Showing papers on "Valence (chemistry) published in 1990"

Energy-adjustedab initio pseudopotentials for the second and third row transition elements

Abstract: Nonrelativistic and quasirelativisticab initio pseudopotentials substituting the M(Z−28)+-core orbitals of the second row transition elements and the M(Z−60)+-core orbitals of the third row transition elements, respectively, and optimized (8s7p6d)/[6s5p3d]-GTO valence basis sets for use in molecular calculations have been generated. Additionally, corresponding spin-orbit operators have also been derived. Atomic excitation and ionization energies from numerical HF as well as from SCF pseudopotential calculations using the derived basis sets differ in most cases by less than 0.1 eV from corresponding numerical all-electron results. Spin-orbit splittings for lowlying states are in reasonable agreement with corresponding all-electron Dirac-Fock (DF) results.

Electronic Band Structure of {Al2O3}, with Comparison to Alon and {AIN}

Abstract: As the uses of Al2O3 and other ceramics expand into new and more demanding applications, it is increasingly important to understand their electronic structure and its relationship to properties. However, compared with metals, semiconductors, or alkali halides, our understanding of the electronic structure of ceramic materials is limited. There has been much recent progress in our understanding of the electronic structure of Al2O3, based on the applications of new experimental and theoretial methods. Vacuum ultraviolet spectroscopy and valence band photoemission spectroscopy coupled with pseudofunction band structure methods provide a comprehensive approach to study a wide variety of electronic structure issues of importance to ceramic materials. The high-temperature electronic structure and its role in determining the high-temperature, intrinsic, electronic conductivity gives us the ability to evaluate high-temperature conductivity data, and supports the conclusion that Al2O3 is predominantly an electronic conductor at high-temperatures. The strain dependence of the electronic structure, as embodied in the deformation potentials, provides a simple method to determine surface stresses and strains. The variation of the electronic structure in the family Al2O3-AION-AIN demonstrates the changes associated with the valence band chemistry of changing the anion from oxygen to nitrogen, and the bonding from mixed ioniccovalent in the direction of greater covalency. These changes in the anion valence bands lead to dramatic changes in the atomic and electronic nature of room-temperature bimaterial interface formation for copper to Al2O3 or AIN. The application of this new methodology to develop our perspective on electronic structure and apply it to problems associated with temperature, stress, composition, or interface formation can improve our understanding of many critical questions in ceramics.

An Electrotopological-State Index for Atoms in Molecules

Abstract: A new method for molecular structure description is presented in which both electronic and topological characteristics are combined. The method makes use of the hydrogen-suppressed graph to represent the structure. The focus of the method is on the individual atoms and hydride groups of the molecular skeleton. An intrinsic atom value is assigned to each atom as I = (delta v + 1)/delta, in which delta v and delta are the counts of valence and sigma electrons of atoms associated with the molecular skeleton. The electrotopological-state value, Si, for skeletal atom i is defined as Si = Ii + delta Ii, for second row atoms, where the influence of atom j on atom i, delta Ii, is given as sigma(Ii-Ij)/rij2; rij is the graph separation between atom i and atom j, counted as the number of atoms. The characteristics of the electrotopological state values are indicated by examples of various types of organic structures, including chain lengthening, branching, heteroatoms, and unsaturation. The relation of the E-state value to NMR chemical shift is investigated for a series of alkyl ethers. The E-state oxygen value gives an excellent correlation with the 17O NMR: r = 0.993 for 10 ethers. A biological application of the E-state values in QSAR analysis is given for the binding of barbiturates to beta-cyclodextrin.

