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Showing papers on "Valence (chemistry) published in 1976"


Journal ArticleDOI
TL;DR: In this paper, an empirical nonlocal pseudopotential scheme was employed to calculate the electronic structure of eleven semiconductors: Si, Ge, $\ensuremath{\alpha}\ensure Math{-}\mathrm{Sn}$, GaP, GaAs, GaSb, InP, InAs, InSb and CdTe.
Abstract: An empirical nonlocal pseudopotential scheme is employed to calculate the electronic structure of eleven semiconductors: Si, Ge, $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$, GaP, GaAs, GaSb, InP, InAs, InSb, ZnSe, and CdTe. Band structures, reflectivity spectra, electronic densities of states, and valence charge densities are presented and compared to experimental results. Improved optical gaps, optical critical-point topologies, valence-band widths, and valence charge distributions are obtained as compared to previous local pseudopotential results.

1,446 citations


Journal ArticleDOI
TL;DR: In this article, a model for the structure and properties of active centers in lone-pair semiconductors, based on the possibility of unique bonding configurations which can arise from the presence of nonbonding orbitals, is presented.
Abstract: A model is presented for the structure and properties of active centers in lone-pair semiconductors, based on the possibility of unique bonding configurations which can arise from the presence of nonbonding orbitals. It is shown that the lowest-energy neutral center is unstable towards the creation of different positively and negatively charged centers, thus resulting in a negative effective correlation energy. These centers yield gap states which explain the unusual properties of lone-pair semiconductors.

1,088 citations


Journal ArticleDOI
TL;DR: In this article, three basis sets (minimal s −p, extended s−p, and minimal s -p with d functions on second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second-row atoms, and d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two-heavy-atom bond lengths even for molecules of normal valence.
Abstract: Three basis sets (minimal s–p, extended s–p, and minimal s–p with d functions on second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second row atoms. d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two‐heavy‐atom bond lengths even for molecules of normal valence.

575 citations


Journal ArticleDOI
TL;DR: In this article, a bond-valence analysis of the repulsion leads to the conclusion that strong and weak hydrogen bonds are different in kind, the stronger ones involve strain and are linear while the weaker ones (O-O greater than 2.7 A) have an extra degree of freedom and are generally bent.
Abstract: The asymmetry of hydrogen bonds arises from the repulsion between the O atoms forming the bonds. A bond-valence analysis of the repulsion leads to the conclusion that strong and weak hydrogen bonds are different in kind, the stronger ones (O-O less than 2.7 A) involve strain and are linear while the weaker ones (O-O greater than 2.7 A) have an extra degree of freedom and are generally bent. The strength of the hydrogen bond is determined by a number of factors such as the requirement that the bond valences around each atom add up to the atomic valence, a tendency for the O-O distance to be close to 2.7 A, and by crystal-packing considerations which often lead to the formation of bent, and hence weaker, hydrogen bonds. The bond-valence analysis correctly predicts the observed correlations between H⋯O distance and O-H-O angle. The frequency with which various hydrogen-bond configurations are observed in crystals is used to propose a method for determining hydrogen-bond energies. This analysis of hydrogen bonding leads to an understanding of the lengthening of hydrogen bonds in high-pressure ices and to proposals for hydrated ion structures which can be used, for example, to predict the acid strengths of anions and to show that in neutral aqueous solutions the oxygen atoms of complex anions each hydrogen-bond to two or three water molecules.

462 citations


Journal ArticleDOI
TL;DR: In this article, a formalism is developed for obtaining ab initio effective core potentials from numerical Hartree-Fock wavefunctions and such potentials are presented for C, N, O, F, Cl, Fe, Br, and I.
Abstract: A formalism is developed for obtaining ab initio effective core potentials from numerical Hartree–Fock wavefunctions and such potentials are presented for C, N, O, F, Cl, Fe, Br, and I. The effective core potentials enable one to eliminate the core electrons and the associated orthogonality constraints from electronic structure calculations on atoms and molecules. The effective core potentials are angular momentum dependent, basis set independent, and stable against variational collapse of their eigenfunctions to core functions. They are derived from neutral atom wavefunctions using a pseudo‐orbital transformation which is motivated by considerations of the expected accuracy of their use and of basis set economy in molecular calculations. Then the accuracy is demonstrated by multiconfiguration Hartree–Fock calculations of potential energy curves for HF, HCl, HBr, HI, F2, Cl2, Br2, and I2 and one‐electron properties for HF and HBr. The differences between valence‐electron calculations employing the present...

421 citations


Journal ArticleDOI
TL;DR: In this article, the metal 2p region spectra of mixed valence spinels, Co3O4, Fe3O 4, Mn 3O4 and related compounds were studied, and the satellite splittings of Co 2p 3 2 for the octahedrally coordinated cobaltous ions are 6.2 eV and those for the tetrahedrically coordinated ones are about 5.3 eV.

