Molecular orbital models of aqueous aluminum-acetate complexes

Molecular orbital calculations with HF/3-21G^∗∗, HF/6-311+G^∗∗, and MP2/6-311+G^∗∗ basis sets (HF = Hartree-Fock approximation; MP2 = 2nd-order Møller-Plesset perturbation theory) have been performed on molecular clusters in the system acetate-aluminum-water. The results model the structures, energetics, and vibrational spectra of Al³⁺ and Al³⁺-acetate complexes in the aqueous phase. An octahedral to tetrahedral coordination change is predicted in the species Al³⁺ (OH)_m⁻ · n (H₂O) where m + n = 6) as m increases from two to three. Calculated reaction energetics for aqueous Al³⁺-acetate complexation compare favorably with experimental enthalpies. In addition, the possible existence of more than one configuration for each Al` -acetate species was investigated. Theoretical vibrational spectra of the Al³⁺-acetate complexes provide predictions for the identification of Al³⁺-acetate species in aqueous solutions.