining infrared spectroscopy and molecular dynamics simulations, we have investigated the structural and dynamical properties of ammonia from liquid state (T = 220 and 303 K) up to the supercritical domain along the isotherm T = 423 K. Infrared spectra show that the N-H stretching and bending modes are significantly perturbed which is interpreted as a signature of the change of the local environment. In order to compare the experimental spectra with those obtained using molecular dynamics simulation, we have used a flexible four sites model which allows to take into account the anharmonicity in all the vibration modes particularly that of the inversion mode of the molecule. A good agreement between our experimental and calculated spectra has been obtained hence validating the intermolecular potential used in this study to simulate supercritical ammonia. The detailed analysis of the molecular dynamics simulation results provides a quantitative insight of the relative importance of hydrogen bonding versus nonhydrogen bonded interactions that governs the structure of fluid ammonia.