Valence Structure Research Articles

Pure tetravalent nickel in γ-type nickel oxyhydroxide as secondary battery electrode

Pure tetravalent nickel in γ-type cobalt substituted nickel oxyhydroxide, Ni0.70Co0.30O2K0.30(H2O)0.42, could be obtained by the “chimie douce” reaction. The presence of tetravalent nickel is confirmed by comparing the Ni K-edge XANES spectrum of the sample with those of reference compounds having various nickel valency and similar layer structure. The Co K-edge XANES spectrum indicates that the trivalent cobalt remains unchanged regardless of the nickel valency. The structural modification during chimie douce reaction observed from XRD patterns and the result of iodometric titration are consistent with the Ni and Co K-edge XANES data.

Mesomeric and π-moments in some hetero-conjugated compounds

A comparison of the dipole moments of adequately chosen compounds with those calculated for their classical valence structures affords more significant values for the mesomeric moments of the N−C = Y mesomeric groups (with Y = C,N,O or S) when taking into account the π-inductive effect of the nitrogen atom on the C= Y bond. In so doing, HMO π-diagrams show a nil charge at the central carbon atom as predicted by the contributing valence structure Z (Fig. 1). The π-molecular diagram was also constructed for acrolein, indicating a small π-charge at the C(2) carbon atom. Ab initio electronic structures (a 6-31G basis set) are also determined for vinylamine, formamidine, formamide and thioformamide and interpreted according to the classical resonance concept.

A study of iso- and alio-valent cation doped Ag 2 SO 4 solid electrolyte

SO4. The solid solubility limits up to x≤3 mole% for monovalent, x≤5.27 mole% for divalent and x≤3.63 mole% for trivalent cation doped Ag2SO4 are set with XRD, SEM, IR and DSC techniques. A predominant dependence of conductivity on the ionic size of iso- and alio-valent cations is observed. In particular, the conductivity enhances in both α and β phases, despite having a lower ionic-size dopant cation (relative to that of Ag+) in the transition element cation doped Ag2SO4. Ca2+, Ba2+, Y3+ and Dy3+ doped samples show depature from the regular behaviour in the β-phase. The conductivity behaviour is discussed considering ionic size, valence and electronic structure of the guest cations.

Electronic communication between carbon chain bridged metals: a theoretical approach. Even Chains

A theoretical investigation (DFT) allowed us to single out the most appropriate valence structure for the [M–Cx–M] unit in a variety of model compounds [{Cp(CO)2M}2(µ-Cx)] (M = Cr, Mn or Fe; x = 2–8) and to foresee it on the basis of the molecular parameters, such as (i) the chain length, (ii) the nature of the metal and its oxidation state, (iii) the dn configuration and the metal co-ordination number. The valence structure of the [M–Cx–M] unit is diagnostic of the metal-to-metal communication. A simple electron-counting scheme has been developed to predict the valence structure, based on the dn configuration of the MLm fragments and the number of pπ electrons of the linear Cx unit.

Study of the Heterometallic Bond Nature in PdCu(111) Surfaces

In this paper a theoretical study of the bonding mechanism between heteroatoms of PdCu alloys using cluster models and several ab initio methods is reported. Models of the Pd8Cu92(111) and Pd40Cu60(111) surfaces have been analyzed. For these Cu-rich alloys, we find that surface Pd centers are negatively charged. The Pd net charge increases with the Cu/Pd atomic ratio of the alloy in parallel with a decrease of the Pd 4d electronic population. The influence of the bimetallic bond on the electronic structure of core, valence, and Fermi levels is discussed, and the apparent contradiction between the net negative charge predicted for Pd and the positive binding energy shift computed and experimentally observed in these alloys for Pd core and Pd-like valence levels is interpreted.

Cobalt valence and crystal structure of La0.5Sr0.5CoO2.25

Cobalt valence and crystal structure of La0.5Sr0.5CoO2.25

A critical survey on the mesomeric moments of nitrogen-oxide compounds

A comparison of the experimental dipole moments of the N-oxides of pyridine, pyrazine, benzylidenemethylamine, benzylideneaniline, benzonitrile, ( Z)- and ( E)-diphenyldiazenes with the corresponding un-oxided nitrogen compounds enabled us to obtain information about the electronic structure of the quoted N-oxides. Assuming these compounds are resonance hybrids between the valence structures A = (A o ⇆ P) with a dative N → O bond and D with an O = N + double bond, whilst P visualizes the π-inductive effect of the N +(O)nitrogen atom on the adjacent π-electron system, the relative contribution from the D structure was determined in most cases from the molecular mesomeric moment, defined as m rarr ∗ = μ rarr exp. − M( A) , and (with a lesser accuracy) from the molecular moment (as a vector) and the values calculated for the A and D structures. Attempts were made to rationalize the mesomeric moments of the N, N-dimethylamino and nitro groups in the 4-substituted pyridines and pyridine N-oxides, as well as the interaction moments of these two groups with the CN or NN bond in the 4-substituted benzylidenemethylamines, benzylideneanilines and ( E)-diphenyldiazenes, and (if any) in the corresponding N-oxides. In addition arguments are given supporting the existence of a mesomeric effect in nitrobenzene.

