CORRELATION CORRECTED PERIODIC HARTREE-FOCK STUDY OF THE INTERACTIONSBETWEEN WATER AND THE (001) MAGNESIUM-OXIDE SURFACE

A theoretical study of water adsorption on the surface of a three-laye r (001) magnesium oxide film has been performed using periodic Hartree -Fock (PHF) theory with density-functional-based correlation correctio ns. The calculations treated two water molecules per MgO unit cell (on e on each side of the film), and for most of the calculations, the siz e of the unit cell was chosen such that the ratio of water molecules t o surface magnesium ions was 1:4. In these configurations the water di poles were aligned parallel and the water-water spacing was 5.95 angst rom between molecules in neighboring cells. Nine geometries were exami ned, three of which were found to be strongly bound to the surface. Th e binding energies for the three bound configurations range from 4.1 t o 8.9 kcal/mol at the PHF level of theory and 6.3 to 12.5 kcal/mol whe n correlation effects were included. For the two cases where the geome try of the bound water molecule was allowed to relax at the equilibriu m water-film distance, the H-O-H angle increased 1-3-degrees from the 6-31G free molecule value of 105.6-degrees and the 0-H bond distance did not change. The six remaining geometries did not show significant binding to the surface. Additional calculations were performed in whic h the dipoles of the water molecules were aligned antiparallel. These calculations indicate that as the coverage increases the water molecul es will tend to form islands on the magnesium oxide surface rather tha n wet the surface. The formation of a fully hydroxylated surface (one hydroxyl group added to every surface magnesium ion and one hydrogen a tom to every surface oxygen ion) was also examined, but was found to b e energetically unfavorable. The energetic bias against dissociative c hemistry on the clean MgO (001) surface, consisting of fully five coor dinated ions, is in agreement with previously published ultraviolet ph otoemission spectroscopy, x-ray photoemission spectroscopy, and IR stu dies.