The Laplacian of the Electronic Charge Distribution

Abstract

<p>The physical basis of chemical concepts are the focal points of this thesis. The chemical concepts of atoms and bonds are equated to real pieces of a molecule and the topology of the electronic charge density which defines their connectivity, respectively. Bader et al's quantum theory of atoms in molecules and topological theory of molecular structure are the theories that justify the above identifications. These theories are reviewed.</p> <p>The physical basis of the electron pair concept proposed by Lewis is the subject of the original work presented in this thesis. It is reiterated that, in general, the charge distributions of atoms and molecules can not be realistically described as a superposition of the separately localized charge distribution of electronic pairs. The physical basis of the electron pair concept is instead argued to be the topological properties of the Laplacian of the total electronic charge distribution, specifically the local concentrations of charge found in the valence shell charge concentrations of atoms in molecules.</p> <p>The postulates of Gillespie's VSEPR model of molecular geometry describe the spatial and interactive properties of valence shell electron pairs. All of these postulates are shown to be a prescription of the corresponding properties of the aforementioned local charge concentrations. The observed number, types, locations and shapes of valence shell charge concentrations are in accord with the corresponding elements of Lewis' and Gillespie's models. The interactive properties of these charge concentrations are related to the effect of Fermi correlation on the electron pair distribution function. A pair of electrons with opposite spin is found to be partially localized and maximally separated from other electron pairs when one of the electrons is located at a position of maximum charge concentration. The properties of the charge concentrations as well as the Fermi correlation of reference electrons located at their positions, are shown to be in consonance with the primary postulate of the VSEPR model. This postulate states that molecular geometries minimizing interpair repulsions within the valence shell of the central atom are the most stable.</p> <p>The relationships between the Laplacion of the electronic charge distribution and the frontier orbital theory of chemical reactivity, as well as the ligand field theory of transition metal complexes, are briefly discussed.</p>