Theoretical exploration of the water exchange mechanism of the polyoxocation GaO 4Al 12(OH) 24(H 2O) 127+ in aqueous solution

Abstract

Reaction pathways, solvent effects and energy barriers have been investigated for the water exchange of the polyoxocation GaO 4Al 12(OH) 24(H 2O) 12 7+ (K–GaAl 12) in aqueous solution by means of supermolecule density functional theory calculations. In the proposed reaction pathway, the supermolecular reactant K–GaAl 12 15H 2O first loses a water ligand to form an intermediate with a five-coordinated aluminum atom, and then the incoming water molecule in the second coordination sphere attacks the intermediate with a five-coordinated aluminum atom to produce the reaction product. Our calculated results indicate that the water exchange of K–GaAl 12 proceeds via a dissociative mechanism, and that the reverse reaction of Step II is the most favorable dissociative pathway, with a barrier height of 31.3 kJ mol −1. The calculated transition-state rate for the favorable dissociative pathway is much larger than the experimental rate constant, but is close to the data calculated for Al 30 by molecular dynamics. The transmission coefficient was also predicted on the basis of both the calculated transition-state rate and the experimental rate. Our calculated results also indicate that both the explicit solvent effect and the bulk solvent effect have obvious effects on the barrier heights of the water exchange reaction of K–GaAl 12. By comparison, the water exchange mechanism for K–GaAl 12 was found to be more similar to that for mineral surfaces than that for monomeric aluminum species.