Valence Structure Research Articles

Angular anisotropy in valence photoionization of Na clusters: theoretical investigation using jellium model

Calculation of the behaviour of photoelectron angular anisotropy in valence ionization of initially neutral Na X (X = 34–58) clusters is provided. The calculations are carried out for 1p, 1d and 1g jellium orbitals as a function of photon energy. The adapted theoretical framework is spherical jellium model using Woods-Saxon potential, which is modified to account for the long-range Coulomb tail in the final state. We discuss on the observed dramatic variations of the angular anisotropy parameter β as a function incident photon energy. It is shown that the behaviour is connected to the oscillation of the valence photoionization cross sections, that is a specific interference property of such metallic clusters whose valence structure can be described using the jellium model.

Pressure-Induced Valence and Structural Changes in YbMn2Ge2—Inelastic X-ray Spectroscopy and Theoretical Investigations

The pressure-induced valence change of Yb in YbMn(2)Ge(2) has been studied by high pressure inelastic X-ray emission and absorption spectroscopy in the partial fluorescence yield mode up to 30 GPa. The crystal structure of YbMn(2)Ge(2) has been investigated by high pressure powder X-ray diffraction experiments up to 40 GPa. The experimental investigations have been complemented by first principles density functional theoretical calculations using the generalized gradient approximation with an evolutionary algorithm for structural determination. The Yb valence and magnetic structures have been calculated using the self-interaction corrected local spin density approximation. The X-ray emission results indicate a sharp increase of Yb valence from v = 2.42(2) to v = 2.75(3) around 1.35 GPa, and Yb reaches a near trivalent state (v = 2.95(3)) around 30 GPa. Further, a new monoclinic P1 type high pressure phase is found above 35 GPa; this structure is characterized by the Mn layer of the ambient (I4/mmm) structure transforming into a double layer. The theoretical calculations yield an effective valence of v = 2.48 at ambient pressure in agreement with experiment, although the pure trivalent state is attained theoretically at significantly higher pressures (above 40 GPa).

Dissociation ofAs4clusters following valence photoionization and3dcore excitation

The VUV-induced photofragmentation of As-4 clusters was studied using photoelectron-photoion-coincidence (PEPICO) spectroscopy. The fragmentation pathways subsequent to outer and inner valence photoionization and resonant photoexcitation were studied by examining the cationic production as a function of electron energy. In addition, regular photoelectron spectroscopy and partial ion yield absorption spectroscopy were employed. In order to enhance the weak signal from the inner valence states, resonant core 3d excitation was used in the PEPICO experiment. The fragmentation pathways were inspected with the aid of ab initio and thermochemical calculations. The many-electron effects were found to play a major role in the observed inner valence structure and resulting photofragmentation. (Less)

π-Electron currents in fixed π-sextet aromatic benzenoids

In view of different patterns of π-electron density currents in benzenoid aromatic compounds it is of interest to investigate the pattern of ring currents in various classes of compounds. Recently such a study using a graph theoretical approach to calculating CC bond currents was reported for fully benzenoid hydrocarbons, that is, benzenoid hydrocarbons which have either π-sextets rings or “empty” rings in the terminology of Clar. In this contribution we consider π-electron currents in benzenoid hydrocarbons which have π-electron sextets and C=C bonds fully fixed. Our approach assumes that currents arise from contributions of individual conjugated circuits within the set of Kekule valence structures of these molecules.

Manganese valence and coordination structure in Mn,Mg-codoped γ-AlON green phosphor

The valence and coordination structure of manganese in a Mn,Mg-codoped γ-AlON spinel-type oxynitride green phosphor were studied by synchrotron X-ray diffraction and absorption fine structure measurements. The absorption edge position of the XANES revealed the bivalency of Mn. Two cation sites are available in the spinel structure for cation doping: a tetrahedral site and an octahedral site. The pre-edge of the XANES and the distance to the nearest neighbor atoms obtained from the EXAFS measurement showed that Mn was situated at the tetrahedral site. Rietveld analysis showed that the vacancy occupied the octahedral site. The preferential occupation of the tetrahedral site by Mn and the roles of N and Mg are discussed in relation to the spinel crystal structure.

