Quantum Chemical Studies of Mononuclear Zinc Species of Hydration and Hydrolysis

Optimal geometries, charge distributions, bond analysis, changes of Gibbs free energy, entropies and enthalpies of hydration, and hydrolysis reactions for mononuclear species of Zn2+ including hydrated and hydrolysis complexes were investigated using quantum chemical calculations in the gas phase. Optimized geometrical structures showed that the stable hydrated and hydrolysis zinc species without outer-sphere water molecules were Zn(H2O)62+, Zn(OH)(H2O)3+, Zn(OH)2(H2O)2, Zn(OH)3-, and Zn(OH)42-. Results of NPA (Natural Population Analysis) indicated that the charge on the Zn atom of the hydrated ions decreased but the charge on the zinc atom of the hydrolysis species increased with the increase of inner-sphere water molecules. NBO (Natural Bond Orbital) analyses demonstrated that hydrated and hydrolysis species of zinc were mainly electrostatic bonding compounds. Calculations of reaction energies indicated that inner-sphere water molecules became more unfavorable as the hydrolysis increased. Stepwise hydrolysis equilibrium constants decreased successively and the order remained unchanged when the inner-sphere dehydration occurred.