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Showing papers on "Valence (chemistry) published in 1995"


Journal ArticleDOI
TL;DR: In this article, the interconnections among orbital interactions, electron delocalization and spin coupling in iron-sulfur clusters are reviewed, with special attention to the complex nature of spin and orbital states in 4Fe4S complexes.

712 citations


Journal ArticleDOI
TL;DR: In this paper, a semi-empirical model was developed which took into account the thermally activated disproportionation of Co 3+ ions into Co 4+ and Co 2+ pairs, and the ionic compensation of oxygen vacancies formed at high temperatures.

667 citations



Journal ArticleDOI
TL;DR: In this article, the bond valence approach is used to model the characteristic out-of-center electronic distortions around d 0 transition metal cations in octahedral coordination, where certain bonds are weighted to take into account the intrinsic inequality of the bonds in such a distorted coordination.

453 citations



Journal ArticleDOI
TL;DR: In this article, the authors synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra, where the analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds.
Abstract: We synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra. The (idealized) stoichiometries of these compounds are RuSr 2 LnCu 2 O 8 (Ru-1212) and RuSr 2 (Ln 1+ x Ce 1− x ) Cu 2 O 10 (Ru-1222), where Ln is one of the three neighboring lanthanides Sm, Eu and Gd. The analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds. Both 1212 and 1222 compounds are semiconducting when prepared in air and rapidly cooled to room temperature. Metallic behavior and superconductivity can be achieved for various compositions upon appropriate annealing.

382 citations


Journal ArticleDOI
TL;DR: A third-neighbor tight-binding model, with spin-orbit coupling included, is developed, to treat the electronic properties of Bi and Sb quantitatively and should be useful for calculations of the electronic property of proposed semimetal-semiconductor systems, including superlattices and resonant-tunneling devices.
Abstract: We have developed a third-neighbor tight-binding model, with spin-orbit coupling included, to treat the electronic properties of Bi and Sb. This model successfully reproduces the features near the Fermi surface that will be most important in semimetal-semiconductor device structures, including (a) the small overlap of valence and conduction bands, (b) the electron and hole effective masses, and (c) the shapes of the electron and hole Fermi surfaces. The present tight-binding model treats these semimetallic properties quantitatively, and it should, therefore, be useful for calculations of the electronic properties of proposed semimetal-semiconductor systems, including superlattices and resonant-tunneling devices.

289 citations


Journal ArticleDOI
TL;DR: In this paper, the room-temperature structural, optical and related properties, in the (low frequency) excitonic spectral region, of some single valence metal-halide compounds based on Pb, Sn and Ge are briefly reviewed and some new results are reported.

256 citations


Journal ArticleDOI
A. Nagashima1, N. Tejima1, Y. Gamou1, T. Kawai1, Chuhei Oshima1 
TL;DR: The strength of the interfacial bond and the influence of this bond upon the electronic states of the monolayer h-BN film is discussed on the basis of the observed band structures for the BN film and a film of monolayers graphite formed on Ni(111).
Abstract: Angle-resolved ultraviolet-photoelectron spectroscopy and angle-resolved secondary-electron emission spectroscopy have been carried out for a film of single-crystalline hexagonal boron nitride (h-BN) formed on the Ni(111) surface to investigate both the valence- and conduction-band structures. The thickness of the film studied in this experiment was 1 ML. The observed electronic dispersion relations were compared with some theoretical ones reported for bulk h-BN. Among these theoretical calculations, the one by Catellani et al. [Phys. Rev. B 36, 6105 (1987)] is in the best agreement with the present results. We have discussed the strength of the interfacial bond and the influence of this bond upon the electronic states of the monolayer h-BN film on the basis of the observed band structures for the BN film and a film of monolayer graphite formed on Ni(111).

253 citations


Journal ArticleDOI
TL;DR: On the basis of the partial structure factors, the origin of the first sharp diffraction peak is investigated and affects differently the localization properties of valence and conduction band states, as suggested by experimental mobilities of electrons and holes.
Abstract: We performed a first-principles molecular dynamics study of liquid ${\mathrm{SiO}}_{2}$ at a temperature of 3500 K, followed by a rapid quench to 300 K obtaining a perfectly chemically ordered amorphous network. Structural and electronic properties of our amorphous sample are in good agreement with neutron diffraction, x-ray photoemission, and optical experiments. On the basis of the partial structure factors, we investigated the origin of the first sharp diffraction peak. Disorder affects differently the localization properties of valence and conduction band states, as suggested by experimental mobilities of electrons and holes.

