Valence Structure Research Articles

On Kekulé structures count

Kekule valence structures offer qualitative description of conjugated cyclic hydrocarbons, suggesting localization of CC single and CC double bonds. Generation of all geometric Kekule structures for families of polyhex tori of a given tube cross-section, provided by the TORUS software, was performed by the new Kekule Count software, developed on a coloring algorithm. The sets of all non-isomorphic Kekule structures for the considered molecules, as the main result of this study, are presented.

Probing the site occupancies of Co-, Ni-, and Mn-substituted biogenic magnetite using XAS and XMCD

Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe 3+ - reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe 3-x O 4 , where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe 3+ -oxyhydroxides containing the appropriate substitutional cations. A combination of L-edge and K-edge X-ray absorption spectroscopy (XAS) and L-edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni 2+ , with at least 80% in Oh coordination. Cobalt ferrites contained only Co 2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn 2+ only on the Oh sites but a mixture of Mn 2+ and Mn 3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.

Synthesis and crystal structure of a mixed valence heteropoly green compound, (PPh4)4[PMo12O40], and its use as a catalyst in olefin epoxidation

A green heteropolyblue compound, (PPh4)4[PMo12O40] · 3DMF(1), has been synthesized from MoO3, H2O2 and H3PO4 in acetylacetone medium and crystallized from N,N-dimethylformamide. Compound 1 was characterized by analytical and spectroscopic methods, and X-ray structure analysis. The compound is a one-electron paramagnet and shows a featureless and cubic EPR spectrum with = 1.95 in DMF glass. The complex shows a Mo(V)–Mo(IV) couple, which has been studied by cyclic voltammetric and coulometric methods. The compound acts as an efficient olefin epoxidation catalyst with H2O2 as oxidant and NaHCO3 as co-catalyst.

π-Electron Partitions, Signatures, and Clar Structures of Selected Benzenoid Hydrocarbons

It is shown for a representative set of isomeric benzenoids that pi-electron partitions and signatures can serve for characterizing and ordering benzenoids. Benzenoid signatures (sequences s6 through s1 where the subscripts correspond to numbers of pi electrons in all rings) are obtained by examining the numbers of assigned pi electrons ranging from 6 to 1 for each ring in all resonance structures. The pi-electron partitions and signatures of all 33 non-isoarithmic peri-condensed benzenoid hydrocarbons with eight rings and four contiguous internal carbon atoms allow an ordering of these benzenoids that agrees fairly well with increasing numbers of Kekulé valence structures and Clar sextets. Interestingly, an excellent correlation (R2 > 0.99) is observed between s6 + s5 + s2 + s1 and s4 + s3, and an explanation for this observation is provided: the number P of pi electrons is divided unequally between two components: s34 = s4 + s3 and s1256 = s6 + s5 + s2 + s1 so that s1256 or the quotient s1256/s34 = Q can serve as a new metric for perfect matchings of polyhexes and a criterion for ordering and for evaluating the complexity of isomeric benzenoids quantitatively.

Correlations between various ways of accounting for the distribution of π-electrons in benzenoids

The π-electrons in polycyclic benzenoid hydrocarbons can be counted in two different ways: (i) as partitions of π-electrons in each ring of the benzenoid (according to the conventions that all resonance structures contribute equally, that shared double bonds resonance structures contribute with one π-electron to each of the condensed rings, and that both π-electrons of an unshared double bond belong to the corresponding ring) and (ii) as benzenoid signature, i.e. a sequence of six real numbers counting for all resonance structures the numbers of times a ring is assigned according to the above conventions si π-electrons, where i takes integer values between 6 and 1. For bridging the gap between these two ways of accounting for the same number of π-electrons, various types of resonance structures (also called Kekule valence structures) are examined and mixed: Fries, Clar, anti-Fries, and quasi-Clar structures.

