Showing papers on "Valence (chemistry) published in 2003"

Optimized Slater-type basis sets for the elements 1-118.

Abstract: Seven different types of Slater type basis sets for the elements H (Z = 1) up to E118 (Z = 118), ranging from a double zeta valence quality up to a quadruple zeta valence quality, are tested in their performance in neutral atomic and diatomic oxide calculations. The exponents of the Slater type functions are optimized for the use in (scalar relativistic) zeroth-order regular approximated (ZORA) equations. Atomic tests reveal that, on average, the absolute basis set error of 0.03 kcal/mol in the density functional calculation of the valence spinor energies of the neutral atoms with the largest all electron basis set of quadruple zeta quality is lower than the average absolute difference of 0.16 kcal/mol in these valence spinor energies if one compares the results of ZORA equation with those of the fully relativistic Dirac equation. This average absolute basis set error increases to about 1 kcal/mol for the all electron basis sets of triple zeta valence quality, and to approximately 4 kcal/mol for the all electron basis sets of double zeta quality. The molecular tests reveal that, on average, the calculated atomization energies of 118 neutral diatomic oxides MO, where the nuclear charge Z of M ranges from Z = 1-118, with the all electron basis sets of triple zeta quality with two polarization functions added are within 1-2 kcal/mol of the benchmark results with the much larger all electron basis sets, which are of quadruple zeta valence quality with four polarization functions added. The accuracy is reduced to about 4-5 kcal/mol if only one polarization function is used in the triple zeta basis sets, and further reduced to approximately 20 kcal/mol if the all electron basis sets of double zeta quality are used. The inclusion of g-type STOs to the large benchmark basis sets had an effect of less than 1 kcal/mol in the calculation of the atomization energies of the group 2 and group 14 diatomic oxides. The basis sets that are optimized for calculations using the frozen core approximation (frozen core basis sets) have a restricted basis set in the core region compared to the all electron basis sets. On average, the use of these frozen core basis sets give atomic basis set errors that are approximately twice as large as the corresponding all electron basis set errors and molecular atomization energies that are close to the corresponding all electron results. Only if spin-orbit coupling is included in the frozen core calculations larger errors are found, especially for the heavier elements, due to the additional approximation that is made that the basis functions are orthogonalized on scalar relativistic core orbitals.

Gaussian basis sets of quadruple zeta valence quality for atoms H–Kr

Abstract: We present Gaussian basis sets of quadruple zeta valence quality with a segmented contraction scheme for atoms H to Kr. This extends earlier work on segmented contracted split valence (SV) and triple zeta valence (TZV) basis sets. Contraction coefficients and orbital exponents are fully optimized in atomic Hartree–Fock (HF) calculations. As opposed to other quadruple zeta basis sets, the basis set errors in atomic ground-state HF energies are less than 1 mEh and increase smoothly across the Periodic Table, while the number of primitives is comparably small. Polarization functions are taken partly from previous work, partly optimized in atomic MP2 calculations, and for a few cases determined at the HF level for excited atomic states nearly degenerate with the ground state. This leads to basis sets denoted QZVP for HF and density functional theory (DFT) calculations, and for some atoms to a larger basis recommended for correlated treatments, QZVPP. We assess the performance of the basis sets in molecular HF, DFT, and MP2 calculations for a sample of diatomic and small polyatomic molecules by a comparison of energies, bond lengths, and dipole moments with results obtained numerically or using very large basis sets. It is shown that basis sets of quadruple zeta quality are necessary to achieve an accuracy of 1 kcal/mol per bond in HF and DFT atomization energies. For compounds containing third row as well as alkaline and earth alkaline metals it is demonstrated that the inclusion of high-lying core orbitals in the active space can be necessary for accurate correlated treatments. The QZVPP basis sets provide sufficient flexibility to polarize the core in those cases. All test calculations indicate that the new basis sets lead to consistent accuracies in HF, DFT, or correlated treatments even in critical cases where other basis sets may show deficiencies.

