Valence Structure Research Articles

On the number of kekulé structures of unbranched cata-condensed benzenoid chains

It is known that an alternative algorithm to the Gordon-Davidson algorithm for counting the Kekulé valence structures of catacondensed non-branched benzenoid hydrocarbons is a reformulation of the original algorithm.

Electronic structure of clusters modeling silica

AbstractSelf‐consistent‐field‐Xα‐scattered wave calculations on clusters Si2O76− and H6Si2O7 modeling silica have been performed. Incorporation of Si 3d orbitals produces significant changes in the overall valence structure. In addition to σ Si — O bonds, there exists a bonding π character due to the participation of O 2p and Si 3d. Hydrogen terminators do not seem to correct edge effects for these π states.

Computer generation of generalized Wheland polynomials

Generalized Wheland polynomials enumerate the valence structures of different degrees of excitation. As such they are structural invariants which may be of interest in discussions of similarities and differences among unsaturated compounds. Brute force enumeration of valence structures is already impractical for graphs having n = 10 vertices (e.g. naphthalene C 10H 8). Here we outline a computer program which enumerates valence structures of any degree of excitation. The results are illustrated on linear chains, cycles, and smaller benzenoid systems. Some properties of the derived polynomials are discussed. The recursive relation for the generalized Wheland polynomials is given. Because of the nonpolynomial character of the problem the computer application is practical only for graphs having n = 16 and less vertices.

Valency and molecular structure

Valency is defined for each molecular orbital. The molecular orbital valency values are shown to be a good measure of the bonding nature of the molecular orbital. Comparisons are made with photoelectron spectral studies and Mulliken overlap population analysis.

A Novel Electron Count and Metal–Metal Multiple Bonding in Trigonal Prismatic Clusters

The anions Tc6Cl and Tc6Cl have more electrons participating in metal-metal bonds than any other hexanuclear cluster known so far. Their metal skeletons (32 and 31 electrons, respectively) can be described in terms of the 30e structure 1. The. surplus electrons occupy a molecular orbital which is strongly antibonding in the TcTc units and weakly bonding between them. The alternative valence structure 2 is conceivable for the hypothetical [Tc6Cl18]6⊖.

Chemical effects induced in alkali Phosphates by 2 keV Ar ion bombardment

Abstract Various alkali phosphates have been bombarded with 2 keV Ar ions at doses ranging between 1015 and 4 × 1016 ions/cm2. The modifications induced at the surfaces have been followed by ESCA. Both stoichiometric and valence structure modifications are induced by the bombardment on the phosphate surfaces. The ESCA spectra indicate that the residual surfaces have been in part transformed into polyphosphate-like compounds. The main parameter which determined the condensation degree has been found to be the (oxygen/ metal) ratio. The results are interpreted in terms of a recently proposed model which links the primary physical processes, mainly considered to be binary colisions between the projectile and the target atoms, to the subsequent chemical reactions.

A statistical approach to resonance energies of large molecules

For large conjugated molecules, resonance energies of SCF MO quality are not available. Here we outline a method of determining molecular resonance energies by combining a graph theoretical approach to aromaticity with a statistical analysis of random Kekule valence structures. The approach involves construction of random Kekule valence forms and subsequent enumeration of conjugated circuits within each such structure.

Valence and molecular structure

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTValence and molecular structureKarl. Jug, Nicolaos D. Epiotis, and Sabine. BussCite this: J. Am. Chem. Soc. 1986, 108, 13, 3640–3644Publication Date (Print):June 1, 1986Publication History Published online1 May 2002Published inissue 1 June 1986https://doi.org/10.1021/ja00273a016RIGHTS & PERMISSIONSArticle Views101Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

On some solved and unsolved problems of chemical graph theory

The development of several novel graph theoretical concepts and their applications in different branches of chemistry are reviewed. After a few introductory remarks we follow with an outline of selected important graph theoretical invariants, introducing some new results and indicating some open problems. We continue with discussing the problem of graph characterization and construction of graphs of chemical interest, with a particular emphasis on large systems. Finally we consider various problems and difficulties associated with special subgraphs, including subgraphs representing Kekulé valence structures. The paper ends with a brief review of structure-property and structure-activity correlations, the topic which is one of prime motivations for application of graph theory to chemistry.