Elastic magnetic form factors of exotic nuclei

How to identify the orbital of the valence nucleon(s) of exotic nuclei is an important problem. The elastic magnetic electron scattering is an excellent probe to determine the valence structure of odd-$A$ nuclei. The relativistic mean-field theory has been successfully applied to systematic studies of the elastic charge electron scattering from even-even exotic nuclei. The extension of this method to investigate the elastic magnetic electron scattering from odd-$A$ exotic nuclei is a natural generalization. The experimental form factors of $^{17}\mathrm{O}$ and $^{41}\mathrm{Ca}$ are reproduced very well with the help of the spectroscopic factors which are introduced into the relativistic treatment of the magnetic electron scattering for the first time. The emphases are put on the magnetic form factors of $^{15,17,19}\mathrm{C}$, $^{23}\mathrm{O}$, $^{17}\mathrm{F}$, and $^{49,59}\mathrm{Ca}$ calculated in the relativistic impulse approximation. Great differences have been found in the form factors of the same nucleus with different configurations. Therefore, the elastic magnetic electron scattering can be used to determine the orbital of the last nucleon of odd-$A$ exotic nuclei. Our results can provide references for the electron scattering from exotic nuclei in the near future.

Z2 structure and gas–solid effect in the stopping of slow ions

A recent claim by Paul of a systematic gas–solid difference in stopping cross sections for ions such as nitrogen and oxygen in the velocity range v ≃ v 0 is studied on the basis of existing experimental data. We find that all existing data support the commonly known Z 2 structure which, by and large, follows the valence structure of the target material. Existing experimental evidence is not found to support a specific gas–solid difference in the velocity range under consideration. The possibility of such an effect due to a gas–solid difference in charge state is rejected on theoretical grounds. Data for compound gases and solids are found to be well described by the Bragg additivity rule. We have also studied nitrogen/helium and oxygen/helium stopping ratios which determine the so-called effective-charge ratio. Taking into account the scatter of experimental data, we do not find clear evidence against Northcliffe’s assumption of a stopping ratio independent of Z 2 and common for gases and solids in the considered velocity range, although the absolute value appears too high.

Frontier electronic structures of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2: A photoelectron spectroscopy study

The frontier electronic structures of Ru(tcterpy)(NCS)3 [black dye (BD)] and Ru(dcbpy)2(NCS)(2) (N719) have been investigated by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). N1s XAS has been used to probe the nitrogen contribution in the unoccupied density of states, and PES, together with RPES over the N1s edge, has been used to delineate the character of the occupied density of states. The experimental findings of the frontier electron structure are compared to calculations of the partial density of states for the nitrogens in the different ligands (NCS and terpyridine/bipyridine) and for Ru4d. The result indicates large similarities between the two complexes. Specifically, the valence level spectra show two well separated structures at low binding energy. The experimental results indicate that the outermost structure in the valence region largely has a Ru4d character but with a substantial character also from the NCS ligand. Interestingly, the second lowest structure also has a significant Ru4d character mixed into the structure otherwise dominated by NCS. Comparing the two complexes the BD valence structures lowest in binding energy contains a large contribution from the NCS ligands but almost no contribution from the terpyridine ligands, while for N719 also some contribution from the bipyridine ligands is mixed into the energy levels.

Perfect and periphrastic passive constructions in Danish

This paper gives an account of the event and argument structure of past participles, and the linking between argument structure and valence structure. It further accounts for how participles form perfect and passive constructions with auxiliaries. We assume that the same participle form is used in both types of construction. Our claim is that the valence structure of a past participle is predictable from its semantic type, and that the valence structure predicts which auxiliary a past participle combines with in perfect constructions and whether the past participle may occur in passive constructions. Our approach sets itself apart from similar approaches, cf. e.g. Heinz & Matiasek (1994), Kathol (1994), Pollard (1994) and Müller (2003), with its strong emphasis on semantics.

Numerical Kekulé Structures of Fullerenes and Partitioning of π-Electrons to Pentagonal and Hexagonal Rings

We calculated the partitioning of pi-electrons within individual pentagonal and hexagonal rings of fullerenes for a collection of fullerenes from C20 to C72 by constructing their Kekulé valence structures and averaging the pi-electron content of individual rings over all Kekulé valence structures. The resulting information is collected in Table 2, which when combined with the Schlegel diagram of fullerenes (illustrated in Figure 7) uniquely characterizes each of the 19 fullerenes considered. The results are interpreted as the basic information on the distributions (variation) of the local (ring) pi-electron density.

