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

Abstract

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.

Computational insights into the O2-evolving complex of photosystem II

“…The resulting structural rearrangements provided insight on the water exchange mechanisms and the relative binding strengths, considering that elongation of the metal-oxygen bond is likely the primary step in water exchange and rate-determining (Lundberg et al 2003;Rotzinger 1997;Rotzinger 2005). These calculations complemented earlier studies of water exchange in transition metal complexes (Cady et al 2006;Helm & Merbach 1999;Houston et al 2006;Rotzinger 1997;Rotzinger 2005;Tagore et al 2006;Tagore et al 2007), including theoretical studies of manganese complexes, based on Hartree-Fock and complete active-space self-consistent field theories (Lundberg et al 2003;Rotzinger 1997;Rotzinger 2005;Tsutsui et al 1999) as well as DFT studies of water exchange in other transition metal complexes (Deeth & Elding 1996;Hartmann et al 1999;Hartmann et al 1997;Lundberg et al 2003;Vallet et al 2001). …”

Ligand‐Exchange Processes on Solvated Lithium Cations: DMSO and Water/DMSO Mixtures

“…In the solvent mixture consisting of DMSO and g-butyrolactone, the latter component is weakly coordinating and can be considered as a diluent for DMSO and alkali-metal ions. Computational studies suggest that ligand exchange on [LiA C H T U N G T R E N N U N G (DMSO) 4 ]…”

Ligand Exchange Processes on Solvated Zinc Cations – DFT Analysis of Hydrogen Cyanide Exchange on [Zn(HCN)6]2+

“…Following the suggestions of Hartmann et al, [13] and for comparison with recent studies on solvent exchange at Li + , [17,18,22] Be 2+ , [19,23] and Al 3+ , [20,21] we fully optimized the structures at the B3LYP/6-311+G** level of theory [24] and characterized by computation of vibration frequencies the structures as local minima or transition states.…”

“…The water exchange mechanism of hydrated zinc(II) was studied theoretically by Hartmann et al [12,13] Structures and hydration energies were calculated using hybrid density functional theory (DFT) for [Zn(H 2 O) n ] 2+ with n = 1-6 and for Hartmann's work a dissociative solvent exchange process for Zn 2+ in contrast to the finding of Wasada-Tsutsui et al [15]. This aspect has therefore been studied in more detail in the present study.…”