Showing papers on "Valence (chemistry) published in 1978"

Electronegativity: The density functional viewpoint

Abstract: Precision is given to the concept of electronegativity. It is the negative of the chemical potential (the Lagrange multiplier for the normalization constraint) in the Hohenberg–Kohn density functional theory of the ground state: χ=−μ=−(∂E/∂N)v. Electronegativity is constant throughout an atom or molecule, and constant from orbital to orbital within an atom or molecule. Definitions are given of the concepts of an atom in a molecule and of a valence state of an atom in a molecule, and it is shown how valence‐state electronegativity differences drive charge transfers on molecule formation. An equation of Gibbs–Duhem type is given for the change of electronegativity from one situation to another, and some discussion is given of certain relations among energy components discovered by Fraga.

The chemical bond

Abstract: Unlike many other books on chemical bonding, this introduction to the subject does not adopt the traditional historical treatment in which the two basic theories of valence, molecular orbital and valence bond, are introduced and applied to increasingly complex molecules. Instead it develops the subject area from fundamental concepts which are important in chemistry as a whole. The validity of these concepts is examined within both the older empirical models and within the more recent ab-initio calculations. In this second edition, the contents have been extended to cover the mathematical basis of ab-initio calculations and the structure of computer programs used to carry these out. The new edition has also allowed the authors to extend the coverage of group theory techniques and update aspects of transition metal chemistry.

The bonding capabilities of transition metal clusters

Abstract: A new classification scheme for transition metal carbonyl cluster compounds is proposed. Class I clusters are defined as those which use all available Cluster Valence Molecular Orbitals, CVMOs, Class II cluster compounds have s and p CVMOs vacant and Class III clusters have vacant d CVMOs. In general the earlier transition elements to the left of the transition metal series will have more of a tendency to form Class III cluster compounds, while the later transition elements to the right of the transition series will tend to form Class II compounds. The more common Class I cluster compounds will be favored by the central Group VIII metals such as Rh. Only the Class I clusters will follow the Polyhedral Skeletal Electron Pair analogy between main group and transition metal clusters and will exhibit the “magic numbers” of Cluster Valence Electrons, CVEs.

Correlation effects in the ionization of hydrocarbons

Abstract: The spectral intensity for ionization as a function of binding energy for the valence electrons of ethylene, allene, butatriene, trans‐butadiene, acetylene, benzene, methane, ethane, and cyclopropane is computed by a many‐body Green’s function method. The results are used to interpret unidentified structures in experimental ionization spectra. For the ionization out of the inner valence orbitals of the unsaturated molecules the spectral intensity is found to be distributed over several lines, in sharp contrast to the ionization out of the inner valence orbitals of the saturated molecules where the greater part of the intensity appears in one main line. The reasons for this behavior are discussed. It is also found that there is a correspondence between the behavior of the spectral intensity in the inner valence region and the satellite structure in the outer valence region. For C6H6, C4H4, and C4H6 interesting satellite lines of considerable intensity are predicted to be situated in the outer valence regio...

Mixed Valence Interactions in Di-μ-oxo Bridged Manganese Complexes. Electron Paramagnetic Resonance and Magnetic Susceptibility Studies

Abstract: EPR examination of the class II (deeply trapped) mixed valence complexes ((bpy)/sub 2/MnO/sub 2/Mn(bpy)/sub 2/)/sup 3 +/ (the bipyridyl(III,IV) dimer) and its phenanthroline analogue in acetonitrile solution verifies that these complexes possess inequivalent Mn ions at room temperature. Isotropic hyperfine structure for two Mn ions is resolved with A/sub 1/ = 167 +- 3G and A/sub 2/ = 79 +- 3G for both complexes. The hyperfine pattern with parallel A/sub 1/ parallel approx. = 2 parallel A/sub 2/ parallel and the small g anisotropy are consistent with high-spin Mn(III) antiferromagnetically coupled to Mn(IV), producing an S = 1/2 ground state. At room temperature a rate of less than 10/sup 8/ s/sup -1/ is estimated for the thermally activated intramolecular electron transfer, consistent with an upper limit of 10/sup 6/ s/sup -1/ calculated from Hush's theory. The magnetic susceptibility of the (III,IV) complexes is characteristic of a strongly antiferromagnetically coupled S = (2, 3/2) pair. The temperature dependence of the data was in good agreement with the isotropic Heisenberg exchange Hamiltonian H = -2JS/sub 1/S/sub 2/, yielding J= -150 +- 7 cm/sup -1/ for the bipyridyl(III,IV) dimer and J = -134 +- 5 cm/sup -1/ for the phenanthroline analogue. 3 figures,more » 2 tables.« less

Divalent surface state on metallic samarium

Abstract: It is shown that the first atomic layer of trivalent metallic samarium has a large divalent component. The valence transition is attributed to a narrowing of the 5d band which populates the low-lying 4f/sup 6/ state.

