Ab initio investigations pertaining to aluminum in tetrahedral versus octahedral sites of clay minerals

Abstract

The structures and atomic charges of several aluminum and silicon oxide systems—[Al(OH) 4] −, Al(OH 3)(H 2O) 3, Al(OH) 2(H 2O) 2(OH) 2Al(OH) 2(H 2O) 2, [Al(OH) 3OSi(OH) 3] −, and Al(OH) 2(H 2O) 3OSi(OH) 3—representing compounds with tetrahedral and octahedral coordination, were investigated by ab initio RHF and MP2 geometry refinements and by density functional calculations. In addition, structures and CHELPG charges were determined for the silicic acid dimer, [Si(OH) 4] 2, at equilibrium and at various displacements from the equilibrium intermolecular separation, in order to determine the effects of molecular association on partial atomic charges. The calculations were performed because the investigated compounds are fragments representative of phyllosilicate soil minerals such as smectite clays, and the results can be used to augment the database available for developing force field parameters for molecular dynamics simulations of adsorption phenomena at the clay mineral/aqueous solution interface. Furthermore, the results make it possible to discuss the effects of electron correlation on structures of this kind.