Conduction and Valence Band Positions of Ta2O5, TaON, and Ta3N5 by UPS and Electrochemical Methods

Abstract: The conduction and valence band edges for electronic band gaps and Fermi levels are determined for Ta2O5, TaON, and Ta3N5 by ultraviolet photoelectron spectroscopy (UPS) and electrochemical analyses. Reasonable agreement between the results of the two methods is obtained at the pH at which the ζ potentials of the particles are zero. The tops of the valence bands are found to be shifted to higher potential energies on the order Ta2O5 < TaON < Ta3N5, whereas the bottoms of the conduction bands are very similar in the range −0.3 to −0.5 V (vs NHE at pH = 0). From the results, it is concluded that TaON and Ta3N5 are promixing catalysts for the reduction and oxidation of water using visible light in the ranges λ < 520 nm and λ < 600 nm, respectively. It is also demonstrated that the proposed UPS technique is a reliable alternative to electrochemical analyses for determining the absolute band gap positions for materials in aqueous solutions that would otherwise be difficult to measure using electrochemical methods.

Systematic behaviour in trivalent lanthanide charge transfer energies

Abstract: Information on the energy that is needed to transfer an electron from the valence band of an inorganic compound to a trivalent lanthanide impurity is presented. The energy is a measure of the location of the ground state of the divalent lanthanide relative to the top of the valence band. A variation with type of lanthanide is found that is the same irrespective of the type of compound (fluorides, chlorides, bromides, iodides, oxides, sulfides). The variation is anti-correlated with the known variation in fd transition energies in divalent lanthanides. Because of the anti-correlation, the energy difference between the first 4fn−15d state and the bottom of the conduction band is relatively invariant with type of lanthanide ion. The difference is largest for Eu2+, and decreases gradually towards the end of the lanthanide series by 0.5 eV for Y b2+. Based on the systematic variation in charge transfer energy and fd energy, a three-parameter model is presented to position the energy levels for each divalent lanthanide relative to valence and conduction band states. Using a similar model the levels of trivalent lanthanides are positioned.

An ab initio model of electron transport in hematite (α-Fe2O3) basal planes

Abstract: Transport of conduction electrons through basal planes of the hematite lattice was modeled as a valence alternation of iron cations using ab initio molecular orbital calculations and electron transfer theory. A cluster approach was successfully implemented to compute electron-transfer rate-controlling quantities such as the reorganization energy and electronic coupling matrix element. Localization of a conduction electron at an iron lattice site is accompanied by large iron–oxygen bond length increases that give rise to a large internal component of the reorganization energy (1.03 eV). The internal reorganization energy calculated directly is shown to differ from Nelsen’s four-point method due to the short-range covalent bridge interaction between the Fe–Fe electron transfer pair in the hematite structure. The external reorganization energy arising from modification of the lattice polarization surrounding the localization site is predicted to contribute significantly to the total reorganization energy. Th...

Geometrical and electronic structures of gold, silver, and gold-silver binary clusters: Origins of ductility of gold and gold-silver alloy formation

Abstract: The structures of pure gold and silver clusters (Auk, Agk, k = 1−13) and neutral and anionic gold−silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination i...

Noncovalent functionalization of carbon nanotubes by aromatic organic molecules

Abstract: The interaction between carbon nanotubes and organic molecules including benzene (C6H6), cyclohexane (C6H12), and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ: C8N2O2Cl2) have been studied using first principles calculations. The equilibrium tube-molecule distance, adsorption energy, and charge transfer are obtained. The hybridization between the DDQ molecular level and nanotube valence bands transforms the semiconducting tube into a metallic one. Coupling of π electrons between tubes and aromatic molecules are observed. Our results show that noncovalent functionalization of carbon nanotubes by aromatic molecules is an efficient way to control the electronic properties of carbon nanotubes.

CO Oxidation Behavior of Copper and Copper Oxides

Abstract: Carbon monoxide oxidation activities over Cu, Cu2O, and CuO were studied to seek insight into the role of the copper species in the oxidation reaction. The activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu2O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enables Cu2O to exhibit higher activities than the other two copper species. The non-stoichiometric metastable copper oxide species formed during reduction are very active in the course of CO oxidation because of its excellent ability to transport surface lattice oxygen. Consequently, the metastable cluster of CuO is more active than CuO, and the activity will be significantly enhanced when non-stoichiometric copper oxides are formed. In addition, the light-off behaviors were observed over both Cu and Cu2O powders. CO oxidation over metallic Cu powders was lighted-off because of a synergistic effect of temperature rises due to heat generation from Cu oxidation as well as CO oxidation over the partially oxidized copper species.