Stable Tetravalent Phosphonium Enolate Zwitterions

Stable Tetravalent Phosphonium Enolate Zwitterions

Near-edge elastic photon scattering in amorphous systems

The structure of valence and unoccupied electron orbitals and the neighbouring electron density distribution of atoms and ions in amorphous systems can be examined through use of resonance in the elastic photon scattering-cross-section in the vicinity of core atomic orbital energies. So-called anomalous X-ray scattering (AXS) is a mode of analysis that offers similar information to that of EXAFS but can be obtained concurrently with diffraction mode imaging. Of interest is whether the dilute-ion aqueous system provides an environment suitable for testing independent particle approximation (IPA) predictions. With the aqueous environment as the reference system for calibrating relative cross-sections, particular challenges include photons scattered by the medium being subsequently absorbed by the ion, limiting the thickness of the attenuating medium and motivating use of bright synchrotron photon sources where tunable X-rays are obtained at sub-eV resolution using a Si 111 monochromator. Measured scattering intensities and fluorescent yields were compared and shown to agree qualitatively with Monte Carlo calculations utilising amplitudes calculated from modified form-factors with anomalous scatter factors at a resolution of several eV determined from the Dirac–Slater exchange potential. Experimentally determined form-factors for pure water were used to calibrate fluorescent yield and elastic scattering intensities for measurement of the energy dependent variation of these quantities near edge and XRF imaging of the Zn concentration in wax mounted, formalin fixed, breast tumour samples. Results indicate the distribution of Zn at higher resolution than sampling dimensions used in previous studies. Shifts in the position and profile of K-edge absorption and elastic scattering features in aqeuous Zn, Zn doped sol–gel glass and Zn in tissue are shown to reflect changes in the atomic charge state and environment and offer support for the presence of non-nutrient Zn bearing components at elevated concentrations in tumour, of which the matrix-metalloprotein MMP-II is a likely contender.

Kekulé Structure Count in Corazulenic Fullerenes

Tiling modification in fullerene modeling can be achieved by some map operations. In this respect, sequences of classical operations, or single generalized operations, were used to obtain corannulene-like azulenic patterns. The aromaticity of such cages tessellated by "corazulenic" supra-faces is discussed in terms of several criteria. Particularly, the number of Kekulé valence structures, was considered as a rough measure of the fullerene aromaticity and implicitely of their stability. The covering was given as a pi-electron partition within some Kekulé valence structures. The well-known geometric index of aromaticity HOMA (harmonic oscillator model of aromaticity) enabled the evaluation of local aromaticity of the discussed supra-faces and brought evidence for several dominant Kekulé valence structures.

Intrinsic Electronically Active Defects in Transition Metal Elemental Oxides

Densities of interfacial and bulk defects in high-κ dielectrics are typically about two orders of magnitude larger than those in Si–SiO2 devices. An asymmetry in electron and hole trapping kinetics, first detected in test capacitor devices with nanocrystalline ZrO2 and HfO2 dielectrics, is a significant potential limitation for Si device operation and reliability in complementary metal oxide semiconductor applications. There are two crucial issues: i) are the electron and hole traps intrinsic defects, or are they associated with processed-introduced impurities?, and ii) what are the local atomic bonding arrangements and electronic state energies of these traps? In this study, thin film nanocrystalline high-κ gate dielectrics, TiO2, ZrO2, and HfO2 (group IVB TM oxides), are investigated spectroscopically to identify the intrinsic electronic structures of valence and conduction band states, as well as those of intrinsic bonding defects. A quantitative/qualitative distinction is made between crystal field and Jahn–Teller (J–T) d-state energy differences in nanocrystralline TM elemental oxides, and noncrystalline TM silicates and Si oxynitrides. It is experimentally shown and theoretically supported that a length scale for nanocrystallite size <2–3 nm i) eliminates J–T d-state term splittings in band edge π-bonded d-states, and ii) represents a transition from the observation of discrete band edge defects to band-tail defects. Additionally, π-state bonding coherence can also be disrupted with similar effects on band edge and defect states in HfO2 films which have been annealed in NH3 at 700 °C, and display Hf–N bonds in N atom K1 edge X-ray absorption spectra.

