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Simulations of a flexible model of the tri-iodide ion have been used to study the effects of temperature and solvent on the vibrational energy relaxation rate and the pure dephasing rate of the two stretching modes. These rates were determined in ethanol and 2-methyltetrahydrofuran at 100 K, 200 K, and 300 K, covering the range between glass and mobile liquid states for both solvents. In agreement with experiment the temperature dependence is small. In ethanol the symmetry of the tri-iodide ion is broken at the three temperatures studied, while in methyltetrahydrofuran symmetry is retained except at the lowest temperature. In order to examine different contributions to the relaxation processes, perturbation theory was applied to simulations with rigid ions. Although the large amplitude of the solvent-induced geometry changes suggests that perturbation theory is at best qualitative, reasonable agreement was obtained between the results from the flexible simulation and perturbation theory in 2-methyltetrahydrofuran. Both solvent-induced curvature and solvent force terms were found to contribute to the solvent frequency shifts and the pure dephasing rates. In ethanolic solutions the time correlation functions of the forces decay rapidly, but the curvature term probes a slow dynamical process. At least in our model, the lines would not be completely motionally narrowed in solution in ethanol. Comparisons are made with experiment and previous work. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the displacement and distortion of the ammonium ion in rotational transition states in ammonium fluoride and ammonium chloride. Computational methods; Equilibrium structure of NH[sub 4]F; Transition states in NH[sub 4]F; Transition states in NH[sub 4]Cl.

Examines immersion in hydrogen-bonding solvents leading to breakage of the centro-symmetry of the tri-iodide ion by computer simulation. Fundamental problems in the simulation of the tri-iodide ion; Calculation of the polarization energy; Stability of solvated ion; Estimation of vibrational energy and phase relaxation.

Abstract: Simulation studies on a model system of a spherical ion with various charges in two imidazolium ionic liquids and in acetonitrile are compared. The average vertical ionisation potentials as a function of the charge on the ion are similar for all three systems. The Landau free energies of each system as a function of the vertical ionisation potential are computed and are close to being parabolic. Results are shown for the solvent reorganisation energies and for the activation free energies. The similarities of all these quantities are interpreted in terms of continuum models. However, the dynamics are likely to be very different in a polar liquid and in an ionic liquid as in the former case screening occurs by reorientation of molecules and in the latter case it occurs by translation of ions. [Copyright &y& Elsevier]

The structure of the gas-liquid surface of dimethylimidazolium chloride has been studied using atomistic simulation. We find that there is a region of enhanced density immediately below the interface in which the cations are oriented with their planes perpendicular to the surface and their dipoles in the surface plane. There is negligible segregation of cations and anions. The temperature dependence of the surface tension is predicted to be anomalously low or be reversed in sign. The vapour-liquid interfaces between mixtures of water and dimethylimidazolium chloride show similar regions of enhanced density and preferential orientation of the cations. Water molecules also show preferential orientation in the interface region and are preferentially adsorbed on the vapour side of the interface. The surface tension decreases with increase in the mole fraction of water. [ABSTRACT FROM AUTHOR]

A molecular dynamics model is developed to investigate the pure vibrational dephasing of I[sub 3, sup -] in solvent at temperatures ranging from room temperature to 100 K. The triiodide ion is described by a valence bond model which responds to the field of the classical solvent molecules. We find that the pure dephasing rate does not change much with temperature. This result can then be interpreted in terms of of Kubo lineshape theory. [ABSTRACT FROM AUTHOR]

Ab initio calculations of the density of states for adsorbed monolayers of water on a MgO(1 0 0) surface are compared with metastable impact electron spectra (MIES) and ultraviolet photoelectron spectra (UPS). The calculations confirm the suggested assignments and in particular show that there is good evidence that some, but not all, molecules have dissociated to hydroxide ions. Density of state calculations are shown for configurations with no dissociated molecules, one third of the molecules dissociated and half the molecules dissociated; the agreement with experiment is best for the configuration with one third of the molecules dissociated. [Copyright &y& Elsevier]

Studies the diffusion coefficients of ions and of uncharged solutes in aqueous solution at 25 degrees Celsius and at infinite dilution by computer simulation using the SPC/E model for water and solute. Friction on an ion; Molecular dynamics simulations; Resolution of the force autocorrelation function.