Ab initio studies of AuH, AuCl, HgH and HgCl2 using relativistic effective core potentials

Abstract: Relativistic effective core potentials (ECP) are derived for Au and Hg atoms, where the ECP incorporates the Coulomb and exchange contributions of the core orbitals, the core‐orthogonality terms for the valence orthogonality terms for the valence orbitals, and the effect of the ’’mass–velocity’’ and ’’Darwin’’ relativistic effects on the valence orbitals. The results of atomic valence‐electron (VE) calculations with the ECP’s compare favorably with relativistic Hartree–Fock and Dirac–Hartree–Fock calculations and with experiment, when the effects of spin–orbit coupling are included in the VE calculations. Nonrelativistic calculations, by contrast, lead to erroneous predictions and to differences in excitation energies of 1.5–3.5 eV. The large relativistic effects in the atoms carry over into the AuH, AuCl, and HgCl2 molecules, as they are important in determining correct bond lengths and bond energies and in influencing the charge distributions. Similarly large relativistic effects are encountered in ioni...

Interpretation of the Mössbauer spectra of the four-iron ferredoxin from Bacillus stearothermophilus.

Abstract: The Mossbauer spectra of both oxidized and reduced ferredoxin from Bacillus stearothermophilus have been analysed using computer fits to theoretical spectra obtained from a spin Hamiltonian. A consistent set of parameters was obtained from fits to spectra obtained over a wide range of temperature and magnetic field. These results are interpreted in terms of a model for the active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the oxidized centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with one pair spin-up and the other pair spin-down. On reduction the extra electron goes predominantly to one pair of iron atoms which become ferrous with the other pair remaining substantially unchanged. Using this model it is possible to obtain relationships between the spin Hamiltonian parameters for individual iron atoms and those for the coupled centre. This can give further insight into the relation between the observed electron paramagnetic resonance and Mossbauer spectra.

Theory of electron-hole liquid in semiconductors

Abstract: The random-phase approximation is generalized to include the effects of band anisotropy, coupling between degenerate valence bands, coupling to optical phonons, and the Hubbard exchange correction. This method is used to calculate the ground-state energy and equilibrium density of the electron-hole liquid in Ge, Si, AgBr, and various II-VI and III-V compounds and the thallous halides and lead chalcogenides. The results are compared to the experiments where they are available. Agreement is excellent for Ge and Si and reasonable for the other materials, considering the large uncertainties in the values of the band masses, etc. Substantial discrepancies remain between theory and experiment, however, in the cases of GaAs and ZnO.

Relativistic effects in ab initio effective core potentials for molecular calculations. Applications to the uranium atom

Abstract: The procedure of deriving ab initio effective core potentials (ECP) to incorporate the Coulomb and exchange effects as well as orthogonality constraints from the inner core electrons is extended to include the dominant relativistic effects on the valence orbitals. An ab initio approach is then described which enables the valence electrons in heavy atoms to be treated in a standard nonrelativistic manner by including the effect of the relativistic core–valence interactions directly into the ECP. The starting point for this procedure is the Pauli Hartree–Fock relativistic treatment of Cowan and Griffin. The pseudo‐orbital transformation and derivation of the l‐dependent effective core potentials are analogous to the nonrelativistic case with certain modifications. Analytic forms for the pseudo‐orbitals and ECP’s are derived for the U atom, and results of valence electron calculations are presented.

Electron energy loss spectroscopy and the optical properties of polymethylmethacrylate from 1 to 300 eV

Abstract: Energy loss spectra of 80 keV electrons transmitted through thin films polymethylmethacrylate (PMMA) were measured with a resolution of 0.1 eV for energy losses from 1 to 300 eV. From the loss spectra, the dielectric response function of PMMA was obtained from 1 to 100 eV and compared with recent synchrotron radiation results. The spectrum of valence excitations from 5 to 13 eV is shown to be characteristic of the pendant group and is compared to experimental gas phase spectra and molecular orbital (CNDO/S) calculations of model molecules. The spectrum of core electron excitations above 285 eV provides a measure of the distribution of empty molecular orbitals and, when the carbon 1s binding energies are taken into account, a qualitatively accurate description of the observed spectrum is made from the CNDO ground state calculation. The energy loss spectra of 20, 40, and 100 eV electrons reflected from the surface of PMMA exhibit a triplet excitation at 4.2 eV and indicate the extreme sensitivity of this ma...

Intervalence Transfer in Mixed-Valence Biferrocene Ions

Abstract: Die Titelkationen (I) bis (III) wurden unter Verwendung des Ru-Komplexes (IV) als Oxidationsmittel ausgehend von Diferrocen bzw. Diferrocenylacetylen in einer Reihe von Losungsmitteln (Aceton, MeCN, CH2Cl2; PhNOz MeNO2, Propylencarbonat) erzeugt.

