Theoretical investigation of the nature of aluminum-containing species present in alkaline solution

The structure, energies, and infrared spectra of a range of possible monome ric and dimeric aluminum-containing neutral and anionic species that may be present in either dilute or concentrated alkaline sodium aluminate solutio ns have been studied using ab initio methods at the MP2/6-31G**, B3LYP/6-31 G**, and B3LYP/6-311++G**//B3LYP/6-311G** levels. Results are presented for the species in vacuo and with a representation of the surrounding environm ent using either a self-consistent polarizable continuum model or a charge- compensating electrostatic potential of varying magnitude to represent high levels of ion concentration. Both predicted energetics and infrared spectr a confirm that the dominant species present under dilute aqueous conditions is the Al(OH)(4)(H2O)(4)(-) anion. At high concentrations of sodium alumin ate, it is predicted that dimerization to the doubly hydroxyl-bridged speci es (OH)(3)Al(OH)(2)Al(OH)(3)(2-) can occur and that this is assisted by the coordination of two or more water molecules.