Maureen I. McCarthy

Water chemistry on surface defect sites: Chemidissociation versus physisorption on MgO(001)

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 corner, 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 corner sites we find a strong preference for hydroxylation. The binding of water to a corner is exothermic...

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

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 A 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 increase...

Periodic Hartree–Fock studies of the properties of the Cl2/MgO (001) interface

The properties of the Cl2/MgO (001) interface were investigated using the ab initio periodic Hartree–Fock LCAO method. The calculations treated a system that was periodic in two dimensions with the adsorbate aligned perpendicular to a three‐layer slab of MgO (001). The total energy was calculated as the periodic net of Cl2 molecules approached either an oxygen or magnesium site on the surface. The finite thickness of the slab was set such that the interlayer atoms resembled those found in the bulk. The basis set for the solid has an optimized split valence form with 8–61G on Mg and 8–51G on O. The binding energy is reported as a function of Cl2 surface coverage, with a 6–21G* basis set on Cl, in the range from 1:1 to 1:8. At the most dilute coverage (1:8), the Cl2 molecule is bound over an oxygen with a binding energy of 4.1 kcal/mol. The energetics of Cl2 approaching a magnesium show a purely repulsive interaction at all coverages. The interfacial properties are probed with calculations of the band struc...

A STUDY OF THE ENERGETICS OF THE CL2/MGO(001) INTERFACE USING CORRELATION CORRECTED PERIODIC HARTREE-FOCK THEORY

The energetics of the Cl2/MgO(001) interface were investigated using the ab initio periodic Hartree–Fock (PHF) method and local density functional correlation corrections to PHF theory, as implemented in the program CRYSTAL92. Estimates of the correlation corrected PHF energies are made by post‐SCF evaluations of three gradient corrected functionals. The correlation energy is calculated from the fully converged ground state PHF charge density and added to the PHF total energy. This is the first study of interfacial energetics using the correlation corrected PHF theory. PHF and correlation corrected molecule/surface binding energies are reported for seven orientations of the adsorbate with respect to the surface plane. Three of the configurations align the intramolecular axes along the surface normal and the remaining geometries arrange the molecules heat‐to‐tail, parallel to the surface plane. The most favorable interaction occurs when chlorine approaches a surface oxygen along the normal direction. This ...

Laser ablation of sodium nitrate: NO desorption following excitation of the π−π* band of the nitrate anion

Abstract We have studied the laser desorption of NO from single crystal sodium nitrate following pulsed laser excitation of theπ* ← π2 absorption band localized on the nitrate anion. The excitation laser irradiance is maintained at low levels (