Showing papers on "Valence (chemistry) published in 1991"

Bond-valence parameters for solids

Abstract: Bond-valence parameters which relate bond valences and bond lengths have been derived for a large number of bonds. It is shown that there is a strong linear correlation between the parameters for bonds from cations to pairs of anions. This correlation is used to develop an interpolation scheme that allows the estimation of bond-valence parameters for 969 pairs of atoms. A complete listing of these parameters is given.

The Spinel Phase of LiMn2 O 4 as a Cathode in Secondary Lithium Cells

Abstract: The electrochemical properties of and were studied for different conditions of sample preparation and different degrees of cation substitution . In the voltage range 3.5–4.5 V, cells of either spinel or (made by leaching the Li from ) reversibly insert 0.4 Li per Mn at an average voltage of 4.1 V, leading to an energy density of 480 Wh/kg of cathode. Cells cycled 50 times lost less than 10% of their initial capacity, suggesting that this material could be used instead of or as the cathode in the new generation of "rocking chair batteries." Replacing Mn with cations of valence 2 (Ni, Zn) or 3 (Fe) reduces the amount of Mn+3 and correspondingly reduces the capacity of the cells at 4.1 V, but does not affect their cycling performance.

Electronic structure of solid C60: Experiment and theory.

Abstract: Synchrotron-radiation and x-ray photoemission studies of the valence states of condensed phase-pure ${\mathrm{C}}_{60}$ showed seventeen distinct molecular fetaures extending \ensuremath{\sim}23 eV below the highest occupied molecular states with intensity variations due to matrix-element effects involving both cluster and free-electron-like final states. Pseudopotential calculations established the orign of these features, and comparison with experiment was excellent. The sharp C 1s main line indicated a single species, and the nine satellite structures were due to shakeup and plasmon features. The 1.9-eV feature reflected transitions to the lowest unoccupied molecular level of the excited state.

Voltage-sensing residues in the S4 region of a mammalian K+ channel.

Abstract: THE ability of ion-channel proteins to respond to a change of the transmembrane voltage is one of the basic mechanisms underlying electrical excitability of nerve and muscle membranes. The voltage sensor has been postulated to be the fourth putative transmembrane segment1 (S4) of voltage-activated Na+, Ca2+ and K+ channels1–5. Mutations of positively charged residues within S4 alter gating of Na4 and Shaker-type K+ channels5, but quantitative correlations between the charge or a residue in S4 and the gating valence of the channel have not yet been established. Here, with improved resolution of the voltage dependence of steady-state activation, we present estimates of the equivalent gating valence with sufficient precision to allow quantitative examination of the contribution of individual charged residues to the gating valence of a mammalian non-inactivating K+ channel. We conclude that at least part of the gating charge associated with channel activation is indeed contributed by charged residues within the S4 segment.

Bonding Patterns in Intermetallic Compounds

Abstract: Intermetallic phases have long been among the black sheep in the family of chemical compounds. Their chemical bonding has eluded description by the valence rules, which otherwise are extremely effective. As a result, understanding of the structure–bonding relationships in these phases to date has remained nebulous, even though they form the largest group of inorganic compounds. Their broad industrial applicability and richly varied structural chemistry call for new approaches for explaining their structures, electronic structures, and physical properties. A combination of new experimental and theoretical investigations has revealed interesting local aspects of chemical bonding in metals and thereby pointed to a route from the valence compounds, through the cluster and electron-deficient compounds, to the intermetallic phases.

Electronic structure of silicon nitride

Abstract: Silicon Nitride is found to have a valence band maximum of nitrogen lone pair p electrons because of the planar nitrogen site. This contrasts with the usual lone pair semiconductors, such as SiO2, caused by a p4 valence configuration. Consequently although the valence band density of electron states shows a lone pair band and a deeper bonding band as usual, impurities have a greater effect in the nitride than in conventional lone pair semiconductors. Hole transport is also discussed.