Formation, oxidation, electronic, and electrical properties of copper silicides

Abstract: The solid state reaction between copper and silicon has been studied using Rutherford backscattering, glancing‐angle x‐ray diffraction, scanning electron microscopy, and x‐ray photoemission spectroscopy. Schottky‐barrier‐height measurements on n‐type Si (100) have also been performed in the temperature range of 95–295 K with the use of a current‐voltage technique. The results show that a metal‐rich compound with a composition in the Cu3Si range forms at low temperatures (473 K). The electronic properties of the compound are dominated by the hybridization between the Cu(d) and Si(p) valence states. A direct consequence of this hybridization is the peculiar oxidation behavior of the compound surface; both Cu and Si have been found to oxidize at room temperature. The oxidation of Si in the silicide is enhanced as compared with the oxidation of the elemental single‐crystalline Si surface. Upon annealing the oxidized surface, a solid state reaction takes place: Cu2O disappears and a thicker SiO2 layer grows, o...

Ab initio Hartree-Fock calculations for periodic compounds: application to semiconductors

Abstract: The ab initio Hartree-Fock crystalline orbital program CRYSTAL is applied to diamond, silicon, BN, BP, SiC and AlP. The effects of the computational parameters controlling the accuracy of the infinite Coulomb and exchange series are analysed; the performances of five standard (but re-optimised in the valence part) molecular basis sets (STO-3G; 3-21G; 3-21G*; 6-21G; 6-21G*) are documented with reference to equilibrium binding energy, lattice parameter and bulk modulus. The analysis is then extended, with the largest basis set, to transverse optical phonon frequencies, band-structure and charge-density data. The results show trends similar to those expected from molecular calculations; typically, the mean lattice parameter and bulk modulus errors obtained at a 6-21G* level are about +1% and +10%, respectively.

AM1 parameters for sulfur

Abstract: AM1 has been parametized for sulfur. Calculations are reported for a wide range of sulfur-containing molecules. The calculated heats of formation and other properties of organosulfur molecules are much superior to those from MNDO and superior overall to those from PM3. AM1 calculations for several reactions agree well which experimental values. The results for compounds of sulfur in its higher valence states are also satisfactory, except for SF 4 , where the error is probably due to the neglect of d AOs

The relationship between bond-valence sums and Tc in cuprate superconductors

Abstract: The recently demonstrated correlation of T c with copper or oxygen bond valence sums for a wide range of superconducting cuprates is shown to contradict the behaviour observed for YBa 2 CU 3 O x as x is varied from 6 to 7. This conflict is removed by introducing two independent variables, namely, the sum of and difference of the copper and oxygen bond valence sums. The latter is a measure of the hole density on the CuO 2 plane and as this increases T c passes through a maximum T c (max). The former indicates the relative distribution of these holes on the oxygen or copper sites for a given compound. T c (max) correlates closely with the preference for hole distribution on the oxygen atoms. A general phase-diagram using these bond sum parameters is presented.

Electronic properties of mixed valence oxide gels

Abstract: Transition metal ions exhibit several valence states. Redox reactions occur during the sol-gel synthesis of transition metal oxides. Mixed valence compounds are obtained. Their electronic properties arise from electron exchanges between metal ions in different valence states. Thermally activated electron hopping leads to semiconducting materials. Optical absorption arising from intervalence transfers gives rise to reversible optical switching. They can be used for making electrochromic display devices. Electron and ion transfers can occur at the oxide-water interface. They seem to be typical of mixed valence oxides having a spinel structure and lead to chemical modifications of both the oxide network and the solution.

Optical gain and gain suppression of quantum-well lasers with valence band mixing

Abstract: The effects of valence band mixing on the nonlinear gains of quantum-well lasers are studied theoretically for the first time. The analysis is based on the multiband effective-mass theory and the density matrix formalism with intraband relaxation taken into account. The gain and the gain-suppression coefficient of a quantum-well laser are calculated from the complex optical susceptibility obtained by the density matrix formulation with the theoretical dipole moments obtained from the multiband effective-mass theory. The calculated gain spectrum shows that there are differences (both in peak amplitude and spectral shape) between this model with valence band mixing and the conventional parabolic band model. The shape of the gain spectrum calculated by the new model becomes more symmetric due to intraband relaxation together with nonparabolic energy dispersions. Optical intensity in the GaAs active region is estimated by solving rate equations for the stationary states with nonlinear gain suppression. >

