Aqueous Solvation of Hg(OH)₂: Energetic\nand Dynamical Density Functional Theory Studies of the Hg(OH)₂–(H₂O)_<i>n</i> (<i>n</i> = 1–24) Structures

Abstract

A systematic\nstudy of the hydration of Hg­(OH)₂ by the\nstepwise solvation approach is reported. The optimized structures,\nsolvation energies, and incremental free energies of 1–24 water\nmolecules interacting with the solute have been computed at the B3PW91\nlevel using 6-31G­(d,p) basis sets for the O and H atoms. The mercury\natom was treated with the Stuttgart–Köln relativistic\ncore potential in combination with an extended optimized valence basis\nset. One to three direct Hg–water interactions appear along\nthe solvation process. The first solvation shell is fully formed with\n24 water molecules. A stable pentacoordinated Hg trigonal bipyramid\nstructure appears for <i>n</i> > 15. Density functional\ntheory (DFT) Born–Oppenheimer molecular dynamics simulations\nshowed the thermal stability of the Hg­(OH)₂–(H₂O)₂₄ structure at room temperature and the persistence\nof the trigonal bipyramid coordination around Hg. The Gibbs free energy\nfor the first solvation shell is significantly larger for the fully\nsolvated Hg­(OH)₂ than the one previously obtained for the\nHgCl₂ case, due to σ-acceptor and π-donor properties\ninvolving the hydroxyl groups of the solute. This suggests that the\ntransmembrane passage of Hg­(OH)₂ into the cell via simple\ndiffusion is less favorable compared to the case when the metal is\ncoordinated with two Cl groups.