Electronic structure considerations for methane activation by third-row transition-metal ions

Abstract: Methane is spontaneously dehydrogenated in the gas phase by many metal ions of the 5d transition series. In most cases, the MCH{sub 2}{sup +} produced undergoes further reactions, leading eventually to products such as WC{sub 8}H{sub 16}{sup +}. The reactivity of the third-row transition-metal ions, both bare and with simple ligands, may be explained in terms of electronic structure considerations. Promotion energy, exchange energy, and the relative and absolute sizes of the valence s and d orbitals all appear to be important.

Recent advances in x‐ray photoelectron spectroscopy studies of oxides

Abstract: X‐ray photoelectron spectroscopy (XPS) has often played the major role in the chemical characterization of select surface species, but the extension of that role to larger classes of compounds has generally been limited by such problems as cleanliness, charging, and relaxation shifts. In this article we suggest that adequate command of these difficulties has now permitted the collective chemical description of quite diverse surfaces. In this regard the progressive bonding differences of representative, simple group A oxides (e.g., BaO and SiO2 ) are analyzed employing various features in the XPS core level and valence band results. Regular progressions in covalency/ionicity are demonstrated. Extensions of these studies to complex (AzMsOt) oxides (e.g., Na2Al2O4 ) are also described in which progressive alterations of the XPS spectra suggest that during this complex formation the A–O bond is generally made more ionic, whereas the M–O bond becomes more covalent. These results also indicate that for many of ...

Medium-size polarized basis sets for high-level-correlated calculations of molecular electric properties: III. Alkali (Li, Na, K, Rb) and alkaline-earth (Be, Mg, Ca, Sr) atoms

Abstract: The basis set polarization method is applied for the generation of medium size polarized basis sets for Li, Na, K, Rb, Be, Mg, Ca, and Sr. The derived basis sets are shown to give satisfactory results in calculations of atomic dipole polarizabilities and in calculations of dipole moments and dipole polarizabilities of diatomic hydrides and hydride ions at both the SCF and highly correlated levels of approximation. The MBPT and CC results for hydrides and hydride ions of the group Ia and IIa metals calculated with polarized basis sets derived in this study indicate the importance of the electron correlation contribution to the electric properties of those systems. The dominant part of this contribution involves excitations from core orbitais of the metal atom and is recognized as the core polarization effect. The valence approximation is found to be completely unsatisfactory for hydrides involving heavier metals of the present series. Details of the basis set data supplement this paper. Some possible applications of those basis sets are surveyed.

The Electrotopological State: An Atom Index for QSAR

Abstract: A new characterization of atoms in molecules is introduced as the electrotopological state index, which combines both the electronic character and the topological environment of each skeletal atom in a molecule. The electrotopological state (E-state) of a skeletal atom is formulated as an intrinsic value Ii plus a perturbation term ΔIi, arising from the electronic interaction within the molecular topological environment of each atom in the molecule. The atom intrinsic value, for first row atoms, is expressed as I = (δv + 1)δ, in which δv and δ are the counts of valence and sigma electrons, respectively, for the atom in the molecular skeleton. The E-state, Si, for atom i is defined as Si = Ij + ΔIj, where the influence of other atoms on atom i, ΔIi, is given as Σ(Ij — Ij)/rij2; rij is the graph separation between atoms i and j, counted as number of atoms, including i and j. Information in the electrotopological state is revealed by examples of various types of organic structures, including skeletal branching and heteroatom variation. The relation of the E-state value to NMR chemical shift is demonstrated for a series of carbonyl compounds. QSAR examples are given for hydrazide inhibition of MAO and for receptor binding of β-carbolines. These examples reveal the power of this approach to QSAR using atom level indexes, computed directly from molecule connection tables, in which it is possible to identify atoms and regions in the molecule which are important for activity.