Neutron diffraction study of Pr valence and oxygen ordering in the Y1−xPrxBa2Cu3O7−δ system

Abstract: The crystallographic changes that occur with increasing Pr concentration in the Y−xPrxBa2Cu3O7−δ system have been studied by neutron diffraction. As x is increased from 0 to 1, the orthorhombic a, b and c lattice parameters expand monotonically, while the orthorhombic distortion, espressed as b a , decreases, reflecting increased occupation of O(5) sites at the expense of O(4) chain site occupancy. The separation of the CuO2 planes in PrBa2Cu3O7−δ indicates that the Pr ions have an intermediate valence of ∼3.3 in this compound. Evidence is presented for increased interaction between the Pr ions and oxygen ions in adjacent CuO2 planes.

Short range resonating valence bond theories and superconductivity

Abstract: We consider the nature of superconductivity near a spin-liquid state with a large spin-excitation-gap. We argue that the quantum-dimer-model with holes is a good approximation in this limit. The insulator is shown to be exactly equivalent to compact quantum electrodynamics, and has a massive spectrum. The doped system is a superconductor with a low density phase characterized by tightly bound pairs and a high density phase with two weakly coupled condensates.

Relative electron donor strengths of tetrathiafulvene derivatives: effects of chemical substitutions and the molecular environment from a combined photoelectron and electrochemical study

Abstract: Interest in organic metals and superconductors has prompted studies of the effects of chemical substituents on the organic electron donor tetrathiafulvalene (TTF). Electron-donating substituents on TTF should lead to reduced ionization potentials and generally greater electron transfer in organic donor/acceptor compounds. However, the relative electron donor abilities may also be influenced by their molecular environment and intermolecular interactions. In order to address these questions, the valence ionization potentials of TTF and two derivatives, bis(ethylenedioxo)tetrathiafulvalene (BEDO-TTF) and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), have been measured in the gas phase by photoelectron spectroscopy and compared with oxidation potentials from solution electrochemical measurements in a variety of solvents. The order of decreasing first ionization potentials is BEDT-TTF {approx} TTF (6.7 eV) > BEDO-TTF (6.46 eV).

Structure, Reactivity, Spectra, and Redox Properties of Cobalt(III) Hexaamines

Abstract: Publisher Summary This chapter discusses the effect of structural properties of cobalt (III) hexa-amines on their other properties. The chapter discusses complexes ranging from cobalt hexa-ammine, to complexes composed of three secondary and three tertiary amine donors. The chapter discusses only those coordination compounds, having a complete CoN 6 core, thus excluding the enormous number of CoN 6 - x L x complexes. Ammonia and organic amines with an aliphatic skeleton are described in the chapter. The chapter discusses complexes, containing just one kind of ligand—that is, homoleptic complexes. Structural properties and their relationship to kinetic stability (toward hydrolysis and racemization), electronic structure, reduction potentials, and electron transfer processes are explained in this chapter. The standard preparative procedure for the Co(III)–amine complexes consist of the reaction of an aqueous or alcoholic solution of a Co(II) salt with the free base of the amine. Oxidation in the solution is achieved by air, oxygen, H 2 0 2 , or other oxidants. In this chapter, the term molecular mechanics describes a method used frequently to predict structures and relative energies of related complexes (usually isomers). The method involves summing the potential energy of all the interatomic interactions in terms of bond deformation, valence angle deformation, torsion angle deformation, and nonbonded interactions. The chapter discusses the effects of structure on the other properties of the amine complexes predominantly in terms of the steric effects.