Can the Low-Lying Electronic States of Benzenoid Hydrocarbons Be Described by the Semiempirical Valence Bond Approach?

The low-lying excited states of benzenoid hydrocarbons with up to 22 π-electrons have been obtained from exact semiempirical valence bond (VB) calculations. The effective valence bond (EVB) model, which implements nonadjacent and cyclic six-body interactions on the basis of the classical valence bond (CVB) model, is shown to give satisfactory predictions of the low-lying excited spectra of small benzenoids with no more than 14 π-electrons. For larger benzenoid systems, the CVB model can account well for their lowest excited spectra, being in good agreement with the available experimental data and semiempirical PPP molecular orbital results. In addition, a good quadratic relationship between the energy difference between the ground and first excited states and the number of Kekulé valence structures is revealed among the isomeric benzenoid systems.

Sulfur Multicenter Bonding and Distortions in a Hypothetical Polydithioquinone

The electronic properties of polydithioquinone, a hypothetical C4S2 carbon-sulfur polymer related to both tetrathiotetracene and thiothiophthene, are examined using approximate molecular orbital calculations. We focus on a curious pattern of sulfur-sulfur bonding interactions suggested by the band structure calculations, a pattern observed in thiothiophthenes and other molecular analogues of our polymer. The ﷿ system of each C4S2 repeat unit contains eight electrons, consistent with a thiothiophthene-like rather than localized polydithioquinone valence structure. A pairing distortion in the carbon sublattice is implied; furthermore, the half-filled sulfur U bands suggest that the sulfur sublattice is also likely to undergo distortions. Restricting our consideration to those distortions that retain planarity of the polymer, we find deformations of the carbon backbone considerably less favorable energetically than those of the sulfur sublattice. Four nicely bond-localized isomers result from the distortions of both carbon and sulfur sublattices; these all are found to be low bandgap semiconductors. We also examine helical distortions and find a stable isomer with an approximately 10-fold helical axis.

Resonance in catacondensed benzenoid hydrocarbons

We consider resonance in cata-condensed benzenoids having six and seven fused benzene rings. The resonance relationship between the Kekule valence structures of the molecules is represented by the resonance graphs in which the vertices represent the Kekule valence structures, and the edges, the presence of the quantum chemical resonance integral involving permutation of three CC double bonds (within a single six-membered ring). The construction of the resonance graphs for large benzenoids is outlined and the properties of the derived resonance graphs are discussed. It is shown that the leading eigenvalue of the resonance graphs correlates with the resonance energy of benzenoids. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 585–600, 1997

Resonance in catacondensed benzenoid hydrocarbons

We consider resonance in cata-condensed benzenoids having six and seven fused benzene rings. The resonance relationship between the Kekule valence structures of the molecules is represented by the resonance graphs in which the vertices represent the Kekule valence structures, and the edges, the presence of the quantum chemical resonance integral involving permutation of three CC double bonds (within a single six-membered ring). The construction of the resonance graphs for large benzenoids is outlined and the properties of the derived resonance graphs are discussed. It is shown that the leading eigenvalue of the resonance graphs correlates with the resonance energy of benzenoids. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 585–600, 1997