Mixed valency systems : applications in chemistry, physics, and biology

Abstract: Mixed Valency Systems: Retrospect And Prospect.- Vibronic Coupling Models Of Mixed Valency: Relation Of The PKS And MO Models For One- And Two-Electron Systems.- Electric Field Perturbation Of Electronic (Vibronic) Absorption Envelopes: Application To Characterization Of Mixed Valence States.- Solvent And Temperature Effects In Mixed Valence Chemistry.- Electron Transfer In Mixed Valence Complexes In The Solid State.- Photoredox Chemistry Of Two-Electron Mixed Valence Systems.- How To Design Fast Two-Electron Transfer Reagents: 37-Electron Mixed Valence Fe(I)Fe(II)-Bisandwiches As Key Intermediates.- Magnetic And Optical Phenomena In Biological Iron-Sulfur Mixed Valence Complexes And Their Chemical Models. A Theoretical Approach.- Mixed Valency In Multinuclear Manganese Enzymes And Clusters.- Mixed Valence Iron And Chromium Fluorides. Bronzes And Related Compounds. Chemistry, Structure And Magnetism.- Mixed Valency Minerals: Influences Of Crystal Structures On Optical And Mossbauer Spectra.- Valency Disproportionation In Inorganic Solids.- Superconductivity By Local Pairs (Bipolarons) In Doped Metal Oxide Semiconductors.- A Comparison Between Conducting C.T. Crystalline Salts And L.B. Films.- Optical Spectroscopic Probes Of Mixed Valence Systems.- Inelastic Neutron Scattering Studies Of Mixed Valency Systems.- Mossbauer Emission Spectroscopy Of Mixed Valency After Nuclear Decay In Iron Complexes.- Intramolecular Electron Transfer. Applications In Molecular Electronics.- Vibronic Coupling Models For Mixed Valence Line Shapes: Going Beyond The PKS Approach.- Mixed Valency Oligomers: Model Pathways For The Control Of Their Properties.- The Effects Of Second Order Vibronic Coupling On The Symmetric Dimer And Trimer.- Semiclassical Approach To The Vibronic Problem For The Creutz-Taube Ion.- A New Spin Hamiltonian For Mixed Valence Systems Including Transfer, Exchange And Exchange-Transfer Terms.- An Example Of The Influence Of The Ligand Held On The Electron Localisation: The Mixed Valence Ferrocene-Ferricinium.- A First Stable Organometallic Analogue [mer(OC)3(PiPr3)2W(?-pyrazine)W(PiPr3)2(CO)3]+ Of The Creutz-Taube Ion. Similarities And Differences.- Mixed Valence Manganese Carboxylates Of Various Nuclearities.- Spectroscopic Studies Of Spin State And Electron Delocalization In A Binuclear Mixed Valence Fe(II/III) Complex.- Bis(Phthalocyaninato) Lanthanide Sandwich Compounds Exhibiting Mixed Valence Ligands.- The Ruthenium Blues.- Optical And Related Properties Of Metal-Halide Chain Compounds: Bulk And Small Particles.- Dielectric Relaxometry: Manganate (VI)-(VII) In Solid And Solution States.- Electronic Instabilities Of Two-Dimensional Metals, K3Cu8S6 And Rb3Cu8S6.- Chemistry And Properties Of New Examples Of Mixed Valence Compounds, Based On Organic ? Donor Molecules And Polyoxometalate Ions.- Mixed Valency In The Organic-Inorganic Compounds (BEDT-TTF)6Cu2Br6 And (BEDT-TTF)6Cu2Cl2Br4.- Electrochemical Generation Of Mixed Valency Copper Complexes In Solution.- Endor Determination Of Proton Hyperfine Tensors As Local Probes Of The Delocalization Of The Spin Population In The 4 Iron - 4 Sulfur Cubanes.- Theory Descriptions Of Mixed Valency.- Electron Transfer Mechanisms, From Dimers To Infinite Solids.- Applications: Energy Storage And Molecular Electronics.- Biology: Models And Proteins.

Soft-x-ray-absorption studies of the location of extra charges induced by substitution in controlled-valence materials.

Abstract: We present high-resolution 2p x-ray-absorption spectra from 3d transition metals M in three series of pseudobinary and pseudoternary oxides. We show how the detailed shape of the multiplet within the spectra can be used to determine the valence of the M ions. The spectra of the early-transition-metal compounds show dramatic changes as a function of substitution due to changes in the M 3d occupancy, indicating that the extra holes or electrons are located predominantly at the 3d-metal sites. This contrasts strongly with the behavior of the spectra of late-transition-metal compounds, for which the extra charges induced by substitution go to oxygen states.

