Polarization and Charge Transfer in the Hydration of Chloride Ions

TL;DR

This study combines molecular dynamics and quantum calculations to analyze the electronic structure, charge transfer, and polarization effects in chloride ion hydration, revealing charge transfer of 0.2 e and insights into water dipole moments.

Contribution

It provides a detailed quantum mechanical analysis of charge transfer and polarization in chloride hydration using combined MD and MP2 calculations, which is novel.

Findings

Charge transfer of 0.2 elementary charges from chloride to water.

Water dipole moments are influenced more by water-water interactions than by the ion.

The AMOEBA model overestimates the chloride ion's dipole moment.

Abstract

A theoretical study of the structural and electronic properties of the chloride ion and water molecules in the first hydration shell is presented. The calculations are performed on an ensemble of configurations obtained from molecular dynamics simulations of a single chloride ion in bulk water. The simulations utilize the polarizable AMOEBA force field for trajectory generation, and MP2-level calculations are performed to examine the electronic structure properties of the ions and surrounding waters in the external field of more distant waters. The ChelpG method is employed to explore the effective charges and dipoles on the chloride ions and first-shell waters. The Quantum Theory of Atoms in Molecules (QTAIM) is further utilized to examine charge transfer from the anion to surrounding water molecules. From the QTAIM analysis, 0.2 elementary charges are transferred from the ion to the first-shell water molecules. The default AMOEBA model overestimates the average dipole moment magnitude of the ion compared with the estimated quantum mechanical value. The average magnitude of the dipole moment of the water molecules in the first shell treated at the MP2 level, with the more distant waters handled with an AMOEBA effective charge model, is 2.67 D. This value is close to the AMOEBA result for first-shell waters (2.72 D) and is slightly reduced from the bulk AMOEBA value (2.78 D). The magnitude of the dipole moment of the water molecules in the first solvation shell is most strongly affected by the local water-water interactions and hydrogen bonds with the second solvation shell, rather than by interactions with the ion.