Requirements for activation of surface oxygen atoms in MgO using the Laplacian of the electron density

This paper demonstrates that the Laplacian of the electron density, the quantity V-2 rho(r), can be used to determine the conditions required for activation of a surface atom and thus guides one in determining surface adsorption sites. It also predicts the geometry of approach of the substrate relative to the surface site and whether the interaction will correspond to physi- or chemisorption. The topology of the electron density and its Laplacian distribution for bulk MgO, its (100), (110) and (140) surfaces, as well as of embedded clusters that model these and other structural features of the surface, are studied to determine the conditions required for chemisorption of carbon monoxide. The unreactive nature of five-coordinated oxygen in the (100) surface is retained when its coordination is decreased to four in a step or to three in a corner. It is shown that electronic excitation can activate a surface oxygen atom towards reaction with a Lewis base through the creation of a ''hole'' in its valence shell of charge concentration, as defined by the topology of the Laplacian distribution. The results determine the site responsible for the formation of the CO2- radical anion on the surface of thermally activated MgO in the presence of CO gas. It also shown how a neighbouring activated oxygen atom in a stepped surface can lead to the formation of carbonate ion.