Many-body valence-bond theory

The classical theory of chemical valence, first, is naturally formalized in mathematics in the area of graph theory and, second, finds an extension in quantum mechanics in terms of the Heitler–London–Pauling “valence-bond” (VB) theory. Thus, VB theory stands in a fairly unique position, although in quantum chemistry, there often has been a preference for the alternative (perhaps even “complementary”) molecular orbital (MO) theory, presumably in large part because of computational efficacy for general molecular structures. Indeed, as formulated by Pauling and others, VB theory was described as a configuration interaction (CI) problem when there were multiple relevant classical valence structures for the same molecular structure. Also, as now recognized, a direct assault on CI is computationally intensive, prone to size-inconsistency problems, and effectively limited to smaller systems—whereas indirect approaches, e.g., via wave-function cluster expansions or renormalization-group theory, often neatly avoid these problems. Thus, what is (and perhaps always has been) needed is “many-body” schemes for VB computations (as well as for higher-order MO-based approaches, too). Here, then, certain such many-body VB-amenable computational schemes are to be discussed, in the context of semiempirical (explicitly correlated) graphical models. The collection of models are described and interrelated in a fairly comprehensive systematic manner. A selection of many-body cluster expansion methods are then discussed with special reference to resonating VB wave functions and the fundamental graph-theoretic nature of the consequent problems (such as also are noted to arise in lattice-discretized statistical-mechanical problems, too). Some examples are described incorporating resonance among exponentially great numbers of VB structures as applied: for large icosahedral-symmetry fullerenic structures, for the (polyacetylenic) linear chain, and for ladderlike conjugated polymers. It is contended that practicable many-body VB-theoretic methods are now available, retaining clear links to classical chemical valence theory. Hopefully, too, these methods may soon find use beyond the semi-empirical framework. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 421–438, 1997

Use of quark wave functions in the calculation of structure functions of mesons.

In the description of deep-inelastic scattering the most extensively used model is the parton model. However, there has been an ongoing effort to understand structure functions of nucleons and mesons in terms of wave functions of quarks. Calculations made using the constituent quark model usually do not fit the experimental data. It has been argued that such calculations refer to some low-momentum scale parameterized by {ital Q}{sup 2}{sub 0}. The calculated distribution is then evolved up to a large scale, {ital Q}{sup 2}, by perturbative QCD evolution. The change needed if the distribution is to fit the data is quite large and, therefore, the scale {ital Q}{sup 2}{sub 0} is taken to be quite small. The large changes required for the distribution makes this procedure suspect. In this work we introduce a class of wave functions that are meant to apply at the scale {ital Q}{sup 2}, so that evolution is not required. These wave functions describe the motion of a current quark, or parton, relative to a residual system. (In order to calculate a cross section the residual system is placed on mass shell in the calculation.) Our procedure is inherently asymmetric in the description of the struck particlemore » and the residual system. We show how we are able to fit the (valence) structure function of the pion at {ital Q}{sup 2}=20 GeV{sup 2} (without QCD evolution) using our scheme. {copyright} {ital 1996 The American Physical Society.}« less

Near-edge x-ray-absorption study of transition-metal-implantedYBa2Cu3O7−δ

We present near-edge measurements for $K$-absorption edges of Ni-, Fe-, and Co-implanted ultrahigh quality single-crystal and polycrystalline $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO). The samples were implanted using a metallized vapor vacuum arc high current ion source with a dose of approximately 1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ions/${\mathrm{cm}}^{2}$, and subsequently annealed in an oxygen atmosphere for approximately 100 h. The results show that upon annealing the Feimplanted sample has a valence structure that is consistent with doped powder studies. This suggests the substitution within the lattice in the Cu(1) and Cu(2) sites for the implanted ions, while the nickel implant case showed a valence structure closer to NiO with no shift in edge energy being observed during the anneal process. This was similar to the Co-implanted result that also showed very little change in valence structure after annealing, with a valence closer to ${\mathrm{Co}}^{3+}$. We also performed Cu $K$-edge absorption measurements of the implanted materials in comparison with YBCO and found no change in edge energy or structure. The results are consistent with previous susceptibility studies.

Influence of water vapor and oxygen on the tribology of carbon materials with sp 2 valence configuration

The tribological behaviors of graphite, non-graphitic carbon, carbon-carbon composite, and unhydrogenated amorphous carbon in nitrogen and air are investigated. The friction and wear of all carbon materials can be similarly influenced by water vapor and oxygen. Carbon materials generally exhibit a high friction and wear behavior when there is a lack of lubricant vapors in the environment. Water vapor, even in small quantities, is effective in lubricating carbon materials-a concentration of 100 ppm in nitrogen can lubricate most of the materials examined in this study. Depending on the water vapor content in the environment and the friction regime involved, oxygen can act either as a lubricant or an agent for tribochemical wear. The tribological properties of carbon materials depend on gases and vapors in the environment that can interact with carbon atoms. In spite of having a wide range of physical properties, the family of sp 2 carbon materials can exhibit similar friction and wear behaviors in various environments owing to their common valence structure.

