Hydration and Water Exchange of Zinc(II) Ions. Application of Density Functional Theory

Details of the hydration and water exchange mechanism of Zn2+ have been studied using density functional calculations with a variety of different basis sets. The computed structures and hydration energies for complexes of the type [Zn(H2O)n]2+ with n = 1−6 are in good agreement with previous results obtained from ab initio calculations and self-consistent reaction field methods. Extension of our investigations to the second coordination (first solvation) sphere and thus to complexes of the general type [Zn(H2O)n]2+·mH2O with n = 5 and m = 1, 2 and n = 6 and m = 1 reveals two types of complexes having either one or two hydrogen bonds between first and second sphere water molecules. The water exchange mechanism of [Zn(H2O)6]2+ is analyzed on the basis of the structures and energies of these complexes. Within the variations due to the different basis sets employed, the Zn−O bond length for water molecules in the first coordination sphere is between 2.0 and 2.1 Å, water molecules in the second coordination sphere between 3.6 and 4.1 Å and at the frontier of both spheres between 2.7 and 3.0 Å. Within the limitations of the present model, in which bulk water, counterions, and more than one exchanging water molecule have not been considered, a limiting dissociative (D) mechanism for the water exchange reaction on [Zn(H2O)6]2+ is suggested. On the basis of the most reliable structures (i.e., those that could be verified with all levels of theory), the energy of activation for the water exchange is between 4.2 and 4.6 kcal/mol, depending on the basis set employed. A transition state for the interchange mechanism could not be localized. All optimizations invariably led to transition state structures that indicate a limiting D mechanism.