Molecular Charge Distributions and Chemical Binding

Abstract

This paper presents an interpretation of the chemical binding found in the first-row homonuclear diatomic molecules. The interpretation is based upon the one-electron density distribution and the forces which it exerts on the nuclei. The general topographical features of the density distributions are discussed in relation to ``molecular size'' and the manner in which the total charge is partitioned between different spatial regions. The binding in these molecules is discussed in terms of the density difference distributions which picture the redistribution of charge which results from the formation of the molecule. It is proposed that the density difference distribution, or Δρ map, may be taken as the pictorial representation of the ``bond density.'' The forces exerted on the nuclei in the molecule are related to the changes in the charge distribution pictured in the ``bond density'' and a quantitative discussion of the manner in which electrostatic equilibrium is attained to give a stable molecule is given in terms of the forces. The concepts of bonding and antibonding are compared with the terms binding and antibinding, terms which are defined in terms of the forces exerted on the nuclei. In particular, both vertical and adiabatic ionization processes are viewed from the standpoint of the change in the electronic force contribution between the molecule and the molecule ion. A definition of ionic and covalent binding based on the density difference distributions is presented. The Δρ map (or bond density) for covalent binding is shown to be characterized by a density increase located between the nuclei and shared equally by each. The Δρ map for ionic binding exhibits an increase in charge density which is localized on a single nucleus. A partitioning of the total electronic force in accordance with the Δρ maps demonstrates that in covalent binding, the nuclei are bound by the density which is shared between them, while in ionic binding the nuclei are bound by the density which is localized on a single nucleus.