Spin polarization and magnetic dichroism in photoemission from core and valence states in localized magnetic systems

Abstract: Using group theory we derive a general model for spin polarization and magnetic dichroism in photoemission in the presence of atomic interactions between the hole created and the valence holes. We predict strong effects in the photoemission from core levels and localized valence levels of transition metal and rare-earth compounds. In the presence of electrostatic interactions between the created hole and magnetically polarized valence electrons, we can distinguish eight fundamental spectra: the isotropic spectrum, spin spectrum, orbit spectrum (magnetic circular dichroism), spin-orbit spectrum, spin-orbit magnetic-quadrupole spectrum, anisotropic spectrum (magnetic linear dichroism), anisotropic spin magnetic-dipole spectrum , and anisotropic spin magnetic-octupole spectrum. Examples are given for the 2p, 3s, 3p, and 3d photoemission from divalent Cu, Co ${\mathit{d}}^{7}$, and Fe ${\mathit{d}}^{6}$.

Contribution to the electron distribution analysis. I. Shell structure of atoms

Abstract: Relativistic spherically averaged numerical all‐electron densities ρ were computed for the atoms Be–Ba, B–Tl, C–Pb, Cu–Au, and Zn–Hg. The Laplacian of these densities is not able to resolve the valence shell from the inner shells in case of heavy atoms, starting with the fourth row. The distribution of the local kinetic energy Ekin shows a valence maximum even for these heavy atoms, unfortunately, in a region of negative kinetic energy; i.e., nonclassically allowed. The quantity −‖∇ρ‖/ρ was also investigated. For all computed atoms, the −‖∇ρ‖/ρ diagrams are capable of describing the complete shell structure. −‖∇ρ‖/ρ is sensitive to basis set quality: poor Gaussian basis sets exhibit spurious oscillations and a premature onset of the linear decay. For the atoms B–Tl, Ba, Au, Hg, and Pb, nonrelativistic numerical calculations were performed to examine the effect of the relativity on the aforementioned quantities. Tests with pseudopotential densities reveal that for pseudopotential calculations, it is advisa...

Aspects of separability in the coupled cluster based direct methods for energy differences

Abstract: In this paper we have discussed in detail the aspects of separability of the energy differences obtained from coupled cluster based “direct” methods such as the open-shell Coupled Cluster (CC) theory and the Coupled Cluster based Linear Response Theory (CC-LRT). It has been emphasized that, unlike the state energiesper se, the energy differences have a semi-local character in that, in the asymptotic limit of non-interacting subsystemsA, B, C, etc., they are separable as ΔE A , ΔE B , ΔE A + ΔE B , etc. depending on the subsystems excited. We classify the direct many-body methods into two categories: core-extensive and core-valence extensive. In the former, we only implicitly subtract the ground state energy computed in a size-extensive manner; the energy differences are not chosen to be valence-extensive (separable) in the semi-local sense. The core-valence extensive theories, on the other hand, are fully extensive — i.e., with respect to both core and valence interactions, and hence display the semi-local separability. Generic structures of the wave-operators for core-extensive and core-valence extensive theories are discussed. CC-LRT is shown to be core-extensive after a transcription to an equivalent wave-operator based form. The emergence of valence disconnected diagrams for two and higher valence problems are indicated. The open-shell CC theory is shown to be core-valence extensive and hence fully connected. For one valence problems, the CC theory and the CC-LRT are shown to be equivalent. The equations for the cluster amplitudes in the Bloch equation are quadratic, admitting of multiple solutions. It is shown that the cluster amplitudes for the main peaks, in principle obtainable as a series inV from the zeroth order roots of the model space, are connected, and hence the energy differences are fully extensive. It is remarkable that the satellite energies obtained from the alternative solutions of the CC equations are not valence-extensive, indicating the necessity of a formal power series structure inV of the cluster amplitudes for the valence-extensivity. The alternative solutions are not obtainable as a power series inV. The CC-LRT is shown to have an effective hamiltonian structure respecting “downward reducibility”. A unitary version of CC-LRT (UCC-LRT) is proposed, which satisfy both upward and downward reducibility. UCC-LRT is shown to lead to the recent propagator theory known as the Algebraic Diagrammatic Construction. It is shown that both the main and the satellite peaks from UCC-LRT for the one valence problems are core-valence extensive owing to the hermitized nature of the underlying operator to be diagonalized.

