Van der Waals contribution to the relative stability of aqueous Zn(2+) coordination states

Manoj Ducher (1) , Fabio Pietrucci (1) , Etienne Balan (1) , Guillaume Ferlat (1) , Lorenzo Paulatto (1) , Marc Blanchard (2)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 GET - Géosciences Environnement Toulouse

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 Zn 2+ , 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[H 2 O] 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 generalised gradient approximation (GGA) or a non-local 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 favours tetraaquo-(Zn[H 2 O] 4) 2+ and pentaaquo-zinc (Zn[H 2 O] 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 kJ mol −1 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, the GGA leads to a reasonable description of the geometry of the aqueous complex.

Domaines: Chimie théorique et/ou physique

Manoj Ducher, Fabio Pietrucci, Etienne Balan, Guillaume Ferlat, Lorenzo Paulatto, et al.. Van der Waals contribution to the relative stability of aqueous Zn(2+) coordination states. Journal of Chemical Theory and Computation, 2017, ⟨10.1021/acs.jctc.7b00455⟩. ⟨hal-01542837⟩