206 citations


Journal ArticleDOI
TL;DR: A detailed electronic structure description of the reduced blue copper active site has now been developed as mentioned in this paper, which is a key determining factor in the high reduction potentials generally observed for blue copper proteins.
Abstract: A detailed electronic structure description of the reduced blue copper active site has now been developed. Photoelectron spectroscopy (PES) of imidazole, dimethyl sulfide, and methanethiolate bound to Cu(I) sites at single crystal surfaces has been used to define normal Cu(I) bonding to ligands relevant to the blue copper site. Variable photon energy PES has been used to assign valence band spectra, assess metal-ligand covalency, and probe specific orbital contributions to Cu(I) bonding. Self Consistent Field-X{alpha}-Scattered Wave (SCF-X{alpha}-SW) molecular orbital calculations calibrated to the photoelectron spectra have been performed to quantitatively complement the experimental bonding descriptions. These calculations have been extended to the reduced blue copper active site in plastocyanin, the prototypical blue copper protein, to detail the electronic structure changes that occur relative to normal Cu(I) bonding and upon oxidation. Ionization energies have been used to estimate the electronic structure contributions to the reduction potential. The long Cu-thioether axial bond present at the active site destabilizes the oxidized state and is therefore a key determining factor in the high reduction potentials generally observed for blue copper proteins. Linear coupling terms have been evaluated for the distortions of a blue copper site unconstrained by the protein backbone. 99 refs., 22 figs.,more » 12 tabs.« less

Journal ArticleDOI
TL;DR: In this paper, the crystal structure of the high-temperature polymorph of α-hexathienyl (α-6T/HT) was determined from a single-crystal structure analysis.
Abstract: α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Huckel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

Journal ArticleDOI
TL;DR: In this article, the ligand pre-edge intensity was used to probe the valence p-orbital covalency of a metal-ligand bond in open shell metal ions.
Abstract: Ligand K-edge X-ray absorption spectroscopy has been shown to be a powerful tool in the study of the electronic structure of ligand-metal interactions in open shell metal ions. The technique was developed previously for application to ligand-Cu[sup II] covalency but has now been extended for application to metal ions with more than one electron (or hole). By use of the expressions derived and the protocol outlined in this study, ligand pre-edge intensity provides a direct probe of the ligand valence p-orbital covalency of a metal-ligand bond. The technique is extended to other d[sup n] systems by application of the analysis presented in this paper. 40 refs., 6 figs., 9 tabs.

Journal ArticleDOI
TL;DR: In this paper, the role of the Auger process in electron-irradiated thin insulating specimens has been analyzed and correlated with atomic displacements, migration and desorption effects.

Journal ArticleDOI
TL;DR: In this paper, an empirical correlation is developed for relating the wavenumbers of (WO) stretching modes to WO bond lengths and bond strength for tungsten oxide reference compounds.
Abstract: An empirical correlation is developed for relating the wavenumbers of (WO) stretching modes to WO bond lengths and bond strengths for tungsten oxide reference compounds. A least-squares exponential fit of crystallographically determined WO bond lengths to the wavenumbers of their Raman stretching modes is presented along with a relation between WO bond strengths, in valence units, and wavenumbers of Raman stretching modes. The empirical bond length/stretching mode wavenumber/bond strength relationships lead to a unique and effective method of interpreting Raman spectra of tungstate species. This method leads to the WO bond lengths and coordination of the tungstate species. The utility of the method is illustrated by estimating the wavenumbers of the Raman stretching modes for the ideal WO4 tetrahedron and WO6 octahedron, and the bonds lengths and coordinations of the tungstate species in the reference compounds Na2WO4, PbWO4, CaWO4 and Bi2WO6. In addition, the WO bond lengths of the WO6 octahedra in crystalline Ba2MgWO6 are reported for the first time. This new approach for evaluating the Raman spectra of tungstate species is expected to be generally applicable to all tungstates, regardless of environment, physical state, or oxidation state.

Journal ArticleDOI
TL;DR: In this paper, the valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies.
Abstract: The valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies. All non-zero dipole oscillator strengths are also computed, at the CASSCF level. For benzene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm–1) are obtained. Of these, 12 singlet and three triplet energies are experimentally known well enough to allow meaningful comparison. The average error is around 0.1 eV. The highest of these singlet states (21 E2g) is the highest valence ππ* state predicted by elementary π-electron theory. Its energy is then considerably lower than has been suggested from laser flash experiments, but in perfect agreement with a reinterpretation of that experiment. For phenol, 27 singlet states are obtained, in the range 4.53–7.84 eV (63 300 cm−1). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.