Reentrant Valence Transition in EuO at High Pressures: Beyond the Bond-Valence Model

The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at ≈45 GPa is accompanied by a reentrant Eu valence transition into a lower valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.

Valence structures of aromatic bioactive compounds: a combined theoretical and experimental study

Valence electronic structures of three recently isolated aryl bioactive compounds, namely 2-phenylethanol (2PE), p-hydroxyphenylethanol (HPE) and 4-hydroxybenzaldehyde (HBA), are studied using a combined theoretical and experimental method. Density functional theory-based calculations indicate that the side chains cause electron charge redistribution and therefore influence the aromaticity of the benzene derivatives. The simulated IR spectra further reveal features induced by the side chains. Solvent effects on the IR spectra are simulated using the polarizable continuum model, which exhibits enhancement of the O-H stretch vibrations with significant red-shift of 464 cm(-1) in 2PE. A significant spectral peak splitting of 94 cm(-1) between O(4)-H and O(8)-H of HPE is revealed in an aqueous environment. Experimental measurements for valence binding energy spectra for 2PE, HPE and HBA are presented and analyzed using outer-valence Green function calculations. The experimental (predicted) first ionization energies are measured as 9.19 (8.79), 8.47 (8.27) and 8.97 (8.82) eV for 2PE, HPE and HBA, respectively. The frontier orbitals (highest occupied molecular orbitals, HOMOs, and lowest unoccupied molecular orbitals, LUMOs) have similar atomic orbital characters although the HOMO-LUMO energy gaps are quite different.

Experimental study of the 4fn → 4fn and 4fn → 4fn−15d1 transitions of the lanthanide diiodides LnI2 (Ln=Nd, Sm, Eu, Dy, Tm, Yb)

The diffuse reflectance and photoluminescence (PL) spectra of NdI2, SmI2, EuI2, DyI2, TmI2 and YbI2 were measured between 225 and 12 500 nm in order to determine their 4fn → 4fn−15d1 optical bandgaps. The results were compared with those obtained using an empirical model of the electronic structure of LnI2. The results can be used to explain the lanthanide valency and crystalline structure changes of other lanthanide diiodides such as PrI2.

Resonance and Aromaticity: An Ab Initio Valence Bond Approach

Resonance energy is one of the criteria to measure aromaticity. The effect of the use of different orbital models is investigated in the calculated resonance energies of cyclic conjugated hydrocarbons within the framework of the ab initio Valence Bond Self-Consistent Field (VBSCF) method. The VB wave function for each system was constructed using a linear combination of the VB structures (spin functions), which closely resemble the Kekulé valence structures, and two types of orbitals, that is, strictly atomic (local) and delocalized atomic (delocal) p-orbitals, were used to describe the π-system. It is found that the Pauling-Wheland's resonance energy with nonorthogonal structures decreases, while the same with orthogonalized structures and the total mean resonance energy (the sum of the weighted off-diagonal contributions in the Hamiltonian matrix of orthogonalized structures) increase when delocal orbitals are used as compared to local p-orbitals. Analysis of the interactions between the different structures of a system shows that the resonance in the 6π electrons conjugated circuits have the largest contributions to the resonance energy. The VBSCF calculations also show that the extra stability of phenanthrene, a kinked benzenoid, as compared to its linear counterpart, anthracene, is a consequence of the resonance in the π-system rather than the H-H interaction in the bay region as suggested previously. Finally, the empirical parameters for the resonance interactions between different 4n+2 or 4n π electrons conjugated circuits, used in Randić's conjugated circuits theory or Herdon's semi-emprical VB approach, are quantified. These parameters have to be scaled by the structure coefficients (weights) of the contributing structures.

