Science.gov

Sample records for chemical bond overlap

  1. Chemical Bonds II

    ERIC Educational Resources Information Center

    Sanderson, R. T.

    1972-01-01

    The continuation of a paper discussing chemical bonding from a bond energy viewpoint, with a number of examples of single and multiple bonds. (Part I appeared in volume 1 number 3, pages 16-23, February 1972.) (AL)

  2. Overlap populations, bond orders and valences for 'fuzzy' atoms

    NASA Astrophysics Data System (ADS)

    Mayer, I.; Salvador, P.

    2004-01-01

    Proper definitions are proposed to calculate interatomic overlap populations, bond order (multiplicity) indices and actual atomic valences from the results of ab initio quantum chemical calculations, in terms of 'fuzzy' atoms, i.e., such divisions of the three-dimensional physical space into atomic regions in which the regions assigned to the individual atoms have no sharp boundaries but exhibit a continuous transition from one to another. The results of test calculations are in agreement with the classical chemical notions, exhibit unexpectedly small basis sensitivity and do not depend too much on the selection of the weight function defining the actual division of the space into 'fuzzy' atomic regions. The scheme is applicable on both SCF and correlated levels of theory. A free program is available.

  3. Chemical bonding technology

    NASA Technical Reports Server (NTRS)

    Plueddemann, E.

    1986-01-01

    Primers employed in bonding together the various material interfaces in a photovoltaic module are being developed. The approach develops interfacial adhesion by generating actual chemical bonds between the various materials bonded together. The current status of the program is described along with the progress toward developing two general purpose primers for ethylene vinyl acetate (EVA), one for glass and metals, and another for plastic films.

  4. Chemical Bonds I

    ERIC Educational Resources Information Center

    Sanderson, R. T.

    1972-01-01

    Chemical bonding is discussed from a bond energy, rather than a wave mechanics, viewpoint. This approach is considered to be more suitable for the average student. (The second part of the article will appear in a later issue of the journal.) (AL)

  5. Strength of Chemical Bonds

    NASA Technical Reports Server (NTRS)

    Christian, Jerry D.

    1973-01-01

    Students are not generally made aware of the extraordinary magnitude of the strengths of chemical bonds in terms of the forces required to pull them apart. Molecular bonds are usually considered in terms of the energies required to break them, and we are not astonished at the values encountered. For example, the Cl2 bond energy, 57.00 kcal/mole, amounts to only 9.46 x 10(sup -20) cal/molecule, a very small amount of energy, indeed, and impossible to measure directly. However, the forces involved in realizing the energy when breaking the bond operate over a very small distance, only 2.94 A, and, thus, f(sub ave) approx. equals De/(r - r(sub e)) must be very large. The forces involved in dissociating the molecule are discussed in the following. In consideration of average forces, the molecule shall be assumed arbitrarily to be dissociated when the atoms are far enough separated so that the potential, relative to that of the infinitely separated atoms, is reduced by 99.5% from the potential of the molecule at the equilibrium bond length (r(sub e)) for Cl2 of 1.988 A this occurs at 4.928 A.

  6. Clarifying Chemical Bonding. Overcoming Our Misconceptions.

    ERIC Educational Resources Information Center

    Hapkiewicz, Annis

    1991-01-01

    Demonstrations to help students change their misconceptions about chemical bond breaking are presented. Students' misconceptions about chemical bonds in both biological and chemical systems are discussed. A calculation for the release of energy from respiration is presented. (KR)

  7. A Model for the Chemical Bond

    ERIC Educational Resources Information Center

    Magnasco, Valerio

    2004-01-01

    Bond stereochemistry in polyatomic hydrides is explained in terms of the principle of bond energies maximization, which yields X-H straight bonds and suggests the formation of appropriate sp hybrids on the central atom. An introduction to the electron charge distribution in molecules is given, and atomic, overlap, gross and formal charges are…

  8. Density Functionals of Chemical Bonding

    PubMed Central

    Putz, Mihai V.

    2008-01-01

    The behavior of electrons in general many-electronic systems throughout the density functionals of energy is reviewed. The basic physico-chemical concepts of density functional theory are employed to highlight the energy role in chemical structure while its extended influence in electronic localization function helps in chemical bonding understanding. In this context the energy functionals accompanied by electronic localization functions may provide a comprehensive description of the global-local levels electronic structures in general and of chemical bonds in special. Becke-Edgecombe and author’s Markovian electronic localization functions are discussed at atomic, molecular and solid state levels. Then, the analytical survey of the main workable kinetic, exchange, and correlation density functionals within local and gradient density approximations is undertaken. The hierarchy of various energy functionals is formulated by employing both the parabolic and statistical correlation degree of them with the electronegativity and chemical hardness indices by means of quantitative structure-property relationship (QSPR) analysis for basic atomic and molecular systems. PMID:19325846

  9. G. N. Lewis and the Chemical Bond.

    ERIC Educational Resources Information Center

    Pauling, Linus

    1984-01-01

    Discusses the contributions of G. N. Lewis to chemistry, focusing on his formulation of the basic principle of the chemical bond--the idea that the chemical bond consists of a pair of electrons held jointly by two atoms. (JN)

  10. Valence-Bond Theory and Chemical Structure.

    ERIC Educational Resources Information Center

    Klein, Douglas J.; Trinajstic, Nenad

    1990-01-01

    Discussed is the importance of valence bond theory on the quantum-mechanical theory of chemical structure and the nature of the chemical bond. Described briefly are early VB theory, development of VB theory, modern versions, solid-state applications, models, treatment in textbooks, and flaws in criticisms of valence bond theory. (KR)

  11. The Chemical Bond in C2.

    PubMed

    Hermann, Markus; Frenking, Gernot

    2016-03-14

    Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for Hn CCHn (n=0-3) and N2 . The quadratic force constants and the stretching potentials of Hn CCHn have been calculated at the CASSCF/cc-pVTZ level. The bond dissociation energies of the C-C bonds of C2 and HC≡CH were computed using explicitly correlated CASPT2-F12/cc-pVTZ-F12 wave functions. The bond dissociation energies and the force constants suggest that C2 has a weaker C-C bond than acetylene. The analysis of the CASSCF wavefunctions in conjunction with the effective bond orders of the multiple bonds shows that there are four bonding components in C2 , while there are only three in acetylene and in N2 . The bonding components in C2 consist of two weakly bonding σ bonds and two electron-sharing π bonds. The bonding situation in C2 can be described with the σ bonds in Be2 that are enforced by two π bonds. There is no single Lewis structure that adequately depicts the bonding situation in C2 . The assignment of quadruple bonding in C2 is misleading, because the bond is weaker than the triple bond in HC≡CH. PMID:26756311

  12. Coulombic Models in Chemical Bonding.

    ERIC Educational Resources Information Center

    Sacks, Lawrence J.

    1986-01-01

    Compares the coulumbic point charge model for hydrogen chloride with the valence bond model. It is not possible to assign either a nonpolar or ionic canonical form of the valence bond model, while the covalent-ionic bond distribution does conform to the point charge model. (JM)

  13. Evaluating Students' Understanding of Chemical Bonding.

    ERIC Educational Resources Information Center

    Tan, Kim-Chwee Daniel; Treagust, David F.

    1999-01-01

    Outlines problems students have with understanding the abstract concept of chemical bonding as revealed in previous research. Describes the development of a two-tier multiple-choice diagnostic instrument for assessing alternative conceptions about chemical bonding held by 14-16 year olds. Discusses the instrument and its findings for a group of…

  14. Coulombic Models in Chemical Bonding.

    ERIC Educational Resources Information Center

    Sacks, Lawrence J.

    1986-01-01

    Describes a bonding theory which provides a framework for the description of a wide range of substances and provides quantitative information of remarkable accuracy with far less computational effort than that required of other approaches. Includes applications, such as calculation of bond energies of two binary hydrides (methane and diborane).…

  15. Compression effects on electrons for chemical bonding

    NASA Astrophysics Data System (ADS)

    Hu, Anguang; Zhang, Fan

    2014-03-01

    How electrons move under compression as chemical bonds between atoms are broken and formed is central to a number of challenges on the performance of materials in extreme conditions. This is not only associated with the fundamental knowledge of material response to compressive loading but also would advance many aspects of material science towards future energy technologies. First-principles simulations of enthalpy minimization, in various target pressures on chemical transformation bonding pathways, reveal that high pressure can push electrons away from their denser regimes where the kinetic energy rises steeply on compression, causing a destabilization of intramolecular bonds. The high-pressure pushing of electrons from one regime to another thus leads to chemical bond destruction and formation with a cell volume collapse accompanied by a drop in stress components. Determination of such electron pathways following bonding conformations of molecular precursors would then result in a number of chemical transformations for novel materials, including high energy density materials.

  16. Melting points and chemical bonding properties of 3d transition metal elements

    NASA Astrophysics Data System (ADS)

    Takahara, Wataru

    2014-08-01

    The melting points of 3d transition metal elements show an unusual local minimal peak at manganese across Period 4 in the periodic table. The chemical bonding properties of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper are investigated by the DV-Xα cluster method. The melting points are found to correlate with the bond overlap populations. The chemical bonding nature therefore appears to be the primary factor governing the melting points.

  17. "Vibrational bonding": a new type of chemical bond is discovered.

    PubMed

    Rhodes, Christopher J; Macrae, Roderick M

    2015-01-01

    A long-sought but elusive new type of chemical bond, occurring on a minimum-free, purely repulsive potential energy surface, has recently been convincingly shown to be possible on the basis of high-level quantum-chemical calculations. This type of bond, termed a vibrational bond, forms because the total energy, including the dynamical energy of the nuclei, is lower than the total energy of the dissociated products, including their vibrational zero-point energy. For this to be the case, the ZPE of the product molecule must be very high, which is ensured by replacing a conventional hydrogen atom with its light isotope muonium (Mu, mass = 1/9 u) in the system Br-H-Br, a natural transition state in the reaction between Br and HBr. A paramagnetic species observed in the reaction Mu +Br2 has been proposed as a first experimental sighting of this species, but definitive identification remains challenging. PMID:25942773

  18. Chemical Bonding Technology: Direct Investigation of Interfacial Bonds

    NASA Technical Reports Server (NTRS)

    Koenig, J. L.; Boerio, F. J.; Plueddemann, E. P.; Miller, J.; Willis, P. B.; Cuddihy, E. F.

    1986-01-01

    This is the third Flat-Plate Solar Array (FSA) Project document reporting on chemical bonding technology for terrestrial photovoltaic (PV) modules. The impetus for this work originated in the late 1970s when PV modules employing silicone encapsulation materials were undergoing delamination during outdoor exposure. At that time, manufacturers were not employing adhesion promoters and, hence, module interfaces in common with the silicone materials were only in physical contact and therefore easily prone to separation if, for example, water were to penetrate to the interfaces. Delamination with silicone materials virtually vanished when adhesion promoters, recommended by silicone manufacturers, were used. The activities related to the direct investigation of chemically bonded interfaces are described.

  19. Teaching Chemical Bonding: A Resource Book for Senior Chemistry.

    ERIC Educational Resources Information Center

    Lindsay, Margaret

    This document presents an instructional strategy for teaching chemical bonding using parables and music. Games, student interactions, and worksheets are included in the lesson plans. Topics include metallic bonding, covalent bonding including molecular and network structure, and ionic bonding. (JRH)

  20. The law of constant rejection. [chemical bonding in crystals

    NASA Technical Reports Server (NTRS)

    Philpotts, J. A.

    1978-01-01

    Improvements in analytical technique, study of multielement partitioning in natural systems, and improvements in ionic radius values have permitted construction of Onuma diagrams, in which the logarithm of partition coefficient is plotted versus ionic radius. The optimum radii, corresponding to crystallographic sites, do change in response to changes in major element composition for any particular mineral-type. In natural systems elements compete for sites rather than substituting for another element. It is proposed on empirical grounds, for equilibrium ionic bonding, that Onuma diagram curves for a particular lattice site are parabolic near optimum radius, linear elsewhere, parallel for different valences, and mirror images on opposite sides of optimum. Deviations may be due to overlapping peaks, liquid structure, polyvalence, bonding differences, contamination, kinetics, alteration, etc. However, dominant crystal-chemical control is indicated.

  1. Learners' Mental Models of Chemical Bonding.

    ERIC Educational Resources Information Center

    Coll, Richard K.; Treagust, David F.

    2001-01-01

    Reports on a study involving two each year-12, undergraduate and postgraduate Australian students. Elicits learners' mental models for chemical bonding using semi-structured interviews comprising a three-phase interview protocol. Concludes that learners across all three academic levels preferred simple, realistic mental models for chemical…

  2. Teaching Chemical Bonding through Jigsaw Cooperative Learning

    ERIC Educational Resources Information Center

    Doymus, Kemal

    2008-01-01

    This study examined the effectiveness of jigsaw cooperative learning in teaching chemical bonding at tertiary level. This study was carried out in two different classes in the Department of Primary Science Education of Ataturk University during the 2005-2006 academic year. One of the classes was the non-jigsaw group (control) and the other was the…

  3. Simultaneous measurement of protein one-bond residual dipolar couplings without increased resonance overlap.

    PubMed

    Vijayan, Vinesh; Zweckstetter, Markus

    2005-06-01

    A NMR strategy designed to measure simultaneously and without increased resonance overlap scalar and dipolar couplings (RDCs) in (13)C-, (15)N-labeled proteins is presented. Contrary to common schemes for simultaneous measurement of RDCs, a single reference experiment is used for the extraction of more than one type of coupling, thereby reducing the required measurement time. This is accomplished by a common reference spectrum followed by a series of interleaved experiments, in which a particular coupling dependent parameter is varied according to the quantitative J-correlation method or using accordion spectroscopy. To illustrate this idea, we have modified the 3D TROSY-HNCO and the 3D CBCA(CO)NH experiment allowing efficient measurement of one-bond (1)D(NH), (1)D(C'N), (1)D(CalphaHalpha), (1)D(CbetaHbeta), and (1)D(CalphaC') couplings in small to medium sized proteins. In addition, the experiments are expected to be useful for largely unfolded proteins, which show strong resonance overlap but have very favorable relaxation properties. Measurement of RDCs is demonstrated on uniformly (15)N-(13)C-labeled ubiquitin and on the sensory domain of the membraneous two-component fumarate sensor DcuS of Escherichia coli (17 kDa). DcuS was found to be unstable and to precipitate in one to two weeks. RDCs obtained from these experiments are in good agreement with the 1.8A X-ray structure of ubiquitin. PMID:15862241

  4. Chemical bonding for precision optical assemblies

    NASA Astrophysics Data System (ADS)

    Green, Katie; Burke, Jan; Oreb, Bozenko

    2011-02-01

    We report on the optimization of precision optical component assemblies for space application with respect to mechanical resilience and retention of optical tolerances such as flatness and angles. Optimized parameters include: the cleaning method of the surfaces to be joined; type, concentration, and quantity of the chemical bonding agent; and post-bonding and curing conditions. Experimental studies and quality assurance are complicated by the large statistical spread in breaking stress, which requires the preparation of a large number of samples. The results previously reported in literature have focused primarily on fused silica, rather than space-qualifiable materials such as Zerodur® and ULE®, and have typically addressed only one or two of the parameters. This study provides a comprehensive picture and a better general understanding of what makes a bond reliably strong.

  5. Chemical Bonding: The Orthogonal Valence-Bond View

    PubMed Central

    Sax, Alexander F.

    2015-01-01

    Chemical bonding is the stabilization of a molecular system by charge- and spin-reorganization processes in chemical reactions. These processes are said to be local, because the number of atoms involved is very small. With multi-configurational self-consistent field (MCSCF) wave functions, these processes can be calculated, but the local information is hidden by the delocalized molecular orbitals (MO) used to construct the wave functions. The transformation of such wave functions into valence bond (VB) wave functions, which are based on localized orbitals, reveals the hidden information; this transformation is called a VB reading of MCSCF wave functions. The two-electron VB wave functions describing the Lewis electron pair that connects two atoms are frequently called covalent or neutral, suggesting that these wave functions describe an electronic situation where two electrons are never located at the same atom; such electronic situations and the wave functions describing them are called ionic. When the distance between two atoms decreases, however, every covalent VB wave function composed of non-orthogonal atomic orbitals changes its character from neutral to ionic. However, this change in the character of conventional VB wave functions is hidden by its mathematical form. Orthogonal VB wave functions composed of orthonormalized orbitals never change their character. When localized fragment orbitals are used instead of atomic orbitals, one can decide which local information is revealed and which remains hidden. In this paper, we analyze four chemical reactions by transforming the MCSCF wave functions into orthogonal VB wave functions; we show how the reactions are influenced by changing the atoms involved or by changing their local symmetry. Using orthogonal instead of non-orthogonal orbitals is not just a technical issue; it also changes the interpretation, revealing the properties of wave functions that remain otherwise undetected. PMID:25906476

  6. Chemically bonded phospho-silicate ceramics

    DOEpatents

    Wagh, Arun S.; Jeong, Seung Y.; Lohan, Dirk; Elizabeth, Anne

    2003-01-01

    A chemically bonded phospho-silicate ceramic formed by chemically reacting a monovalent alkali metal phosphate (or ammonium hydrogen phosphate) and a sparsely soluble oxide, with a sparsely soluble silicate in an aqueous solution. The monovalent alkali metal phosphate (or ammonium hydrogen phosphate) and sparsely soluble oxide are both in powder form and combined in a stochiometric molar ratio range of (0.5-1.5):1 to form a binder powder. Similarly, the sparsely soluble silicate is also in powder form and mixed with the binder powder to form a mixture. Water is added to the mixture to form a slurry. The water comprises 50% by weight of the powder mixture in said slurry. The slurry is allowed to harden. The resulting chemically bonded phospho-silicate ceramic exhibits high flexural strength, high compression strength, low porosity and permeability to water, has a definable and bio-compatible chemical composition, and is readily and easily colored to almost any desired shade or hue.