Performance of coupled cluster theory in thermochemical calculations of small halogenated compounds

Abstract: Atomization energies at 0 K and heats of formation at 298 K were obtained for a collection of small halogenated molecules from coupled cluster theory including noniterative, quasiperturbative triple excitations calculations with large basis sets (up through augmented septuple zeta quality in some cases). In order to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, we adopted a composite theoretical approach which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory energies and (up to) five corrections: (1) a core/valence correction; (2) a Douglas–Kroll–Hess scalar relativistic correction; (3) a first-order atomic spin–orbit correction; (4) a second-order spin–orbit correction for heavy elements; and (5) an approximate correction to account for the remaining correlation energy. The last of these corrections is based on a recently proposed approximation to full configuration interaction via a continued fraction approximant for coupled cluster theory [CCSD(T)-cf]. Failure to consider corrections (1) to (4) can introduce errors significantly in excess of the target accuracy of ±1 kcal/mol. Although some cancellation of error may occur if one or more of these corrections is omitted, such a situation is by no means universal and cannot be relied upon for high accuracy. The accuracy of the Douglas–Kroll–Hess approach was calibrated against both new and previously published four-component Dirac Coulomb results at the coupled cluster level of theory. In addition, vibrational zero-point energies were computed at the coupled cluster level of theory for those polyatomic systems lacking an experimental anharmonic value.

First-principles study of optical properties of barium titanate

Abstract: The optical properties of perovskite barium titanate in the core-level spectra are investigated by the first principles under scissor approximation. There are nine peaks at the curve of the imaginary part of dielectric function. The optical spectra are assigned to interband contribution from O 2p valence bands to Ti 3d conduction bands in the low-energy region and outer core electron excitation (core level excitation) from near valence band semicore levels Ba 5p and O 2s to conduction band in the high-energy region. In contrast to the calculated results by the tight-binding linear muffin-tin orbitals method, our results are in better agreement with the experimental results.

Electronic structure and magnetism in half-Heusler compounds

Abstract: In this paper we have applied the full-potential linearized muffin tin orbital method and the tight-binding linearized muffin tin orbital method to investigate in detail the electronic structure and magnetism of a series of half-Heusler compounds XMZ with X = Fe,Co,Ni, M = Ti,V,Nb,Zr,Cr,Mo,Mn and Z = Sb,Sn. Our detailed analysis of the electronic structure using various indicators of chemical bonding suggests that covalent hybridization of the higher-valent transition element X with the lower-valent transition element M is the key interaction responsible for the formation of the d–d gap in these systems. However, the presence of the sp-valent element is crucial to provide stability to these systems. The influence of the relative ordering of the atoms in the unit cell on the d–d gap is also investigated. We have also studied in detail some of these systems with more than 18 valence electrons which exhibit novel magnetic properties, namely half-metallic ferro- and ferrimagnetism. We show that the d–d gap in the paramagnetic state, the relatively large X–Sb hybridization and the large exchange splitting of the M atoms are responsible for the half-metallic property of some of these systems.

First-principles study on structures and energetics of intrinsic vacancies in SrTiO 3

Abstract: We have performed first-principles plane-wave pseudopotential calculations to study the electronic structures, structural optimization, and formation energies of intrinsic vacancies in bulk ${\mathrm{SrTiO}}_{3}.$ The anion and cation vacancy-induced levels appeared near the valence- and conduction-band edges in the band gap. The formation energies of isolated vacancies with different charge states were obtained, and the defect reaction energies, such as Sr partial Schottky ${(V}_{\mathrm{Sr}}^{2\ensuremath{-}}{+V}_{\mathrm{O}}^{2+}),$ Ti partial Schottky ${(V}_{\mathrm{Ti}}^{4\ensuremath{-}}{+2V}_{\mathrm{O}}^{2+}),$ and full Schottky ${(V}_{\mathrm{Sr}}^{2\ensuremath{-}}{+V}_{\mathrm{Ti}}^{4\ensuremath{-}}{+3V}_{\mathrm{O}}^{2+})$ were also evaluated. It was found that depending on the atomic chemical potentials, the relative stability of the defect species or reactions is different. The overall trend of the stable defect structures can explain the electrical conductivity of ${\mathrm{SrTiO}}_{3}$ for different chemical environments experimentally observed.

