Valence Structure Research Articles

Characteristic polynomials of alternant edge weighted linear chains with subsequent application to some linear poly (p-phenylene) graphs

The algorithms for the calculation of the characteristic polynomial coefficients for the alternant edge weighted graphs of linear chains have been developed. These algorithms have been utilized to calculate the characteristic polynomials of the linear poly (p-phenylene) and the methylene substituted linear poly (p-phenylene) compounds. These compounds are found to be important materials for electro-optical and electronic applications. The recurrence relation to obtain the sum of the CP coefficients \({\left({S_{\rm CP}^{N_r }}\right)}\) of any such poly (p-phenylene) in terms of the respective sums of its lower analogs has been derived. The number of Kekule valence structures (K) for such molecular graphs have been presented. Excellent linear correlations of logarithm of sum of CP coefficients \(({\rm log} \, {S_{\rm CP}^{N_r }})\) and logarithm of Kekule valence structures count (log K) with the ambient conditions density and bulk modulus of linear poly (p-phenylene) have also been found.

ChemInform Abstract: Maximum Valence Structures in Nonbenzenoid Polycyclic Hydrocarbons

A number of polycyclic nonbenzenoids obtained by fusion benzene and cyclobutadiene rings were examined using the maximum valence structure model. The model assumes that only Kekule valence structures associated with the largest weight contribute to the overall molecular description. The weights of individual valence structures were determined by the smallest Pauling bond order found for any of CC double bonds occurring in a particular structure. Similarities and differences between the maximum valence structure model and models using all Kekule valence structures are discussed.

Auger Electron Angular Distribution of Double Core-Hole States in the Molecular Reference Frame

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×10(11) photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.

On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

The degree of p-electron (de)localization and aromaticity of a series of polybenzenoid hydrocarbons (PBHs) has been analyzed through the π-contribution to the electron localization function (ELFπ), calculated at the B3LYP/6-311G(d,p) hybrid density functional theory level. The extent of p-electron delocalization in the various hexagons of a PBH was determined through analysis of the bifurcation values of the ELFp basins (BV(ELFp)), the spans in the bifurcation values in each hexagon (ΔBV(ELFπ)), and the ring-closure bifurcation values of the ELFπ (RCBV(ELFπ)). These computed results were compared to the qualitative description of local aromaticities of the different hexagons in terms of Clar structures with p-sextets. Benzene, [18]annulene, and thirty two PBHs were analyzed at their equilibrium geometries, and benzene and triphenylene were also analyzed at bond length distorted structures. In general, the description of PBHs in terms of Clar valence structures is supported by the ELFp properties, although there are exceptions. For PBHs at their equilibrium geometries there is a clear sigmoidal relationship between the CC bond lengths and the amount of p-electron (de)localization at these bonds, however, this relationship is lost for bond distorted geometries. In the latter cases, we specifically examined benzene in D3h symmetric “1,3,5-cyclohexatriene” structures and triphenylene in eight different structures. From the distorted benzenes and triphenylenes it becomes clear that there is a distinct tendency for the p-electron network to retain delocalization (aromaticity). The ELFp analysis thus reveals an antidistortive rather than a distortive behavior of the p-electrons in these investigated compounds.

Spin12+, spin32+, and transition magnetic moments of low lying and charmed baryons

Magnetic moments of the low lying and charmed spin 1/2^+ and spin 3/2^+ baryons have been calculated in the SU(4) chiral constituent quark model (\chiCQM) by including the contribution from c \bar c fluctuations. Explicit calculations have been carried out for the contribution coming from the valence quarks, "quark sea" polarizations and their orbital angular momentum. The implications of such a model have also been studied for magnetic moments of the low lying spin 3/2^+ \to 1/2^+ and 1/2^+ \to 1/2^+ transitions as well as the transitions involving charmed baryons. We are able to achieve an excellent agreement with data for the case of low lying spin 1/2^+ baryons and spin 3/2^+ baryons. For the spin 1/2^+ and spin 3/2^+ charmed baryon magnetic moments, our results are consistent with the predictions of the QCD sum rules, Light Cone sum rules and Spectral sum rules. For the cases where "light" quarks dominate in the valence structure, the sea and orbital contributions are found to be fairly significant however, they cancel in the right direction to give the correct magnitude of the total magnetic moment. On the other hand, when there is an excess of "heavy" quarks, the contribution of the "quark sea" is almost negligible. The effects of configuration mixing and quark masses have also been investigated.

