Gy. I. Szász

A molecular dynamics study of the translational and rotational motions in an aqueous LiI solution

The spectral densities of the hindered translations and librations for the ions and the water molecules in a 2.2 m LiI solution have been derived from an MD simulation with the ST2 model for water. The reorientational times of the dipole moment vector, the intramolecular proton–proton vector as well as the ion–oxygen and ion–hydrogen vectors are also reported. The separate calculation of various properties for the three water subsystems—hydration water of the cation, of the anion and bulk water—contributes significantly to the understanding of macroscopically measured quantities on a molecular level.

On the orientation of water molecules in the hydration shell of the ions in a MgCl2 solution

The orientation of the water molecules in the hydration shells of Mg2+ and Cl− have been calculated from a molecular dynamics simulation of a 1.1 molal MgCl2 solution with the central force model for water. The results are compared with information obtained from neutron diffraction studies with isotopic substitution.

The structure of an aqueous ammonium chloride solution

X‐ray measurements have been performed on a 2 molal NH4 Cl solution at 25 °C. The scattering data have been evaluated with the first neighbor model (FNM). These results are compared with molecular dynamics (MD) simulations where the ST2 model for water is employed. The NH4+ is described by a rigid tetrahedron with point charges at the positions of the hydrogen atoms and the Cl− by a Lennard‐Jones sphere with an elementary charge at the center. Good agrement has been found between the measurements and the simulation. Contrary to previous results the nearest neighbor distance for NH4+–water is found to be significantly different from the one for water–water. The structure of the solvent described by the nearest neighbor distance and its root mean square deviation is in the limits of error the same as in pure water and significantly different from the solvent structure in alkali halide solutions.

The structure of the hydration shell of the lithium ion

Abstract Structural data on the hydrated lithium ion are derived from the MD simulation of a 2.2 molal aqueous Lil solution. The results are in good agreement with recent neutron diffraction measurements with isotopic substitution and show additionally the octahedral arrangement of the water molecules around the lithium ion.

Structural Properties of an Aqueous LiI Solution Derived from a Molecular Dynamics Simulation

In the molecular dynamics simulation of the aqueous LiI solution the ST2 water model was employed and the ions were described as Lennard-Jones spheres with an elementary charge at the center. The basic periodic cube contained 200 water molecules and 8 ions of each kind equivalent to a 2.2 molal solution. The simulation extended over 10 ps with an average temperature of 305 K. The structure is discussed by means of ion-water, ion-ion and water-water radial pair distribution functions and the orientation of the water molecules. Potential energy relationships are reported and the heat of solution has been calculated and found in agreement with experimental values.

The Second Hydration Shell of Li+ in Aqueous LiI from X-Ray and MD Studies

Indications from a molecular dynamics simulation of a 2.2 molal LiI solution of the existence of a second hydration shell of Li+ have been checked by an x-ray investigation of the same solution. The scattering data are analysed via partial structure functions and radial distribution functions which have been obtained from a model fitted to the total structure function. Experiment and simulation agree on first neighbor ion-water distances. An octahedral arrangement of six water molecules in the first hydration shell of Li+ and additional twelve water molecules in the second shell have been verified by the experiment.

On the variational calculation of energies and widths of resonances

Abstract An approximation, based on a direct method of the calculus of variations, is applied in order to compute energies and widths of resonances (and energies of bound states).

Molecular Dynamics Study of Aqueous Solutions IX. Dynamical Properties of an NH4Cl Solution

Abstract The dynamical properties of an aqueous ammonium chloride solution have been calculated from a molecular dynamics simulation over 3.5 picoseconds at a temperature of 301 K where the basic periodic cube contained 200 water molecules, 8 ammonium ions and 8 chloride ions. The effective pair potentials are based on the ST 2 water model, a rigid tetrahedral four point charge model for NH4+, and a single point charge model for CI-. The coefficients of self-diffusion and of rotational diffusion, the spectral densities of hindered translations and of librations, the correlation times of the dipole moment vector and the vector connecting the two protons in a water molecule are reported separately for the various subsystems - NH4+ , Cl-, bulk water, hydration water of NH4+ and CI- - and are compared with the available experimental data. Implications with respect to the structure breaking ability of the ions are discussed.

A Molecular Dynamics Study of Aqueous Solutions. VIII. Improved Simulation and Structural Properties of a NH4Cl Solution

A molecular dynamics simulation of an aqueous ammonium chloride solution has been carried out at a mass density of 1.027 g/cm3, corresponding to a 2.2 m solution, and at a temperature of T = 301 K. The effective pair potentials are based on the ST2 model of water, a rigid tetrahedral four point charge model of NH4+ and a single point charge model of Cl-. The calculated structural properties of this solution are discussed and compared with those obtained for alkali halide solutions. The first hydration shell of the ammonium ion contains on the average eight water molecules, it has a more complex structure than the hydration shells of cations with spherical symmetry. The static orientational order of water dipole moments indicate a structure breaking ability of NH4+.