Journal ArticleDOI
TL;DR: Linear carbon-chain radicals CnH for n = 2-7 have been studied with correlation consistent valence and core-valence basis sets and the coupled cluster method RCCSD(T) and the C2H- and C3H- anions have also been characterized.



Journal ArticleDOI
TL;DR: In this paper, the x-ray absorption near-edge structure (XANES) spectra of a selection of silicate and aluminosilicate minerals have been measured using synchrotron radiation (SR).
Abstract: Silicon K-edge x-ray absorption near-edge structure (XANES) spectra of a selection of silicate and aluminosilicate minerals have been measured using synchrotron radiation (SR). The spectra are qualitatively interpreted based on MO calculation of the tetrahedral SiO44−cluster. The Si K-edge generally shifts to higher energy with increased polymerization of silicates by about 1.3 eV, but with considerable overlap for silicates of different polymerization types. The substitution of Al for Si shifts the Si K-edge to lower energy. The chemical shift of Si K-edge is also sensitive to cations in more distant atom shells; for example, the Si K-edge shifts to lower energy with the substitution of Al for Mg in octahedral sites. The shifts of the Si K-edge show weak correlation with average Si-O bond distance (dSi-O), Si-O bond valence (sSi-O) and distortion of SiO4 tetrahedra, due to the crystal structure complexity of silicate minerals and multiple factors effecting the x-ray absorption processes.

Journal ArticleDOI
TL;DR: In this paper, high-resolution core and valence band photoemission as well as photoinduced Auger spectra of metallic Sn, polycrystalline SnO and SnO2 ((110) natural single-crystal cassiterite and polycrystine) samples were measured in order to give information on local electronic and geometric structures around the core-ionized atoms and far providing data for facilitating quantitative analysis of systems containing these species.
Abstract: High-resolution core and valence band photoemission as well as photoinduced Auger spectra of metallic Sn, polycrystalline SnO and SnO2 ((110) natural single-crystal cassiterite and polycrystalline) samples were measured in order to give information on local electronic and geometric structures around the core-ionized atoms and far providing data for facilitating quantitative analysis of systems containing these species, The SnO surfaces were obtained without the use of ion sputtering by using a special preparation procedure based on in vacuo surface scraping, The respective experimental valence land spectra could be utilized in quantitative analysis of layered tin oxide structures. Valence band spectral shapes are compared to spectra calculated by using a cluster-type (DV-X alpha) MO model. From the Sn Auger parameter measurements, using a simple electrostatic model, the similarity of the initial-state effects for the two oxides was deduced, while the oxygen Auger parameter values indicated large polarizability and the respective final-state hole-hole repulsion energies were determined.


Journal ArticleDOI
TL;DR: In this paper, the potential relevance of polyoxometalate chemistry for providing compounds that can be of interest in molecular magnetism and in the preparation of new classes of molecular materials is discussed.
Abstract: The potential relevance of polyoxometalate chemistry for providing compounds that can be of interest in molecular magnetism and in the preparation of new classes of molecular materials is discussed. We show that this kind of inorganic complex provides remarkable examples of magnetic clusters for which the number and type of interacting metal atoms can be varied in a controlled manner. They also provide striking examples of high nuclearity mixed valence clusters in which delocalized electrons can interact with the localized moments of the metal centers. Finally, we illustrate the use of polyoxoanions as inorganic components of new molecular materials based on organic donors of the tetrathiafulvalene type. In particular, we discuss the possibility of synthesizing molecular materials coupling localized magnetic moments with delocalized electrons taking advantage of the magnetic character in the polyanion.

Journal ArticleDOI
TL;DR: In this paper, a series of new bis(tetrathiafulva1ene) [bis(TTF)] compounds were synthetized with the TTF-like synthon 8a,b, previously used to prepare the pyrazine-fused bis(F) 7a, b. These compounds exhibit four reversible sequential one-electron oxidation steps, on the basis of cyclic voltammetry.
Abstract: The series of new bis(tetrathiafulva1ene) [bis(TTF)] compounds 10-12a,b have been synthetized with the TTF-like synthon 8a,b, previously used to prepare the pyrazine-fused bis("F) 7a,b. These compounds exhibit four reversible sequential one-electron oxidation steps, on the basis of cyclic voltammetry. For the cation radicals 7b.+,10 .+,and 12.+, generated electrochemically, rather intense and broad bands in the near-IR region, specific of class 11 mixed valence compounds, were found. Theoretical calculations at the semiempirical AM1 level as well as at the ab-initio level indicated that these cation radicals are charge-distributions localized as related to mixed valence species. These calculations also allowed the low-energy absorption bands to be assigned to intervalence transitions; these transitions imply the existence of multiple low-lying excited states that are non-adiabatically coupled to the ground-state of these open-shell species.

