Intrinsic acidity of aluminum, chromium (III) and iron (III) mu(3)-hydroxofunctional 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 de termination of the relative acidities of the mu(3)-hydroxo and aquo functio nal groups. Contrary to previous predictions and rationalizations, =Fe3OH a nd =Al3OH groups have nearly the same intrinsic acidity, while =Cr3OH group s are significantly more acidic. The gas-phase acidity of the Fe3OH site is in good agreement with the value predicted by the molecular mechanics mode l previously used to estimate the relative acidities of surface sites on ir on 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 c harging 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. Copyri ght (C) 2000 Elsevier Science Ltd.