On the molecular electron structure of three phosphinine‐containing macrocycles

A broad set of structural models and theoretical methods has been successfully used for studying both the molecular electron structure of the silacalix[3]phosphinine and the changes of the macrocycle core under the conditions that frequently correspond to its complexes with metals. The macrocycle core of the silacalix[3]phosphinine and its neutral derivatives are strongly deviated from the main molecular plane. The phosphorous electron lone pairs and the pi-cloud of the phosphinine units give the main contribution to the electron valence structure in the silacalix[3]phosphinine and also in its both oxidized and reduced derivatives. Although the electron lone pairs of the P atoms tend to be strongly repulsive, they are either totally or partially extended above all the fragment of the phosphorous atoms depending on geometrical factors or even strongly coupled with the pi-cloud of the phosphinine units. Electronic processes that take away part of the electron density from the macrocyle favor both its planar configuration and the asymmetric distribution of the valence electrons in the silacalix[3]phosphinine and its derivatives. The limit condition to this effect is the appearance of a new in-plane sigma molecular orbital between the P atoms of two neighboring phosphinine units.

Temperature-dependent valence bond structure study of tantalum

Based on energy and shape method for determining the VB structures of crystal, the valence bond structure of tantalum is redetermined at room temperature and extended to the whole temperature range. The outer shell valence electronic distribution of Ta at room temperature is e c 2.9011 (d c 2.2711+s c 0.1580+p c 0.4720) e f 2.0989 in crystal. The single-bond radius value is 1.2812 Å. The temperature-dependent valence structure of tantalum is not a drastic change. The bond length and single-bond radius increases while the bond energy and bond valence decreases with increasing temperature. The valence electrons e c (d c, s c and p c) decrease while e f increases with increasing temperature.

Energy and shape method for determining the valence and bond structures of crystals

Based on Xie's the one-atom self-consistency method for determining the valence and bond (VB) structures of crystal, the energy and shape (ES) method has been brought forward. New progress has been made in the theory. The VB structures are determined with the unity of lattice constant and energy minimization. To take Mg as an example, the ES method is introduced in detail. The VB structures of Na, Mg, Al, Ti, Zr and Hf are determined. The results are more refined and reliable.

Retro-Leapfrog and Related Retro Map Operations

Operations on maps are well-known theoretical tools for transforming a given polyhedral tessellation. Several theoretical investigations of fullerenes, such as their pi-electronic structure and stability, need information on the original map which was transformed into a larger molecular structure. In this respect, retro-operations, particularly those of the most used leapfrog, chamfering, and capra operations, appear particularly useful in searching the associate graphs of fullerenes. A series of analyzed cages proved to be leapfrog transforms of smaller cages. This information was useful in understanding their closed pi-electronic structure and related properties including the local aromaticity. An index based on the optimized geometries enabled the evaluation of aromaticity of their various substructures. Pictorial images of the pi-electron distribution as the main Kekulé valence structures have been performed by the aid of the JSCHEM software package.

Band structure and chemical bond in alkali metal carbonates

The band structure, state density, optical functions, and distribution of valence and difference density in alkali-metal carbonates are calculated within the local electron-density functional theory using the method of pseudopotential in the basis of numerical pseudoorbitals. When passing from a lithium cation to a potassium one, the character of hybridization between the crystal sublattices changes to result in an increase in the valence-band width, a decrease in the forbidden-band width, a complication of the structure of state-density spectrum, and a shift of the maxima of optical functions to the low-energy range. It is found that the electron overflow between the σ-and π-orbitals of crystallographically nonequivalent oxygen atoms can occur in different ways, hence their interaction force with the surrounding atoms is different. The role of cations in stabilization of anion chains resulting from the electron-cloud overlapping in lithium and sodium carbonates is shown.

Valance electron structure of carbide-diamond interface and catalytic mechanism for diamond synthesis under high-pressure and high-temperature

Abstract The metallic films surrounding a synthetic diamond formed under high-pressure and high-temperature (HPHT) in the presence of Fe-based and Ni-based catalysts were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was showed that the carbide was the primary carbon source for the nucleation and growth of diamond. Based on the EET (empirical electron theory in solid and molecules) theory, the valence electron structure of interface between carbide (Fe3C, Ni3C, (Fe, Ni) 3C) and diamond was calculated using the bonding length difference (BLD) method. The boundary criterion of Thomas-Fermi-Dirac-Cheng (TFDC): the electron density being equal on the contacting surfaces of atoms' was applied to analyze the valence structure of carbide-diamond interface. The result based on the calculation valance electron structure is in good accordance with the experimental result. This study is very helpful to reveal the catalytic mechanism of diamond...