Building Blocks for the Molecular Expression of Quantum Cellular Automata. Isolation and Characterization of a Covalently Bonded Square Array of Two Ferrocenium and Two Ferrocene Complexes

Abstract: The suitability of [{(η5-C5H5)Fe(η5-C5H4)}4(η4-C4)Co(η5-C5H5)][PF6]2, [1][PF6]2, for use as a molecular quantum cellular automata (QCA) cell is demonstrated. To this end the structure of 1 in the solid state and the conversion of 1 to mono- and dicationic mixed-valence complexes have been accomplished. The latter compounds have been isolated as pure materials and characterized by IR, EPR, and Mossbauer spectroscopies and single-crystal XRD (monocation only) and magnetic susceptibility measurements. Near-IR spectra demonstrate the mixed valence character of the cations (valence trapped on the IR, EPR and Mossbauer time scales), and the energies of the intervalence charge-transfer bands provide a measure of the hole hopping frequency.

Quantum phase transition in organic charge-transfer complexes.

Abstract: A phase transition in an organic charge-transfer complex, which originates from the neutral-ionic valence instability, can be tuned toward zero kelvin with use of external pressure or chemical modification as a control parameter. The phase diagram and observed dielectric behaviors are typical of quantum paraelectricity, yet this zero-kelvin transition point namely, the quantum critical point, accompanies large quantum fluctuation of the molecular charge, as demonstrated by the molecular vibrational mode spectra. The result indicates that the π-electron transfer between donor and acceptor molecules is coupled with the zero-point lattice dynamics around the quantum critical point.

Spectroscopic methods applied to zircon

Abstract: Natural and synthetic (pure and doped) zircon (ZrSiO4) have been studied with a variety of spectroscopic techniques. These techniques are based on different physical phenomena, for instance transitions between spin states of nuclei and electrons, energetic transitions of valence electrons, intra-molecular vibrations, or vibrations of atoms and molecular units in the lattice. All of the diverse spectroscopic techniques, however, have in common that they probe energy differences between a ground and excited states, mostly upon interaction of the mineral with incident radiation. Such interactions are not only determined by the excited elementary particles or molecules themselves but depend greatly on their local environments (i.e. number, type, valence and geometrical arrangement of neighboring atoms). Spectroscopic techniques are thus sensitive to the local structure and provide information on the short-range order. Most research on zircon crystals using spectroscopic techniques was done to study their “real structures,” that is the characterization of deviations from “perfect” zircon. Such features include the incorporation of non-formula elements, structural defects and the presence of inclusions and other impurities. Correspondingly, most of the spectroscopic investigations can be assigned to two major groups. The first group represents studies done to characterize the structural position and local environment of non-formula elements when incorporated in the zircon lattice, and accompanying effects on physical properties. The second group comprises studies subjected to the real structures of “metamict” zircon samples, i.e., changes of the zircon structure caused by the impact of self-irradiation and upon recovery from radiation damage (Ewing et al., this volume). It is most obvious that a spectroscopic bulk or point analysis will first of all yield a spectrum (i.e. a plot of the intensity of the respective physical parameter versus wavelength, frequency or wavenumber), and this is what is used in most studies. In addition, image generation based on …

All-electron and pseudopotential study of MgO: Equation of state, anharmonicity, and stability

Abstract: static and ab initio molecular dynamics simulations. The simulations were performed using the projector augmented-wave and pseudopotential methods with different descriptions of the Mg atom ~‘‘small core’’ and ‘‘large core’’ !. The errors of large-core pseudopotentials increase with pressure and are mainly due to the overlap between the Mg semicore ~2p! orbitals and the valence orbitals, both of the same Mg atom and of the neighboring O atoms, rather than core deformation or core-core overlap effects. In agreement with previous works, we find that MgO remains in the B1~‘‘NaCl’’ ! structure at all pressures existing within the Earth, and transforms into the CsCl-type structure at 509 GPa. Direct ab initio calculations avoid the simplifying assumptions inherent to many empirical treatments of thermoelasticity and allowed us to assess some of the common assumptions. We present a detailed qualitative analysis of the effects of intrinsic anharmonicity and analyze the

Tuning the valence of the cerium center in (Na)phthalocyaninato and porphyrinato cerium double-deckers by changing the nature of the tetrapyrrole ligands.

