Density functional theory study of the aluminium(III) hydrolysis in aqueous solution.

Supermolecule density functional calculations were performed on the hydrolysis of aluminum(III) and prediction of pK(a) in aqueous solution. The optimization results showed that the most stable structures for the first, second and third hydrolysis products were hexacoordinate, hexacoordinate and pentacoordinate, respectively. The different coordination geometries could easily transform into each other due to the small energy gaps (within 2.5 kcal mol(-1)). The calculated value of 4.6 for the first hydrolysis constant agreed well with the experimental value of 5.0. The results from the different thermodynamic cycles have been compared, which revealed that the cycle involving the solvation of H(2)O and H(3)O(+) could reasonably predict the first hydrolysis constant, while the other cycle involving the solvation of H(+) acquired a fairly good correlation of the hydrolysis reaction free energies and the experimental pK(a).