Polarizations of atomic and molecular charge distributions

Abstract

It is shown that the dominant polarization of a molecular charge distribution in the region of a nucleus of an atom which employs p orbitals in its bonding (Be -> F, Mg -> C1) is quadrupolar in nature, and dipolar for an atom which employs s orbitals (H, He, Li, Na). That these polari~ations are of a fundamental nature is demonstrated by showing that they represent the primary response of a charge distribution to an electric field, whether it be internal o r external, static or dynamic.Canadian Journal of Chemistry, 47, 2308 (1969) The polarization of a molecular charge distribution measured relative to the charge densities of its constituent separated atoms is determined by a density difference distribution ApsA(r). Such a distribution is constructed by subtracting from the molecular distribution, one obtained from the overlap of the undistorted atomic densities, the atomic densities being centered at the same nuclear positions as in the molecule. An extensive investigation of such ApsA(r) distributions (1-4)' for diatomic molecules formed from atoms in the first two rows of the periodic table, has revealed that the component atomic densities exhibit one of two dominant polarizations 011 the formation of a chemical bond. The polarization of atoms which employ principally s orbitals in their bonding, e.g., H, He, Li, and Na, is dbolar in character. The ApsA(r) distributioil in the vicinity of such a nucleus shows that the change in the atomic charge density corresponds to a simple transfer of charge from one side of the nucleus to the other. This is exemplified by the Aps,(r) map in the region of the proton for the OH molecule ( Fig. l(a)). The redistributioil of charge found in the Aps,(r) distributions for Be -> F or Mg + C1 atoms is characterized by an increase in charge density along the bond axis on both the bonded and nonbonded sides of the nucleus, and in its removal from a torus-like region perpendicular to the bond axis at the position of the nucleus. Such a q~radrupolar polarization, i.e., charge increase along one axis and its removal from another perpendicular to it, results in a gross In this note we wish to point out that the dipolar, and in particular, the quadrupolar polarizations, are the dominant ones in the formation of polyatomic systems as well, and offer evidence that such polarizations represent the primary response of a charge distribution to an imposed field.Figure l(b) shows a ApsA(r) map for the water molecule in the plane of the nuclei3. The pattern of the charge rearrangement in the vicinity of the oxygen nucleus is quadrupolar, and dipolar in the region of each proton. In fact, there is a striking similarity between this ApsA(r) map and the map for the OH diatomic molecule. The principal increase in charge density again occurs along a single axis, the two-fold symmetry axis, and charge is again concentrated in both the binding and antibinding regions of the oxygen nucleus. As in the OH molecule, the charge decrease occurs in a belt perpendicular to the axis of charge increase at ...

Comment on the Comparative Use of the Electron Density and Its Laplacian

“…This pattern of charge reorganization is the universal response of an atom to an axial electric field, be it an applied static field or a dynamic one arising from nuclear displacements, vibrational or bond formation. [33] As an example, one notes that the density of a F atom 13 au from an approaching Li atom exhibits a quadrupolar polarization along the axis of approach consistent with the valence state configuration 2ps 1 2pp 4 , [34] the same configuration that yields a build-up of density in the binding region of the bond density map for F 2 . [26] This is the valence state that is defined by the energy of interaction of arising from the approach of the atoms and the consistent use of physics avoids all contradictions in the response of the density to a particular perturbation.…”