An index of electrotopological state for atoms in molecules

Abstract: A new method for molecular structure quantitation is described, in which both electronic and topological attributes are united. The method uses the hydrogen-suppressed skeleton to represent the structure and leads to a graph invariant index for the individual atoms and hydride groups of the molecular skeleton. An intrinsic atom value is calculated for each atom asI = (δ ν + 1)/δ, in whichδ ν andδ are the counts of valence and sigma electrons of atoms in the molecular skeleton, that is, exclusive of bonds to hydrogen atoms. The electrotopological state valueS i for an atomi is defined asS i =I i + ΔI i, where the influence of atom j on atom i, ΔI i, is given as Σ(I i-j j)/r 2;r is the graph separation between atoms i and j, counted as number of atoms, includingi andj. The information in the electrotopological state values is revealed by examples of various types of organic structures, including chain branching and heteroatom variation. The relation of the E-state value to NMR chemical shift is demonstrated for a series of carbonyl compounds.

Ab initio model potential study of the equilibrium geometry of alkaline earth dihalides: MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I)

Abstract: The ground state equilibrium geometry of alkaline earth dihalides MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I) has been optimized at the Hartree–Fock (HF) level using the Cowan–Griffin relativistic ab initio model potential method and a uniformly good, extended, spd valence basis set. The results show that, according to the method, all magnesium dihalides and CaCl2, CaBr2, and CaI2 are linear, SrF2 and all barium dihalides are bent, and CaF2 and SrCl2, SrBr2, and SrI2 are quasilinear molecules. The alkaline earth (n−1)d orbitals are shown to be responsible for the bending of the heavier molecules while their (n−1)p orbitals contribute considerably to the final quantitative prediction of the apex angle and the relative stability of the bent structures. Relativistic effects are shown to be very small on the bond distances and vibrational frequencies; they are important on the size of the bending barrier of the bent molecules. The results obtained are compared to previous theoretical studies and provide some insig...

Ab initio quantum chemical calculations of aluminum substitution in zeolite ZSM-5

Abstract: The authors have performed ab initio quantum mechanical calculations in monomeric clusters modeling the 12 different T sites of zeolite ZSM-5. By comparing the results of calculations that use minimum basis sets with those that employ valence double-[zeta] bases, the authors conclude that minimum basis sets are unreliable for predicting relative replacement energies for the substitution of silicon by aluminum atoms at the T sites of the zeolite. From these calculations, it is also concluded that small differences in the bond lengths and angles can significantly alter the order of the sites with respect to the replacement energies. From calculations using valence double-[zeta] basis sets on T(OH)[sub 4] monomers, it is concluded that in the absence of protons or other ions, the most favorable sites for Al substitution in zeolite ZSM-5 are the T[sub 6], T[sub 12], and T[sub 9] sites, whereas the least favorable site is T[sub 3]. However, the least favorable and most favorable sites only differ by 3.3 kcal/mol. The authors also present a simple empirical model that is capable of reproducing the results of the ab initio calculations. This model gives the replacement energy in terms of the bond lengths and bond angles about each site.

Studies of mixed-valence states in three-dimensional halogen-bridged gold compounds, Cs2AuIAuIIIX6, (X = Cl, Br or I). Part 2. X-Ray photoelectron spectroscopic study

Abstract: The mixed-valence states in Cs2AuIAuIIIX6(X = Cl, Br or I) were systematically studied by X-ray photoelectron spectroscopy of the Au 4f region for the first time. The gold valence states in halogeno complexes of AuI and AuIII were also studied. In Cs2Au2X6 the difference between the oxidation state of AuI and that of AuIII decreases as the covalency of the –AuI–X–AuIII–X– bonds increases in the order X = Cl < Br < I, because the charge-transfer interaction between AuI and AuIII through the bridging halogen becomes stronger in this order. In the non-stoichiometric CsAu0.6Br2.6 the oxidation state of Au is considered to be almost homogeneous.