Abstract: A series of 7 cerium double-decker complexes with various tetrapyrrole ligands including porphyrinates, phthalocyaninates, and 2,3-naphthalocyaninates have been prepared by previously described methodologies and characterized with elemental analysis and a range of spectroscopic methods. The molecular structures of two heteroleptic \[(na)phthalocyaninato](porphyrinato) complexes have also been determined by X-ray diffraction analysis which exhibit a slightly distorted square antiprismatic geometry with two domed ligands. Having a range of tetrapyrrole ligands with very different electronic properties, these compounds have been systematically investigated for the effects of ligands on the valence of the cerium center. On the basis of the spectroscopic (UV−vis, near-IR, IR, and Raman), electrochemical, and structural data of these compounds and compared with those of the other rare earth(III) counterparts reported earlier, it has been found that the cerium center adopts an intermediate valence in these complexes. It assumes a virtually trivalent state in cerium bis(tetra-tert-butylnaphthalocyaninate) as a result of the two electron rich naphthalocyaninato ligands, which facilitate the delocalization of electron from the ligands to the metal center. For the rest of the cerium double-deckers, the cerium center is predominantly tetravalent. The valences (3.59−3.68) have been quantified according to their LIII-edge X-ray absorption near-edge structure (XANES) profiles.

Electronic structure of conducting polymers: Limitations of oligomer extrapolation approximations and effects of heteroatoms

Abstract: Density-functional methods are used to analyze the scaling of discrete oligomeric π-electron conducting molecules towards idealized isolated polymer chains, treated in periodic boundary conditions. The band gaps of a series of conjugated oligomers of incrementally increasing lengths exactly fit a nearly-free-electron molecular-orbital picture and exhibit a smooth deviation from the classical empirical "I/N" trend for long oligomers and infinite polymers. The calculations also show a smooth convergence of bond lengths. The full band structures and densities of states of a polyacetylene, polypyrrole, polyfuran, and polythiophene show that band crossing, localized bands, and other effects cannot be accurately determined from simple extrapolation of oligomer electronic structures. Systematic comparisons of the electronic structure variations of the polymers investigated indicate that the electron affinity, rather than the electronegativity of the heteroatom or the bond-length alternation of the conjugated backbone, significantly affects the band gap of the resulting polymer as indicated by the presence of heteroatom states in the partial density of states of the conduction band, requiring revision of previous semiempirical analyses. Consequences for doping processes are also studied, along with a comparison of valence bandwidths, conduction bandwidths, and carrier effective masses as a function of heteroatom.

Phonon Dispersions of fcc δ-Plutonium-Gallium by Inelastic X-ray Scattering

Abstract: We report an experimental determination of the phonon dispersion curves in a face-centered cubic (fcc) delta-plutonium-0.6 weight % gallium alloy. Several unusual features, including a large elastic anisotropy, a small-shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes, are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory calculations for delta-plutonium.

First Radical Cation Salt of Paramagnetic Transition Metal Complex Containing TTF as Ligand, [CuII(hfac)2(TTF-py)2](PF6)·2CH2Cl2 (hfac = Hexafluoroacetylacetonate and TTF-py = 4-(2-Tetrathiafulvalenyl-ethenyl)pyridine)

Abstract: The preparation, X-ray crystal structure, EPR data, and magnetic measurement of [Cu(II)(hfac)(2)(TTF-py)(2)](PF(6)).2CH(2)Cl(2), a novel material where the conducting and the localized spin systems are covalently linked through conjugated bridges, are reported. The partial oxidation of the TTF-type organic donor ligand yielded the first radical cation salt of a paramagnetic transition metal complex. Moreover, this compound shows a mixed valence state at the unimolecular level, and additionally, the arrangement of the molecules in the crystal structure revealed the presence of isolated mixed valence TTF dimers.

