Correlation corrected periodic Hartree–Fock study of the interactions between water and the (001) magnesium oxide surface

Abstract

A theoretical study of water adsorption on the surface of a three-layer (001) magnesium oxide film has been performed using periodic Hartree–Fock (PHF) theory with density-functional-based correlation corrections. The calculations treated two water molecules per MgO unit cell (one on each side of the film), and for most of the calculations, the size of the unit cell was chosen such that the ratio of water molecules to surface magnesium ions was 1:4. In these configurations the water dipoles were aligned parallel and the water–water spacing was 5.95 Å between molecules in neighboring cells. Nine geometries were examined, three of which were found to be strongly bound to the surface. The binding energies for the three bound configurations range from 4.1 to 8.9 kcal/mol at the PHF level of theory and 6.3 to 12.5 kcal/mol when correlation effects were included. For the two cases where the geometry of the bound water molecule was allowed to relax at the equilibrium water–film distance, the H–O–H angle increased 1–3° from the 6-31G* free molecule value of 105.6° and the O–H bond distance did not change. The six remaining geometries did not show significant binding to the surface. Additional calculations were performed in which the dipoles of the water molecules were aligned antiparallel. These calculations indicate that as the coverage increases the water molecules will tend to form islands on the magnesium oxide surface rather than wet the surface. The formation of a fully hydroxylated surface (one hydroxyl group added to every surface magnesium ion and one hydrogen atom to every surface oxygen ion) was also examined, but was found to be energetically unfavorable. The energetic bias against dissociative chemistry on the clean MgO (001) surface, consisting of fully five coordinated ions, is in agreement with previously published ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and IR studies.