This paper outlines some personal reminiscences of my first experience of simulations emphasizing the way the computational results were related to theories of spectroscopic line shapes in liquids. [ABSTRACT FROM PUBLISHER]

Studies the kinetics of passage of carbon dioxide and nitrogen through the liquid-vapor interface of water. Initiation of trajectories in the gas phase; Average lifetime of molecules in the surface determined to be longer than the inverse of the energy relaxation rate; Return of molecules to the gas phase.

The temperature dependence of the vibrational relaxation of a flexible model of triiodide in a Lennard-Jones solvent (xenon) has been studied using equilibrium molecular dynamics simulations. The internal dynamics of the ion is calculated from a previously published semiempirical valence bond model with a limited number of basis states. Vibrational decorrelation rates of the symmetric and antisymmetric stretching modes were found from the time correlation functions of the normal coordinate velocities and the vibrational energy relaxation rates from the time correlation functions of the kinetic energy in each mode. The vibrational dephasing rates and the energy relaxation rates decrease slowly as the temperature is lowered and do not show a discontinuity when the fluid solidifies, although the reorientational diffusion rates change rapidly at low temperatures. In order to interpret the results, perturbation theory expressions for the relaxation rates were evaluated for simulations of a rigid model of the ion and found to agree well with the direct observations. These showed that, unusually, both the solvent force and its derivative, the solvent potential curvature, contribute to the dephasing of the symmetric mode. The relevant fluctuation correlation times are very short, which may explain the insensitivity of the vibrational relaxation to the state of the solvent. [ABSTRACT FROM AUTHOR]

Recent discovery of the anomalous crystallization in n-alkanes and n-alcohols at the free surfaces of melts has revealed the necessity for much deeper understanding of the molecular mechanism of crystallization. With the aid of large-scale molecular simulation, and by use of a simplified molecular model of bead spring, the molecular processes of melting and crystallization in a thin film of n-alkane are directly observed. It is found that melting of the thin film is greatly influenced by its surface state. The film is more stable when the surface chains are lying perpendicular to the surface, on which the chain ends have marked preference. Crystallization by both rapid and slow cooling of the melt is shown to give rise to a formation of monolayers on the free surfaces. By slow stepwise cooling, the layer-by-layer growth of stacked lamellae is clearly reproduced. Detailed inspection of the molecular processes involved shows that there are considerable differences in the molecular mechanisms of monolayer formation and 3D crystallization. Furthermore, the molecular mobility and diffusivity in the surface monolayer are discussed. It is shown that longitudinal displacements of the chains along their axes are much larger in the monolayer than in the crystals, and that the calculated rate of transverse diffusion in the monolayer shows good comparison with experimental values. The interchanges of the molecules between the melt and the surface monolayers are observed directly; dynamical stability of the surface monolayer is clearly demonstrated. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on the transfer of a carbon dioxide molecule from the gas phase through a thin liquid water film supported on ionic single crystal surfaces of sodium chloride and magnesium oxide. Free energy profile for the carbon monoxide; Use of a constrained molecular dynamics technique to evaluate the free energy profile of the dynamics of mass transport of molecules.

Molecular dynamics simulations of solutions of benzene in dimethylimidazolium chloride and dimethylimidazolium hexafluorophosphate have been performed with a view to answering the question posed in the title. The difference between the chemical potential of a normal model of benzene and one with no charges was found to depend on the solvent but is at least 4 kBT. This difference is sufficient to account for the observed solubility differences. There are substantial changes in the local structure around benzene with and without charges. [Copyright &y& Elsevier]

The structure and energetics of a monolayer of water on a perfect MgO substrate is investigated by ab initio calculations. Several minima in the potential energy surface were found including both physisorbed and chemisorbed states in which one third of the water molecules were dissociated. In the more stable of the physisorbed states, the water molecules were not all parallel to the surface, but some showed hydrogen bonding with surface oxygen ions. Even in the physisorbed state, the geometry of these surface H-bonded water molecules were distorted relative to the bulk. One of the stable chemisorbed states was the same as that found in earlier work by Giordano et al. [Phys. Rev. Lett. 81, 1271 (1998)]. Two more stable chemisorbed structures were found in which strong hydrogen bonds to the hydroxide ions in the water layer were formed. The existence of different minima appears to be the result of a subtle interplay between hydrogen bonding between adsorbed species and with the surface oxide ions. Harmonic vibrational frequencies were determined for both a chemisorbed and a physisorbed states and spectroscopic features which should discriminate between chemisorbed and physisorbed states are identified. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]