Mössbauer and electron paramagnetic resonance study of the double-exchange and Heisenberg-exchange interactions in a novel binuclear Fe(II/III) delocalized-valence compound

Abstract: In this paper we present the characterization by UV‐VIS, Mossbauer, and EPR spectroscopy of [L2Fe2(μ‐OH)3](ClO4)2⋅2CH3OH⋅2H2O, with L=N,N’,N‘‐trimethyl‐1,4,7‐triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange‐coupled delocalized‐valence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mossbauer spectroscopy. Mossbauer, EPR, and magnetic susceptibility data imply a system spin St =9/2 for the ground state. This finding is explained as being a consequence of the double‐exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV‐VIS, Mossbauer, and EPR spectroscopy are for the double‐exchange parameter B=1300 cm−1, the g factors gx,y =2.04 and gz =2.3, the parameters for zero‐field splitting D=4 cm−1 and E≊0 cm−1, and for the hyperfine parameters ΔEQ =−2.14 mm s−1, Ax,y =−21.2 T, Az =−27 T, and δ=0.74 mm s−1. From our temperature‐dependent studies we assign to the first excited state a spin‐octet with an excitation energy Δ>175 cm−1. From this value a lower bound of −235 cm−1 has been deduced for the exchange‐coupling constant J. In the framework of a simplified description of the iron atoms by unperturbed 3d orbitals, the values of the A tensor components as well as the quadrupole splitting ΔEQ can be interpreted in a consistent manner by assuming the excess electron being delocalized over two dσ orbitals centered at the two iron sites of the dimer and directed along the iron–iron axis as the z direction.

Simulation of ionic crystals: The ab initio perturbed-ion method and application to alkali hydrides and halides

Abstract: A new theoretical scheme appropriate for studying the electronic structure of ionic crystals is presented and applied to nine AB-type cubic lattices (A=Li,Na,K; B=H,F,Cl). The scheme, called the ab initio perturbed-ion (PI) method, is based on the theory of electronic separability and the ab initio model-potential approach of Huzinaga. In the PI method, the self-consistent-field (SCF) equations for each different lattice ion are first solved in a lattice potential that contains nuclear attraction, Coulombic, and nonlocal exchange operators, and lattice projectors enforcing the required ion-lattice orthogonality. The ionic SCF solutions are then used to compute the lattice potential, and the process is repeated until ion-lattice consistency is achieved. The lattice energy and other equilibrium properties are immediately obtained from the PI wave functions. The most remarkable ideas suggested by this crystal simulation are the following: (a) The crystal potential produces a contraction of the free-ion valence radial density, large for the anions but very small for the cations, that works as a bonding mechanism able to describe accurately the stability and equilibrium elastic constants of simple ionic crystals; (b) the PI method explains well the variation of several crystal properties with hydrostatic pressure; (c) the crystal bonding can be clearly analyzed in terms of simple cationic and anionic contributions. Moreover, the PI code may be used as an efficient source of environment-consistent ionic wave functions and energies.

X-ray absorption near-edge studies of substitution for Cu in YBa2(Cu1-xMx)3O7- delta (M=Fe, Co, Ni, and Zn).

Abstract: We report x-ray absorption near-edge measurements of YBa{sub 2}(Cu{sub 1{minus}{ital x}M{ital x}}){sub 3}O{sub 7{minus}{delta}} ({ital M}=Fe, Co, Ni, and Zn). Our study is primarily to determine several important material parameters, i.e., the location of dopants, the valence of dopants, structural changes, modifications of electronic densities of states, and the distribution of holes, which are thought to be key factors for understanding mechanisms causing the suppression of superconductivity via metal doping. Our results indicate that each metal substituent has a preference for a specific Cu site or combination of sites. We find that Fe and Co preferentially substitute for the Cu(1) atom at the linear-chain site; Ni resides in both Cu(1) and Cu(2) (the plane site); Zn occupies only the plane position Cu(2). In all cases, the metal-oxygen bond lengths (Fe-O, Co-O, Ni-O, and Zn-O) and valence states of dopant (mainly Fe{sup 3+}, Co{sup 3+}, Ni{sup 2+}, and Zn{sup 2+}) show little dependence on dopant concentration. Metal doping also has little effect on the valence of copper. However, each dopant affects to a different degree the oxygen 2{ital p} states. The changes of the oxygen 2{ital p} states for Fe-, Co-, and Ni-doped samples may be related to a reduction ofmore » some oxygen hole states. This reduction is attributed to a redistribution of charge carriers in the chain layer and possibly an inhibition of charge transfer from the chains to the planes. However, Zn substitutions show no observable change in its valence and oxygen hole states, indicating a different mechanism for suppression of superconductivity. Several possible explanations are offered that may elucidate the large suppression of {ital T}{sub {ital c}} in the Zn-doped case.« less