  7. Chemical bonding technology for terrestrial photovoltaic modules

    NASA Technical Reports Server (NTRS)

    Coulter, D. R.; Cuddihy, E. F.; Plueddeman, E. P.

    1983-01-01

    Encapsulated photovoltaic modules must hold together for 20 years, reliably resisting delamination and separation of any of the component materials. Delamination of encapsulation materials from each other, or from solar cells and interconnects, can create voids for accumulation of water, promoting corrosive failure. Delamination of silicone elastomers from unprimed surfaces was a common occurrence with early modules, but the incidences of silicone delamination with later modules decreased when adhesion promoters recommended by silicone manufacturers were used. An investigation of silicone delamination from unprimed surfaces successfully identified the mechanism, which was related to atmospheric oxygen and moisture. This early finding indicated that reliance on physical bonding of encapsulation interfaces for long life in an outdoor environment would be risky. For long outdoor life, the material components of a module must therefore be held together by weather-stable adhesion promoters that desirably form strong, interfacial chemical bonds.

  8. Mechanical and chemical bonding of artificial joints.

    PubMed

    Oonishi, H

    1990-01-01

    Biomaterials which create chemical and mechanical bonds with tissue, i.e. (1) non-porous materials with or without a hydroxyapatite coating, (2) porous titanium alloy (beads) with or without a hydroxyapatite coating, (3) alpha-tricalcium phosphate bioactive bone cement and PMMA cement, and (4) interface bioactive bone cement made by interposing hydroxyapatite granules between polymethylmethacrylate cement and the bone, were used in animal experiments and clinical applications. The common problem with cementless fixation is that some patients complain of slight pain on weight-bearing, because a complete initial fixation is not obtained and micro-movement of the component may occur. Porous metal with hydroxyapatite coating is found to be better than that without coating for producing earlier and stronger fixation, and problems with fatigue and peeling of hydroxyapatite from the base metal are eliminated when the beads are coated with hydroxyapatite. As hydroxyapatite bonds chemically to the bone, pain on weight-bearing due to micromovement should never occur. In order to obtain long-term and stable fixation for severe bony atrophy, bioactive bone cement or interface bioactive bone cement (interposing hydroxyapatite at the bone interface) is desirable. PMID:10147505

  9. Bonding pathways of high-pressure chemical transformations

    NASA Astrophysics Data System (ADS)

    Hu, Anguang; Zhang, Fan

    2013-09-01

    A three-stage bonding pathway towards high-pressure chemical transformations from molecular precursors or intermediate states has been identified by first-principles simulations. With the evolution of principal stress tensor components in the response of chemical bonding to compressive loading, the three stages can be defined as the van der Waals bonding destruction, a bond breaking and forming reaction, and equilibrium of new bonds. The three-stage bonding pathway leads to the establishment of a fundamental principle of chemical bonding under compression. It reveals that during high-pressure chemical transformation, electrons moving away from functional groups follow anti-addition, collision-free paths to form new bonds in counteracting the local stress confinement. In applying this principle, a large number of molecular precursors were identified for high-pressure chemical transformations, resulting in new materials.

  10. Integrated defense system overlaps as a disease model: with examples for multiple chemical sensitivity.

    PubMed Central

    Rowat, S C

    1998-01-01

    The central nervous, immune, and endocrine systems communicate through multiple common messengers. Over evolutionary time, what may be termed integrated defense system(s) (IDS) have developed to coordinate these communications for specific contexts; these include the stress response, acute-phase response, nonspecific immune response, immune response to antigen, kindling, tolerance, time-dependent sensitization, neurogenic switching, and traumatic dissociation (TD). These IDSs are described and their overlap is examined. Three models of disease production are generated: damage, in which IDSs function incorrectly; inadequate/inappropriate, in which IDS response is outstripped by a changing context; and evolving/learning, in which the IDS learned response to a context is deemed pathologic. Mechanisms of multiple chemical sensitivity (MCS) are developed from several IDS disease models. Model 1A is pesticide damage to the central nervous system, overlapping with body chemical burdens, TD, and chronic zinc deficiency; model 1B is benzene disruption of interleukin-1, overlapping with childhood developmental windows and hapten-antigenic spreading; and model 1C is autoimmunity to immunoglobulin-G (IgG), overlapping with spreading to other IgG-inducers, sudden spreading of inciters, and food-contaminating chemicals. Model 2A is chemical and stress overload, including comparison with the susceptibility/sensitization/triggering/spreading model; model 2B is genetic mercury allergy, overlapping with: heavy metals/zinc displacement and childhood/gestational mercury exposures; and model 3 is MCS as evolution and learning. Remarks are offered on current MCS research. Problems with clinical measurement are suggested on the basis of IDS models. Large-sample patient self-report epidemiology is described as an alternative or addition to clinical biomarker and animal testing. Images Figure 1 Figure 2 Figure 3 Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 PMID:9539008

  11. The electronic structure and chemical bonding of vitamin B12

    NASA Astrophysics Data System (ADS)

    Kurmaev, E. Z.; Moewes, A.; Ouyang, L.; Randaccio, L.; Rulis, P.; Ching, W. Y.; Bach, M.; Neumann, M.

    2003-05-01

    The electronic structure and chemical bonding of vitamin B12 (cyanocobalamin) and B12-derivative (methylcobalamin) are studied by means of X-ray emission (XES) and photoelectron (XPS) spectroscopy. The obtained results are compared with ab initio electronic structure calculations using the orthogonalized linear combination of the atomic orbital method (OLCAO). We show that the chemical bonding in vitamin B12 is characterized by the strong Co-C bond and relatively weak axial Co-N bond. It is further confirmed that the Co-C bond in cyanocobalamin is stronger than that of methylcobalamin resulting in their different biological activity.

  12. The Pairwise Correlated Generalized Valence Bond Model of Electronic Structure I; The Estimation of Pair Energies from Orbital Overlaps

    PubMed Central

    Petersson, G. A.

    1974-01-01

    A new method for the accurate a priori calculation of atomic and molecular energies is proposed. The new method agrees with experiment to within less than 1 kcal/mole in all cases examined thus far, and is applicable to excited states and to transition states for chemical reactions. Since the new method corrects the results of generalized valence bond calculations for the effects of electron pair correlations, we call the new method the pairwise correlated generalized valence bond method. PMID:16592172

  13. Upper Secondary Teachers' Knowledge for Teaching Chemical Bonding Models

    ERIC Educational Resources Information Center

    Bergqvist, Anna; Drechsler, Michal; Chang Rundgren, Shu-Nu

    2016-01-01

    Researchers have shown a growing interest in science teachers' professional knowledge in recent decades. The article focuses on how chemistry teachers impart chemical bonding, one of the most important topics covered in upper secondary school chemistry courses. Chemical bonding is primarily taught using models, which are key for understanding…

  14. Teaching and Learning the Concept of Chemical Bonding

    ERIC Educational Resources Information Center

    Levy Nahum, Tami; Mamlok-Naaman, Rachel; Hofstein, Avi; Taber, Keith S.

    2010-01-01

    Chemical bonding is one of the key and basic concepts in chemistry. The learning of many of the concepts taught in chemistry, in both secondary schools as well as in the colleges, is dependent upon understanding fundamental ideas related to chemical bonding. Nevertheless, the concept is perceived by teachers, as well as by learners, as difficult,…

  15. A corpuscular picture of electrons in chemical bond

    NASA Astrophysics Data System (ADS)

    Ando, Koji

    2016-03-01

    We introduce a theory of chemical bond with a corpuscular picture of electrons. It employs a minimal set of localized electron wave packets with "floating and breathing" degrees of freedom and the spin-coupling of non-orthogonal valence-bond theory. Its accuracy for describing potential energy curves of chemical bonds in ground and excited states of spin singlet and triplet is examined.

  16. A corpuscular picture of electrons in chemical bond.

    PubMed

    Ando, Koji

    2016-03-28

    We introduce a theory of chemical bond with a corpuscular picture of electrons. It employs a minimal set of localized electron wave packets with "floating and breathing" degrees of freedom and the spin-coupling of non-orthogonal valence-bond theory. Its accuracy for describing potential energy curves of chemical bonds in ground and excited states of spin singlet and triplet is examined. PMID:27036429

  17. Unicorns in the world of chemical bonding models.

    PubMed

    Frenking, Gernot; Krapp, Andreas

    2007-01-15

    The appearance and the significance of heuristically developed bonding models are compared with the phenomenon of unicorns in mythical saga. It is argued that classical bonding models played an essential role for the development of the chemical science providing the language which is spoken in the territory of chemistry. The advent and the further development of quantum chemistry demands some restrictions and boundary conditions for classical chemical bonding models, which will continue to be integral parts of chemistry. PMID:17109434

  18. Closing in on chemical bonds by opening up relativity theory.

    PubMed

    Whitney, Cynthia K

    2008-03-01

    This paper develops a connection between the phenomenology of chemical bonding and the theory of relativity. Empirical correlations between electron numbers in atoms and chemical bond stabilities in molecules are first reviewed and extended. Quantitative chemical bond strengths are then related to ionization potentials in elements. Striking patterns in ionization potentials are revealed when the data are viewed in an element-independent way, where element-specific details are removed via an appropriate scaling law. The scale factor involved is not explained by quantum mechanics; it is revealed only when one goes back further, to the development of Einstein's special relativity theory. PMID:19325749

  19. Closing in on Chemical Bonds by Opening up Relativity Theory

    PubMed Central

    Whitney, Cynthia Kolb

    2008-01-01

    This paper develops a connection between the phenomenology of chemical bonding and the theory of relativity. Empirical correlations between electron numbers in atoms and chemical bond stabilities in molecules are first reviewed and extended. Quantitative chemical bond strengths are then related to ionization potentials in elements. Striking patterns in ionization potentials are revealed when the data are viewed in an element-independent way, where element-specific details are removed via an appropriate scaling law. The scale factor involved is not explained by quantum mechanics; it is revealed only when one goes back further, to the development of Einstein’s special relativity theory. PMID:19325749

  20. Electronegativity, Bond Energy, and Chemical Reactivity.

    ERIC Educational Resources Information Center

    Myers, R. Thomas

    1979-01-01

    Discusses the Pauling electronegativity concept which rationalizes several kinds of chemical reactions of covalent substances. Electronegativity differences applied to some reactions are demonstrated. (SA)

  1. The Bondons: The Quantum Particles of the Chemical Bond

    PubMed Central

    Putz, Mihai V.

    2010-01-01

    By employing the combined Bohmian quantum formalism with the U(1) and SU(2) gauge transformations of the non-relativistic wave-function and the relativistic spinor, within the Schrödinger and Dirac quantum pictures of electron motions, the existence of the chemical field is revealed along the associate bondon particle B̶ characterized by its mass (mB̶), velocity (vB̶), charge (eB̶), and life-time (tB̶). This is quantized either in ground or excited states of the chemical bond in terms of reduced Planck constant ħ, the bond energy Ebond and length Xbond, respectively. The mass-velocity-charge-time quaternion properties of bondons’ particles were used in discussing various paradigmatic types of chemical bond towards assessing their covalent, multiple bonding, metallic and ionic features. The bondonic picture was completed by discussing the relativistic charge and life-time (the actual zitterbewegung) problem, i.e., showing that the bondon equals the benchmark electronic charge through moving with almost light velocity. It carries negligible, although non-zero, mass in special bonding conditions and towards observable femtosecond life-time as the bonding length increases in the nanosystems and bonding energy decreases according with the bonding length-energy relationship Ebond[kcal/mol]×Xbond[A0]=182019, providing this way the predictive framework in which the B̶ particle may be observed. Finally, its role in establishing the virtual states in Raman scattering was also established. PMID:21151435

  2. The Nature of the Chemical Bond--1990.

    ERIC Educational Resources Information Center

    Ogilvie, J. F.

    1990-01-01

    Three aspects of quantum mechanics in modern chemistry are stressed: the fundamental structure of quantum mechanics as a basis of chemical applications, the relationship of quantum mechanics to atomic and molecular structure, and the consequent implications for chemical education. A list of 64 references is included. (CW)

  3. In-silico bonding schemes to encode chemical bonds involving sharing of electrons in molecular structures.

    PubMed

    Punnaivanam, Sankar; Sathiadhas, Jerome Pastal Raj; Panneerselvam, Vinoth

    2016-05-01

    Encoding of covalent and coordinate covalent bonds in molecular structures using ground state valence electronic configuration is achieved. The bonding due to electron sharing in the molecular structures is described with five fundamental bonding categories viz. uPair-uPair, lPair-uPair, uPair-lPair, vPair-lPair, and lPair-lPair. The involvement of lone pair electrons and the vacant electron orbitals in chemical bonding are explained with bonding schemes namely "target vacant promotion", "source vacant promotion", "target pairing promotion", "source pairing promotion", "source cation promotion", "source pairing double bond", "target vacant occupation", and "double pairing promotion" schemes. The bonding schemes are verified with a chemical structure editor. The bonding in the structures like ylides, PCl5, SF6, IF7, N-Oxides, BF4(-), AlCl4(-) etc. are explained and encoded unambiguously. The encoding of bonding in the structures of various organic compounds, transition metals compounds, coordination complexes and metal carbonyls is accomplished. PMID:27041446

  4. Chemical Case Studies: Science-Society "Bonding."

    ERIC Educational Resources Information Center

    Hofstein, Avi; Nae, Nehemia

    1981-01-01

    Describes a unit designed to illustrate the "science-society-technology connection," in which three case studies of the chemical industry in Israel are presented to high school chemistry students. Chosen for the unit are case studies on copper production in Timna, on plastics, and on life from the Dead Sea. (CS)

  5. Upper Secondary Teachers' Knowledge for Teaching Chemical Bonding Models

    NASA Astrophysics Data System (ADS)

    Bergqvist, Anna; Drechsler, Michal; Rundgren, Shu-Nu Chang

    2016-01-01

    Researchers have shown a growing interest in science teachers' professional knowledge in recent decades. The article focuses on how chemistry teachers impart chemical bonding, one of the most important topics covered in upper secondary school chemistry courses. Chemical bonding is primarily taught using models, which are key for understanding science. However, many studies have determined that the use of models in science education can contribute to students' difficulties understanding the topic, and that students generally find chemical bonding a challenging topic. The aim of this study is to investigate teachers' knowledge of teaching chemical bonding. The study focuses on three essential components of pedagogical content knowledge (PCK): (1) the students' understanding, (2) representations, and (3) instructional strategies. We analyzed lesson plans about chemical bonding generated by 10 chemistry teachers with whom we also conducted semi-structured interviews about their teaching. Our results revealed that the teachers were generally unaware of how the representations of models they used affected student comprehension. The teachers had trouble specifying students' difficulties in understanding. Moreover, most of the instructional strategies described were generic and insufficient for promoting student understanding. Additionally, the teachers' rationale for choosing a specific representation or activity was seldom directed at addressing students' understanding. Our results indicate that both PCK components require improvement, and suggest that the two components should be connected. Implications for the professional development of pre-service and in-service teachers are discussed.

  6. The nature of chemical bonds from PNOF5 calculations.

    PubMed

    Matxain, Jon M; Piris, Mario; Uranga, Jon; Lopez, Xabier; Merino, Gabriel; Ugalde, Jesus M

    2012-06-18

    Natural orbital functional theory (NOFT) is used for the first time in the analysis of different types of chemical bonds. Concretely, the Piris natural orbital functional PNOF5 is used. It provides a localization scheme that yields an orbital picture which agrees very well with the empirical valence shell electron pair repulsion theory (VSEPR) and Bent's rule, as well as with other theoretical pictures provided by valence bond (VB) or linear combination of atomic orbitals-molecular orbital (LCAO-MO) methods. In this context, PNOF5 provides a novel tool for chemical bond analysis. In this work, PNOF5 is applied to selected molecules that have ionic, polar covalent, covalent, multiple (σ and π), 3c-2e, and 3c-4e bonds. PMID:22615195

  7. Stability of Cu-Nb layered nanocomposite from chemical bonding

    NASA Astrophysics Data System (ADS)

    Saikia, Ujjal; Sahariah, Munima B.; Pandey, Ravindra

    2016-07-01

    The potential use of layered metallic nanocomposites in radiation-resistant materials has been recognized with ultra-high mechanical strengths. Here we present results on layered Cu-Nb composite examining its stability in terms of chemical bond via charge density and transfer analysis, QTAIM, electron localization function and density of states using DFT. An intermediate character of bonding with a significant amount of charge transfer at the interface has been predicted. Shortening of intraplanar bond length is a good manifestation of their observed structural stability which may be due to electron promotion of 3 d → (4 s, 4 p) orbitals associated with the constituent atoms of the composite.

  8. Is There a Need to Discuss Atomic Orbital Overlap When Teaching Hydrogen-Halide Bond Strength and Acidity Trends in Organic Chemistry?