On the stability of conjugated hydrocarbon ions

We consider the question of predicting the stability of hypothetical structures that are too large for ab initio calculations, too complex for simple MO calculations, and too few for reliable semiempirical SCFMO schemes. The problem is illustrated on a polycyclic conjugated cation having 21 carbon atoms. The analysis is general and applies to conjugated hydrocarbons and their ions for which it is not apparent whether they exhibit aromatic characteristics (and, if they do, to what extent). For the investigated polycyclic cation we find that to a large extent its aromatic components are cancelled by its antiaromatic components resulting in an essentially nonaromatic system. The analysis, which is based on close examination of various contributing conjugated circuits, allows one to examine the partial role of the individual Kekule valence structures as well as to foretell the nature of both the positively and the negatively charged system. We conclude that the particular cation should be stable but represents a challenge as it would be a cation with a rather small resonance energy. In contrast, the corresponding anion is relatively stable and, at least from a theoretical point of view, is less interesting.

The exact one‐electron model of molecular structure

AbstractThe Schrödinger equation satisfied by the electron density is derived without approximation from the theory of marginal and conditional amplitudes. The equation arises from a factorization of the total N‐electron wavefunction defined by the normalization appropriate to a conditional amplitude. This Schrödinger equation is an exact dynamical model for computing effective one‐electron potentials from known N‐electron wavefunctions. Results are presented for several small molecules. They display the shell structure of atoms, and the valence structure of both ionic and covalent molecules.

Supramolecular phenomena in organic redox films at electrodes: Part I. The methylene blue/leucomethylene blue redox couple at the platinum electrode

It is possible to generate conductive multilayer structures by reduction of methylene blue cations at the interface: clean surface of a platinum electrode/1 M aqueous electrolyte solution. The characteristics of the multilayer ordered phase generated in fluoride and nitrate solutions are presented and the mechanism of a fast charge transfer between electrode/organic film and organic film/aqueous solution interfaces is discussed. The conductivity of the film is interpreted by postulating formation of a mixed valence structure with the generation of a cation radical intermediate which is favoured in the solid state at characteristic potentials.

The electronic structure of the calcium monohalides. A ligand field approach

The electronic structure of the calcium monohalides is addressed using a ligand field model which approximates the halide as a polarizable negative charge perturbing the one electron valence structure of the Ca+ ion. A simple, zero-free-parameter model is shown to predict accurately electronic energies, transition moments, permanent dipole moments, and several other molecular constants that have been experimentally determined. The molecular properties and electronic wave functions are interpreted in terms of the polarization (s/p/d/f mixing) and radial expansion (nl/n+1l mixing) of the low lying, free ion, basis functions caused by the electric field of the ligand.

2,5,7,10‐Tetrakis(trimethylsilyl)‐1,6‐methano[10]annulene, a Fluxional Valence Tautomer with Cyclopolyolefin Structure

Nullification of the aromatic structure of 1,6-methano[10]annulene by substitution is observed in the title compound. In solution at room temperature, single and double bonds fluctuate with concomitant mutual change of conformation of the seven-membered rings, whereas in the crystal a valence structure is frozen in.

