The following paper presents the results of a theoretical study that probed the chemistry of water at structural defects on the MgO (001) surface. The computational technique used was periodic Hartree-Fock (PHF) theory with density functional based correlation corrections. The adsorption energies for water adsorbed on isolated comer, edge, and surface sites on the MgO surface were compared to the hydroxylation energies for the same sites. As stated in a previous paper, the binding of water to the perfect surface is exothermic by 4.1-5.6 kcal/mol whereas hydroxylating the perfect surface was endothermic by 24.5 kcal/mol. At step-edge sites, the process of water adsorption is exothermic and comparable in magnitude to the hydroxylation of these sites. The binding energies associated with water bound to the step-edge are 6.5-10.5 kcal/mol, and hydroxylation of this site is exothermic by 7.3 kcal/mol. At comer sites we find a strong preference for hydroxylation. The binding of water to a comer is exothermic by 20.7 kcal/mol, and hydroxylation is exothermic by 67.3 kcaymol. Mulliken populations indicate that the formation of a hydroxylated surface is governed by the stability of the hydroxyl bond where a hydrogen is bonded to a surface oxygen ion. As the coordination number of this oxygen binding site decreases, its ionic character also decreases, and it forms a more stable bond with the incoming hydrogen. This trend is confirmed by the densities of states for these sites. Finally, hydroxylation of the perfect (001) surface was examined as a function of lattice dilation. It was determined that, as the lattice constant increases, hydroxylation becomes more energetically favorable. This may be important in interpreting experimental thin-film results where the lattice constant of the substrate upon which the MgO film is deposited is slightly larger than that of bulk MgO.

A pairwise additive potential energy expression for the water/MgO interaction was obtained by fitting the parameters to ab initio electronic structure energy data, computed using correlation-corrected periodic HartreeFock (PHF) theory, at selected adsorbate/surface geometries. This potential energy expression was used in molecular dynamics and Monte Carlo simulations to elucidate the water/MgO interaction. Energy minimization reveals a nearly planar adsorbate/surface equilibrium geometry (-15°from the surface plane with the hydrogens pointing toward the surface oxygens) for an isolated water on perfect MgO (001), with a binding energy of 17.5 kcal/mol; subsequent PHF calculations on this system confirmed that this is a potential minimum. Rate constants for desorption (k dsorb ), intersite hopping (k hop ), intrasite rotation (k rot ), and intrasite flipping (k flip ) were estimated for an isolated water on the surface using simple transition state theory. The computed rates (at T ) 300 K) are k dsorb ) 1.1 × 10 5 s -1 , k hop ) 3.7 × 10 10 s -1 , k rot ) 5.7 × 10 11 s -1 , and k flip ) 4.6 × 10 11 s -1 . The motion of a single water on the surface is described by an effective diffusion constant (D eff ) 8.0 × 10 -6 cm 2 /s), computed from the surface rate constants combined with Monte Carlo simulations. The structure of the liquid water/MgO interface was determined from simulations with 64 and 128 water molecules on the surface. Simulations (at T ) 300 K) of the two-dimensional water overlayers reveal a densely packed first layer, Z(O w -surf) ) 2-3 Å, with one water per surface magnesium, with a nearly equal distribution of water molecules aligned -17°and +30°with respect to the surface plane. A more diffuse second layer exists, Z(O w -surf) ) 4-5.5 Å, with a much broader distribution of water angular orientations with respect to the surface plane. The region Z(O w -surf) > 6 Å resembles bulk water, with the density profile approaching a constant as a function of distance above the surface and a uniform distribution in water/surface angular orientations. At the water/vacuum interface (top of the multilayer) the waters assume a "planar orientation" (0°with respect to the surface plane). During the timescale of these simulations very little interlayer exchange of water molecules occurs between the first monolayer (n ) 1) and the additional overlayers (n g 2). In contrast, the water molecules in the multilayers (n g 2) display motion similar to bulk liquid water at this temperature.

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.

We report a neutron diffraction study of the liquid structure of YC13 and combine