Aqueous microsolvation of bivalent Cu, Zn and Cd. Quantum chemical topology analyses of cooperativity, anticooperativity and covalency

Different anthropogenic activities have lead to an increase in the exposure of transition metal ions such as Cu2+, Zn2+ and Cd2+. In this regard, we have only a limited knowledge about the chemical bonding scenario in the aqueous solvation of these cations. Hence, we employed genetic algorithms, DFT calculations and state-of-the-art methods for the analysis of wave function and DFT electron densities, namely the Quantum Theory of Atoms in Molecules (QTAIM) and the Interacting Quantum Atoms (IQA) energy partition to address the microhydration of Cu2+, Zn2+ and Cd2+. We quantified the charge transfer occurring between the investigated metallic ions and the surrounding water molecules and we related such charge transfer with the stabilisation of these ions within these clusters. The QTAIM and IQA analyses revealed marked cooperative and anticooperative effects in the M2+center dot center dot center dot OH2 interactions which involve the metal cation as well as the first and the second solvation shells. As opposed to other ion -dipole contacts, the QTAIM and IQA approaches indicate that covalency plays a preponderant role in the examined interactions. Finally, our results indicate that the simulations of the hydration of the ions considered in this research should take into account, at least to some extent, non-additivity as well as the covalent contributions of the transition metal-water interactions described herein.