Solvent effect on Al(III) hydrolysis constants from density functional theory

Density functional theory was used to compute proton affinity constants (pK _a values) for the four step-wise hydrolysis reactions of the aluminum cation (Al ³⁺ through [Al(OH) ₄ ] ⁻ ). The hydrolysis species were modelled as (Formula presented.) complexes. Solvation effects were evaluated combining two approaches; initially, microsolvation of the Al-species was achieved by varying the number of explicit solvation water molecules surrounding the Al-complexes and reaction partner H ₂ O/H ₃ O ⁺ species. In the second approach, the long-range polar solvent effect (global solvation) was included by using the COSMO continuum solvation model in the DFT calculations. pK _a values for the hydrolysis reactions were calculated using a thermodynamic cycle, and were calculated directly from the global solvation data. The computed energies, optimised coordination structures, bond lengths, charge from natural population analyses, ionic potential, and H-bonding for the Al-hydrolysis species varied depending on the solvation approach. Similarly, calculated pK _a values varied for each solvation approach. Overall cumulative pK _a values calculated directly from the global solvation showed good agreement with experimental data.