Conjugated circuits currents in hexabenzocoronene and its derivatives formed by joining proximal carbons

We report on calculated CC bond currents for a dozen derivatives of hexabenzocoroenene in which one or more proximal carbon atoms at the molecular periphery have been bridged. The approach that we use is graph-theoretical in nature, following our outline of this method in 2003, which is based on finding all conjugated circuits in all Kekulé valence structures of these molecules. To the π-electrons having 4n + 2 π-electrons are assigned anticlockwise π-electron currents and to conjugated circuits having 4n π-electrons are assigned π-electron currents. One may summarize the results reported in this work by stating that CC bond currents in the compounds considered decrease on going from peripheral rings to the central ring of the molecule, and also that CC bond currents decrease by insertion of bridges to proximal peripheral benzenoid rings.

The Status of Generalizations: Valency and Argument Structure Constructions

This article outlines why valency theory and Goldberg's theory of argument structure constructions should be combined. In the light of empirical valency research it is argued that the amount of item-specificity to be observed in language must be accounted for in a theory of complementation and that generalizations concerning parallels between semantic properties of verbs and their occurrence in particular valency patterns must be treated with a certain degree of caution. It is suggested that one way of combining the two approaches is to make use of specific formal categories and to introduce a valency realisation principle in a theory of argument structure constructions.

π‐electron currents in polycyclic conjugated hydrocarbons: Coronene and its isomers having five and seven member rings

AbstractWe have outlined novel graph theoretical model for computing π‐electron currents in π‐electron polycyclic conjugated hydrocarbons. We start with Kekulé valence structures of a polycyclic conjugated hydrocarbon and their conjugated circuits. To each 4n+2 conjugated circuits we assign counter clockwise current i and to each 4n conjugated circuit we assign clockwise current i. By adding the contributions from all conjugated circuits in a single Kekulé valence structure one obtains π‐electron current pattern for the particular Kekulé valence structure. By adding the conjugated circuit currents in all Kekulé valence structure one obtains the pattern of π‐electron currents for considered molecule. We report here π‐electron current patters for coronene and 17 its isomers, which have been recently considered by Balaban et al., obtained by replacing one or more pairs of peripheral benzene rings with five and seven member rings. Our results are compared with their reported π‐electron current density patters computed by ab initio molecular orbital (MO) computations and satisfactory parallelism is found between two so disparate approaches. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Single-Molecule FRET Studies of Counterion Effects on the Free Energy Landscape of Human Mitochondrial Lysine tRNA

The folding energy landscape of RNA is greatly affected by interactions between the RNA and counterions that neutralize the backbone negative charges and may also participate in tertiary contacts. Valence, size, coordination number, and electron shell structure can all contribute to the energetic stabilization of specific RNA conformations. Using single-molecule fluorescence resonance energy transfer (smFRET), we have examined the folding properties of the RNA transcript of human mitochondrial tRNA(Lys), which possesses two different folded states in addition to the unfolded one under conditions of thermodynamic equilibrium. We have quantitatively analyzed the degree of RNA tertiary structure stabilization for different types of cations based on a thermodynamic model that accounts for multiple conformational states and RNA-ion interactions within each state. We have observed that small monovalent ions stabilize the tRNA tertiary structure more efficiently than larger ones. More ions were found in close vicinity of compact RNA structures, independent of the type of ion. The largest conformation-dependent binding specificity of ions of the same charge was found for divalent ions, for which the ionic radii and coordination properties were responsible for shaping the folding free energy.

Total-Reflection Inelastic X-Ray Scattering from a 10-nm ThickLa0.6Sr0.4CoO3Thin Film

To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total-reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5 edge with momentum transfer as an example.

Applying the conjugated circuits method to Clar structures of [n]phenylenes for determining resonance energies

We consider the aromaticity of biphenylene and structurally related linear or angular [n]phenylenes for which the direct application of the model of conjugated circuits does not offer valid expressions for resonance energy and aromaticity. We located the cause of this problem as being due to Kekulé valence structures in which neighboring benzenoid rings are connected by two CC double bonds. By restricting the selection of Kekulé valence structures to those that contribute to Clar structures of such systems, we were able to show that linear and angular [n]phenylenes have approximately similar resonance energies, with angular [n]phenylenes being slightly more stable due to second order contributions arising from disjoint conjugated circuits. Expressions for resonance energies of [n]phenylenes up to n = 8 are listed and recursion expressions for higher n values are outlined.