Zinc-Doped TiO<sub>2</sub> Nanotube Arrays

Zinc-doped TiO2 nanotube arrays were fabricated by immersing TiO2 nanotube arrays in zinc-containing solution for hours. And subsequent heat-treatment was crucial for Zn2+ coming into the crystal lattice of TiO2 nanotubes. TEM analysis was used as main technique to investigate the structure of zinc-doped TiO2 nanotubes, and found that the Zn2+ ions only combine into the lattice of TiO2 nanotubes. This kind of doping can change the valence structure in the surface of TiO2 nanotube array. The obtained zinc-doped TiO2 nanotube arrays have potential application in photocatalysis.

Stopping of ∼0.2–3.4MeV/amu 1H+ and 4He+ ions in polyvinyl formal

Abstract The stopping powers of polyvinyl formal resin for protons and alpha particles have been measured over the energy range ∼(0.2–3.4) MeV/amu with an overall relative uncertainty of less than 2.5% using the ion beam transmission method. The obtained results are discussed in comparison to scarce experimental data from the literature and to values calculated by the SRIM-2008 computer code assuming Bragg-Kleeman’s additivity rule of stopping powers. Departures from additivity attaining ∼5.5% and ∼3.0% for the proton and alpha particle data, respectively, are observed at moderate projectile velocities, increasingly as one evolves towards the stopping power maximum dominated by charge exchange effects. They are presumed to be mainly due to valence structure effects involving the C–H bonds of the studied polymer compound. The less pronounced deviation for alpha particles is probably due to the projectile screening effect that occurs in distant collisions tending to reduce the contribution of valence electrons in the low projectile velocity regime.

Ab Initio Thermochemistry with High-Level Isodesmic Corrections: Validation of the ATOMIC Protocol for a Large Set of Compounds with First-Row Atoms (H, C, N, O, F)

The recently proposed ATOMIC protocol is a fully ab initio thermochemical approach designed to provide accurate atomization energies for molecules with well-defined valence structures. It makes consistent use of the concept of bond-separation reactions to supply high-level precomputed bond increments which correct for errors of lower-level models. The present work extends the approach to the calculation of standard heats of formation and validates it by comparison to experimental and benchmark level ab initio data reported in the literature. Standard heats of formation (298 K) have been compiled for a large sample of 173 neutral molecules containing hydrogen and first-row atoms (C, N, O, F), resorting to several previous compilations and to the original experimental literature. Statistical evaluation shows that the simplest implementation of the ATOMIC protocol (composite model C) achieves an accuracy comparable to the popular Gaussian-3 approach and that composite models A and B perform better. Chemical accuracy (1-2 kcal/mol) is normally achieved even for larger systems with about 10 non-hydrogen atoms and for systems with charge-separated valence structures, bearing testimony to the robustness of the bond-separation reaction model. Effects of conformational averaging have been examined in detail for the series of n-alkanes, and our most refined composite model A reproduces experimental heats of formation quantitatively, provided that conformational averaging is properly accounted for. Several cases of larger discrepancy with respect to experiment are discussed, and potential weaknesses of the approach are identified.

Sensitive Valence Structures of [(pap)2Ru(Q)]n(n= +2, +1, 0, −1, −2) with Two Different Redox Noninnocent Ligands, Q = 3,5-Di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine and pap = 2-Phenylazopyridine