333 citations


Journal ArticleDOI
TL;DR: The structure of BaBiO3 at 25°C has been solved and refined with the use of neutron diffraction powder data as mentioned in this paper, and the valence situation is found to be BaBi3+ 1 2 Bi5+1 2 O3 rather than BaBi4+O3.

324 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a valence density multipole model for pseudoatoms restricted to single exponential radial functions, which can be used for determination of static charge physical properties and aspects of chemical bonding.
Abstract: The one-electron density function for a group of atoms within the asymmetric region of a unit cell is represented by a finite multipole expansion of the charge density about each atomic center. Each atomic expansion is called a pseudoatom. If the pseudoatom charge density is effectively rigid with nuclear motion, then the model may be used for a static charge density analysis of X-ray diffraction data. A valence density multipole model for pseudoatoms is restricted to single exponential radial functions. The representation is rotationally invariant. The model may be used for determination of static charge physical properties as well as aspects of chemical bonding. These results can be a critical test of the X-ray diffraction experiment for the determination of electron density distributions. The pseudoatoms discussed are primarily intended for crystals comprised of first and second-row atoms. The valence scattering model demands extensive data sets (probably at low temperatures) or an independent determination of atomic positions and mean square amplitudes of vibration.

255 citations


Journal ArticleDOI
TL;DR: In this paper, a two-parameter tight-binding theory of the electronic structure of 4:2-coordinated materials is proposed, and the parameters, a covalent and a polar energy, are fitted to the optical absorption spectra.
Abstract: A two-parameter tight-binding theory of the electronic structure of 4:2-coordinated materials is proposed. The parameters, a covalent and a polar energy, are fitted to the optical absorption spectra. The valence energy bands and density of states are calculated. In terms of these a consistent interpretation of all the observed photoemission and x-ray-emission spectra of ${\mathrm{SiO}}_{2}$ is obtained. The x-ray-absorption spectra are also analyzed. A bond-orbital approximation allows a simple calculation of the refractive index (or dielectric constant) of the various allotropic forms of silica and germania. Finally, the variation in total energy and charge distribution with local distortion is analyzed in order to study structural stability, elastic rigidity, and the effective charges (including dynamic contributions) which determine the piezoelectric constants and infrared absorption intensities.

244 citations


Journal ArticleDOI
TL;DR: In this article, a tight-binding framework using the bond orbital approximation was proposed to simplify the total energy calculation. But the reconstruction of semiconductor surfaces was not considered, and it was shown that the 7 × 7 reconstruction is a pattern of add-atoms rather than (and topologically unrelated to) a patterns of vacancies.

201 citations



Journal ArticleDOI
TL;DR: In this article, relativistic and non-relativistic Hartree-Fock one-centre expansion calculations including valence s and p orbitals are reported for CuH, AgH and AuH molecules.

Journal ArticleDOI
TL;DR: In this paper, the semi-empirical tight-binding method was used to construct accurate valence bands for bulk Si and Ge using Hamiltonian parameters through second neighbors, and the density of surface bands and resonances associated with atomic layers near the surface was calculated.
Abstract: The semiempirical tight-binding method is used to construct accurate valence bands for bulk Si and Ge using Hamiltonian parameters through second neighbors. One additional parameter is used to describe surface relaxation (back-bond contraction). The density of surface bands and resonances associated with atomic layers near the surface is calculated. Surprisingly, the resonances are found to contribute to the surface density of states almost as effectively as proper surface states. Comparison with ion-neutralization, electron-energy loss, and ultraviolet photoemission data is made.

Journal ArticleDOI
TL;DR: In this paper, the polarization configuration interaction (POL-CI) wavefunction is described and a generalized valence bond (GVB) calculation is performed to insure proper dissociation.
Abstract: The polarization configuration interaction (POL–CI) wavefunction is described. Calculations are reported for the X 3Σ−, a 1Δ, b 1Σ+, A 3Π, and c 1Π states of the NH molecules in a double zeta plus polarization Gaussian basis. Calculated excitation energies and spectroscopic constants are compared with results from more extensive calculations in this basis and with previously reported calculations. The starting point for POL–CI calculations is a generalized valence bond (GVB) calculation to insure proper dissociation. Results of using both ground state and excited state GVB orbitals show that a single set of (ground state) orbitals may be used for all states without loss of accuracy. Dipole moments and electronic transition moments as a function of R are also given.

Journal ArticleDOI
W. Ey1
TL;DR: In this article, an ansatz for states of an open shell A -body Fermi system is derived using as far as possible data of the ( A -1)-body system.

