Intrinsic acidity of aluminum, chromium (III) and iron (III) μ3-hydroxo functional groups from ab initio electronic structure calculations

Density functional calculations are performed on M-3(OH)(7)(H2O)(6)(2+) and M3O(OH)(6)(H2O)(6)(+) clusters for M=Al, Cr(III), and Fe(III), allowing determination of the relative acidities of the mu(3)-hydroxo and aquo functional groups. Contrary to previous predictions and rationalizations, =Fe3OH and =Al3OH groups have nearly the same intrinsic acidity, while =Cr3OH groups are significantly more acidic. The gas-phase acidity of the Fe3OH site is in good agreement with the value predicted by the molecular mechanics model previously used to estimate the relative acidities of surface sites on iron oxides. [J. R. Rustad et al. (1996) Geochim. Cosmochim. Acta 60, 1563]. Acidities of aquo functional groups were also computed for Al and Cr. The =AlOH2 site is more acidic than the =Al3OH site, whereas the =Cr3OH site is more acidic than the =CrOH2 site. These findings predict that the surface charging behavior of chromium oxides/oxyhydroxides should be distinguishable from their Fe, Al counterparts. The calculations also provide insight into why the lepidocrocite/boehmite polymorph is not observed for CrOOH. Copyright (C) 2000 Elsevier Science Ltd.