A correlation for establishing solvolysis rates of aqueous Al(III) complexes: a possible strategy for colloids and...

We here examine whether rates of solvolysis for a range of aluminum complexes can be predicted semiempirically by correlating calculated values of Al-O bond lengths with rate coefficients. We focus on a series of mono- and bis-ligated aqueous aluminum monomers and three epsilon-Keggin-like aluminum polyoxocations, and we make no attempt to simulate transition states. The Al-O bond lengths were calculated by performing ab initio geometry optimizations using the polarizable-continuum model to estimate solvation effects. Both Hartree-Fock and density functional methods (B3LYP) were tested using several basis sets up to 6-31+G(d). We find a strong correlation between rate coefficients for mono-ligated aluminum monomers and bond lengths to hydration waters. We cannot extrapolate the correlation, however, to large epsilon-Keggin-like multimers or to bis-ligated complexes, which suggests that the activated equilibrium for exchange of a water molecule in these molecules is different than that in the simple monomers.