The direct and indirect effects in the relativistic modification of atomic valence orbitals

Abstract: The relative importance of the direct and indirect relativistic effects on the behaviour of atomic valence electrons in the Dirac-Fock scheme is investigated for the ground configurations of gold, thallium and lutetium which give information on s, p, p, d and d orbitals. The direct effect, which is always found to contract the relativistic orbital, is completely responsible for the relativistic contraction of the 6s orbital in gold, and the effect is found to become progressively smaller in the order s>p>d approximately d; for none of the systems studied is the direct relativistic effect negligible. The indirect effect is always found to expand the orbital and increases according to the order s

X-ray photoemission study of Ce-pnictides

Abstract: All the Ce pnictides crys allize in the NaCl structure with lattice constant increasing systematically from CeN to CeBi. Since Ce has the largest $4f$-orbital radius in the entire lanthanide series, it is quite interesting to follow the evolution of his level as the distance between the Ce atoms is varied. The core levels and the valence-band region of CeN, CeP, CeAs, and CeSb have been studied by high-resolution x-ray photoemission spectroscopy. The $4d$ and $3d$ core-level spectra of Ce show clearly that the $4f$ state remains strictly localized in all compounds. In CeN a superposition of lines corresponding to two valences is found which reveals the intermediate valence character of this compound. In the valence-band spectrum, the $4f$ level is found to be pinned at the Fermi level and superimposed on the $5d$ states. The extended valence states originating from the anion $p$ states are located at higher binding energies. In the compounds of heavier rare-earth elements the $4f$ state moves away from the Fermi level and gradually overlaps in energy with $p$ states, so that the probability for interatomic Auger processes in the photoemission final state increases rapidly. The natural width of the $4f$ peak is already 0.8 eV in CeP, and becomes so large in CeAs and CeSb that this line can no longer be unambiguously identified in the other valence-band structure. Finally, it can be concluded from the similarity of the $4d$ spectrum of metallic Ce and CeN that the $4f$ electron remains localized in the $\ensuremath{\alpha}$ phase of this metal.

Phonon dispersion in intermediate-valence Sm 0.75 Y 0.25 S

Abstract: Phonon dispersion has been measured in an intermediate-valence-state system. The phonons of intermediate valence Sm/sub 0.75/Y/sub 0.25/S are vastly different from those of the SmS and YS constituents. The LA phonons are unusually soft compared to the TA and have anomalous widths, particularly for the (111) direction. The LO phonons lie below the TO. Temperature-dependent measurements show that the anomalous phonon properties change with changing valence.

The effect of charged additives on the carrier concentrations in lone-pair semiconductors

Abstract: The Street—Mott model of gap states, interpreted in terms of valence alternation defects, is used to calculate the temperature dependence of the carrier concentrations and of the conductivity as a function of the concentration of charged additives in chalcogenide semiconductors. When the charged additives are allowed to equilibrate with the valence alternation defects at elevated temperatures we find that the conductivity activation energy remains essentially unchanged although the conductivity may be increased by a few orders of magnitude for large concentrations of additives. Very much larger increases in conductivity result when the charged additives are prevented from equilibrating with the valence alternation defects. At high additive concentrations the chalcogenide material is expected to behave like a partially compensated and nearly degenerate semiconductor in agreement with observations of Ovshinsky (1977). The gradual unpinning of the Fermi level with increasing additive concentration i...

Molecular geometries of acetylene and ethylene chemisorbed on Cu, Ni, Pd, and Pt surfaces

Abstract: Ultraviolet photoemission measurements of the valence orbital electronic structure of acetylene and ethylene chemisorbed on Cu(100) or Cu(111), Ni(111), Pd(111), and Pt(111) are presented. We compare the measured energy levels of these chemisorbed species to those of the free molecule and use a similar comparison of the relative changes in ground state energy levels of distorted free molecules calculated with a SCF-LCAO (Self Consistent Field--Linear Combination of Atomic Orbitals) method to determine the molecular geometries of these chemisorbed species. The limitations and accuracies of such an approach are discussed. From the determined geometries we identify two trends in the structure of these chemisorbed molecules on these surfaces: first, increasingly greater molecular distortions occur with increasing atomic number of the substrate atom, and secondly, greater molecular distortions occur for ethylene than for acetylene on the same metal. These trends are consistent with a π-d bonding interaction and can be accounted for by the electronic structure of the substrate and of the molecule, respectively. With the exception of ethylene on Pd or Pt, we determine molecular geometries characteristic of small rehybridization. The molecular geometry of ethylene on Pd or Pt is generally characteristic of rehybridization to an sp3 configuration.

Local order as determined by electronic and vibrational spectroscopy: Amorphous semiconductors

Abstract: Raman and infrared spectroscopy yield detailed information about the vibrational modes of solids. Photoelectron spectroscopy gives corresponding information on valence and core electronic states. From these spectroscopies the nature of the local atomic order can be inferred. Applications of these methods are illustrated for prototype tetrahedral and chalcogenide amorphous semiconductors. The retention of the crystalline short range order is manifested in the vibrational spectrum of pure amorphous silicon and the III–V compounds. Similar conclusions are obtained from the photoelectron spectra along with additional information on ring configurations. For Se and chalcogenide compounds (e.g. GeTe) vibronic and electronic spectral reveal differences between amorphous and crystalline short range orders. In addition to these intrinsic structures, the local environments and effects of additives, such as hydrogen and conventional dopants in amorphous silicon, are discussed.