Electronic structure of poly(tetrafluoroethylene) studied by UPS, VUV absorption, and band calculations

Abstract: The electronic structure of poly(tetrafluoroethylene) (PTFE) was studied by UPS, VUV absorption, and ab-initio MO calculations. The UPS spectra give a photoemission threshold energy of 10.6eV, with deeper valence band features consistent with the reported XPS and the oligomer vapour UPS spectra. The UPS spectra are also consistent with the density of states derived from the calculated band structure, which indicates that the uppermost part of the valence band is formed from the C and F 2p orbitals with C-C bonding and C-F antibonding combinations. The VUV absorption spectrum shows an intense peak at 7.7eV, which most probably corresponds to the valence excitation from the top of the valence band to the bottom of the conduction band. With these data, the structure of the occupied and vacant states are deduced.

Atomic Magnetic Moment and Exchange Interaction between Mn Atoms in Intermetallic Compounds in Mn–Ge System

Abstract: Magnetic properties of four intermetallic compounds, e, e 1 , κ and η, in manganese-germanium system are studied by means of magnetization measurement and neutron diffraction. Exchange coefficients of the interaction between Mn atoms in the compounds are estimated quantitatively using the molecular field approximation, based on the localized moment model. The exchange coefficient between Mn atoms on different sublattices is found to depend largely on the interatomic distance: The exchange coefficient is found to be negative for smaller interatomic distances than r c =2.9 A and positive for larger distances. The magnitude of atomic magnetic moment in the compounds is consistently discussed using the Pauling valence, after Mori and Mitsui (J. Phys. Soc. Jpn. 25 (1968) 82).

XPS studies on cerium, zirconium and yttrium valence states in plasma‐sprayed coatings

Abstract: X-ray photoelectron spectroscopy (XPS) is used for investigating the valence state of the chemical elements present in 25.5 wt.% CeO2−2.5 Y2O3−72 ZrO2 plasma-sprayed coatings as a function of the deposition temperature. Unlike the other chemical elements, cerium changes its valence state during and after the spraying process. The analysis of Ce 3d binding energies and satellite structures shows that CeO2 after spraying is either partially reduced to Ce(III) at low deposition temperature, or remains essentially unchanged at high deposition temperature. In both cases, the presence of ZrO2−x suboxides is found, which does not depend apparently on the cooling rate.

Electrochemistry and spectroelectrochemistry of 1,8-naphthalene- and 1,8-anthracene-linked cofacial binuclear metallophthalocyanines. New mixed-valence metallophthalocyanines

Abstract: : 1,8-Napthalene or 1,8-anthracene-linked cofacial dizinc, dicopper, and dicobalt diphthalocyanines have been studied by solution and surface electrochemistry, spectroelectrochemistry, and electron spin resonance (ESR). These derivatives are mixtures of syn and anti isomers which have very similar electrochemistry except where we comment specifically. The phthalocyanine ring first oxidation phthalocyanine, the Cobalt(III)/Cobalt(II) and Cobalt(II)/ cobalt(I) redox couples split into two couples as a consequence of intra-ring exchange interactions. The spectra of the electrochemically oxidised or reduced species, and in particular, those of the mixed valence species are recorded. Exciton coupling energies are derived and are seen to be related to the ground state mixed valence splitting energies. These are discussed in terms of structure and inter-ring distance. Cobalt derivatives immobilized onto ordinary pyrolytic graphite catalyze the electroreduction of oxygen by two electrons to hydrogen peroxide. (JG)