The complexes [(pap)(2)Ru(Q)]ClO(4), [1]ClO(4)-[4]ClO(4), with two different redox noninnocent ligands, Q = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine (-aryl = m-(Cl)(2)C(6)H(3) (1(+)), C(6)H(5) (2(+)), m-(OCH(3))(2)C(6)H(3) (3(+)), and m-((t)Bu)(2)C(6)H(3) (4(+))) and pap = 2-phenylazopyridine, have been synthesized and characterized using various analytical techniques. The single-crystal X-ray structure of the representative [2]ClO(4).C(7)H(8) exhibits multiple intermolecular C-H...O hydrogen bondings and C-H...pi interactions. The C1-O1 = 1.287(4) (density functional theory, DFT, 1.311) and C6-N1 = 1.320(4) (DFT, 1.353) A and intraring bond distances associated with the sensitive quinine (Q) moiety along with the azo(pap) bond distances, N3-N4 = 1.278(4) (DFT, 1.297) and N6-N7 = 1.271(4) (DFT, 1.289) A, in 2(+) justify the [(pap)(2)Ru(II)(Q(*-))](+) valence configuration at the native state of 1(+)-4(+). Consequently, Mulliken spin densities on Q, pap, and Ru in 2(+) are calculated to be 0.8636, 0.1040, and 0.0187, respectively, and 1(+)-4(+) exhibit free radical sharp EPR spectra and one weak and broad transition around 1000 nm in CH(3)CN due to interligand transition involving a singly occupied molecular orbital (SOMO) of Q(*-) and the vacant pi* orbital of pap. Compounds 1(+)-4(+) undergo a quasi-reversible oxidation and three successive reductions. The valence structure of the electron paramagnetic resonance (EPR)-inactive oxidized state in 1(2+)-4(2+) has been established as [(pap)(2)Ru(II)(Q degrees )](2+) instead of the alternate formalism of antiferromagnetically coupled [(pap)(2)Ru(III)(Q(*-))](2+) on the basis of the DFT calculations on the optimized 2(+), which predict that the singly occupied molecular orbital is primarily composed of Q with 77% contribution. Accordingly, the optimized structure of 2(2+) predicts shorter C1-O1 (1.264) and C6-N1 (1.317 A) distances and longer Ru1-O1 (2.080) and Ru1-N1 (2.088 A) distances. Compounds 1(2+)-4(2+) exhibit the lowest energy transitions around 600 nm, corresponding to Ru(dpi)/Q(pi) --> pap(pi*). The presence of two sets of strongly pi-acceptor ligands, pap and Q, in 1(2+)-4(2+) stabilizes the Ru(II) state to a large extent such that the further oxidation of {Ru(II)-Q degrees } --> {Ru(III)-Q degrees } has not been detected within +2.0 V versus a saturated calomel electrode. The EPR-inactive reduced states 1-4 have been formulated as [(pap)(2)Ru(II)(Q(2-))] over the antiferromagnetically coupled alternate configuration, [(pap)(pap(*-))Ru(II)(Q(*-))]. The optimized structure of 2 predicts sensitive C1-O1 and C6-N1 bond distances of 1.337 and 1.390 A, respectively, close to the doubly reduced Q(2-) state, whereas the N horizontal lineN distances of pap, N3-N4 = 1.299 and N6-N7 = 1.306 A, remain close to the neutral state. In corroboration with the doubly reduced Q(2-) state, 1-4 exhibit a moderately strong interligand pi(Q(2-)) --> pi*(pap) transition in the near-IR region near 1300 nm. The subsequent two reductions are naturally centered around the azo functions of the pap ligands.

Novel behaviors in rare-earth-filled skutterudites studied by bulk-sensitive photoemission spectroscopy

Abstract The electronic structures of rare-earth-filled skutterudites have been investigated by high-resolution photoemission spectroscopy. In order to reveal their bulk nature, the bulk-sensitive photoemission spectroscopy with soft X-ray (hv∼800 – 1100 eV) and hard X-ray (hv∼8000 eV) is a very powerful tool since the rare-earths' valence and electronic structures on the surface are often different from those in the bulk. Our experimental results clearly show the coexistence of the strongly mixed-valence state and the heavy-fermion state in SmOs4Sb12 and the quasi-particle peak near the Fermi level in PrFe4P12 induced by the hybridization between the rare-earth 4f and conduction electrons.

Identifying valence structure in LiFeAs and NaFeAs with core-level spectroscopy

Resonant x-ray emission spectroscopy (XES) measurements at FeL2,3 edges and electronic structure calculations for LiFeAs and NaFeAs are presented.Experiment and theory show that in the vicinity of the Fermi energy, the density ofstates is dominated by contributions from Fe 3d states. The comparison of FeL2,3 XESwith spectra of related FeAs compounds reveals similar trends in energy and the ratio of intensities of theL2 andL3 peaks (I(L2)/I(L3) ratio).The I(L2)/I(L3) ratio for all FeAs-based superconductors is found to be closer to that of metallic Fe thanthat of the strongly correlated FeO. We conclude that iron-based superconductors areweakly or, at most, moderately correlated systems.