Localized molecular orbital studies on transition-metal complexes. III. Localized valence structures of transition-metal carbene complexes

Localized molecular orbital studies on transition-metal complexes. III. Localized valence structures of transition-metal carbene complexes

The transition from physisorbed to chemisorbed oxygen on silver films studied by photoemission

The O 1s and valence electronic levels of physisorbed O2 show a homogeneous upward shift of about 2.2 eV compared to free O2. Under O2 pressures of 10−6 Torr, the shift is about 1.4 eV, probably reflecting O2 adsorbed as a “second layer”. Physisorbed O2 induces a structure at 1.1 eV below EF. After high O2 exposures at 45 K ionization potentials are observed at 7.0 eV, 11.4 eV and 534.5 eV below EF, assigned to chemisorbed dioxygen. No firm assignment has been reached for the oxygen induced valence structure, especially for the structure above the Ag 4d band at temperatures above 100 K. The difference of 7.1 eV in the O 1s ionization potential of physisorbed O2 and chemisorbed O is equal to the energetic difference of satellites of the O 1s and C 1s photoemission structure of CO on Ag(110) [Krause et al., Surface Sci. 138 (1984) 305].

Structure of Valence and Conduction Levels in NiO

Photoemission measurements on cleaved NiO(100) identify both the O $2p$ $\ensuremath{\pi}$ valence orbitals and the previously undetected O $2p$ $\ensuremath{\sigma}$ orbitals. An upper limit of 0.8\ifmmode\pm\else\textpm\fi{}0.1 eV is set on the width of the Ni $3d$ ${t}_{2g}\ensuremath{\downarrow}$ emission. A large maximum in the empty density of states is detected in the secondary-electron spectrum, explaining previously debated photoabsorption data. We conclude from the available data that NiO is not a conventional band-gap insulator, as has been suggested recently, but is in fact a Mott insulator.

Metal oxide analogue of metal alloy catalysts

Because of ion size, valence and oxide structure, hafnium and zirconium mixed oxide catalysts are ideal for study as a metal alloy analogue. Furthermore, hafnium oxide and zirconium oxide exhibit quite different selectivity for the conversion of 2-alcohols to alkene products. The alkene selectivity changed abruptly from that of pure HfO 2 to that of pure ZrO 2 at a composition with ca. 90 mol% Zr. Characterization of the catalysts by ESCA, Auger, and ISS spectroscopy show that the surface composition is very similar to the bulk composition and, thus, changes uniformly throughout the composition range rather than abruptly as does the alkene selectivity. Pure ZrO 2 impregnated with Hf to produce catalysts with the full range of Hf-Zr surface compositions showed a catalytic selectivity that paralleled the selectivity of the bulk composition catalysts. Zr or Th impregnated onto alumina serve as catalyst poisons; however, these two metals alter the selectivity in a quite different manner. The data indicate that the catalytic selectivity of these mixed oxide catalysts is determined by ligand, rather than bulk electronic, effects.

Metal oxide analogue of metal alloy catalysts

Because of ion size, valence and oxide structure, hafnium and zirconium mixed oxide catalysts are ideal for study as a metal alloy analogue. Furthermore, hafnium oxide and zirconium oxide exhibit quite different selectivity for the conversion of 2-alcohols to alkene products. The alkene selectivity changed abruptly from that of pure HfO2 to that of pure ZrO2 at a composition with ca. 90 mol% Zr. Characterization of the catalysts by ESCA, Auger, and ISS spectroscopy show that the surface composition is very similar to the bulk composition and, thus, changes uniformly throughout the composition range rather than abruptly as does the alkene selectivity. Pure ZrO2 impregnated with Hf to produce catalysts with the full range of Hf-Zr surface compositions showed a catalytic selectivity that paralleled the selectivity of the bulk composition catalysts. Zr or Th impregnated onto alumina serve as catalyst poisons; however, these two metals alter the selectivity in a quite different manner. The data indicate that the catalytic selectivity of these mixed oxide catalysts is determined by ligand, rather than bulk electronic, effects.

Fine structures of valence of elements in periodic table from group AI...group B...group AIII, La series RE and a semi-quantitative electron theory of metallic phase formations and isotopic phase transitions

Fine structures of valence of elements in periodic table from group AI...group B...group AIII, La series RE and a semi-quantitative electron theory of metallic phase formations and isotopic phase transitions