Synthesis of the defect perovskite series LaCu03–δ with copper valence varying from 2+ to 3+

Abstract: MANY important questions about the origin of superconductivity in the high-temperature copper oxide superconductors could be addressed by the study of a copper oxide compound exhibiting a wide range of oxygen stoichiometry while maintaining the basic perovskite structure. A number of compounds are known in which copper occurs formally as Cu3+ (refs 1, 2), but most are structurally unrelated to the superconductors, so that a direct comparison of their chemical and physical properties is not appropriate. Here we report the synthesis, structure and preliminary characterization of a new oxygen-deficient perovskite series, LaCuO3–δ. Although superconductivity has not been observed in this system, its oxygen stoichiometry range, 0.0 < δ < 0.5, is the widest yet found in a copper oxide compound, allowing the formal copper valence to be varied from 2+ to 3+. This series may thus serve as a model system with which to address such questions as the nature of the Cu/O hole states in the superconductors, and the way in which magnetic and transport properties vary with carrier concentration.

Distance dependence of the effective coupling parameters through conjugated ligands of the polyene type

Abstract: Experimental results on the decay of the effective coupling parameter in mixed valence ruthenium complexes bridged by bipyridylpolyenes are discussed. Absolute values of the couplings are satisfactorily reproduced by extended Huckel calculations. An exponential decrease of the effective coupling is found, with a particularly small exponent, 0.078 A−1, corresponding to a decrease by a factor of 2 for each 9 A in the metal-metal distance. The results are also discussed in the frame of a simplified valence bond taking into account the main structural features of the system. It is concluded that, in the present compounds, the transmission of electronic couplings over large distances is limited by the Peierls distortion of the polyene chain.

Palladium clusters on graphite: evidence of resonant hybrid states in the valence and conduction bands

Abstract: The electronic structure of Pd clusters deposited on polycrystalline graphite has been investigated by x-ray photoemission, Auger spectroscopy, and bremsstrahlung isochromat spectroscopy. Initial- and final-state shifts of energy levels as a function of the average cluster size R are small, but important modifications of the density of filled and empty states at the Fermi level are observed. With decreasing particle size, the d portion of the conduction band broadens and shifts to higher energy while its valence counterpart shows the opposite shift. A simple analytical model is developed which accounts in some detail for the measured shifts and linewidths and also explains earlier x-ray-absorption data. The size dependences are explained in terms of mixing of Pd 4d and graphite * continua with a squared coupling constant V2 proportional to R-1. As a consequence a gap opens up across the Fermi level in the cluster density of states and unoccupied hybrid resonances arise as observed experimentally. Initial-state energy shifts and cluster charging are estimated and found to play a minor role for the present system.

Electrochromism in the near-infrared absorption spectra of bridged ruthenium mixed-valence complexes

Abstract: Many experimental and theoretical approaches have been developed to characterize the chemical and physical properties of mixed-valence complexes. These molecules may possess metals in differing oxidation states which participate in intervalence charge-transfer transitions. In principle, these transitions should be strongly affected by an external electric field. Such electrochromism can provide a direct and sensitive approach to investigating the electronic properties of molecules. The authors report the first measurements of the effects of an externally applied electric field on the near-infrared absorption spectra of ((NH{sub 3}){sub 5}Ru){sub 2}L{sup 5+} (L = pyrazine or 4,4{prime}-bipyridine). Significant differences are observed between the two complexes, illustrating the range of electronic interactions between the metal centers.

Pseudospectral generalized valence‐bond calculations: Application to methylene, ethylene, and silylene

Abstract: The pseudospectral (PS) method for self‐consistent‐field calculations is extended for use in generalized valence‐bond calculations and is used to calculate singlet–triplet excitation energies in methylene, silylene, and ethylene molecules and bond dissociation and twisting energies in ethylene. We find that the PS calculations lead to an accuracy in total energies of ≤0.1 kcal/mol and excitation energies to ≤0.01 kcal/mol for all systems. With effective core potentials on Si, we find greatly improved accuracy for PS.