Solvation of Cu2+ in water and ammonia insight from static and dynamical density functional theory

We have studied the solvation of divalent copper by water and ammonia throu gh 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 d ynamics simulations. We found that as the number of solvent molecules incre ases to more than four, the additional ligands prefer to be hydrogen-bonded to the planar tetragonal primary hydration shell of [Cu(solvent)(4)](2+) i nstead of filling the vacant axial position. The energetic preference of wa ter is about 20-35 kJ/mol for the hydrogen bond compared to the axial posit ion, whereas ammonia shows preference of only a few kJ/mol. Dynamical simul ations were successful in reaching the lowest energy conformations. Especia lly 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 f our 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 o f eight hydrogen bonds between the solvent molecules in the first and secon d solvation shells. Each secondary water and ammonia is hydrogen-bonded to two adjacent molecules in the primary solvation shell. It is remarkable tha t 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 stabilit y of the eight-coordinate clusters in previous mass spectrometry experiment s 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 shell s.