    ERIC Educational Resources Information Center

    Devarajan, Deepa; Gustafson, Samantha J.; Bickelhaupt, F. Matthias; Ess, Daniel H.

    2015-01-01

    Undergraduate organic chemistry textbooks and Internet websites use a variety of approaches for presenting and explaining the impact of halogen atom size on trends in bond strengths and/or acidity of hydrogen halides. In particular, several textbooks and Internet websites explain these trends by invoking decreasing orbital overlap between the…

  9. Chemical bonds and vibrational properties of ordered (U, Np, Pu) mixed oxides

    NASA Astrophysics Data System (ADS)

    Yang, Yu; Zhang, Ping

    2013-01-01

    We use density functional theory +U to investigate the chemical bonding characters and vibrational properties of the ordered (U, Np, Pu) mixed oxides (MOXs), UNpO4,NpPuO4, and UPuO4. It is found that the 5f electronic states of different actinide elements keep their localized characters in all three MOXs. The occupied 5f electronic states of different actinide elements do not overlap with each other and tend to distribute over the energy band gap of the other actinide element's 5f states. As a result, the three ordered MOXs all show smaller band gaps than those of the component dioxides, with values of 0.91, 1.47, and 0.19 eV for UNpO4,NpPuO4, and UPuO4, respectively. Through careful charge density analysis, we further show that the U-O and Pu-O bonds in MOXs show more ionic character than in UO2 and PuO2, while the Np-O bonds show more covalent character than in NpO2. The change in covalencies in the chemical bonds leads to vibrational frequencies of oxygen atoms that are different in MOXs.

  10. Chemical strategies for die/wafer submicron alignment and bonding.

    SciTech Connect

    Martin, James Ellis; Baca, Alicia I.; Chu, Dahwey; Rohwer, Lauren Elizabeth Shea

    2010-09-01

    This late-start LDRD explores chemical strategies that will enable sub-micron alignment accuracy of dies and wafers by exploiting the interfacial energies of chemical ligands. We have micropatterned commensurate features, such as 2-d arrays of micron-sized gold lines on the die to be bonded. Each gold line is functionalized with alkanethiol ligands before the die are brought into contact. The ligand interfacial energy is minimized when the lines on the die are brought into registration, due to favorable interactions between the complementary ligand tails. After registration is achieved, standard bonding techniques are used to create precision permanent bonds. We have computed the alignment forces and torque between two surfaces patterned with arrays of lines or square pads to illustrate how best to maximize the tendency to align. We also discuss complex, aperiodic patterns such as rectilinear pad assemblies, concentric circles, and spirals that point the way towards extremely precise alignment.

  11. Remediating Misconceptions Concerning Chemical Bonding through Conceptual Change Text

    ERIC Educational Resources Information Center

    Pabuccu, Aybuke; Geban, Omer

    2006-01-01

    The purpose of this study was to explore the effects of conceptual change texts oriented instruction on 9th grade students' understanding of chemical bonding concepts. In this study, the main aim of the preparation of conceptual change texts was to activate students' prior knowledge and misconceptions and to help them to understand the chemical…

  12. Characterizing and Representing Student's Conceptual Knowledge of Chemical Bonding

    ERIC Educational Resources Information Center

    Yayon, Malka; Mamlok-Naaman, Rachel; Fortus, David

    2012-01-01

    Chemical bonding knowledge is fundamental and essential to the understanding of almost every topic in chemistry, but it is very difficult to learn. While many studies have characterized some of the central elements of knowledge of this topic, these elements of knowledge have not been systematically organized. We describe the development and…

  13. Morphological and Chemical Characterization of Bonding Hydrophobic Adhesive to Dentin Using Ethanol Wet Bonding Technique

    PubMed Central

    Shin, T. Phillip; Yao, Xiaomei; Huenergardt, Robin; Walker, Mary P; Wang, Yong

    2009-01-01

    Objective BisGMA, a widely used component in dentin adhesive has very good mechanical properties after curing, but is relatively hydrophobic and thus, does not adequately infiltrate the water wet demineralized dentin collagen. Developing techniques that would lead to optimum infiltration of the hydrophobic component into the demineralized dentin matrix is very important. The purpose of this study was to evaluate interfacial morphological and chemical characteristics of the resultant adhesive-dentin interface when the ethanol wet bonding technique is used with hydrophobic adhesives. Materials and methods The occlusal one-third of the crown was removed from six unerupted human third molars; a uniform smear layer was created with 600 grit SiC. The dentin surface was etched with 35% phosphoric acid for 15 seconds before applying BisGMA/HEMA model adhesive using either water wet or ethanol wet bonding technique. Five-micro-thick sections of adhesive/dentin interface specimens were cut and stained with Goldner’s trichrome for light microscopy. Companion slabs were analyzed with SEM and micro-Raman spectroscopy. Results The presence of ethanol in the demineralized dentin increased adhesive collagen encapsulation as indicated by trichrome staining. The SEM results confirmed that the ethanol wet bonding improved the quality of the interface. Micro-Raman spectral analysis of the dentin/adhesive interface indicated there was a gradual decrease in penetration of BisGMA component for specimens using water wet bonding, while relatively homogeneous distribution of the hydrophobic BisGMA component was noted in the interface with ethanol wet bonding. Significance Wet bonding with ethanol instead of water permits better BisGMA infiltration improving the quality of interface. We speculate that the higher infiltration of hydrophobic BisGMA and better collagen encapsulation observed from the specimens using ethanol wet bonding would lead to more durable bonds because of improved

  14. Chemical Trends for Transition Metal Compound Bonding to Graphene

    NASA Astrophysics Data System (ADS)

    Lange, Bjoern; Blum, Volker

    2015-03-01

    Transition metal compounds are of interest as catalysts for the hydrogen evolution reaction (HER). However, a perfect candidate to replace expensive platinum has not yet been identified. To tailor a specific compound, several properties come into play. One is the bonding to the underlying substrate, for which π-bonded carbon nanostructures are promising candidates. Here we analyze the bonding of small transition metal compound nanoclusters to a graphene layer for a range of chemical compositions: MxAy (M = Mo, Ti; A = S, O, B, N, C). The clusters are generated by an unbiased random search algorithm. We perform total energy calculations based on density functional theory to identify lowest energy clusters. We calculate binding energies using the PBE and HSE functionals with explicit van der Waals treatment and benchmark those against RPA cluster calculations. Our results indicate that molybdenum-carbides and -nitrides tend to bond tightly to graphene. Mo-oxides and -sulfides show small binding energies, indicating van der Waals bonding.

  15. Nanohardness and chemical bonding of Boron Nitride films

    SciTech Connect

    Jankowski, A F

    1998-07-08

    Boron-nitride (BN) films are deposited by the reactive sputter deposition of fully dense, boron targets utilizing a planar magnetron source and an argon-nitrogen working gas mixture. Near-edge x-ray absorption fine structure analysis reveals distinguishing features of chemical bonding within the boron is photoabsorption cross-section. The hardness of the BN film surface is measured using nanoindentation. The sputter deposition conditions as well as the post-deposition treatments of annealing and nitrogen-ion implantation effect the chemical bonding and the film hardness. A model is proposed to quantify the film hardness using the relative peak intensities of the p*-resonances to the boron 1s spectra.

  16. Fast and accurate predictions of covalent bonds in chemical space.

    PubMed

    Chang, K Y Samuel; Fias, Stijn; Ramakrishnan, Raghunathan; von Lilienfeld, O Anatole

    2016-05-01

    We assess the predictive accuracy of perturbation theory based estimates of changes in covalent bonding due to linear alchemical interpolations among molecules. We have investigated σ bonding to hydrogen, as well as σ and π bonding between main-group elements, occurring in small sets of iso-valence-electronic molecules with elements drawn from second to fourth rows in the p-block of the periodic table. Numerical evidence suggests that first order Taylor expansions of covalent bonding potentials can achieve high accuracy if (i) the alchemical interpolation is vertical (fixed geometry), (ii) it involves elements from the third and fourth rows of the periodic table, and (iii) an optimal reference geometry is used. This leads to near linear changes in the bonding potential, resulting in analytical predictions with chemical accuracy (∼1 kcal/mol). Second order estimates deteriorate the prediction. If initial and final molecules differ not only in composition but also in geometry, all estimates become substantially worse, with second order being slightly more accurate than first order. The independent particle approximation based second order perturbation theory performs poorly when compared to the coupled perturbed or finite difference approach. Taylor series expansions up to fourth order of the potential energy curve of highly symmetric systems indicate a finite radius of convergence, as illustrated for the alchemical stretching of H2 (+). Results are presented for (i) covalent bonds to hydrogen in 12 molecules with 8 valence electrons (CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, GeH4, AsH3, H2Se, HBr); (ii) main-group single bonds in 9 molecules with 14 valence electrons (CH3F, CH3Cl, CH3Br, SiH3F, SiH3Cl, SiH3Br, GeH3F, GeH3Cl, GeH3Br); (iii) main-group double bonds in 9 molecules with 12 valence electrons (CH2O, CH2S, CH2Se, SiH2O, SiH2S, SiH2Se, GeH2O, GeH2S, GeH2Se); (iv) main-group triple bonds in 9 molecules with 10 valence electrons (HCN, HCP, HCAs, HSiN, HSi

  17. Fast and accurate predictions of covalent bonds in chemical space

    NASA Astrophysics Data System (ADS)

    Chang, K. Y. Samuel; Fias, Stijn; Ramakrishnan, Raghunathan; von Lilienfeld, O. Anatole

    2016-05-01

    We assess the predictive accuracy of perturbation theory based estimates of changes in covalent bonding due to linear alchemical interpolations among molecules. We have investigated σ bonding to hydrogen, as well as σ and π bonding between main-group elements, occurring in small sets of iso-valence-electronic molecules with elements drawn from second to fourth rows in the p-block of the periodic table. Numerical evidence suggests that first order Taylor expansions of covalent bonding potentials can achieve high accuracy if (i) the alchemical interpolation is vertical (fixed geometry), (ii) it involves elements from the third and fourth rows of the periodic table, and (iii) an optimal reference geometry is used. This leads to near linear changes in the bonding potential, resulting in analytical predictions with chemical accuracy (˜1 kcal/mol). Second order estimates deteriorate the prediction. If initial and final molecules differ not only in composition but also in geometry, all estimates become substantially worse, with second order being slightly more accurate than first order. The independent particle approximation based second order perturbation theory performs poorly when compared to the coupled perturbed or finite difference approach. Taylor series expansions up to fourth order of the potential energy curve of highly symmetric systems indicate a finite radius of convergence, as illustrated for the alchemical stretching of H 2+ . Results are presented for (i) covalent bonds to hydrogen in 12 molecules with 8 valence electrons (CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, GeH4, AsH3, H2Se, HBr); (ii) main-group single bonds in 9 molecules with 14 valence electrons (CH3F, CH3Cl, CH3Br, SiH3F, SiH3Cl, SiH3Br, GeH3F, GeH3Cl, GeH3Br); (iii) main-group double bonds in 9 molecules with 12 valence electrons (CH2O, CH2S, CH2Se, SiH2O, SiH2S, SiH2Se, GeH2O, GeH2S, GeH2Se); (iv) main-group triple bonds in 9 molecules with 10 valence electrons (HCN, HCP, HCAs, HSiN, HSi

  18. Chemically bonded ceramic matrix composites: Densification and conversion to diffusion bonding

    SciTech Connect

    Johnson, B.R.; Guelguen, M.A.; Kriven, W.M.

    1995-10-01

    Chemically bonded ceramics appear to be a promising alternative route for near-net shape fabrication of multi-phase ceramic matrix composites (CMC`s). The hydraulic (and refractory) properties of fine mono-calcium aluminate (CaAl{sub 2}O{sub 4}) powders were used as the chemically bonding matrix phase, while calcia stabilized zirconia powders were the second phase material. Samples containing up to 70 wt% (55 vol%) zirconia have been successfully compacted and sintered. Various processing techniques were evaluated. Processing was optimized based on material properties, dilatometry and simultaneous thermal analysis (DTA/TGA). The physical characteristics of this novel CMC were characterized by hardness, density, and fracture toughness testing. Microstructures were evaluated by SEM and phase identification was verified using XRD.

  19. Students' Understandings of Chemical Bonds and the Energetics of Chemical Reactions.

    ERIC Educational Resources Information Center

    Boo, Hong Kwen

    1998-01-01

    Investigates Grade 12 students' understandings of the nature of chemical bonds and the energetics elicited across five familiar chemical reactions following a course of instruction. Discusses the many ways in which students can misconstruct concepts and principles. Contains 63 references. (DDR)

  20. Nature of the chemical bond and prediction of radiation tolerance in pyrochlore and defect fluorite compounds

    SciTech Connect

    Lumpkin, Gregory R. Pruneda, Miguel; Rios, Susana; Smith, Katherine L.; Trachenko, Kostya; Whittle, Karl R.; Zaluzec, Nestor J.

    2007-04-15

    The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation-anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation-anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis of the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature. - Graphical abstract: Three-dimensional representation of the predicted critical amorphization temperature in pyrochlores.

  1. Atom-specific look at the surface chemical bond using x-ray emission spectroscopy

    SciTech Connect

    Nilsson, A.; Wassdahl, N.; Weinelt, M.

    1997-04-01

    CO and N{sub 2} adsorbed on the late transition metals have become prototype systems regarding the general understanding of molecular adsorption. It is in general assumed that the bonding of molecules to transition metals can be explained in terms of the interaction of the frontier HOMO and LUMO molecular orbitals with the d-orbitals. In such a picture the other molecular orbitals should remain essentially the same as in the free molecule. For the adsorption of the isoelectronic molecules CO and N{sub 2} this has led to the so called Blyholder model i.e., a synergetic {sigma} (HOMO) donor and {pi} (LUMO) backdonation bond. The authors results at the ALS show that such a picture is oversimplified. The direct observation and identification of the states related to the surface chemical bond is an experimental challenge. For noble and transition metal surfaces, the adsorption induced states overlap with the metal d valence band. Their signature is therefore often obscured by bulk substrate states. This complication has made it difficult for techniques such as photoemission and inverse photoemission to provide reliable information on the energy of chemisorption induced states and has left questions unanswered regarding the validity of the frontier orbitals concept. Here the authors show how x-ray emission spectroscopy (XES), in spite of its inherent bulk sensitivity, can be used to investigate adsorbed molecules. Due to the localization of the core-excited intermediate state, XE spectroscopy allows an atomic specific separation of the valence electronic states. Thus the molecular contributions to the surface measurements make it possible to determine the symmetry of the molecular states, i.e., the separation of {pi} and {sigma} type states. In all the authors can obtain an atomic view of the electronic states involved in the formation of the chemical bond to the surface.

  2. Site-selective chemical cleavage of peptide bonds.

    PubMed

    Elashal, Hader E; Raj, Monika

    2016-05-01

    Site-selective cleavage of extremely unreactive peptide bonds is a very important chemical modification that provides invaluable information regarding protein sequence, and it acts as a modulator of protein structure and function for therapeutic applications. For controlled and selective cleavage, a daunting task, chemical reagents must selectively recognize or bind to one or more amino acid residues in the peptide chain and selectively cleave a peptide bond. Building on this principle, we have developed an approach that utilizes a chemical reagent to selectively modify the serine residue in a peptide chain and leads to the cleavage of a peptide backbone at the N-terminus of the serine residue. After cleavage, modified residues can be converted back to the original fragments. This method exhibits broad substrate scope and selectively cleaves various bioactive peptides with post-translational modifications (e.g. N-acetylation and -methylation) and mutations (d- and β-amino acids), which are a known cause of age related diseases. PMID:27087443

  3. Vitrified chemically bonded phosphate ceramics for immobilization of radioisotopes

    DOEpatents

    Wagh, Arun S.

    2016-04-05

    A method of immobilizing a radioisotope and vitrified chemically bonded phosphate ceramic (CBPC) articles formed by the method are described. The method comprises combining a radioisotope-containing material, MgO, a source of phosphate, and optionally, a reducing agent, in water at a temperature of less than 100.degree. C. to form a slurry; curing the slurry to form a solid intermediate CBPC article comprising the radioisotope therefrom; comminuting the intermediate CBPC article, mixing the comminuted material with glass frits, and heating the mixture at a temperature in the range of about 900 to about 1500.degree. C. to form a vitrified CBPC article comprising the radioisotope immobilized therein.

  4. X-ray charge density study of chemical bonding in skutterudite CoSb{sub 3}

    SciTech Connect

    Ohno, Atsuko; Sasaki, Satoshi; Nishibori, Eiji; Aoyagi, Shinobu; Sakata, Makoto; Iversen, Bo Brummerstedt

    2007-08-01

    The experimental charge density of nondoped CoSb{sub 3} has been determined by the maximum entropy method (MEM) using low temperature (10 K), short wavelength (0.42 A), and high-resolution (d{sub min}=0.33 A) synchrotron x-ray powder diffraction data measured at SPring-8, Japan. The MEM charge density clearly reveals three types of charge density overlap between atoms in CoSb{sub 3}. The four Sb atoms form an Sb{sub 4} ring, and the MEM charge densities at two types of Sb-Sb midpoints are 0.35 and 0.50 e A{sup -3}. The charge density overlap between the Co and Sb atoms at the midpoint is 0.52 e A{sup -3}, which is larger than the values observed at the Sb-Sb midpoints. The nature of the chemical bonding observed in the present MEM charge density is consistent with a previous theoretical study by Lefebvre-Devos et al. [Phys. Rev. B 63, 125110 (2001)].

