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 M2+(H2O)(5)(OH-), which yields a relative higher gas- phase acidity for a Zn2+- coordinated as compared to a Mg2+- coordinated water molecule, Delta pK(a)(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 pK(a) Mg2+ = 11.2 and pK(a) 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 (Delta pK(a)(gp) = 5.3) to the solvated (Delta pK(a) = 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.