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, M 2+ = Mg 2+, Zn 2+ 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 M 2+ (H 2 O) 5 (OH - ), which yields a relative higher gas-phase acidity for a Zn 2+ -coordinated as compared to a Mg 2+ -coordinated water molecule, Δp = 5.3. We explain this difference on the basis of a gain in stabilization energy of the Zn 2+ (H 2 O) 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 p K a Mg 2+ = 11.2 and p K a Zn 2+ = 8.4, which compare favorably to experimental data. Finally, we examine the factors responsible for the apparent decrease in the relative Zn 2+ -coordinated water acidity in going from the gas-phase (Δp = 5.3) to the solvated (Δp K a = 2.8) regime. We propose two simultaneously occurring solvation-induced processes affecting the relative stability of Zn 2+ (H 2 O) 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 - → Zn 2+ charge transfer, either by making it energetically unfavorable or by delocalizing the ligand charge density over several water molecules.