Influence of Explicit Hydration Waters in Calculating the Hydrolysis Constants for Geochemically Relevant Metals

The effect of including a second, explicit solvation shell of water molecules is examined on the DFT calculation of a selection of aquo-metal ions' pK(a) values (deprotonation constants). Our goal is to gauge the accuracy of how certain cluster approximations and implicit solvation models (PCM or COSMO) affect the results. The ions in this study include: Al-(aq)(3+); Fe-(aq)(3+); Cr-(aq)(3+); Mn-(aq)(3+); Ga-(aq)(3+); Fe-(aq)(2+); Mg-(aq)(2+); and Be-(aq)(2+). Overall, we find that experimental pK(a) constants call be calculated succcssfully to within 1-2 pH units, provided a consistent hydration structure is maintained for products and reactants.