  5. Absence of X-point band overlap in divalent hexaborides and variability of the surface chemical potential

    SciTech Connect

    Denlinger, Jonathan D.; Gweon, Gey-Hong; Mo, Sung-Kwan; Allen, James W.; Sarrao, John L.; Bianchi, Adrian D.; Fisk, Zachary

    2001-11-04

    Angle-resolved photoemission measurements of divalent hexaborides reveals a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. While the global ARPES band structure and gap size observed are consistent with the results of bulk-sensitive soft x-ray absorption and emission boron K-edge spectroscopy, the surface-sensitive photoemission measurements also show a variation with cation, surface and time of the position of the surface chemical potential in the band structure.

  6. Chemical bonding and stability of multilayer graphene oxide layers

    NASA Astrophysics Data System (ADS)

    Gong, Cheng; Kim, Suenne; Zhou, Si; Hu, Yike; Acik, Muge; de Heer, Walt; Berger, Claire; Bongiorno, Angelo; Riedo, Eliso; Chabal, Yves

    2014-03-01

    The chemistry of graphene oxide (GO) and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is however crucial to enable future applications of the material that typically are subject to environmental conditions. The nature of the initial GO is also highly dependent on the preparation and the form of the initial carbon material. Here, we consider both standard GO made from oxidizing graphite and layered GO made from oxidizing epitaxial graphene on SiC, and examine their evolution under different stimuli. The effect of the solvent on the thermal evolution of standard GO in vacuum is first investigated. In situ infrared absorption measurements clearly show that the nature of the last solvent in contact with GO prior to deposition on a substrate for vacuum annealing studies substantially affect the chemical evolution of the material as GO is reduced. Second, the stability of GO derived from epitaxial graphene (on SiC) is examined as a function of time. We show that hydrogen, in the form of CH, is present after the Hummers process, and that hydrogen favors the reduction of epoxide groups and the formation of water molecules. Importantly, this transformation can take place at room temperature, albeit slowly (~ one month). Finally, the chemical interaction (e.g. bonding) between GO layers in multilayer samples is examined with diffraction (XRD) methods, spectroscopic (IR, XPS, Raman) techniques, imaging (APF) and first principles modeling.

  7. Effect of silica coating on bond strength between a gold alloy and metal bracket bonded with chemically cured resin

    PubMed Central

    Ryu, Min-Ju; Lim, Sung-Hoon

    2014-01-01

    Objective The purpose of this study was to evaluate the effects of three different surface conditioning methods on the shear bond strength (SBS) of metal brackets bonded directly to gold alloy with chemically cured resin. Methods Two hundred ten type III gold alloy specimens were randomly divided into six groups according to the combination of three different surface conditioning methods (aluminum oxide sandblasting only, application of a metal primer after aluminum oxide sandblasting, silica coating and silanation) and thermocycling (with thermocycling, without thermocycling). After performing surface conditioning of specimens in accordance with each experimental condition, metal brackets were bonded to all specimens using a chemically cured resin. The SBS was measured at the moment of bracket debonding, and the resin remnants on the specimen surface were evaluated using the adhesive remnant index. Results Application of metal primer after aluminum oxide sandblasting yielded a higher bond strength than that with aluminum oxide sandblasting alone (p < 0.001), and silica coating and silanation yielded a higher bond strength than that with metal primer after aluminum oxide sandblasting (p < 0.001). There was no significant change in SBS after thermocycling in all groups. Conclusions With silica coating and silanation, clinically satisfactory bond strength can be attained when metal brackets are directly bonded to gold alloys using a chemically cured resin. PMID:24892023

  8. Method of waste stabilization with dewatered chemically bonded phosphate ceramics

    DOEpatents

    Wagh, Arun; Maloney, Martin D.

    2010-06-29

    A method of stabilizing a waste in a chemically bonded phosphate ceramic (CBPC). The method consists of preparing a slurry including the waste, water, an oxide binder, and a phosphate binder. The slurry is then allowed to cure to a solid, hydrated CBPC matrix. Next, bound water within the solid, hydrated CBPC matrix is removed. Typically, the bound water is removed by applying heat to the cured CBPC matrix. Preferably, the quantity of heat applied to the cured CBPC matrix is sufficient to drive off water bound within the hydrated CBPC matrix, but not to volatalize other non-water components of the matrix, such as metals and radioactive components. Typically, a temperature range of between 100.degree. C.-200.degree. C. will be sufficient. In another embodiment of the invention wherein the waste and water have been mixed prior to the preparation of the slurry, a select amount of water may be evaporated from the waste and water mixture prior to preparation of the slurry. Another aspect of the invention is a direct anyhydrous CBPC fabrication method wherein water is removed from the slurry by heating and mixing the slurry while allowing the slurry to cure. Additional aspects of the invention are ceramic matrix waste forms prepared by the methods disclosed above.

  9. Learners' Use of Analogy and Alternative Conceptions for Chemical Bonding: A Cross-Age Study.

    ERIC Educational Resources Information Center

    Coll, Richard K.; Treagust, David F.

    2002-01-01

    Describes Australian secondary school, undergraduate, and postgraduate students' use of analogy and anthropomorphism along with their alternative conceptions for mental models of chemical bonding. Finds that learners make use of analogy and anthropomorphism to aid their explanations of chemical bonding, and in addition reveals that learners across…

  10. Dynamics of the chemical bond: inter- and intra-molecular hydrogen bond.

    PubMed

    Arunan, Elangannan; Mani, Devendra

    2015-01-01

    In this discussion, we show that a static definition of a 'bond' is not viable by looking at a few examples for both inter- and intra-molecular hydrogen bonding. This follows from our earlier work (Goswami and Arunan, Phys. Chem. Chem. Phys. 2009, 11, 8974) which showed a practical way to differentiate 'hydrogen bonding' from 'van der Waals interaction'. We report results from ab initio and atoms in molecules theoretical calculations for a series of Rg∙∙∙HX complexes (Rg=He/Ne/Ar and X=F/Cl/Br) and ethane-1,2-diol. Results for the Rg∙∙∙HX/DX complexes show that Rg∙∙∙DX could have a 'deuterium bond' even when Rg∙∙∙HX is not 'hydrogen bonded', according to the practical criterion given by Goswami and Arunan. Results for ethane-1,2-diol show that an 'intra-molecular hydrogen bond' can appear during a normal mode vibration which is dominated by the OO stretching, though a 'bond' is not found in the equilibrium structure. This dynamical 'bond' formation may nevertheless be important in ensuring the continuity of electron density across a molecule. In the former case, a vibration 'breaks' an existing bond and in the later case, a vibration leads to 'bond' formation. In both cases, the molecule/complex stays bound irrespective of what happens to this 'hydrogen bond'. Both these cases push the borders on the recent IUPAC recommendation on hydrogen bonding (Arunan et al. Pure. Appl. Chem. 2011, 83 1637) and justify the inclusive nature of the definition. PMID:25627627

  11. Classification of compounds with distinct or overlapping multi-target activities and diverse molecular mechanisms using emerging chemical patterns.

    PubMed

    Namasivayam, Vigneshwaran; Hu, Ye; Balfer, Jenny; Bajorath, Jürgen

    2013-06-24

    The emerging chemical patterns (ECP) approach has been introduced for compound classification. Thus far, only very few ECP applications have been reported. Here, we further investigate the ECP methodology by studying complex classification problems. The analysis involves multi-target data sets with systematically organized subsets of compounds having distinct or overlapping target activities and, in addition, data sets containing classes of specifically active compounds with different mechanism-of-action. In systematic classification trials focusing on individual compound subsets or mechanistic classes, ECP calculations utilizing numerical descriptors achieve moderate to high sensitivity, dependent on the data set, and consistently high specificity. Accurate ECP predictions are already obtained on the basis of very small learning sets with only three positive training instances, which distinguishes the ECP approach from many other machine learning techniques. PMID:23692475

  12. Electronic structure and chemical bonding in PuO2

    NASA Astrophysics Data System (ADS)

    Teterin, Yu. A.; Maslakov, K. I.; Teterin, A. Yu.; Ivanov, K. E.; Ryzhkov, M. V.; Petrov, V. G.; Enina, D. A.; Kalmykov, St. N.

    2013-06-01

    Quantitative analysis of the x-ray photoelectron spectra structure in the binding energy (BE) range of 0 eV-˜35 eV for plutonium dioxide (PuO2) valence electrons was done. The BEs and structure of the core electronic shells (35 eV-1250 eV BE), as well as the relativistic discrete variation calculation results for the finite fragments of the PuO2 lattice and the data of other authors, were taken into account. The experimental data show that the many-body effects and the multiplet splitting contribute to the spectral structure much less than the outer (0 eV-˜15 eV) and the inner (˜15 eV-˜35 eV) valence molecular orbitals (OVMO and IVMO, respectively). The filled Pu 5f electronic states were shown to form in the PuO2 valence band. The Pu 6p electrons participate in the formation of both the IVMO and the OVMO (bands). The filled Pu 6p3/2 and the O 2s electronic shells were found to take maximum part in the IVMO formation. The MO composition and the sequence order in the BE range of 0 eV-˜35 eV in PuO2 were established. The experimental and theoretical data allowed a quantitative MO scheme for PuO2, which is fundamental for understanding both the chemical bond nature in plutonium dioxide and the interpretation of other x-ray spectra of PuO2.

  13. Natural Bond Critical Point analysis: quantitative relationships between natural bond orbital-based and QTAIM-based topological descriptors of chemical bonding.

    PubMed

    Weinhold, Frank

    2012-11-15

    We have developed a "Natural Bond Critical Point" (NBCP) module for the natural bond orbital (NBO) program that allows mutual analysis of NBO-based versus Bader-type quantum theory of atoms in molecules (QTAIM) topological descriptors of chemical bonding interactions. Conventional QTAIM bond path and bond critical point (BCP) descriptors deduced from total electron density ρ(r) can thereby be compared with analogous "natural" (NBCP) descriptors for idealized densities ρ(NAIM)(r) composed solely from NBO-based "natural atoms in molecules" (NAIM) at the terminal nuclei. Standard ρ(r(BCP)) and [nabla](2)ρ(r(BCP)) descriptors can also be decomposed into unique contributions from individual NBOs or other localized, semilocalized, or delocalized orbital components. These results allow one to recognize many relationships between QTAIM and NBO analyses, showing why close correlations are often found between NBO-based versus ρ(r(BCP))-based characterizations of chemical bonding interactions, despite strongly divergent conceptions of "the atom in the molecule." PMID:22837020

  14. Organohelium compounds: structures, stabilities and chemical bonding analyses.

    PubMed

    Fourré, Isabelle; Alvarez, Elsa; Chaquin, Patrick

    2014-02-24

    This paper deals with the possibility of forming short and relatively strong carbon-helium bonds in small typical organic molecules through substitution of one or several H atoms by He(+). A structural and energetics study (based on high-level calculations) of this unusual bonding, as well as a topological characterization of the resulting cations, is undertaken. Stable species generally requires substitution of about half of the hydrogen atoms for formation. Under these conditions, the number of such species appears to be potentially unlimited. "True" C-He bonds exhibit equilibrium distances ranging from 1.327 (C2H2He2(2+)) to 1.129 Å (He2CO(2+)). The energies of neutral He releasing range from approximately 5 kcal mol(-1) [He2CO(2+), (Z)-C2H2He2(2+)] to 25 kcal mol(-1) (C2HHe3(3+)), but remain most frequently around 10 kcal mol(-1). However, most of He(+)-substituted hydrocarbons are metastable with respect to C-C cleavage, except derivatives of ethene. Atoms in molecules (AIM) and electron localization function (ELF) topological descriptors classify the C-He bond as a weak charge-shift interaction [S. Shaik, D. Danovich, B. Silvi, D. L. Lauvergnat, P. C. Hiberty, Chem. Eur. J. 2005, 11, 6358-6371] in agreement with a recent publication by Rzepa [S. H. Rzepa, Nat. Chem. 2010, 2, 390-393]. He2CO(2+) is the only investigated compound that presents a C-He bonding ELF basin, which indicates a non-negligible covalent contribution to the bond. Other modifications in the electronic structure, such as the breaking of the triple bond in ethyne derivatives or the loss of aromaticity in C6H3He3(3+), are also nicely revealed by the ELF topology. PMID:24488791

  15. Hydrogen Bond Migration between Molecular Sites Observed with Ultrafast 2D IR Chemical Exchange Spectroscopy

    PubMed Central

    Rosenfeld, Daniel E.; Kwak, Kyungwon; Gengeliczki, Zsolt

    2010-01-01

    Hydrogen bonded complexes between phenol and phenylacetylene are studied using ultrafast two-dimensional infrared (2D IR) chemical exchange spectroscopy. Phenylacetylene has two possible π hydrogen bonding acceptor sites (phenyl or acetylene) that compete for hydrogen bond donors in solution at room temperature. The OD stretch frequency of deuterated phenol is sensitive to which acceptor site it is bound. The appearance of off-diagonal peaks between the two vibrational frequencies in the 2D IR spectrum reports on the exchange process between the two competitive hydrogen bonding sites of phenol-phenylacetylene complexes in the neat phenylacetylene solvent. The chemical exchange process occurs in ∼5 ps, and is assigned to direct hydrogen bond migration along the phenylacetylene molecule. Other non-migration mechanisms are ruled out by performing 2D IR experiments on phenol dissolved in the phenylacetylene/carbon tetrachloride mixed solvent. The observation of direct hydrogen bond migration can have implications for macromolecular systems. PMID:20121275

  16. Electronic structure imperfections and chemical bonding at graphene interfaces

    NASA Astrophysics Data System (ADS)

    Schultz, Brian Joseph

    ) fabricate graphene/metal interfaces and metal/graphene/metal sandwich structures evidencing classical anisotropic umpolung chemistry from carbon pz-orbrital charge pinning, and (Chapter 5) engineer graphene/dielectric interfaces showing electron depletion from carbon atoms at the HfO2/graphene interface. The fabrication of graphene interfaces remains a critical gap for successful commercialization of graphene-based devices, yet we demonstrate that interfacial hybridization, anisotropic charge redistribution, local chemical bonding, and discrete electronic hybridization regimes play a critical role in the electronic structure at graphene interfaces.

  17. Beyond the Halogen Bond: Examining the Limits of Extended Polybromide Networks through Quantum-Chemical Investigations.

    PubMed

    Easton, Max E; Chan, Bun; Masters, Anthony F; Radom, Leo; Maschmeyer, Thomas

    2016-03-01

    The bonding environments of some polybromide monoanions and networks were examined by quantum-chemical methods to investigate electronic interactions between dibromine-dibromine contacts. Examination of thermodynamic parameters and a bond critical point analysis give strong evidence for such bonding modes, which have been previously treated disparately in the literature. The thermodynamic stability of large polybromides up to [Br37 ](-) was also predicted by these methods. PMID:26833846

  18. Representational Classroom Practices that Contribute to Students' Conceptual and Representational Understanding of Chemical Bonding

    ERIC Educational Resources Information Center

    Hilton, Annette; Nichols, Kim

    2011-01-01

    Understanding bonding is fundamental to success in chemistry. A number of alternative conceptions related to chemical bonding have been reported in the literature. Research suggests that many alternative conceptions held by chemistry students result from previous teaching; if teachers are explicit in the use of representations and explain their…

  19. Bond Order and Chemical Properties of BF, CO, and N[subscript 2

    ERIC Educational Resources Information Center

    Martinie, Ryan J.; Bultema, Jarred J.; Vander Wal, Mark N.; Burkhart, Brandon J.; Vander Griend, Douglas A.; DeKock, Roger L.

    2011-01-01

    The traditional chemical approaches, Lewis electron dot structures and molecular orbital theory, predict the relative bond orders of boron monofluoride, carbon monoxide, and dinitrogen to be BF less than CO less than N[subscript 2]. This is quantified by quantum mechanical, theoretical studies that show the bond orders to be approximately 1.4,…

  20. Describing the chemical bonding in C70 and C70O3 - A quantum chemical topology study

    NASA Astrophysics Data System (ADS)

    Bil, Andrzej; Latajka, Zdzisław; Hutter, Jürg; Morrison, Carole A.

    2014-03-01

    Cc-Cc and Ca-Cb bonds in C70 have dominant characteristics of double bonds, whereas the remaining six other types of bonds are single bonds with contributions from π-electron density. 'Single' bonds can act as active sites in chemical reactions which would typically require a multiple bond, such as addition of an ozone molecule, due to the fact that all adjacent bonds can serve as an efficient source of π-electron density. Thus any alteration in the electron density distribution following functionalization has far-reaching impact. We note that formation of the most stable ozonide isomer causes the smallest total perturbation in the electron density of the parent fullerene and C-C bond evolution correlates well with the shape of the minimum energy path for the ozone ring opening reaction on the fullerene surface. Finally, we observe that the O-O bond in C70O3 is protocovalent, and as such resembles the O-O bond in H2O2.

  1. EVALUATION OF CHEMICALLY BONDED PHOSPHATE CERAMICS FOR MERCURY STABILIZATION OF A MIXED SYNTHETIC WASTE

    EPA Science Inventory

    This experimental study was conducted to evaluate the stabilization and encapsulation technique developed by Argonne National Laboratory, called the Chemically Bonded Phosphate Ceramics technology for Hg- and HgCl2-contaminated synthetic waste materials. Leachability ...