Journal ArticleDOI
15 Aug 1976
TL;DR: In this article, the Hartree-Fock-Slater Discrete Variational Method was used to investigate the electronic structures of all the comlexes of MnO−4, MnO2−4 and MnO3−4.
Abstract: The electronic structures of MnO−4, MnO2−4, MnO3−4, CrO2−4, CrO3−4, VO3−4, RuO4, RuO−4, RuO2−4, TcO−4 and MoO2−4 have been investigated using the Hartree-Fock-Slater Discrete Variational Method. The calculated ordering of the valence orbitals of all the comlexes is: t1, 4t2, 3a1, 1c, 3t2, with t1 the orbital of highest energy. The calculated single transition energies are in good agreement with experimental values and indicate the uniform assignment: t1 → 2e(v1), 4t2 → 2e(v2). t1 → 5t2(v3), and 4t2 → 5t2(v4). A/D values, calculated from the theory of magnetic circular dichroism (MDC) also support this assignment. Population analyses reveal that all complexes, whether d0, d1 or d2, have d-orbital populations close to those of the corresponding M2+ ions in which two electrons have been removed from the (n + 1)s orbital of M. This is also true of the excited states, such as t1 → 2e and 4t2 → 2e, where a transfer of charge from the ligands to the metal has previously been assumed. It is shown that, instead of a transfer of charge from ligands to metal, electronic excitation consists of a rearrangement of electron density both at the ligands and at the metal.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the sensitivity of the transverse-acoustic modes to covalent bonding is related to the magnitude of the effect from zero to the melting temperature.
Abstract: The direct and indirect gaps between valence and conduction bands in semiconductors usually decrease with temperature. This effect is related by thermodynamic identities to the influence of electron-hole pairs on the lattice vibration frequencies. We show that the surprisingly large magnitude of the effect in Si and similar semiconductors is related to the sensitivity of the transverse-acoustic modes to covalent bonding. We are able to account for the magnitude of the effect from zero to the melting temperature. We also account for anomalous temperature variation in HgTe and related cases and mention other applications of the theory.

Journal ArticleDOI
TL;DR: In this paper, the electronic structure of Hg has been studied by means of high resolution ESCA, using monochromatized AlKα excitation, and the atomic spectrum from the gas phase is compared with the solid state spectrum from frozen Hg.

Journal ArticleDOI
J. E. Demuth1, D. E. Eastman1
TL;DR: In this paper, the authors compared the photoemission valence orbital spectra of adsorbed acetylene, ethylene, propylene, and benzene on Ni(lll) with gas-phase spectra as well as Hartree-Fock ground-state orbital energies calculated using a self-consistent field linear-combination-of-atomic-orbitals method for a range of possible hybridizations of the carbon atoms.
Abstract: The uv photoemission valence orbital spectra of adsorbed acetylene, ethylene, propylene, and benzene on Ni(lll) are compared with gas-phase spectra as well as Hartree-Fock ground-state orbital energies calculated using a self-consistent-field linear-combination-of-atomic-orbitals method for a range of possible hybridizations of the carbon atoms. We conclude that the chemisorption bonding of these unsturated hydrocarbons involves predominantly $\ensuremath{\pi}\ensuremath{-}d$ bonding without any significant rehybridization relative to their gas-phase counterparts.

Journal ArticleDOI
TL;DR: In this paper, the x-ray photo-electron spectra of transition-metal carbides with the rock-salt structure were analyzed and the band structures and densities of states (DOSs) of ZrC, HfC, and TaC were calculated by the augmented plane-wave method.
Abstract: The band structures and densities of states (DOSs) of ZrC, HfC, and TaC were calculated by the augmented-plane-wave method, and the x-ray photoelectron spectra of valence bands of these compounds were observed. The theoretical energy distribution curves (EDCs) were in good agreement with the experimental EDCs. These band structures resemble each other and also those of TiC obtained by our previous work. This fact suggests that the rigid-band model is applicable to the transition-metal carbides with the rock-salt structure. Their DOSs are divided into three parts. Peak I derived from the $\mathrm{C} 2s$ state is isolated from the higher valence-band peak II arising from the $\mathrm{C} 2p$ and the valence electrons of the metal atom. Peak III derived from the $d$ and $s$ states of the metal atom is separated by the Fermi level from peak II. The Fermi level lies at the minimum point of the DOS for the group IV carbides, but for TaC it lies at a relatively large DOS point. The DOS at the Fermi level of ZrC, HfC, and TaC are 0.18, 0.16, and 0.65 electrons/(eV primitive cell), respectively. The characteristic mutual differences among these compounds are a stronger localization of $d$ electrons in ZrC and HfC compared with TiC and an enhancement of the photoelectron spectrum intensity of TaC around the Fermi level.

