No abstract

An interpretation of the binding in the first-row diatomic hydrides AH is presented, based upon the molecular charge distribution and the forces which it exerts on the nuclei. The density distributions are discussed in relation to ``molecular'' size and the partitioning of the total charge between different spatial regions. Density difference maps are employed to compare the molecular charge distribution with that of the separated atomic charge densities and the charge density of the corresponding united atom. The molecular-orbital charge densities are classified as binding, nonbinding, or antibinding with respect to the forces which they exert on the nuclei. The orbital forces provide a quantitative assessment of the relative binding abilities of the orbital charge densities; for a given molecule, or through the complete series of molecules. On the basis of the total charge distributions, the density difference distributions and the forces exerted on the nuclei, the bonding in LiH is classified as ionic, that in BH→HF as covalent. The binding in BeH is found to be transitional between the limiting classifications of ionic and covalent.

An interpretation of the binding in the second-row diatomic hydrides NaH, MgH, AlH, SiH, PH, SH, and HCl is presented based on the molecular charge distributions and the forces exerted on the nuclei. The total density distributions are discussed in relation to “molecular” size and the arbitrary partitioning of the total charge between different spatial regions. Density difference maps are employed to compare the Hartree–Fock molecular charge distribution with the appropriate Hartree–Fock separated-atom charge densities and also the corresponding Hartree–Fock united-atom charge density. In addition, for SiH, PH, SH, and HCl, two-center Hartree–Fock molecular charge distributions are compared with extensive one-center charge distributions. The molecular orbital charge densities are classified as binding, nonbinding, or antibinding on the basis of their partial contributions to the force acting on each nucleus. The orbital forces provide a quantitative assessment of the relative binding abilities of the orbital charge densities for a given molecule, through a complete series of molecules, or between homologous series of molecules. In terms of the total charge distributions, the various density difference maps, and the partial forces exerted on the nuclei, a qualitative and quantitative comparison is made between the bonding in the first- and second-row hydrides. The bonding in NaH is classified as ionic, that in SiH, PH, SH, and HCl as covalent, and for the second row both MgH and AlH appear transitional between the limiting classifications of ionic or covalent. Particular attention is paid to the role of the large and diffuse K L-shell core on A and the Increased role of the proton in determining the details of the molecular charge distribution in the second-row hydrides. The latter two features account for the major differences in AH bonds between the first- and second-row hydride congeners.

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 ...

A continuum of sets of five equivalent d orbitals is derived and discussed. A discussion of five equivalent sd orbitals is also presented.