  2. NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and Solid State

    NASA Astrophysics Data System (ADS)

    Stevens, J. S.; Gainar, A.; Suljoti, E.; Xiao, J.; Golnak, R.; Aziz, E. F.; Schroeder, S. L. M.

    2016-05-01

    Solid-state and solution pH-dependent NEXAFS studies allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment, revealing the chemical state and bonding of the chemical species. Variations in the ionization potential (IP) and 1s→π* resonances unequivocally identify the chemical species (neutral, cationic, or anionic) present and the varying local environment. Shifts in σ* shape resonances relative to the IP in the NEXAFS spectra vary with C-N bond length, and the important effect of minor alterations in bond length is confirmed with nitrogen FEFF calculations, leading to the possibility of bond length determination in solution.

  3. The Chemical Bond and Solid-state Physics

    ERIC Educational Resources Information Center

    Phillips, James C.

    1970-01-01

    Proposes a new scale of ionicity, with which the ionic character of bonding in crystals can be predicted and measured. This new scale of ionicity has led to improved understanding of such crystalline properties as lattice structure, heats of formation, elastic constants, and nonlinear optical properties. Bibliography. (LC)

  4. Chemical bonding of hydrogen molecules to transition metal complexes

    SciTech Connect

    Kubas, G.J.

    1990-01-01

    The complex W(CO){sub 3}(PR{sub 3}){sub 2}(H{sub 2}) (CO = carbonyl; PR{sub 3} = organophosphine) was prepared and was found to be a stable crystalline solid under ambient conditions from which the hydrogen can be reversibly removed in vacuum or under an inert atmosphere. The weakly bonded H{sub 2} exchanges easily with D{sub 2}. This complex represents the first stable compound containing intermolecular interaction of a sigma-bond (H-H) with a metal. The primary interaction is reported to be donation of electron density from the H{sub 2} bonding electron pair to a vacant metal d-orbital. A series of complexes of molybdenum of the type Mo(CO)(H{sub 2})(R{sub 2}PCH{sub 2}CH{sub 2}PR{sub 2}){sub 2} were prepared by varying the organophosphine substitutent to demonstrate that it is possible to bond either dihydrogen or dihydride by adjusting the electron-donating properties of the co-ligands. Results of infrared and NMR spectroscopic studies are reported. 20 refs., 5 fig.

  5. A New "Bottom-Up" Framework for Teaching Chemical Bonding

    ERIC Educational Resources Information Center

    Nahum, Tami Levy; Mamlok-Naaman, Rachel; Hofstein, Avi

    2008-01-01

    Traditional curriculum for teaching bonding often fosters the use of over-simplifications and over-generalizations. Therefore, there is a need for a presentation that is consistent with current scientific knowledge and that provides the student with the proper intellectual infrastructure for further studies. In this article, we present a general…

  6. Scientists Get First Glimpse of a Chemical Bond Being Born

    SciTech Connect

    Nilsson, Anders

    2015-02-12

    Scientists have used an X-ray laser at the Department of Energy’s SLAC National Accelerator Laboratory to get the first glimpse of the transition state where two atoms begin to form a weak bond on the way to becoming a molecule.

  7. A Descriptive Analysis of the Chemical Bond Approach, the Chemical Education Material Study, and a Representative Traditional Chemistry Course.

    ERIC Educational Resources Information Center

    Crilly, Alice Roth

    This study compared three one-year courses in high school chemistry. One of these, entitled Modern Chemistry, represents the "traditional" precollegiate chemistry course. The others are the Chemical Bond Approach (CBA) and the Chemical Education Material Study (CHEM Study). These are the two major revisions in high school chemistry which emerged…

  8. Orbital Exponent Optimization in Elementary VB Calculations of the Chemical Bond in the Ground State of Simple Molecular Systems

    ERIC Educational Resources Information Center

    Magnasco, Valerio

    2008-01-01

    Orbital exponent optimization in the elementary ab-initio VB calculation of the ground states of H[subscript 2][superscript +], H[subscript 2], He[subscript 2][superscript +], He[subscript 2] gives a fair description of the exchange-overlap component of the interatomic interaction that is important in the bond region. Correct bond lengths and…

  9. Development and validation of a tool for diagnosing conceptual knowledge of chemical bonding

    NASA Astrophysics Data System (ADS)

    Yayon, Malka

    Chemical bonding knowledge is fundamental and essential to the understanding of almost every topic in chemistry, but it is very difficult to learn. This study describes the development of an automatic on-line tool that diagnoses basic conceptual knowledge of chemical bonding among high school students. While many studies have characterized and diagnosed some of the central concepts of this topic, these concepts have not been systematically organized nor tested at a fine grain size. The design of the tool consisted of two main stages: A) the systematic characterization of the canonical conceptual knowledge of chemical bonding deemed relevant for high school chemistry majors in the form of a matrix, and B) the design of items and tests, their online version, and automatic feedback for two topics in chemical bonding, that represent the student's conceptual knowledge of chemical bonding compared with and linked to the matrix. The matrix contains three strands: the structure of matter at the nanoscopic level, electrostatic interactions between charged entities, and energy aspects related to bonding. In each strand there are hierarchically ordered cells that contain fine grain concepts. The matrix represents what Reif called the "Final performance (Sf)" - what a student should know after learning high school chemistry. The tests were designed to assess the student's knowledge of this canon at different stages along the learning trajectory. In this way, representing the student's knowledge with the matrix can show changes from the "Initial performance (Si)" as learning is taking place. Understanding of chemical bonding is more than just knowledge of the basic concepts, but without these basic concepts, a deep understanding of chemical bonding is impossible. Therefore, a diagnostic tool that can pinpoint the concepts that are used correctly, incorrectly, or not at all could be a helpful instructional and research tool. The development process described herein could serve

  10. Valence-bond description of chemical reactions on Born-Oppenheimer molecular dynamics trajectories

    NASA Astrophysics Data System (ADS)

    Noguchi, Nao; Nakano, Haruyuki

    2009-04-01

    The nature of chemical bonds on dynamic paths was investigated using the complete active space valence-bond (CASVB) method and the Born-Oppenheimer dynamics. To extract the chemical bond picture during reactions, a scheme to collect contributions from several VB (resonance) structures into a small numbers of indices was introduced. In this scheme, a tree diagram for the VB structures is constructed with the numbers of the ionic bonds treated as generation. A pair of VB structures is related to each other if one VB structure is transferred into the other by changing a covalent bond to an ionic bond. The former and latter VB structures are named parent and child structures, respectively. The weights of the bond pictures are computed as the sum of the CASVB occupation numbers running from the top generation to the bottom along the descent of the VB structures. Thus, a number of CASVB occupation numbers are collected into a small number of indices, and a clear bond picture may be obtained from the CASVB wave function. The scheme was applied to the hydrogen exchange reaction H2+F→H+HF and the Diels-Alder reaction C5H6(cyclopentadiene)+CH2=CH2(ethylene)→C7H10(norbornene). In both the reactions, the scheme gave a clear picture for the Born-Oppenheimer dynamics trajectories. The reconstruction of the bonds during reactions was well described by following the temporal changes in weight.

  11. Developing and validating a chemical bonding instrument for Korean high school students

    NASA Astrophysics Data System (ADS)

    Jang, Nak Han

    The major purpose of this study was to develop a reliable and valid instrument designed to collect and investigate on Korean high school students' understanding about concepts regarding chemical bonding. The Chemical Bonding Diagnostic Test (CBDT) was developed by the procedure by previously relevant researches (Treagust, 1985; Peterson, 1986; Tan, 1994). The final instrument consisted of 15 two-tier items. The reliability coefficient (Cronbach alpha) for the whole test was 0.74. Also, the range of values for the discrimination index was from 0.38 to 0.90 and the overall average difficulty index was 0.38. The test was administered to 716 science declared students in Korean high school. The 37 common misconceptions on chemical bonding were identified through analysis of the items from the CBDT. The grade 11 students had slightly more misconceptions than the grade 12 students for ionic bonding, covalent bonding, and hydrogen bonding while the grade 12 students had more misconceptions about octet rule and hydrogen bonding than the grade 11 students. From the analysis of ANCOVA, there was no significant difference in grades, and between grade levels and gender on the mean score of CBDT. However, there was a significant difference in gender and a significant interaction between grade levels and chemistry preference. In conclusion, Korean high school students had the most common misconception about the electron configuration on ionic bonding and the water density on hydrogen bonding. Korean students' understanding about the chemical bonding was dependent on the interaction between grade levels and the chemistry preference. Consequently, grade 12 chemistry-preferred students had the highest mean scores among student groups concerned by this study.

  12. Effects of Mechanical and Chemical Pretreatments of Zirconia or Fiber Posts on Resin Cement Bonding

    PubMed Central

    Li, Rui; Zhou, Hui; Wei, Wei; Wang, Chen; Sun, Ying Chun; Gao, Ping

    2015-01-01

    The bonding strength between resin cement and posts is important for post and core restorations. An important method of improving the bonding strength is the use of various surface pretreatments of the post. In this study, the surfaces of zirconia (fiber) posts were treated by mechanical and/or chemical methods such as sandblasting and silanization. The bonding strength between the zirconia (fiber) post and the resin cement was measured by a push-out method after thermocycling based on the adhesion to Panavia F 2.0 resin cement. The zirconia and fiber posts exhibited different bonding strengths after sandblasting and/or silanization because of the different strengths and chemical structures. The zirconia post showed a high bonding strength of up to 17.1 MPa after a combined treatment of sandblasting and silanization because of the rough surface and covalent bonds at the interface. This effect was also enhanced by using 1,2-bis(trimethoxysilyl)ethane for the formation of a flexible layer at the interface. In contrast, a high bonding strength of 13.9 MPa was obtained for the fiber post treated by silane agents because the sandblasting treatment resulted in damage to the fiber post, as observed by scanning electron microscopy. The results indicated that the improvement in the bonding strength between the post and the resin cement could be controlled by different chemical and/or mechanical treatments. Enhanced bonding strength depended on covalent bonding and the surface roughness. A zirconia post with high bonding strength could potentially be used for the restoration of teeth in the future. PMID:26066349

  13. The Trouble with Chemical Energy: Why Understanding Bond Energies Requires an Interdisciplinary Systems Approach

    ERIC Educational Resources Information Center

    Cooper, Melanie M.; Klymkowsky, Michael W.

    2013-01-01

    Helping students understand "chemical energy" is notoriously difficult. Many hold inconsistent ideas about what energy is, how and why it changes during the course of a chemical reaction, and how these changes are related to bond energies and reaction dynamics. There are (at least) three major sources for this problem: 1) the way biologists talk…

  14. Coulombic Interaction in Finnish Middle School Chemistry: A Systemic Perspective on Students' Conceptual Structure of Chemical Bonding

    ERIC Educational Resources Information Center

    Joki, Jarkko; Lavonen, Jari; Juuti, Kalle; Aksela, Maija

    2015-01-01

    The aim of this study was to design a novel and holistic way to teach chemical bonding at the middle school level according to research on the teaching and learning of bonding. A further aim was to investigate high achieving middle school students' conceptual structures concerning chemical bonding by using a systemic perspective. Students in one…

  15. Analysis of Thermal and Chemical Effets on Negative Valve Overlap Period Energy Recovery for Low-Temperature Gasoline Combustion

    SciTech Connect

    Ekoto, Dr Isaac; Peterson, Dr. Brian; Szybist, James P; Northrop, Dr. William

    2015-01-01

    A central challenge for efficient auto-ignition controlled low-temperature gasoline combustion (LTGC) engines has been achieving the combustion phasing needed to reach stable performance over a wide operating regime. The negative valve overlap (NVO) strategy has been explored as a way to improve combustion stability through a combination of charge heating and altered reactivity via a recompression stroke with a pilot fuel injection. The study objective was to analyze the thermal and chemical effects on NVO-period energy recovery. The analysis leveraged experimental gas sampling results obtained from a single-cylinder LTGC engine along with cylinder pressure measurements and custom data reduction methods used to estimate period thermodynamic properties. The engine was fueled by either iso-octane or ethanol, and operated under sweeps of NVO-period oxygen concentration, injection timing, and fueling rate. Gas sampling at the end of the NVO period was performed via a custom dump-valve apparatus, with detailed sample speciation by in-house gas chromatography. The balance of NVO-period input and output energy flows was calculated in terms of fuel energy, work, heat loss, and change in sensible energy. Experiment results were complemented by detailed chemistry single-zone reactor simulations performed at relevant mixing and thermodynamic conditions, with results used to evaluate ignition behavior and expected energy recovery yields. For the intermediate bulk-gas temperatures present during the NVO period (900-1100 K), weak negative temperature coefficient behavior with iso-octane fueling significantly lengthened ignition delays relative to similar ethanol fueled conditions. Faster ethanol ignition chemistry led to lower recovered fuel intermediate yields relative to similar iso-octane fueled conditions due to more complete fuel oxidation. From the energy analysis it was found that increased NVO-period global equivalence ratio, either from lower NVOperiod oxygen

  16. Analytic projection from plane-wave and PAW wavefunctions and application to chemical-bonding analysis in solids.

    PubMed

    Maintz, Stefan; Deringer, Volker L; Tchougréeff, Andrei L; Dronskowski, Richard

    2013-11-01

    Quantum-chemical computations of solids benefit enormously from numerically efficient plane-wave (PW) basis sets, and together with the projector augmented-wave (PAW) method, the latter have risen to one of the predominant standards in computational solid-state sciences. Despite their advantages, plane waves lack local information, which makes the interpretation of local densities-of-states (DOS) difficult and precludes the direct use of atom-resolved chemical bonding indicators such as the crystal orbital overlap population (COOP) and the crystal orbital Hamilton population (COHP) techniques. Recently, a number of methods have been proposed to overcome this fundamental issue, built around the concept of basis-set projection onto a local auxiliary basis. In this work, we propose a novel computational technique toward this goal by transferring the PW/PAW wavefunctions to a properly chosen local basis using analytically derived expressions. In particular, we describe a general approach to project both PW and PAW eigenstates onto given custom orbitals, which we then exemplify at the hand of contracted multiple-ζ Slater-type orbitals. The validity of the method presented here is illustrated by applications to chemical textbook examples-diamond, gallium arsenide, the transition-metal titanium-as well as nanoscale allotropes of carbon: a nanotube and the C60 fullerene. Remarkably, the analytical approach not only recovers the total and projected electronic DOS with a high degree of confidence, but it also yields a realistic chemical-bonding picture in the framework of the projected COHP method. PMID:24022911

  17. Bonding and reactivity of clean and chemically-modified semiconductor surfaces probed with STM

    SciTech Connect

    Hamers, R.J.

    1995-12-01

    Scanning tunneling microscopy has been used to directly study the interrelationships between chemical composition, chemical reactivity, electronic structure, and surface morphology at the atomic level on clean and chemically-modified semiconductor surfaces. Our work has focused recently on understanding the atomic-level interactions of dopants such as phosphorus and boron on the Si(001) surface, and the influence of these atoms on other chemical reactions such as the thermal decomposition of disilane to grow epitaxial silicon. Using STM to study how these atoms modify the surface in combination with tunneling spectroscopy to reveal the occupied and unoccupied molecular orbitals of these structures provides direct insight into the nature of chemical bonding on these surfaces. This talk will discuss the application of STM to understand chemical bonding at silicon surfaces.

  18. Effect of interfacial chemical bonding and surface topography on adhesion in carbon fiber/epoxy composites

    SciTech Connect

    Drzal, L.T.; Sugiura, N.; Hook, D. |

    1994-12-31

    A series of PAN-based IM6 carbon fibers having varying amounts of surface treatment were, pretreated with compounds representing the constituents encountered in epoxy composites to pre-react any groups on the fiber surface before composite fabrication in order to determine the effect of chemical bonding on fiber-matrix adhesion. Chemical bonding was quantified using XPS. Chemical bonding between reactive groups in amine cured epoxy matrices and the surface groups present on IN46 carbon fibers as a result of commercial surface treatments has been detected although the absolute amount of chemical bonding is low (1-3%). It was found that reaction with monofunctional epoxy groups having hydrocarbon functionalities blocked the surface from further reaction and reduced the adhesion that could be attained to its lowest value. Prereaction with difunctional amines had little effect on adhesion when compared to normal composite fabrication procedures. Prereaction with difunctional epoxy groups did enhance adhesion levels over the level attained in normal composite fabrication methods. These results showed that chemical bonding between epoxy and the carbon fiber surface could increases the adhesion between fiber and matrix about 25% while between the amino group and the carbon fiber surface about 15%. Quantitative measurements of the fiber surface microtopography were made with scanning tunneling microscopy. An increase in roughness was detected with increasing surface treatment. It was concluded that surface roughness also accounted for a significant increase in fiber-matrix adhesion.

  19. Local spin density functional investigations of the chemical bonding and of the magnetism in some uranium ternary intermetallic systems: How physics and chemistry can meet in the solid state

    SciTech Connect

    Matar, S.F.

    2000-04-20

    The electronic and magnetic structures of different uranium-based ternary intermetallic systems are self-consistently calculated within local spin density functional theory using the augmented spherical wave method. The influence of hybridization on the chemical bonding and on the magnetic behavior is discussed from the densities of states as well as from the crystal orbital overlap population. From this the author addresses the mechanisms of chemical bonding and of the onset of magnetism. The original concept of building blocks between different intermetallic systems is discussed.

