Ab initio calculation of the deprotonation constants of an atomistically defined nanometer-sized, aluminium hydroxide oligomer

Abstract

Aluminium possesses significant and diverse chemistry. Numerous compounds have been defined, and the elucidation of their chemistry is of significant geochemical interest. In this paper, a brucite-like, eight-aluminium aqueous cluster is modelled with density functional theory to identify its primary site of deprotonation and the associated pK _a constant using both explicit (a full first solvent shell) and implicit solvent. Two methods for calculating the pK _a are compared. We found that a bond density approach is better than a direct energy calculation for ions with large charge and high symmetry. The terminal aluminium atoms have equatorial ligated waters that in solvent have one long O–H bond. This site is more reactive than any of the other protons on the particle. Insights into the experimental crystal structure and Bader's Atoms in Molecules density analysis are presented as routes to reduce the computational time required for the identification of protonation sites.

Keywords:

• atoms in molecules
• surface reactivity
• DFT
• pK _a

Acknowledgements

This research was performed using the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, operated for the Department of Energy by Battelle. Matthew Wander thanks Barry Bickmore for comments and discussion.