First-Principles Calculations of PuO2±x

Abstract: The electronic structure of PuO2± x was studied using first-principles quantum mechanics, realized with the self-interaction corrected local spin density method. In the stoichiometric PuO2 compound, Pu occurs in the Pu(IV) oxidation state, corresponding to a localized f4 shell. If oxygen is introduced onto the octahedral interstitial site, the nearby Pu atoms turn into Pu(V) (f3) by transferring electrons to the oxygen. Oxygen vacancies cause Pu(III) (f5) to form by taking up electrons released by oxygen. At T = 0, the PuO2 compound is stable with respect to free oxygen, but the delicate energy balance suggests the possible deterioration of the material during long-term storage.

Temperature-induced valence transition and associated lattice collapse in samarium fulleride

Abstract: The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-Tc) copper oxide superconductors. Metal fulleride salts1—metal intercalation compounds of C60—and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system—they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + e) + to nearly 2 + . The negative thermal expansion—heating from 4.2 to 32 K leads to contraction rather than expansion—occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour2. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm2+ towards the smaller Sm(2+e)+, analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).

Valence Charge Concentrations, Electron Delocalization and β‐Agostic Bonding in d0 Metal Alkyl Complexes

Abstract: In this paper we describe a range of model d(0) metal ethyl compounds and related complexes, studied by DFT calculations and high resolution X-ray diffraction. The concept of ligand-opposed charge concentrations (LOCCs) for d(0) metal complexes is extended to include both cis-and trans-ligand-induced charge concentrations (LICCs) at the metal, which arise as a natural consequence of covalent metal-ligand bond formation in transition metal alkyl complexes. The interplay between locally induced sites of increased Lewis acidity and an ethyl ligand is crucial to the development of a beta-agostic interaction in d(0) metal alkyl complexes, which is driven by delocalization of the M-C bonding electrons. Topological analysis of theoretical and experimental charge densities reveals LICCs at the metal atom, and indicates delocalization of the M-C valence electrons over the alkyl fragment, with depletion of the metal-directed charge concentration (CC) at the alpha-carbon atom, and a characteristic ellipticity profile for the C(alpha)-C(beta) bond. These ellipticity profiles and the magnitude of the CC values at C(alpha) and C(beta) provide experimentally observable criteria for assessing quantitatively the extent of delocalization, with excellent agreement between experiment and theory. Finally, a concept is proposed which promises systematic control of the extent of C-H activation in agostic complexes.

Understanding Materials Compatibility

Abstract: ▪ Abstract The use of chemical potential diagrams to examine interface chemistry is discussed in terms of the chemical reactions among oxides and associated interdiffusion across the interface. The driving force for both processes can be determined from the chemical potential values. The geometrical features of the chemical potential diagrams can be related to the valence stability of binary oxides and the stabilization energy of double oxides from the constituent oxides. The materials compatibility in solid oxide fuel cell materials is discussed with a focus on a lanthanum manganite cathode and a yttria-stabilized zirconia (YSZ) electrolyte. Emphasis is placed on the valence numbers of manganese in the fluorite solid solution and the perovskite oxides, which have been derived by thermodynamic analysis of the magnitude of the stabilization energy/interaction parameters as a function of ionic size for respective valence numbers. The change of manganese valence on La2Zr2O7 formation and Mn dissolution in YS...

Sigma aromaticity of the bimetallic Au5Zn+ cluster.

Abstract: Experimental and theoretical evidence for a "sigma aromatic" bimetallic cluster is presented. A mass spectrometric analysis of AuNZn+ (N = 2-44) photofragments shows Au5Zn+ to be very abundant, proving its high stability. Calculations predict that Au5Zn+ has a planar geometry and six valence s electrons occupying delocalized sigma-bonded molecular orbitals in a manner similar to that of aromatic organic compounds, except for their nodal properties in the molecular plane. The delocalized sigma electrons provide diamagnetic ring currents, suggesting Au5Zn+ is aromatic.