Pressing some boundaries in Mendeleev’s chart

We can generally understand the way simple chemical combinations arise by assuming a standard valence structure for the elements, but a high-pressure study suggests that the rules for hydrogen may not be so easily pinned down.

Europäisch eingestellt – Valenzforschung mit Parallelkorpora

The aim of this research is to demonstrate with a case study the significance of corpus linguistics within the field of verb valency and bilingual lexicography. Specifically, we will introduce a corpus-based process that determines context-sensitive translations of polysemous word forms. Three steps are considered here in detail. First, text evidences of the verb einstellen in the monolingual Deutsches Referenzkorpus (DeReKo) will be examined with a collocation analysis. With help of the analytical instrument COSMAS II, the collocation profiles will then be summarized into a typology (senses and subsenses, valency structures and typical collocations). In a further step, the determined senses can be attributed to the corresponding translations of the word form einstellen in other languages (English, French and Italian) by means of the multilingual parallel corpus Europarl (Open Source Parallel Corpus OPUS). Finally, the results will be compared to the codifications of commonly used bilingual dictionaries.

The effect of emotional valence and body structure on emotional empathy to humanoid robot: an fMRI study

The effect of emotional valence and body structure on emotional empathy to humanoid robot: an fMRI study

Ab initio thermochemistry using optimal-balance models with isodesmic corrections: The ATOMIC protocol

A theoretical composite approach, termed ATOMIC for Ab initio Thermochemistry using Optimal-balance Models with Isodesmic Corrections, is introduced for the calculation of molecular atomization energies and enthalpies of formation. Care is taken to achieve optimal balance in accuracy and cost between the various components contributing to high-level estimates of the fully correlated energy at the infinite-basis-set limit. To this end, the energy at the coupled-cluster level of theory including single, double, and quasiperturbational triple excitations is decomposed into Hartree-Fock, low-order correlation (MP2, CCSD), and connected-triples contributions and into valence-shell and core contributions. Statistical analyses for 73 representative neutral closed-shell molecules containing hydrogen and at least three first-row atoms (CNOF) are used to devise basis-set and extrapolation requirements for each of the eight components to maintain a given level of accuracy. Pople's concept of bond-separation reactions is implemented in an ab initio framework, providing for a complete set of high-level precomputed isodesmic corrections which can be used for any molecule for which a valence structure can be drawn. Use of these corrections is shown to lower basis-set requirements dramatically for each of the eight components of the composite model. A hierarchy of three levels is suggested for isodesmically corrected composite models which reproduce atomization energies at the reference level of theory to within 0.1 kcal/mol (A), 0.3 kcal/mol (B), and 1 kcal/mol (C). Large-scale statistical analysis shows that corrections beyond the CCSD(T) reference level of theory, including coupled-cluster theory with fully relaxed connected triple and quadruple excitations, first-order relativistic and diagonal Born-Oppenheimer corrections can normally be dealt with using a greatly simplified model that assumes thermoneutral bond-separation reactions and that reduces the estimate of these corrections to the simple task of adding up bond increments. Preliminary validation with experimental enthalpies of formation using the subset of neutral closed-shell (HCNOF) species contained in the G3/99 test set indicates that the ATOMIC protocol performs slightly better than the popular G3 approach. The newly introduced protocol does not require empirical calibration, however, and it is still efficient enough to be applied routinely to molecules with 10 or 20 nonhydrogen atoms.

Aqueous solvent effects on structure and lowest electronic transition of methylene peroxide in an explicit solvent model

The electronic structure of methylene peroxide is a balance between zwitterionic and biradical valence structures. In this work we study how this balance is changed by an aqueous solvent, and what effect this has on the nuclear structure. A combined quantum chemical statistical mechanical method with an explicit representation of the solvent is used. The multiconfigurational quantum chemical method used, is very similar in quality to the complete active space self-consistent-field (CASSCF) method. The optimized structure of methylene peroxide in aqueous solution is reported along with properties of the solvation such as radial distribution functions for the surrounding water. The solvent increases the contribution from the zwitterionic form in the wave function. Moreover the lowest energy absorption of methylene peroxide in water is computed. Our best estimate is 2.94 eV.