Nucleon structure functions at moderate Q2: Relativistic constituent quarks and spectator mass spectrum

We present a model description of the nucleon valence structure function applicable over the entire region of the Bjorken variable x, and above moderate values of Q 2 (∼ 1 GeV 2). We stress the importance of describing the complete spectrum of intermediate states which are spectator to the deep-inelastic collision. At a scale of 1 GeV 2 the relevant degrees of freedom are constituent quarks and pions. The large- x region is then described in terms of scattering from constituent quarks in the nucleon, while the dressing of constituent quarks by pions plays an important role at intermediate x values. The correct small- x behavior, which is necessary for the proper normalization of the valence distributions, is guaranteed by modeling the asymptotic spectator mass spectrum according to Regge phenomenology.

Contrastive verb valency and conceptual structures in the verbal lexicon

The CONTRAGRAM Contrastive Verb Valency Dictionary of Dutch, French and English not only describes interlingual differences between different types of verb complementation exhaustively, it also provides a semantic picture of the entry verbs. Next to a lexical approach to verb valency, a multidirectional and non-reductivist contrastive approach therefore offers a suitable basis for describing conceptual differences between various languages. These conceptual structures in the verbal lexicon are illustrated by means of a concrete contrastive lexicological analysis of the Dutch verb beslissen and its proto-translatious, French décider and English decide.

Linear Free Energy Relationships in Dinuclear Compounds. 2. Inductive Redox Tuning via Remote Substituents in Quadruply Bonded Dimolybdenum Compounds.

Syntheses and characterizations are reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr(-)) ligand, where Ar is XC(6)H(4)(-), with X as p-OMe (1), H (2), m-OMe (3), p-Cl (4), m-Cl (5), m-CF(3) (6), p-COMe (7), p-CF(3) (8), or Ar is 3,4-Cl(2)C(6)H(3)(-) (9) or 3,5-Cl(2)C(6)H(3)(-) (10). The (quasi)reversible oxidation potentials measured for the Mo(2)(5+)/Mo(2)(4+) couple were found to correlate with the Hammett constant (sigma(X)) of the aryl substituents according to the following equation: DeltaE(1/2) = E(1/2)(X) - E(1/2)(H) = 87(8sigma(X)) mV. Molecular structure determinations of compounds 1, 2, 5, and 10 revealed an invariant core geometry around the Mo(2) center, with statistically identical Mo-Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Å, respectively. Magnetic anisotropies for compounds 1-10 estimated on the basis of (1)H NMR data were similar and unrelated to sigma(X). Similarity in UV-vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and (1)H NMR spectra, was interpreted as the existence of a constant upper valence structure across the series. Results of Fenske-Hall calculations performed for several model compounds paralleled the experimental observations.

A critical survey on the mesomeric moments of amidic compounds

Amides and amidines contain an NCY group which can be regarded as a resonance hybrid between a classical and a zwitterion valence structure (I and II), whose calculated dipole moments (as vectors) are M → ( I) and M → ( II) . The mesomeric moment of these compounds ( m → ) can be obtained from their their dipole moment μ → ) in two ways: (a) from M → ( I) assuming the direction of m → , indicated by the angle τ between NC and m → ; (b) by using M → ( I) and M → ( II) , with no assumption about the value of τ. Methods (a) and (b) were applied to the amides quoted in Tables 1 and 3, whilst first procedures have been used for two aliphatic amidines and for N, N-dimethyl-benzamide, -thiobenzamide and -benzamidine. The conformations in solution of sterically crowded tetramethyl-urea and -thiourea were predicted in the light of their mesomeric moments by application of an equation relating the m-values with the shape of the nitrogen pyramids (not quite flattened as in uncrowded amides and amidines), and their rotation about the relevant NC axes. The relative contribution of II was determined for all the amides quoted in Table 3 and for four amides, by taking into account Proutiére's induction terms. Finally the relation between the π-moments and mesomeric moments is outlined, and the method used for the calculation of M → ( I) and M → ( II) is presented in the Appendix.

Relativistic electronic structure, effective masses, and inversion-asymmetry effects of cubic silicon carbide (3C-SiC).

We present relativistic linear-muffin-tin-orbital calculations of the electronic structure of zinc-blende-type silicon carbide (3C-SiC) within the local-density approximation. Information about matrix elements, effective masses, Luttinger parameters, as well as linear and cubic spin splittings due to inversion-asymmetry effects is extracted by comparison with k\ensuremath{\cdot}p calculations. Ionicity and transverse effective charge are also discussed. The parameters determined in this way are subsequently used as input in an extended 16\ifmmode\times\else\texttimes\fi{}16 k\ensuremath{\cdot}p calculation so as to obtain the detailed band structure of the higher valence and the lower conduction band states around the \ensuremath{\Gamma} point in the [100], [110], and [111] directions.