Fine structure of the Ca 2p x-ray-absorption edge for bulk compounds, surfaces, and interfaces.

Abstract: The fine structure of the Ca 2p soft-x-ray-absorption edge is studied for a variety of bulk compounds (Ca metal, CaSi2, CaO, and CaF2), for surfaces and interfaces [CaF2(111), BaF2 on CaF2(111), Ca and CaF2 on Si(111)], and for defects (F centers in CaF2). The observed multiplet structure is explained by atomic calculations in a crystal field [cubic O(h) for the bulk and threefold C3-nu for the (111) surfaces and interfaces]. While the bulk spectra are isotropic, the surface and interface spectra exhibit a pronounced polarization dependence, which is borne out by the calculations. This effect can be used to become surface and/or interface selective via polarization-modulation experiments, even for buried interfaces. A change in valence from Ca2+ to Ca1+ causes a downwards energy shift and extra multiplet lines according to the calculation. The energy shift is observed for F centers at the CaF2 surface and for the CaF2/Si(111) interface.

Electronic structure of C60 and the alkali atom containing compounds MC60, M = Li, Na, K, Rb, Cs and K2C60

Abstract: Calculations have been performed in a study of the electronic structure of the truncated icosahedral C60-"Buckminsterfullerene" or "follene-60" and intercalated compounds of C60 with alkali atoms within the local density approximation. Certain carbon-carbon bond lengths were assumed and the evaluated molecular eigenvalues were found to be rather insensitive to small changes in the carbon-carbon bond lengths. Theoretical ionization potentials and electron affinities calculated with the von Barth-Hedin exchange-correlation potential for C60 were found to be in good agreement with available experimental data, which was not the case when using the Xα exchange potential with α = 0.7. Encouraged by this agreement between theoretical and experimental data, calculations were performed for alkali-containing compounds MC60, where M = Li, Na, K, Rb, Cs and K2C60 and the evaluated ionization energies were used for the identification of the locations of the alkali atoms. The electronic structure of these carbon- and metal-containing clusters was also analyzed by means of a Mulliken charge density analysis, a partial density of state analysis and contour plots of the electronic charge density of the valence levels. Energies of optical transitions were evaluated and compared with available experimental data and with predictions from other calculations.

Valence‐electron contributions to the electric‐field gradient and the crystal field at rare‐earth sites in intermetallic compounds

Abstract: From first‐principles band‐structure calculations, the electric‐field gradient Vzz at Gd nuclei in Gd2Fe14B, Gd2Co14B, Gd2Fe17, Gd2Co17, GdCo5, GdNi5, GdFe3, and elemental Gd has been calculated. The crystal‐field parameter A02 has been calculated for Gd2Fe14B. A02 and Vzz are mainly determined by the asphericity of the charge density of the 5d and 6p valence shells of the Gd atoms. We discuss the validity of a frequently used proportionality relation between A02 and Vzz.

Dependence of hole concentration on oxygen vacancy order in YBa2Cu3O7−δ: A chemical valence model

Abstract: Using a simple chemical model, the hole concentration is calculated for YBa 2 Cu 3 O 7−δ as a function of oxygen stoichiometry δ. For the calculations, it is assumed that 2-coordinated Cu is monovalent while 3- and 4-coordinated Cu's are nominally divalent. Because monovalent Cu's bind more electrons than do divalent Cu's, the degree of hole doping of the conduction band depends on the oxygen vacancy configuration. We examine extreme ordering conditions and apply the chemical valence model to an extended phase field of the double-cell (Ortho II) structure. For δ > 0.2, the dependence of the calculated hole concentration on δ, obtained for the (defect) double-cell structure, is similar to the observed dependence of T c on δ suggesting a simple relationship between T c and hole concentration. If T c is assumed to depend quadratically on hole concentration (as observed for other oxide superconductors), then the calculated T c versus δ corresponds closely with experiment. Low temperature aging behavior is also simulated in the model.