A química orgânica na consolidação dos conceitos de átomo e molécula

The present work discusses the appearance of the concepts of valence and molecular structure, and describes the appropriation and evolution of the concept of molecule in the period following the publication of Avogadro's Hypothesis. The point of reference is the development of what became known as Organic Chemistry, which encompassed Pharmacy, Physiological Chemistry, Animal and Plant Chemistry, Chemistry of Dyestuffs, Agricultural Chemistry, and the fledgling Organic Synthesis industry in the early 19th century. The theories formulated in these areas and the quest for accurate atomic weights led to those concepts of valence and molecular structure and to a precise differentiation between atom and molecule.

Valence structures of the diastereomeric complexes meso- and rac-[Ru2(acac)4(μ-Q)]n (n = 2−, 1−, 0, 1+, 2+) with the multiple quinonoid bridging ligand Q = 1,2,4,5-tetraimino-3,6-diketocyclohexane

Meso- and rac-configurated diastereoisomers [Ru(2)(acac)(4)(mu-Q)] have been separated and identified as Ru(II)-Q(0) species through a crystal structure analysis of the meso form. The presence of two redox-active {Ru(acac)(2)} groups (acac(-) = 2,4-pentanedionate) and quinonoid Q with two equivalent pi-conjugated alpha-diimine chelate sites and one p-quinone function allowed for the full cyclic voltammetric and spectroelectrochemical (UV-vis-NIR, IR, EPR) characterisation of the five accessible states (2-, 1-, 0, 1+ and 2+ forms) for both isomers. Oxidation occurs at the metal ions to produce Ru(II)Ru(III) mixed-valent states [Ru(2)(acac)(4)(mu-Q)](+) (K(c) approximately 10(4.5)) with corresponding EPR features but without detectable intervalence absorption in the near infrared region. IR-spectroelectrochemistry reveals opposite frequency shifts for the nu(C=O) and nu(NH) stretching vibrations on reduction and oxidation, in agreement with the assumed electronic structure. Reduction leads to strongly stabilised [Ru(2)(acac)(4)(mu-Q)](-) states (K(c) approximately 10(11)) which show weak NIR shoulders around 1040 nm. The EPR characteristics are remarkably different for the two isomeric monoanions, reflecting presumably flexible geometry and electronic structure. The observation of broad but detectable EPR resonance at room temperature in solution and the g factor anisotropy in the glassy frozen state at 110 K suggest a rather evenly metal-ligand mixed singly occupied MO. Together with the ZINDO calculations and the partial experimental results reported previously by Masui et al. (Inorg. Chem., 2000, 39, 141) for [Ru(2)(bpy)(4)(mu-Q)](n+) (n = 2, 3, 4), the characteristic differences provide an insight into the electronic features such as mixed valency manifestations and the variable extent of mixing of the metal-quinone frontier orbitals of these systems involving Ru(II)-stabilised Q which is unknown as a free ligand.

Electronic structures of Al-based transition metal compounds and effective valence of transition elements

Abstract Electronic structures of Al-TM (transition metal) compounds with rather simple B2, L12 and A15 structures are studied. Focusing on the energy bands, which are not hybridized strongly with the TM-d bands, we estimate the number of electrons occupying the states below the gap induced by periodic potentials and hence obtain the average number of valence electrons per atom e/a. Although the values of e/a obtained for the different structures are scattered in spite of combination of the same chemical species, these values are obtained if one assumes similar negative values for effective valence of TM. Roles of the interference effect opening the gap in the nearly-free-electron bands are discussed.

Development and Characterization of New Cyclodextrin Polymer-Based DNA Delivery Systems

In this study, we investigated whether a cyclodextrin polymer (polybetaCD) complexed with cationic adamantyl derivatives (Ada) could be used as a vector for gene delivery. DNA compaction as a function of adamantyl/DNA phosphate ratio (A/P) by this new class of vector was demonstrated using surface enhanced Raman spectroscopy, zeta potential measurements, and DNA retardation assays. Transfection data highlight the relationship between in vitro gene delivery efficiency and the combination of several physical properties of the polybetaCD/Ada/DNA polyplexes, including cationic polar headgroup valency and chemical structure of the spacer arm of Ada connectors, the adamantyl/DNA phosphate ratio (A/P) of the polybetaCD/Ada/DNA polyplexes, and the ionic strength of the medium. Finally, when associating the best formulation with a fusogenic peptide, we reached transfection levels which were of the same order as those obtained with DOTAP.