Laplacian of the electronic charge density and heat of adsorption of O2 and CO molecules on 3d transition metals

A topological analysis of the Laplacian of the electronic charge density of bulk 3d transition metals has been performed. This analysis shows that the atomic graph for the atoms of the fee metals (Ni and Cu) is an octahedron with six vertexes (corresponding to local charge concentrations) linked by twelve edges and bounded by eight faces (corresponding to local charge minima). For the bcc case (V, Cr, and Fe), the corresponding graph is a cube with eight vertexes, twelve edges, and six faces. For the early hcp metals (Sc and Ti), the graph is a trigonal bipyramid with five vertexes, nine edges, and six faces, whereas for Co, it is a trigonal prism with six vertexes, nine edges, and five faces. Despite the different kinds of the graphs obtained, we have been able to correlate the value of the Laplacian at the local minima with the experimental heats of adsorption (which show a systematic decrease) for the O-2 and CO molecules on the 3d transition metals. This result suggests that the bends formed by these molecules with 3d transition metals become stronger for cases in which the local charge minima of the metal atom decreases.