Extending the VSEPR model through the properties of the Laplacian of the charge density

Abstract

The Laplacian of the electronic charge density demonstrates the presence of local concentrations of charge in the valence shell of an atom in a molecule. These local maxima faithfully duplicate in number, location, and size the spatially localized electron pairs of the VSEPR model. Thus the Laplacian of the charge density provides a physical basis for the Lewis and VSEPR models. The VSEPR model can fail to predict the observed geometry when a transition metal is the central atom in the system. In such a case there is no basis within the model itself to predict the existence of the nonbonded pair or pairs of electrons whose presence would account for the observed geometry. By appealing to the Laplacian of the charge density in one such system, namely VOCl3, one observes that the penultimate shell of charge concentration of this transition metal atom exhibits nonbonded concentrations of charge with the locations and properties required to account for the observed geometry of the molecule. Thus the properties of the Laplacian distribution, which are model independent, can be used to extend the applicability of the VSEPR model to those cases where the usual arguments fail to account for the number and type of charge concentrations that exist within the outer shell of an atom. Keywords: Laplacian of the charge density, VSEPR model, transition metal.