Solvation of Cu2+ in Water and Ammonia. Insight from Static and Dynamical Density Functional Theory

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Abstract

We have studied the solvation of divalent copper by water and ammonia through the optimization of the structures of [Cu(H2O)n]2+ and [Cu(NH3)n]2+, n = 3−8, by static density functional theory and ab initio molecular dynamics simulations. We found that as the number of solvent molecules increases to more than four, the additional ligands prefer to be hydrogen-bonded to the planar tetragonal primary hydration shell of [Cu(solvent)4]2+ instead of filling the vacant axial position. The energetic preference of water is about 20−35 kJ/mol for the hydrogen bond compared to the axial position, whereas ammonia shows preference of only a few kJ/mol. Dynamical simulations were successful in reaching the lowest energy conformations. Especially remarkable is the dynamics of [Cu(H2O)8]2+, which has evolved from an eight-coordinate structure to a planar structure with four primary and four secondary solvent molecules in a short 10 ps simulation. Both [Cu(H2O)8]2+ and [Cu(NH3)8]2+ prefer a quasi-planar structure with a total of eight hydrogen bonds between the solvent molecules in the first and second solvation shells. Each secondary water and ammonia is hydrogen-bonded to two adjacent molecules in the primary solvation shell. It is remarkable that ammonia can form two hydrogen bonds with only one lone electron pair. The strong network of hydrogen bonds stabilizes the tetragonal planar primary hydration shell. These calculations indicate that the high kinetic stability of the eight-coordinate clusters in previous mass spectrometry experiments is related to the stabilization of the planar primary solvation shell by the network of hydrogen bonds. We found a correlation between experimental ion signals in the gas phase and the planarity of the first solvation shells.

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