Book ChapterDOI
01 Jan 1995
TL;DR: In this paper, the authors analyzed the valence distributions of Fe-S polymetallic centers in proteins and model systems, and showed that it is possible to transfer the electron delocalization from one polymetric center to another.
Abstract: Iron-sulfur polymetallic centers pose interesting questions on the oxidation state of each metal ion, on the possibility of electron delocalization over the metal ions versus the possibility of existence of equilibria among different localized-valence states, and on the factors determining if, and to what extent, electron delocalization is present. By interpreting the hyperfine coupling of unpaired electrons with 57Fe as well as with 1H, and by analyzing the EPR spectra, the valence distributions of Fe-S polymetallic centers in proteins and model systems are obtained. The systems analyzed here are: [Fe2S2]2+/+, [Fe3S4]+/0, [Fe4S4]3+/2+/+. The understanding of the parameters describing the exchange coupling within pairs of spin vectors, as well as the double exchange coupling, is attempted on the basis of the oxidation states of the various ions. Upon proper scaling of these parameters, we show that it is possible to transfer them from one polymetallic center to another. The electronic properties are tentatively related to the microscopic reduction potentials of each iron in the center.


Journal ArticleDOI
TL;DR: In this paper, an equation-of-motion coupled cluster method was used to describe the electronic excitation spectra of the model carbonyl compounds, formaldehyde, acetaldehyde, and acetone.

Journal ArticleDOI
TL;DR: In this paper, the authors performed full-potential linearized augmented-plane-wave band calculations based on the local density approximation for three sulphide spinels CuM2S4 (M=Co,Rh,Ir).
Abstract: Full-potential linearized augmented-plane-wave band calculations based on the local density approximation are performed for three sulphide spinels CuM2S4 (M=Co,Rh,Ir). The electronic states near the Fermi level consist mainly of the M ndT (n=3 for Co,4 for Rh,5 for Ir) and the S 3p orbitals. A large d gamma -d$54T splitting of the M nd bands is attributed mainly to the effects of hybridization between the M nd gamma and the S 3p orbitals. The Co 3d orbitals are more localized compared with the Rh 4d and the Ir 5d orbitals. Among the three compounds, the density of states or the partial density of states of the M n/sub /components at the Fermi level is largest in CuCo2S4. The Cu 3d orbitals form relatively narrow bands. Judging from the number of Cu 3d electrons in the muffin-tin sphere the valence of the Cu ions is Cu1+ rather than Cu2+. Hence the Cu ions are expected to be non-magnetic.

Journal ArticleDOI
TL;DR: In this article, the dependence of the length of the N −H length on the H −N distance was studied using neutron diffraction data; this dependence is the same for both charged and uncharged N−H donors; geometry predictions for strong hydrogen bonds and values for the shortest possible N −N and N −O separations were deduced on the basis of the bond valence concept.
Abstract: For N–H ⋯ N hydrogen bonds, the dependence of the N–H length on the H ⋯ N distance is studied using neutron diffraction data; this dependence is the same for both charged and uncharged N–H donors; geometry predictions for strong hydrogen bonds and values for the shortest possible N ⋯ N and N ⋯ O separations are deduced on the basis of the bond valence concept.

Journal ArticleDOI
TL;DR: In this article, the authors show that the c-axis and the interplanar distances related to the position of Ba and O(2, 3) exhibit a sigmoidal dependence on rR.
Abstract: RBa2Cu3O7 and [R0.5Pr0.5]Ba2Cu3O7 compounds with RY and lanthanides have been prepared and studied at ambient temperature by means of XRD and infrared-reflectance spectroscopy. In Pr free compounds, increasing the Shannon radius of R, rR, leaves the puckering of the Cu(2)−O(2, 3) sheet unaffected. In the Pr containing compounds, the PrO coordination polyhedron is typical of R+3 ions in eight-fold coordination and argues against differentiating Pr in terms of valence or hybridization. Instead, a repositioning of Ba within the cuprate block is observed, leading to the flattening of the sheet and the anomalously high frequency of the Ba and Cu(2)−O(2, 3) B1u modes. In RPr compounds the c-axis and the interplanar distances related to the position of Ba and O(2, 3) are found to exhibit a sigmoidal dependence on rR. It is argued that this sigmoidal dependence is related to the two-mode behavior exhibited by several phonon modes of these compounds.