  20. Bond strength: a comparison between chemical coated and mechanical interlock bases of ceramic and metal brackets.

    PubMed

    Wang, W N; Meng, C L; Tarng, T H

    1997-04-01

    Two types of chemically coated bases, two types of mechanical interlock base polycrystalline ceramic brackets, as well as one type of mechanical interlock base metal bracket were selected for bonding with Concise orthodontic resin on 60 extracted premolars. Bond strength was measured with an Instron testing machine and the debonded interface and enamel detachment were examined with scanning electron microscope and energy dispersive x-ray spectrometer. The results showed the greater bond strength with a chemically coated base of ceramic brackets had a greater debonded interface between enamel and resin, and the weaker bond strength of mechanical interlock base of ceramic and metal brackets had a greater debonded interfaces between bracket and resin. There was no significant statistical difference in bond strengths with mechanically interlock bases between ceramic and metal brackets. The enamel detachment was found on only the stronger bond strength in which there was a chemically coated base on the ceramic bracket. Ceramic bracket fractures were not found during debonding in this specially designed specimen with 1 mm/min speed of crosshead. The mechanical interlock base of the ceramic bracket combines the strength, durability and retention of a metal bracket along with an aesthetic advantage and no enamel detachment after debonding. PMID:9109582

  1. The nature of chemical bonding in actinide and lanthanide ferrocyanides determined by X-ray absorption spectroscopy and density functional theory.

    PubMed

    Dumas, Thomas; Guillaumont, Dominique; Fillaux, Clara; Scheinost, Andreas; Moisy, Philippe; Petit, Sébastien; Shuh, David K; Tyliszczak, Tolek; Den Auwer, Christophe

    2016-01-28

    The electronic properties of actinide cations are of fundamental interest to describe intramolecular interactions and chemical bonding in the context of nuclear waste reprocessing or direct storage. The 5f and 6d orbitals are the first partially or totally vacant states in these elements, and the nature of the actinide ligand bonds is related to their ability to overlap with ligand orbitals. Because of its chemical and orbital selectivities, X-ray absorption spectroscopy (XAS) is an effective probe of actinide species frontier orbitals and for understanding actinide cation reactivity toward chelating ligands. The soft X-ray probes of the light elements provide better resolution than actinide L3-edges to obtain electronic information from the ligand. Thus coupling simulations to experimental soft X-ray spectral measurements and complementary quantum chemical calculations yields quantitative information on chemical bonding. In this study, soft X-ray XAS at the K-edges of C and N, and the L2,3-edges of Fe was used to investigate the electronic structures of the well-known ferrocyanide complexes K4Fe(II)(CN)6, thorium hexacyanoferrate Th(IV)Fe(II)(CN)6, and neodymium hexacyanoferrate KNd(III)Fe(II)(CN)6. The soft X-ray spectra were simulated based on quantum chemical calculations. Our results highlight the orbital overlapping effects and atomic effective charges in the Fe(II)(CN)6 building block. In addition to providing a detailed description of the electronic structure of the ferrocyanide complex (K4Fe(II)(CN)6), the results strongly contribute to confirming the actinide 5f and 6d orbital oddity in comparison to lanthanide 4f and 5d. PMID:26733312

  2. Chemically bonded graphene/BiOCl nanocomposites as high-performance photocatalysts.

    PubMed

    Gao, Feidan; Zeng, Dawen; Huang, Qingwu; Tian, Shouqin; Xie, Changsheng

    2012-08-14

    After the successful solvothermal synthesis of graphene (GR) from ethanol and sodium, we obtained chemically bonded graphene/BiOCl (GR/BiOCl) nanocomposite photocatalysts via a facile chemical-bath method. A significant enhancement was observed in the photodegradation of methylbenzene, which was largely ascribed to the chemical coupling effects between Bi and C, as shown by X-ray photoelectron spectroscopy. Raman spectroscopy also indicated an increased size of the sp(2) ring clusters and decreased disorder in the graphitic structure, as substitutions of defects like vacancies as well as oxygen containing carbonaceous groups with C-Bi attachment take place. Overall, information about chemical coupling effects between GR and BiOCl might take us a step further in GR-based hybrid materials, providing a very good reference to the fabrication of chemically bonded GR/semiconductor compounds and facilitating their applications in environmental protection, photo-electrochemical conversion and photocatalytic decomposition of water. PMID:22744132

  3. Effective solidification/stabilisation of mercury-contaminated wastes using zeolites and chemically bonded phosphate ceramics.

    PubMed

    Zhang, Shaoqing; Zhang, Xinyan; Xiong, Ya; Wang, Guoping; Zheng, Na

    2015-02-01

    In this study, two kinds of zeolites materials (natural zeolite and thiol-functionalised zeolite) were added to the chemically bonded phosphate ceramic processes to treat mercury-contaminated wastes. Strong promotion effects of zeolites (natural zeolite and thiol-functionalised zeolite) on the stability of mercury in the wastes were obtained and these technologies showed promising advantages toward the traditional Portland cement process, i.e. using Portland cement as a solidification agent and natural or thiol-functionalised zeolite as a stabilisation agent. Not only is a high stabilisation efficiency (lowered the Toxicity Characteristic Leaching Procedure Hg by above 10%) obtained, but also a lower dosage of solidification (for thiol-functionalised zeolite as stabilisation agent, 0.5 g g(-1) and 0.7 g g(-1) for chemically bonded phosphate ceramic and Portland cement, respectively) and stabilisation agents (for natural zeolite as stabilisation agent, 0.35 g g(-1) and 0.4 g g(-1) for chemically bonded phosphate ceramic and Portland cement, respectively) were used compared with the Portland cement process. Treated by thiol-functionalised zeolite and chemically bonded phosphate ceramic under optimum parameters, the waste containing 1500 mg Hg kg(-1) passed the Toxicity Characteristic Leaching Procedure test. Moreover, stabilisation/solidification technology using natural zeolite and chemically bonded phosphate ceramic also passed the Toxicity Characteristic Leaching Procedure test (the mercury waste containing 625 mg Hg kg(-1)). Moreover, the presence of chloride and phosphate did not have a negative effect on the chemically bonded phosphate ceramic/thiol-functionalised zeolite treatment process; thus, showing potential for future application in treatment of 'difficult-to-manage' mercury-contaminated wastes or landfill disposal with high phosphate and chloride content. PMID:25568090

  4. Chemical bonding and electronic structures of microcline, orthoclase and the plagioclase series by X-ray photoelectron spectroscopy.

    PubMed

    Kloprogge, J Theo; Wood, Barry J

    2015-02-25

    A detailed analysis was undertaken of the X-ray photoelectron spectra obtained from microcline, orthoclase and several samples of plagioclase with varying Na/Ca ratio. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each specimen. The spectra for Si 2p and Al 2p vary with the change in symmetry between microcline and orthoclase, while in plagioclase an increase in Al-O-Si linkages results in a small but observable decrease in binding energy. The overall shapes of the O 1s peaks observed in all spectra are similar and show shifts similar to those observed for Si 2p and Al 2p. The lower-VB spectra for microcline and orthoclase are similar intermediate between α-SiO2 and α-Al2O3 in terms of binding energies. In the plagioclase series increasing coupled substitution of Na and Si for Ca and Al results in a change of the overall shape of the spectra, showing a distinct broadening associated with the presence of two separate but overlapping bands similar to the 21 eV band observed for quartz and the 23 eV band observed for corundum. The bonding character for microcline and orthoclase is more covalent than that of α-Al2O3, but less than that of α-SiO2. In contrast, the plagioclase samples show two distinct bonding characters that are comparable with those of α-SiO2 and α-Al2O3. PMID:25261735

  5. Chemical bonding in view of electron charge density and kinetic energy density descriptors.

    PubMed

    Jacobsen, Heiko

    2009-05-01

    Stalke's dilemma, stating that different chemical interpretations are obtained when one and the same density is interpreted either by means of natural bond orbital (NBO) and subsequent natural resonance theory (NRT) application or by the quantum theory of atoms in molecules (QTAIM), is reinvestigated. It is shown that within the framework of QTAIM, the question as to whether for a given molecule two atoms are bonded or not is only meaningful in the context of a well-defined reference geometry. The localized-orbital-locator (LOL) is applied to map out patterns in covalent bonding interaction, and produces results that are consistent for a variety of reference geometries. Furthermore, LOL interpretations are in accord with NBO/NRT, and assist in an interpretation in terms of covalent bonding. PMID:19090572

  6. Chemical activation of molecules by metals: Experimental studies of electron distributions and bonding

    NASA Astrophysics Data System (ADS)

    Lichtenberger, D. L.

    1991-10-01

    The formal relationship between measured molecular ionization energies and thermodynamic bond dissociation energies has been developed into a single equation which unifies the treatment of covalent bonds, ionic bonds, and partially ionic bonds. This relationship has been used to clarify the fundamental thermodynamic information relating to metal-hydrogen, metal-alkyl, and metal-metal bond energies. We have been able to obtain a direct observation and measurement of the stabilization energy provided by the agostic interaction of the C-H bond with the metal. The ionization energies have also been used to correlate the rates of carbonyl substitution reactions of (eta sup 5-C5H4)Rh(CO)2 complexes, and to reveal the electronic factors that control the stability of the transition state. The extent that the electronic features of these bonding interactions transfer to other chemical systems is being investigated in terms of the principle of additivity of ligand electronic effects. Specific examples under study include metal- phosphines, metal-halides, and metallocenes. Especially interesting has been the recent application of these techniques to the characterization of the soccer-ball shaped C60 molecule, buckminsterfullerene, and its interaction with a metal surface. The high resolution valence ionizations in the gas phase reveal the high symmetry of the molecule, and studies of thin films of C60 reveal weak intermolecular interactions. Scanning tunneling and atomic force microscopy reveal the arrangement of spherical molecules on gold substrates, with significant delocalization of charge from the metal surface.

  7. Generating giant and tunable nonlinearity in a macroscopic mechanical resonator from a single chemical bond

    NASA Astrophysics Data System (ADS)

    Huang, Pu; Zhou, Jingwei; Zhang, Liang; Hou, Dong; Lin, Shaochun; Deng, Wen; Meng, Chao; Duan, Changkui; Ju, Chenyong; Zheng, Xiao; Xue, Fei; Du, Jiangfeng

    2016-05-01

    Nonlinearity in macroscopic mechanical systems may lead to abundant phenomena for fundamental studies and potential applications. However, it is difficult to generate nonlinearity due to the fact that macroscopic mechanical systems follow Hooke's law and respond linearly to external force, unless strong drive is used. Here we propose and experimentally realize high cubic nonlinear response in a macroscopic mechanical system by exploring the anharmonicity in chemical bonding interactions. We demonstrate the high tunability of nonlinear response by precisely controlling the chemical bonding interaction, and realize, at the single-bond limit, a cubic elastic constant of 1 × 1020 N m-3. This enables us to observe the resonator's vibrational bi-states transitions driven by the weak Brownian thermal noise at 6 K. This method can be flexibly applied to a variety of mechanical systems to improve nonlinear responses, and can be used, with further improvements, to explore macroscopic quantum mechanics.

  8. Generating giant and tunable nonlinearity in a macroscopic mechanical resonator from a single chemical bond

    PubMed Central

    Huang, Pu; Zhou, Jingwei; Zhang, Liang; Hou, Dong; Lin, Shaochun; Deng, Wen; Meng, Chao; Duan, Changkui; Ju, Chenyong; Zheng, Xiao; Xue, Fei; Du, Jiangfeng

    2016-01-01

    Nonlinearity in macroscopic mechanical systems may lead to abundant phenomena for fundamental studies and potential applications. However, it is difficult to generate nonlinearity due to the fact that macroscopic mechanical systems follow Hooke's law and respond linearly to external force, unless strong drive is used. Here we propose and experimentally realize high cubic nonlinear response in a macroscopic mechanical system by exploring the anharmonicity in chemical bonding interactions. We demonstrate the high tunability of nonlinear response by precisely controlling the chemical bonding interaction, and realize, at the single-bond limit, a cubic elastic constant of 1 × 1020 N m−3. This enables us to observe the resonator's vibrational bi-states transitions driven by the weak Brownian thermal noise at 6 K. This method can be flexibly applied to a variety of mechanical systems to improve nonlinear responses, and can be used, with further improvements, to explore macroscopic quantum mechanics. PMID:27225287

  9. Generating giant and tunable nonlinearity in a macroscopic mechanical resonator from a single chemical bond.

    PubMed

    Huang, Pu; Zhou, Jingwei; Zhang, Liang; Hou, Dong; Lin, Shaochun; Deng, Wen; Meng, Chao; Duan, Changkui; Ju, Chenyong; Zheng, Xiao; Xue, Fei; Du, Jiangfeng

    2016-01-01

    Nonlinearity in macroscopic mechanical systems may lead to abundant phenomena for fundamental studies and potential applications. However, it is difficult to generate nonlinearity due to the fact that macroscopic mechanical systems follow Hooke's law and respond linearly to external force, unless strong drive is used. Here we propose and experimentally realize high cubic nonlinear response in a macroscopic mechanical system by exploring the anharmonicity in chemical bonding interactions. We demonstrate the high tunability of nonlinear response by precisely controlling the chemical bonding interaction, and realize, at the single-bond limit, a cubic elastic constant of 1 × 10(20) N m(-3). This enables us to observe the resonator's vibrational bi-states transitions driven by the weak Brownian thermal noise at 6 K. This method can be flexibly applied to a variety of mechanical systems to improve nonlinear responses, and can be used, with further improvements, to explore macroscopic quantum mechanics. PMID:27225287

  10. Alternative Conceptions of Chemical Bonding Held by Upper Secondary and Tertiary Students.

    ERIC Educational Resources Information Center

    Coll, Richard K.; Taylor, Neil

    2001-01-01

    Examination of senior secondary and tertiary level chemistry students' descriptions of their mental models for chemical bonding revealed prevalent alternative conceptions. Some 20 alternative conceptions were revealed, the most common being belief that continuous ionic or metallic lattices were molecular in nature, and confusion over ionic size…

  11. Reflections on the Electron Theory of the Chemical Bond: 1900-1925.

    ERIC Educational Resources Information Center

    Stranges, Anthony N.

    1984-01-01

    Traces the history of the electron theory of the chemical bond. Nineteenth-century ideas on electrical combination, early twentieth-century theories of electrical attraction, and the contribution of G. N. Lewis's shared electron pair are among the topics considered. (JN)

  12. Bench-Scale Evaluation Of Chemically Bonded Phosphate Ceramic Technology To Stabilize Mercury Waste Mixtures

    EPA Science Inventory

    This bench-scale study was conducted to evaluate the stabilization of mercury (Hg) and mercuric chloride-containing surrogate test materials by the chemically bonded phosphate ceramics technology. This study was performed as part of a U.S. EPA program to evaluate treatment and d...

  13. Students' Reasoning about Basic Chemical Thermodynamics and Chemical Bonding: What Changes Occur during a Context-based Post-16 Chemistry Course?

    ERIC Educational Resources Information Center

    Barker, Vanessa; Millar, Robin

    2000-01-01

    A longitudinal study of students (n=250) following the Salters Advanced Chemistry course probed a range of chemical ideas including the exothermicity of bond formation and the development of thinking about covalent, ionic, and intermolecular bonds. At the start, many students demonstrated misunderstandings about these chemical ideas, but their…

  14. Chemical activation of molecules by metals: Experimental studies of electron distributions and bonding

    SciTech Connect

    Lichtenberger, D.L.

    1992-01-01

    Purpose of this research program is to obtain experimental information on the different fundamental ways metals bond and activate organic molecules. Our approach has been to directly probe the electronic interactions between metals and molecules through a wide variety of ionization spectroscopies and other techniques, and to investigate the relationships with bonding modes, structures, and chemical behavior. During this period, we have (1) characterized the electronic features of diphosphines and monophosphines in their coordination to metals, (2) carried out theoretical and experimental investigations of the bonding capabilities of C[sub 60] to transition metals, (3) developed techniques for the imaging of single molecules on gold substrates that emphasizes the electronic backbonding from the metal to the molecule, (4) obtained the high resolution photoelectron spectrum of pure C[sub 70] in the gas phase, (5) compared the bonding of [eta][sup 3]- acetylide ligands to the bonding of other small organic molecules with metals, and (6) reported the photoelectron spectra and bonding of [eta][sup 3]-cyclopropenyl groups to metals.

  15. Intramolecular hydrogen bonding in 5-nitrosalicylaldehyde: IR spectrum and quantum chemical calculations

    NASA Astrophysics Data System (ADS)

    Moosavi-Tekyeh, Zainab; Taherian, Fatemeh; Tayyari, Sayyed Faramarz

    2016-05-01

    The structural parameters, and vibrational frequencies of 5-nitrosalicylaldehyde (5NSA) were studied by the FT-IR and Raman spectra and the quantum chemical calculations carried out at the B3LYP/6-311++G(d,p) level of theory in order to investigate the intramolecular hydrogen bonding (IHB) present in its structure. The strength and nature of IHB in the optimized structure of 5NSA were studied in detail by means of the atoms in molecules (AIM) and the natural bond orbital (NBO) approaches. The results obtained were then compared with the corresponding data for its parent molecule, salicylaldehyde (SA). Comparisons made between the geometrical structures for 5NSA and SA, their OH/OD stretching and out-of-plane bending modes, their enthalpies for the hydrogen bond, and their AIM parameters demonstrated a stronger H-bonding in 5NSA compared with that in SA. The calculated binding enthalpy (ΔHbind) for 5NSA was -10.92 kcal mol-1. The observed νOH and γOH appeared at about 3120 cm-1 and 786 cm-1 respectively. The stretching frequency shift of H-bond formation was 426 cm-1 which is consistent with ΔHbind and the strength of H-bond in 5NSA. The delocalization energies and electron delocalization indices derived by the NBO and AIM approaches indicate that the resonance effects were responsible for the stronger IHB in 5NSA than in SA.