On Oxygen Deficiency in Nanocrystallites La1-xSrxCoO3

The crystal lattice and the bond valence structure of perovskite system La1-xSrxCoO3 are reported. The atomic valences for the given perovskites were determined on the basis of bond-valence theory. For the studied compounds the failure of distortion theorem was observed, and the non-stoichiometry of oxygen was estimated. The model for oxygen distribution is then suggested. The relationship between the grain diameter and oxygen content is discussed. [DOI: 10.1380/ejssnt.2011.469]

Topological Characterization of the Electron Density Laplacian in Crystals. The Case of the Group IV Elements

We discuss the rigorous characterization of the electron density Laplacian of crystals in terms of its topological properties: critical points (CPs), zero flux surfaces, and accumulation and depletion basins. Comparison with the atomic shell structure is exploited to characterize the numerous core critical points so that the important effort is applied to the more significant valence structure. Efficient algorithms are adapted or newly developed for the main tasks of topological study: finding the critical points, determining the 1D and 2D bundles of (3, − 1) and (3, + 1) CPs, and integrating well-defined properties within the accumulation and depletion basins. As an application of the tools and concepts developed we perform a quantitative analysis of chemical bonding on group IV semiconductors, mainly devoted to the properties of the diamond phase but also including the main effects of allotropy influences on these elements. The topological analysis of the Laplacian provides a complementary and very diff...

Generation of Kekulé valence structures and the corresponding valence bond wave function

A new scheme, called "list of nonredundant bonds", is presented to record the number of bonds and their positions for the atoms involved in Kekulé valence structures of (poly)cyclic conjugated systems. Based on this scheme, a recursive algorithm for generating Kekulé valence structures has been developed and implemented. The method is general and applicable for all kinds of (poly)cyclic conjugated systems including fullerenes. The application of the algorithm in generating Valence Bond (VB) wave functions, in terms of Kekulé valence structures, is discussed and illustrated in actual VB calculations. Two types of VBSCF calculations, one involving Kekulé valence structures only and the second one involving all covalent VB structures, were performed for benzene, pentalene, benzocyclobutadiene, and naphthalene. Both strictly local and delocalised p-orbitals were used in these calculations. Our results show that, when the orbitals are restricted to their own atoms, other VB structures (Dewar structures) also have a significant contribution in the VB wave function. When removing this restriction, the other VB structures (Dewar and also the ionic structures) are accommodated in the Kekulé valence structures, automatically. Therefore, at VBSCF delocal level, the ground states of these systems can be described almost quantitatively by considering Kekulé valence structures only at a considerable saving of time.

Graph theoretical approach to π-electron currents in polycyclic conjugated hydrocarbons

We consider calculation of induced currents of π-electrons over the molecular network when molecules are placed in a magnetic field. The approach is based on consideration of current contributions arising from conjugated circuits within each individual Kekulé valence structure. The calculation is illustrated for an isomer of coronene C 24H 12 for which ab initio computations of π-electron current densities are available. Agreement is found between the quantum chemical ab initio computations that make no assumptions on ring contributions and the graph theoretical models based on conjugated circuit currents.

Ultraviolet photoelectron spectra of mono-metal endohedral fullerene Er@C 82 (I)

Ultraviolet photoelectron spectra (UPS) of Erbium endohedral fullerene Er@C 82 (I) were measured using a synchrotron radiation light source. The spectral onset of Er@C 82 (I) was 0.4 eV, which is smaller than those of divalent atom endohedral metallofullerenes and analogous to those of trivalent atom endohedral metallofullerenes. The upper valence structure ( E b < 5 eV) of Er@C 82 (I) was almost identical with those of C 2v-Tb@C 82 and C 2v-La@C 82. The oxidation states of Er in Er@C 82 from the analogy of the oxidation state of Tb in Tb@C 82 and La in La@C 82 is estimated to be +3. The UPS of Er@C 82 (I) differs from those of Tm@C 82 isomers. The reason of the difference is attributed to differences in their cage structures and oxidation states. The upper valence UPS of Er@C 82 (I) could be well reproduced by a simulated spectrum obtained with a Gaussian03 program module assuming C 82–C 2v cage structure with three additional electrons.