Molecular Charge Distributions and Chemical Binding. IV. The Second-Row Diatomic Hydrides AH

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.