  16. Duration of an intense laser pulse can determine the breakage of multiple chemical bonds.

    PubMed

    Xie, Xinhua; Lötstedt, Erik; Roither, Stefan; Schöffler, Markus; Kartashov, Daniil; Midorikawa, Katsumi; Baltuška, Andrius; Yamanouchi, Kaoru; Kitzler, Markus

    2015-01-01

    Control over the breakage of a certain chemical bond in a molecule by an ultrashort laser pulse has been considered for decades. With the availability of intense non-resonant laser fields it became possible to pre-determine femtosecond to picosecond molecular bond breakage dynamics by controlled distortions of the electronic molecular system on sub-femtosecond time scales using field-sensitive processes such as strong-field ionization or excitation. So far, all successful demonstrations in this area considered only fragmentation reactions, where only one bond is broken and the molecule is split into merely two moieties. Here, using ethylene (C2H4) as an example, we experimentally investigate whether complex fragmentation reactions that involve the breakage of more than one chemical bond can be influenced by parameters of an ultrashort intense laser pulse. We show that the dynamics of removing three electrons by strong-field ionization determines the ratio of fragmentation of the molecular trication into two respectively three moieties. We observe a relative increase of two-body fragmentations with the laser pulse duration by almost an order of magnitude. Supported by quantum chemical simulations we explain our experimental results by the interplay between the dynamics of electron removal and nuclear motion. PMID:26271602

  17. Duration of an intense laser pulse can determine the breakage of multiple chemical bonds

    NASA Astrophysics Data System (ADS)

    Xie, Xinhua; Lötstedt, Erik; Roither, Stefan; Schöffler, Markus; Kartashov, Daniil; Midorikawa, Katsumi; Baltuška, Andrius; Yamanouchi, Kaoru; Kitzler, Markus

    2015-08-01

    Control over the breakage of a certain chemical bond in a molecule by an ultrashort laser pulse has been considered for decades. With the availability of intense non-resonant laser fields it became possible to pre-determine femtosecond to picosecond molecular bond breakage dynamics by controlled distortions of the electronic molecular system on sub-femtosecond time scales using field-sensitive processes such as strong-field ionization or excitation. So far, all successful demonstrations in this area considered only fragmentation reactions, where only one bond is broken and the molecule is split into merely two moieties. Here, using ethylene (C2H4) as an example, we experimentally investigate whether complex fragmentation reactions that involve the breakage of more than one chemical bond can be influenced by parameters of an ultrashort intense laser pulse. We show that the dynamics of removing three electrons by strong-field ionization determines the ratio of fragmentation of the molecular trication into two respectively three moieties. We observe a relative increase of two-body fragmentations with the laser pulse duration by almost an order of magnitude. Supported by quantum chemical simulations we explain our experimental results by the interplay between the dynamics of electron removal and nuclear motion.

  18. An Investigation into the Relationship between Students' Conceptions of the Particulate Nature of Matter and Their Understanding of Chemical Bonding

    ERIC Educational Resources Information Center

    Othman, Jazilah; Treagust, David F.; Chandrasegaran, A. L.

    2008-01-01

    A thorough understanding of chemical bonding requires familiarity with the particulate nature of matter. In this study, a two-tier multiple-choice diagnostic instrument consisting of ten items (five items involving each of the two concepts) was developed to assess students' understanding of the particulate nature of matter and chemical bonding so…

  19. Developing a New Teaching Approach for the Chemical Bonding Concept Aligned with Current Scientific and Pedagogical Knowledge

    ERIC Educational Resources Information Center

    Nahum, Tami Levy; Mamlok-Naaman, Rachel; Hofstein, Avi; Krajcik, Joseph

    2007-01-01

    The traditional pedagogical approach for teaching chemical bonding is often overly simplistic and not aligned with the most up-to-date scientific models. As a result, high-school students around the world lack fundamental understanding of chemical bonding. In order to improve students' understanding of this concept, it was essential to propose a…

  20. Electronic structure and chemical bonding of amorphous chromium carbide thin films

    NASA Astrophysics Data System (ADS)

    Magnuson, Martin; Andersson, Matilda; Lu, Jun; Hultman, Lars; Jansson, Ulf

    2012-06-01

    The microstructure, electronic structure and chemical bonding of chromium carbide thin films with different carbon contents have been investigated with high-resolution transmission electron microscopy, electron energy loss spectroscopy and soft x-ray absorption-emission spectroscopies. Most of the films can be described as amorphous nanocomposites with non-crystalline CrCx in an amorphous carbon matrix. At high carbon contents, graphene-like structures are formed in the amorphous carbon matrix. At 47 at.% carbon content, randomly oriented nanocrystallites are formed creating a complex microstructure of three components. The soft x-ray absorption-emission study shows additional peak structures exhibiting non-octahedral coordination and bonding.

  1. Mapping lipid and collagen by multispectral photoacoustic imaging of chemical bond vibration

    NASA Astrophysics Data System (ADS)

    Wang, Pu; Wang, Ping; Wang, Han-Wei; Cheng, Ji-Xin

    2012-09-01

    Photoacoustic microscopy using vibrational overtone absorption as a contrast mechanism allows bond-selective imaging of deep tissues. Due to the spectral similarity of molecules in the region of overtone vibration, it is difficult to interrogate chemical components using photoacoustic signal at single excitation wavelength. Here we demonstrate that lipids and collagen, two critical markers for many kinds of diseases, can be distinguished by multispectral photoacoustic imaging of the first overtone of C-H bond. A phantom consisting of rat-tail tendon and fat was constructed to demonstrate this technique. Wavelengths between 1650 and 1850 nm were scanned to excite both the first overtone and combination bands of C-H bonds. B-scan multispectral photoacoustic images, in which each pixel contains a spectrum, were analyzed by a multivariate curve resolution-alternating least squares algorithm to recover the spatial distribution of collagen and lipids in the phantom.

  2. Studying Chemical Reactions, One Bond at a Time, with Single Molecule AFM Techniques

    NASA Astrophysics Data System (ADS)

    Fernandez, Julio M.

    2008-03-01

    The mechanisms by which mechanical forces regulate the kinetics of a chemical reaction are unknown. In my lecture I will demonstrate how we use single molecule force-clamp spectroscopy and protein engineering to study the effect of force on the kinetics of thiol/disulfide exchange. Reduction of disulfide bond via the thiol/disulfide exchange chemical reaction is crucial in regulating protein function and is of common occurrence in mechanically stressed proteins. While reduction is thought to proceed through a substitution nucleophilic bimolecular (SN2) reaction, the role of a mechanical force in modulating this chemical reaction is unknown. We apply a constant stretching force to single engineered disulfide bonds and measure their rate of reduction by dithiothreitol (DTT). We find that while the reduction rate is linearly dependent on the concentration of DTT, it is exponentially dependent on the applied force, increasing 10-fold over a 300 pN range. This result predicts that the disulfide bond lengthens by 0.34 å at the transition state of the thiol/disulfide exchange reaction. In addition to DTT, we also study the reduction of the engineered disulfide bond by the E. coli enzyme thioredoxin (Trx). Thioredoxins are enzymes that catalyze disulfide bond reduction in all organisms. As before, we apply a mechanical force in the range of 25-450 pN to the engineered disulfide bond substrate and monitor the reduction of these bonds by individual enzymes. In sharp contrast with the data obtained with DTT, we now observe two alternative forms of the catalytic reaction, the first requiring a reorientation of the substrate disulfide bond, causing a shortening of the substrate polypeptide by 0.76±0.07 å, and the second elongating the substrate disulfide bond by 0.21±0.01 å. These results support the view that the Trx active site regulates the geometry of the participating sulfur atoms, with sub-ångström precision, in order to achieve efficient catalysis. Single molecule

  3. Chemical role of oxygen plasma in wafer bonding using borosilicate glasses

    NASA Astrophysics Data System (ADS)

    Hansen, D. M.; Albaugh, C. E.; Moran, P. D.; Kuech, T. F.

    2001-11-01

    Plasma-treated oxide layers are commonly used in wafer bonding applications. Borosilicate glass (BSG) layers deposited by low-pressure chemical vapor deposition treated with an O2 plasma in reactive ion etching mode for 5 min at 0.6 W/cm2 and rinsed with DI H2O readily bond to GaAs and Si. The chemical role of this prebonding treatment was investigated using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The peak intensities for both the Si-O and B-O absorbance bands decreased in intensity as a result of the plasma treatment is consistent with the uniform sputtering of 9.8 nm±0.8 nm of BSG. Polarization dependent ATR-FTIR revealed that the H2O/OH absorbance bands decreased in peak intensity with the OH groups being preferentially oriented perpendicular to the sample surface after the plasma treatment. The subsequent DI H2O rinse restores the water to the surface while removing B2O3 from the BSG layer. This prebonding treatment, therefore, results in a hydrophilic bond, but alters the composition of the BSG film at the bonded interface.

  4. Chemical activation of molecules by metals: Experimental studies of electron distributions and bonding

    SciTech Connect

    Lichtenberger, D.L.

    1991-10-01

    The formal relationship between measured molecular ionization energies and thermodynamic bond dissociation energies has been developed into a single equation which unifies the treatment of covalent bonds, ionic bonds, and partially ionic bonds. This relationship has been used to clarify the fundamental thermodynamic information relating to metal-hydrogen, metal-alkyl, and metal-metal bond energies. We have been able to obtain a direct observation and measurement of the stabilization energy provided by the agostic interaction of the C-H bond with the metal. The ionization energies have also been used to correlate the rates of carbonyl substitution reactions of ({eta}{sup 5}-C{sub 5}H{sub 4}X)Rh(CO){sub 2} complexes, and to reveal the electronic factors that control the stability of the transition state. The extent that the electronic features of these bonding interactions transfer to other chemical systems is being investigated in terms of the principle of additivity of ligand electronic effects. Specific examples under study include metal- phosphines, metal-halides, and metallocenes. Especially interesting has been the recent application of these techniques to the characterization of the soccer-ball shaped C{sub 60} molecule, buckminsterfullerene, and its interaction with a metal surface. The high-resolution valence ionizations in the gas phase reveal the high symmetry of the molecule, and studies of thin films of C{sub 60} reveal weak intermolecular interactions. Scanning tunneling and atomic force microscopy reveal the arrangement of spherical molecules on gold substrates, with significant delocalization of charge from the metal surface. 21 refs.

  5. Wetting Properties of Chemically Modified Surfaces: The role of hydrogen bonding

    NASA Astrophysics Data System (ADS)

    Bekele, Selemon; Tsige, Mesfin

    2015-03-01

    Many industrial processing operations involve the spreading of a liquid on a solid material. Controlling the wetting of one material by another is of crucial importance in such applications as adhesion, coating and oil recovery. A strategy often employed to control the wettability of solid surfaces is a combination of surface patterning and chemical surface modification. In order to understand the effect of surface chemistry on the wetting process, we have carried out all-atom molecular dynamics (MD) simulations of a water droplet spreading on pure and oxidized polystyrene surfaces. Our previous results show that the contact angle generally decreases with increasing oxygen concentration and there is a correlation between the spreading and hydrogen bonding. In this talk, we will present results on the structure and dynamics of the hydrogen bonds in the interfacial region between water and the polystyrene substrate. We will discuss our findings on hydrogen bond lifetimes, time correlations functions and number of hydrogen bonds per water molecule for the hydrogen bonds around the water/polystyrene interface which are found to play a role in the spreading process. This work was supported by NSF Grant DMR0847580.

  6. Developing density functional theory for Bose-Einstein condensates. The case of chemical bonding

    NASA Astrophysics Data System (ADS)

    Putz, Mihai V.

    2015-01-01

    Since the nowadays growing interest in Bose-Einstein condensates due to the expanded experimental evidence on various atomic systems within optical lattices in weak and strong coupling regimes, the connection with Density Functional Theory is firstly advanced within the mean field framework at three levels of comprehension: the many-body normalization condition, Thomas-Fermi limit, and the chemical hardness closure with the inter-bosonic strength and universal Hohenberg-Kohn functional. As an application the traditional Heitler-London quantum mechanical description of the chemical bonding for homopolar atomic systems is reloaded within the non-linear Schrödinger (Gross-Pitaevsky) Hamiltonian; the results show that a two-fold energetic solution is registered either for bonding and antibonding states, with the bosonic contribution being driven by the square of the order parameter for the Bose-Einstein condensate density in free (gas) motion, while the associate wave functions remain as in classical molecular orbital model.

  7. Developing density functional theory for Bose-Einstein condensates. The case of chemical bonding

    SciTech Connect

    Putz, Mihai V.

    2015-01-22

    Since the nowadays growing interest in Bose-Einstein condensates due to the expanded experimental evidence on various atomic systems within optical lattices in weak and strong coupling regimes, the connection with Density Functional Theory is firstly advanced within the mean field framework at three levels of comprehension: the many-body normalization condition, Thomas-Fermi limit, and the chemical hardness closure with the inter-bosonic strength and universal Hohenberg-Kohn functional. As an application the traditional Heitler-London quantum mechanical description of the chemical bonding for homopolar atomic systems is reloaded within the non-linear Schrödinger (Gross-Pitaevsky) Hamiltonian; the results show that a two-fold energetic solution is registered either for bonding and antibonding states, with the bosonic contribution being driven by the square of the order parameter for the Bose-Einstein condensate density in free (gas) motion, while the associate wave functions remain as in classical molecular orbital model.

  8. Multi-layered, chemically bonded lithium-ion and lithium/air batteries

    SciTech Connect

    Narula, Chaitanya Kumar; Nanda, Jagjit; Bischoff, Brian L; Bhave, Ramesh R

    2014-05-13

    Disclosed are multilayer, porous, thin-layered lithium-ion batteries that include an inorganic separator as a thin layer that is chemically bonded to surfaces of positive and negative electrode layers. Thus, in such disclosed lithium-ion batteries, the electrodes and separator are made to form non-discrete (i.e., integral) thin layers. Also disclosed are methods of fabricating integrally connected, thin, multilayer lithium batteries including lithium-ion and lithium/air batteries.

  9. Effect of raw material ratios on the compressive strength of magnesium potassium phosphate chemically bonded ceramics.

    PubMed

    Wang, Ai-juan; Yuan, Zhi-long; Zhang, Jiao; Liu, Lin-tao; Li, Jun-ming; Liu, Zheng

    2013-12-01

    The compressive strength of magnesium potassium phosphate chemically bonded ceramics is important in biomedical field. In this work, the compressive strength of magnesium potassium phosphate chemically bonded ceramics was investigated with different liquid-to-solid and MgO-to-KH2PO4 ratios. X-ray diffractometer was applied to characterize its phase composition. The microstructure was imaged using a scanning electron microscope. The results showed that the compressive strength of the chemically bonded ceramics increased with the decrease of liquid-to-solid ratio due to the change of the packing density and the crystallinity of hydrated product. However, with the increase of MgO-to-KH2PO4 weight ratio, its compressive strength increased firstly and then decreased. The low compressive strength in lower MgO-to-KH2PO4 ratio might be explained by the existence of the weak phase KH2PO4. However, the low value of compressive strength with the higher MgO-to-KH2PO4 ratio might be caused by lack of the joined phase in the hydrated product. Besides, it has been found that the microstructures were different in these two cases by the scanning electron microscope. Colloidal structure appeared for the samples with lower liquid-to-solid and higher MgO-to-KH2PO4 ratios possibly because of the existence of amorphous hydrated products. The optimization of both liquid-to-solid and MgO-to-KH2PO4 ratios was important to improve the compressive strength of magnesium potassium phosphate chemically bonded ceramics. PMID:24094224

  10. Halogen bonded supramolecular capsules: a challenging test case for quantum chemical methods.

    PubMed

    Sure, Rebecca; Grimme, Stefan

    2016-08-01

    Recently, Diederich et al. synthesized the first supramolecular capsule with a well-defined four-point halogen bonding interaction [Angew. Chem., Int. Ed., 2015, 54, 12339]. This interesting system comprising about 400 atoms represents a challenging test case for accurate quantum chemical methods. We investigate it with our new density functional based composite method for structures and noncovalent interactions (PBEh-3c) as well as our standard protocol for supramolecular thermochemistry and give predictions for chemical modifications to improve the binding strength. PMID:27416814

  11. Chemically-bonded brick production based on burned clay by means of semidry pressing

    NASA Astrophysics Data System (ADS)

    Voroshilov, Ivan; Endzhievskaya, Irina; Vasilovskaya, Nina

    2016-01-01

    We presented a study on the possibility of using the burnt rocks of the Krasnoyarsk Territory for production of chemically-bonded materials in the form of bricks which are so widely used in multistory housing and private house construction. The radiographic analysis of the composition of burnt rock was conducted and a modifier to adjust the composition uniformity was identified. The mixing moisture content was identified and optimal amount at 13-15% was determined. The method of semidry pressing has been chosen. The process of obtaining moldings has been theoretically proved; the advantages of chemically-bonded wall materials compared to ceramic brick were shown. The production of efficient artificial stone based on material burnt rocks, which is comparable with conventionally effective ceramic materials or effective with cell tile was proved, the density of the burned clay-based cell tile makes up to 1630-1785 kg m3, with compressive strength of 13.6-20.0 MPa depending on the compression ratio and cement consumption, frost resistance index is F50, and the thermal conductivity in the masonry is λ = 0,459-0,546 W m * °C. The clear geometric dimensions of pressed products allow the use of the chemically-bonded brick based on burnt clay as a facing brick.