Journal ArticleDOI
TL;DR: In this article, it was shown that the radius of the point at which the radial density function R (r) = 4πr2ρ(r) defines a physically meaningful boundary surface separating core and valence regions of first-row atoms.
Abstract: It is proposed that the minimum in the radial density function R (r) = 4πr2ρ (r) defines a physically meaningful boundary surface separating core and valence regions of first‐row atoms. The point rm at which this minimum occurs has been found to fall within the interval in which the linear lnρ (r) vs r plots of these atoms undergo a significant change in slope. When the valence region is defined in the proposed manner, a valid estimate of its electronic energy can be obtained with the formula E=−12π/7 (Z−−i−i) F∞rmρ (r) r dr, in which Ni is the number of electrons in the core region of an atom with nuclear charge Z. This equation is a modified form of an expression previously found to be accurate for atoms. For Be, C, O, F, and Ne, Ni ranges from 2.05 to 2.20 electrons. The core‐valence separation in larger atoms may be treated in a similar manner; rm is then the outermost minimum in the radial density function.

Journal ArticleDOI
TL;DR: In this article, the valence states of CO2, CO2+, CO2+ and CO2− were compared with polarized and unpolarized basis sets at each level.
Abstract: Correlated and uncorrelated ab initio vertical spectra are reported for the valence states of CO2, CO2+, and CO2−. Calculations with polarized and unpolarized basis sets are compared at each level. Ground state quadrupole moments are computed. Linear molecule to bent molecule SCF correlation diagrams are also reported.

Journal ArticleDOI
TL;DR: In this paper, an all-valence-electron CI treatment is reported for the low-lying valence and Rydberg states of butadiene, and the agreement between calculated and experimental transition energies is invariably better than 0.2 eV where comparison is possible, with all low-liding valence triplet and Ryszkowski singlet excited states being unambiguously assigned.

Journal ArticleDOI
TL;DR: In this paper, the core and valence levels of poly(alkyl acrylates) have been studied by ESCA and the results suggest that ESCA may be applied to the study of surface compositions.
Abstract: The core and valence levels of a series of poly(alkyl acrylates) have been studied by ESCA. From an analysis of the individual component peaks for the C1s and O1s core levels and from comparison of relative area ratios it is shown that ESCA may be applied to the study of surface compositions. The evidence presented strongly suggests that on the ESCA depth-profiling scale the technique statistically sample the repeat units with no evidence for preferential orientation of side chains at the surface. For some samples, ESCA provides evidence for a degree of surface oxidation and hydrocarbon contamination. The valence energy levels are shown to be characteristic of the polymer system. The measured absolute and relative binding energies of the core levels have been compared with model calculations using the charge-potential model in the CNDO/2 SCF MO formalism.

Journal ArticleDOI
TL;DR: In this paper, the authors determined the pressure-volume relationship for cerium monophosphide (CeP) to 200kbar pressure and concluded that there is an electronic transition involving a change in the valence state of Ce from 3 + + to 4 + + near 100kbar.
Abstract: The pressure-volume relationship for cerium monophosphide (CeP) has been determined to 200-kbar pressure. In this range the substance maintains the NaCl-type structure and exhibits abnormal compression. From the data we conclude that there is an electronic transition involving a change in the valence state of Ce from ${3}^{+}$ towards the ${4}^{+}$ state near 100-kbar pressure. Qualitative observations of the reflectivity of CeP under pressure support the valence transition. CeP under high pressure may be expected to exhibit superconductivity.


Journal ArticleDOI
TL;DR: In this article, the potential energy surface of the hydrogen fluoride dimer was studied with ab initio LCAO-SCF theory using minimal, split valence, and polarized basis sets.

Journal ArticleDOI
TL;DR: The analysis indicates that the first, third, and fifth photoelectron bands in adenine and the two methyl substituted derivatives arise from pi orbitals, and the six uppermost orbitals of 9-methyladenine and 6-methylaminopurine have lower ionization potentials.
Abstract: The UV photoelectron spectra of adenine, 9-methyladenine, and 6-methylaminopurine contain highly resolved bands arising from the six highest occupied molecular orbitals. The spectra have been analyzed using UV absorption data, photoelectron data from previous studies of heterocyclic compounds, and results from both semi-empirical and ab initio molecular orbital calculations. The analysis indicates that the first, third, and fifth photoelectron bands in adenine and the two methyl substituted derivatives arise from pi orbitals. The second, fourth, and sixth bands arise from nitrogen atom lone-pair orbitals. Compared to adenine, the six uppermost orbitals of 9-methyladenine and 6-methylaminopurine have lower ionization potentials. This destabilization of the valence electrons is expected to play an important role in causing the increase in base stacking forces observed in methyl substituted adenines.

Journal ArticleDOI
TL;DR: In this article, the valence ionization potentials of furan and thiophene were studied by an ab initio many-body approach which includes the effects of electron correlation and reorganization beyond the Hartree-Fock approximation.