Molecular Dynamic Modeling of Pertechnate Ion in Aqueous Solution

Abstract

A molecular dynamics simulation of an aqueous solution of pertechnate ion was performed. To determine the thermodynamic, structural, and dynamic properties of an infinitely dilute solution of pertechnate ion, the following systems were simulated at 25ºC using the MDNAES software package: 400 H2O molecules; 1+399 H2O. The SPC/E water model was used. A classical description of intermolecular interactions via paired (site-site) potentials was chosen, consisting of a sum of short-range Lennard-Jones potentials (12–6) and a Coulomb component.

Atomic charges within the ion were obtained from a quantum chemical calculation using the Gaussian package at the level of second-order Møller-Plesset excitation theory, using the def2QZVP basis set. To verify the obtained model potential parameters, quantum chemical calculations in Gaussian were performed for both pertechnate and perchlorate ions using the method described above, reproducing literature data on atomic charges for which the calculation procedure was considered correct. Atomic charges for the pertechnate ion were then obtained using this method and used in subsequent MD simulations.

The ion's enthalpy of solvation, radial distribution functions, current coordination numbers, and autocorrelation functions were calculated. It was found that the absolute value of the pertechnate ion's enthalpy of solvation is lower than that of the perchlorate ion, as the larger pertechnate ion hydrates less readily.

Based on the calculations, a model for the arrangement of water molecules in the immediate environment of the pertechnate ion was proposed. It was found that the pertechnate ion is located in a deformed octahedral environment of six water molecules. It has been shown that water molecules behave identically in the context of translational dynamics in the first and second solvation shells and in the bulk solution. The dynamics of water molecules are fundamentally the same in the bulk solution and in the solvation shells of the ion, and also differ little in the nature of the ion's dynamics. Thus, the integration of the anion into the water structure occurs with minimal changes in the latter.