Quantum Mechanical and Molecular Dynamical Simulations on Thorium(IV) Hydrates in Aqueous Solution

Abstract

We report the combined quantum mechanical and molecular dynamical simulations on thorium(IV) hydrates in aqueous solution. Hydration of the Th⁴⁺ ion in aqueous system was first investigated using the B3LYP hybrid density functional theoretical calculations. The results show that the first shell hydration number of Th⁴⁺ ion in liquid phase is 9 at the bond distance of Th−O_I 2.54(1) Å and Th−H_I 3.22(1) Å. Second, the second shell hydration properties of the Th⁴⁺ ion in aqueous solution were studied by the molecular dynamical simulation using AMBER force field. The concept of the hydrated ion was used, [Th(H₂O)₉]⁴⁺ being the cationic entity interacting in solution. The [Th(H₂O)₉]⁴⁺−water interaction potential was developed by ab intio B3LYP calculations. The partial atomic charge of [Th(H₂O)₉]⁴⁺ is derived from the ESP method. The MD calculated results show a well-defined second coordination shell and an ill-defined third shell around the [Th(H₂O)₉]⁴⁺ ion. The strong hydrogen bonding due to the polarization of the first coordination sphere water molecules leads to a mean coordination number of 18.9 water molecules in the second shell at the bond distance of Th−O_II 4.75 Å and Th−H_II 5.35 Å. The residence time of a water molecule in the second hydration shell is 423.4 ps. Our simulated results indicate that the hydrated ion concept for simulating the Th⁴⁺ ion in aqueous solution is appropriate.