Long-range solvent effects on the orbital interaction mechanism of water acidity enhancement in metal ion solutions: a comparative study of the electronic structure of aqueous Mg and Zn dications

We study the dissociation of water coordinated to a divalent metal ion center, M2+ = Mg2+, Zn2+ using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. First, the proton affinity of a coordinated OH- group is computed from gas-phase Mg2+(H2O)5(OH-), which yields a relative higher gas-phase acidity for a Zn2+-coordinated as compared to a Mg2+-coordinated water molecule, DeltapKa(gp) = 5.3. We explain this difference on the basis of a gain in stabilization energy of the Zn2+(H2O)5(OH-) system arising from direct orbital interaction between the coordinated OH- and the empty 4s state of the cation. Next, we compute the acidity of coordinated water molecules in solution using free-energy thermodynamic integration with constrained AIMD. This approach yields pKa Mg2+ = 11.2 and pKa Zn2+ = 8.4, which compare favorably to experimental data. Finally, we examine the factors responsible for the apparent decrease in the relative Zn2+-coordinated water acidity in going from the gas-phase (DeltapKa(gp) = 5.3) to the solvated (DeltapKa = 2.8) regime. We propose two simultaneously occurring solvation-induced processes affecting the relative stability of Zn2+(H2O)5(OH-), namely: (a) reduction of the Zn 4s character in solution states near the bottom of the conduction band; (b) hybridization between OH- orbitals and valence-band states of the solvent. Both effects contribute to hindering the OH- --> Zn2+ charge transfer, either by making it energetically unfavorable or by delocalizing the ligand charge density over several water molecules.