van der Waals Contribution to the Relative Stability of Aqueous Zn(2+) Coordination States

Many properties of aqueous cations depend on their coordination state. However, the lack of long-range order and the dynamic character of aqueous solutions make it difficult to obtain information beyond average coordination parameters. A thorough understanding of the molecular-scale environment of aqueous cations usually requires a combination of experimental and theoretical approaches. In the case of Zn²⁺, significant discrepancies occur among theoretical investigations based on first-principles molecular dynamics (FPMD) or free-energy calculations, although experimental data consistently point to a dominant hexaaquo-zinc complex (Zn[H₂O]₆)²⁺ in pure water. In the present study, the aqueous speciation of zinc is theoretically investigated by combining FPMD simulations and free-energy calculations based on metadynamics and umbrella-sampling strategies. The simulations are carried out within the density functional theory (DFT) framework using for the exchange-correlation functional either a standard generalized gradient approximation (GGA) or a nonlocal functional (vdw-DF2) which includes van der Waals interactions. The theoretical environment of Zn is confronted to experiment by comparing calculated and measured X-ray absorption spectra. It is shown that the inclusion of van der Waals interactions is crucial for the correct modeling of zinc aqueous speciation, whereas GGA incorrectly favors tetraaquo- (Zn[H₂O]₄)²⁺ and pentaaquo-zinc (Zn[H₂O]₅)²⁺ complexes, results obtained with the vdW-DF2 functional show that the hexaaquo-zinc complex is more stable than the tetraaquo and pentaaquo-zinc complexes by 13 and by 4 kJ mol^-1, respectively. These results highlight the critical importance of even subtle interactions for the correct balance of different coordination states in aqueous solutions. However, for a given coordination state, GGA leads to a reasonable description of the geometry of the aqueous complex.