Polarization and charge transfer in the hydration of chloride ions

摘要

A theoretical study of the structural and electronic properties of the chloride ion and water moleculesin the first hydration shell is presented. The calculations are performed on an ensemble ofconfigurations 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-levelcalculations are performed to examine the electronic structure properties of the ions and surroundingwaters in the external field of more distant waters. The ChelpG method is employed to explore theeffective charges and dipoles on the chloride ions and first-shell waters. The quantum theory ofatoms in molecules (QTAIM) is further utilized to examine charge transfer from the anion tosurrounding water molecules. The clusters extracted from the AMOEBA simulations exhibit highprobabilities of anisotropic solvation for chloride ions in bulk water. From the QTAIM analysis, 0.2elementary 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 to thequantum mechanical value. The average magnitude of the dipole moment of the water molecules inthe 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 thedipole moment of the water molecules in the first solvation shell is most strongly affected by thelocal water-water interactions and hydrogen bonds with the second solvation shell, rather than byinteractions with the ion.