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 Al^(3+) and Al^(3+)-acetate complexes in the aqueous phase. An octahedral to tetrahedral coordination change is predicted in the species Al^(3+) (OH)_m−· n (H_2O) [where m + n = 6) as m increases from two to three. Calculated reaction energetics for aqueous Al^(3+)-acetate complexation compare favorably with experimental enthalpies. In addition, the possible existence of more than one configuration for each Al^(3+)-acetate species was investigated. Theoretical vibrational spectra of the Al^(3+)-acetate complexes provide predictions for the identification of Al^(3+)-acetate species in aqueous solutions