  12. Development of chemically bonded phosphate ceramics for stabilizing low-level mixed wastes

    NASA Astrophysics Data System (ADS)

    Jeong, Seung-Young

    1997-11-01

    Novel chemically bonded phosphate ceramics have been developed by acid-base reactions between magnesium oxide and an acid phosphate at room temperature for stabilizing U.S. Department of Energy's low-level mixed waste streams that include hazardous chemicals and radioactive elements. Newberyite (MgHPOsb4.3Hsb2O)-rich magnesium phosphate ceramic was formed by an acid-base reaction between phosphoric acid and magnesium oxide. The reaction slurry, formed at room-temperature, sets rapidly and forms stable mineral phases of newberyite, lunebergite, and residual MgO. Rapid setting also generates heat due to exothermic acid-base reaction. The reaction was retarded by partially neutralizing the phosphoric acid solution by adding sodium or potassium hydroxide. This reduced the rate of reaction and heat generation and led to a practical way of producing novel magnesium potassium phosphate ceramic. This ceramic was formed by reacting stoichiometric amount of monopotassium dihydrogen phosphate crystals, MgO, and water, forming pure-phase of MgKPOsb4.6Hsb2O (MKP) with moderate exothermic reaction. Using this chemically bonded phosphate ceramic matrix, low-level mixed waste streams were stabilized, and superior waste forms in a monolithic structure were developed. The final waste forms showed low open porosity and permeability, and higher compression strength than the Land Disposal Requirements (LDRs). The novel MKP ceramic technology allowed us to develop operational size waste forms of 55 gal with good physical integrity. In this improved waste form, the hazardous contaminants such as RCRA heavy metals (Hg, Pb, Cd, Cr, Ni, etc) were chemically fixed by their conversion into insoluble phosphate forms and physically encapsulated by the phosphate ceramic. In addition, chemically bonded phosphate ceramics stabilized radioactive elements such U and Pu. This was demonstrated with a detailed stabilization study on cerium used as a surrogate (chemically equivalent but nonradioactive

  13. Four chemical methods of porcelain conditioning and their influence over bond strength and surface integrity

    PubMed Central

    Stella, João Paulo Fragomeni; Oliveira, Andrea Becker; Nojima, Lincoln Issamu; Marquezan, Mariana

    2015-01-01

    OBJECTIVE: To assess four different chemical surface conditioning methods for ceramic material before bracket bonding, and their impact on shear bond strength and surface integrity at debonding. METHODS: Four experimental groups (n = 13) were set up according to the ceramic conditioning method: G1 = 37% phosphoric acid etching followed by silane application; G2 = 37% liquid phosphoric acid etching, no rinsing, followed by silane application; G3 = 10% hydrofluoric acid etching alone; and G4 = 10% hydrofluoric acid etching followed by silane application. After surface conditioning, metal brackets were bonded to porcelain by means of the Transbond XP system (3M Unitek). Samples were submitted to shear bond strength tests in a universal testing machine and the surfaces were later assessed with a microscope under 8 X magnification. ANOVA/Tukey tests were performed to establish the difference between groups (α= 5%). RESULTS: The highest shear bond strength values were found in groups G3 and G4 (22.01 ± 2.15 MPa and 22.83 ± 3.32 Mpa, respectively), followed by G1 (16.42 ± 3.61 MPa) and G2 (9.29 ± 1.95 MPa). As regards surface evaluation after bracket debonding, the use of liquid phosphoric acid followed by silane application (G2) produced the least damage to porcelain. When hydrofluoric acid and silane were applied, the risk of ceramic fracture increased. CONCLUSIONS: Acceptable levels of bond strength for clinical use were reached by all methods tested; however, liquid phosphoric acid etching followed by silane application (G2) resulted in the least damage to the ceramic surface. PMID:26352845

  14. Revealing halogen bonding interactions with anomeric systems: an ab initio quantum chemical studies.

    PubMed

    Lo, Rabindranath; Ganguly, Bishwajit

    2015-02-01

    A computational study has been performed using MP2 and CCSD(T) methods on a series of O⋯X (X=Br, Cl and I) halogen bonds to evaluate the strength and characteristic of such highly directional noncovalent interactions. The study has been carried out on a series of dimeric complexes formed between interhalogen compounds (such as BrF, BrCl and BrI) and oxygen containing electron donor molecule. The existence and consequences of the anomeric effect of the electron donor molecule has also been investigated through an exploration of halogen bonding interactions in this halogen bonded complexes. The ab initio quantum chemical calculations have been employed to study both the nature and directionality of the halogen molecules toward the sp(3) oxygen atom in anomeric systems. The presence of anomeric nO→σ*CN interaction involves a dominant role for the availability of the axial and equatorial lone pairs of donor O atom to participate with interhalogen compounds in the halogen-bonded complexes. The energy difference between the axial and equatorial conformers with interhalogen compounds reaches up to 4.60 kJ/mol, which however depends upon the interacting halogen atoms and its attaching atoms. The energy decomposition analysis further suggests that the total halogen bond interaction energies are mainly contributed by the attractive electrostatic and dispersion components. The role of substituents attached with the halogen atoms has also been evaluated in this study. With the increase of halogen atom size and the positive nature of σ-hole, the halogen atom interacted more with the electron donor atom and the electrostatic contribution to the total interaction energy enhances appreciably. Further, noncovalent interaction (NCI) studies have been carried out to locate the noncovalent halogen bonding interactions in real space. PMID:25522359

  15. The active site of hen egg-white lysozyme: flexibility and chemical bonding

    SciTech Connect

    Held, Jeanette Smaalen, Sander van

    2014-04-01

    Chemical bonding at the active site of lysozyme is analyzed on the basis of a multipole model employing transferable multipole parameters from a database. Large B factors at low temperatures reflect frozen-in disorder, but therefore prevent a meaningful free refinement of multipole parameters. Chemical bonding at the active site of hen egg-white lysozyme (HEWL) is analyzed on the basis of Bader’s quantum theory of atoms in molecules [QTAIM; Bader (1994 ▶), Atoms in Molecules: A Quantum Theory. Oxford University Press] applied to electron-density maps derived from a multipole model. The observation is made that the atomic displacement parameters (ADPs) of HEWL at a temperature of 100 K are larger than ADPs in crystals of small biological molecules at 298 K. This feature shows that the ADPs in the cold crystals of HEWL reflect frozen-in disorder rather than thermal vibrations of the atoms. Directly generalizing the results of multipole studies on small-molecule crystals, the important consequence for electron-density analysis of protein crystals is that multipole parameters cannot be independently varied in a meaningful way in structure refinements. Instead, a multipole model for HEWL has been developed by refinement of atomic coordinates and ADPs against the X-ray diffraction data of Wang and coworkers [Wang et al. (2007), Acta Cryst. D63, 1254–1268], while multipole parameters were fixed to the values for transferable multipole parameters from the ELMAM2 database [Domagala et al. (2012), Acta Cryst. A68, 337–351] . Static and dynamic electron densities based on this multipole model are presented. Analysis of their topological properties according to the QTAIM shows that the covalent bonds possess similar properties to the covalent bonds of small molecules. Hydrogen bonds of intermediate strength are identified for the Glu35 and Asp52 residues, which are considered to be essential parts of the active site of HEWL. Furthermore, a series of weak C

  16. The trouble with chemical energy: why understanding bond energies requires an interdisciplinary systems approach.

    PubMed

    Cooper, Melanie M; Klymkowsky, Michael W

    2013-06-01

    Helping students understand "chemical energy" is notoriously difficult. Many hold inconsistent ideas about what energy is, how and why it changes during the course of a chemical reaction, and how these changes are related to bond energies and reaction dynamics. There are (at least) three major sources for this problem: 1) the way biologists talk about chemical energy (which is also the way we talk about energy in everyday life); 2) the macroscopic approach to energy concepts that is common in physics and physical sciences; and 3) the failure of chemistry courses to explicitly link molecular with macroscopic energy ideas. From a constructivist perspective, it is unlikely that students can, without a coherent understanding of such a central concept, attain a robust and accurate understanding of new concepts. However, changes are on the horizon, guided by the increasing understanding that difficult concepts require coherent, well-designed learning progressions and the new National Research Council Framework for K-12 Science Education. We provide supporting evidence for our assertions and suggestions for an interdisciplinary learning progression designed to better approach the concept of bond energies, a first step in an understanding chemical energy and behavior of reaction systems that is central to biological systems. PMID:23737636

  17. The Trouble with Chemical Energy: Why Understanding Bond Energies Requires an Interdisciplinary Systems Approach

    PubMed Central

    Cooper, Melanie M.; Klymkowsky, Michael W.

    2013-01-01

    Helping students understand “chemical energy” is notoriously difficult. Many hold inconsistent ideas about what energy is, how and why it changes during the course of a chemical reaction, and how these changes are related to bond energies and reaction dynamics. There are (at least) three major sources for this problem: 1) the way biologists talk about chemical energy (which is also the way we talk about energy in everyday life); 2) the macroscopic approach to energy concepts that is common in physics and physical sciences; and 3) the failure of chemistry courses to explicitly link molecular with macroscopic energy ideas. From a constructivist perspective, it is unlikely that students can, without a coherent understanding of such a central concept, attain a robust and accurate understanding of new concepts. However, changes are on the horizon, guided by the increasing understanding that difficult concepts require coherent, well-designed learning progressions and the new National Research Council Framework for K–12 Science Education. We provide supporting evidence for our assertions and suggestions for an interdisciplinary learning progression designed to better approach the concept of bond energies, a first step in an understanding chemical energy and behavior of reaction systems that is central to biological systems. PMID:23737636

  18. Theory of chemical bonds in metalloenzymes II: Hybrid-DFT studies in iron-sulfur clusters

    NASA Astrophysics Data System (ADS)

    Shoji, M.; Koizumi, K.; Kitagawa, Y.; Yamanaka, S.; Kawakami, T.; Okumura, M.; Yamaguchi, K.

    Important chemical reactions for life often require multistep electron transfers (ET) and strong reducing forces. In these reactions, electron transfer proteins as ferredoxins (Fds) play a key role. For elucidation of the core electronic states in these electron transfer processes, an inorganic model compound [Fe2S2(S2-o-xyl)2] is used as our first study. It was experimentally characterized that the model compound is in a similar electronic state to the active site core in Fds. On the reduced form, the diiron core exists in a characteristic mixed-valence state that has mobile electron (spin). Hybrid density functional theory (HDFT) calculations are performed to investigate the chemical bond nature, electronic structures, and magnetic interactions. The spin states and energy levels are further discussed with spin Hamiltonians, which contain Heisenberg exchange term and double exchange term to describe the mixed-valence state. We have determined their effective exchange integrals (J) and resonance parameters (B) from (HDFT) calculations in several procedures. These magnetic interactions are in good agreement with experiments. To estimate B values, we propose a new procedure using molecular orbital energies. The B values are properly evaluated compared with other procedures, using total energies. The chemical bond natures and the ground electronic structures are elucidated in terms of chemical indices defined by the occupation number of natural orbitals. Finally, implications of the computational results are discussed in relation to rational design of biomolecular devices.

  19. A theoretical study of the structures and chemical bonds of neptunium (III) molecules by a density functional method

    NASA Astrophysics Data System (ADS)

    Yin, Yao-Peng; Dong, Chen-Zhong; Du, Lei-Qiang; Wu, Fang-Xian; Ding, Xiao-Bin

    2014-10-01

    In this paper, equilibrium structures and chemical bond characteristics of neptunium trihalide molecules NpX3 (X = F, Cl, Br and I) have been investigated by using density functional theory (DFT). The influences of the size of the relativistic effective core potential (RECP) have been examined on the molecular structures. The chemical bond characteristics have also been systematically studied by calculating the density of states (DOS), bond length differences and electronic charge distributions. We have determined that the chemical bonds are mainly ionic in those molecules, and the covalency is enhancing while ionicity decreases from NpF3 to NpI3. The calculated bond energies show that the interaction strength in NpX3 molecules becomes weaker as the halogen atoms becoming heavier.

  20. Chemical bonding motifs from a tiling of the many-electron wavefunction.

    PubMed

    Liu, Yu; Frankcombe, Terry J; Schmidt, Timothy W

    2016-05-11

    A method is presented to partition the 3N-dimensional space of a many-electron wavefunction into hyper-regions related by permutation symmetry. These hyper-regions represent unit cells, or "tiles" of the wavefunction from which the wavefunction may be regenerated in its entirety upon application of the set of permutations of like-spin electrons. The method, wherein a Voronoi diagram is constructed from the (even permutations of the) average position of a swarm of Monte Carlo walkers sampling |Ψ|(2), determines a self-consistent partitioning of the wavefunction. When one of the identical 3N-dimensional Voronoi sites is projected onto the coordinates of each electron, chemical motifs naturally appear, such as core electrons, lone-pairs, single-bonds and banana-bonds. The structures determined for N2, O2, F2, and other molecules correspond to the double-quartet theory of Linnett. When the procedure is applied to C2, we arrive at an interpretation of its bonding in terms of a near triple bond with singlet-coupled outer electrons. PMID:27122062

  1. Hybrid density functional study on lattice vibration, thermodynamic properties, and chemical bonding of plutonium monocarbide

    NASA Astrophysics Data System (ADS)

    Rong, Yang; Bin, Tang; Tao, Gao; BingYun, Ao

    2016-06-01

    Hybrid density functional theory is employed to systematically investigate the structural, magnetic, vibrational, thermodynamic properties of plutonium monocarbide (PuC and PuC0.75). For comparison, the results obtained by DFT, DFT + U are also given. For PuC and PuC0.75, Fock-0.25 hybrid functional gives the best lattice constants and predicts the correct ground states of antiferromagnetic (AFM) structure. The calculated phonon spectra suggest that PuC and PuC0.75 are dynamically stable. Values of the Helmholtz free energy ΔF, internal energy ΔE, entropy S, and constant-volume specific heat C v of PuC and PuC0.75 are given. The results are in good agreement with available experimental or theoretical data. As for the chemical bonding nature, the difference charge densities, the partial densities of states and the Bader charge analysis suggest that the Pu–C bonds of PuC and PuC0.75 have a mixture of covalent character and ionic character. The effect of carbon vacancy on the chemical bonding is also discussed in detail. We expect that our study can provide some useful reference for further experimental research on the phonon density of states, thermodynamic properties of the plutonium monocarbide. Project supported by the National Natural Science Foundation of China (Grant Nos. 21371160 and 21401173).

  2. Strongly correlated valence electrons and core-level chemical bonding of Lithium at terapascal pressures

    NASA Astrophysics Data System (ADS)

    Hu, Anguang; Zhang, Fan

    2015-03-01

    As the simplest pure metal, lithium exhibits some novel properties on electrical conductivity and crystal structures under high pressure. All-electron density functional theory simulations, recently developed by using the linear combination of localized Slater atomic orbitals, revealed that the bandwidth of its valence bands remains almost unchanged within about 3.5 eV even up to a terapascal pressure range. This indicates that the development from delocalized to strongly correlated electronic systems takes place under compression, resulting in metal-semiconductor and superconductivity transitions together with a sequence of new high-pressure crystal phases, discovered experimentally. In contrast to the valence bands, the core-level bands become broadening up to about 10 eV at terapascal pressures. It means the transformation from chemical non-bonding to bonding for core electrons. Thus, dense lithium under compression can be characterized as core-level chemical bonding and a completely new class of strongly correlated materials with narrow bands filled in s-electron shells only.

  3. Weak Intermolecular Hydrogen Bonds with Fluorine: Detection and Implications for Enzymatic/Chemical Reactions, Chemical Properties, and Ligand/Protein Fluorine NMR Screening.

    PubMed

    Dalvit, Claudio; Vulpetti, Anna

    2016-05-23

    It is known that strong hydrogen-bonding interactions play an important role in many chemical and biological systems. However, weak or very weak hydrogen bonds, which are often difficult to detect and characterize, may also be relevant in many recognition and reaction processes. Fluorine serving as a hydrogen-bond acceptor has been the subject of many controversial discussions and there are different opinions about it. It now appears that there is compelling experimental evidence for the involvement of fluorine in weak intramolecular or intermolecular hydrogen bonds. Using established NMR methods, we have previously characterized and measured the strengths of intermolecular hydrogen-bond complexes involving the fluorine moieties CH2 F, CHF2 , and CF3 , and have compared them with the well-known hydrogen-bond complex formed between acetophenone and the strong hydrogen-bond donor p-fluorophenol. We now report evidence for the formation of hydrogen bonds involving fluorine with significantly weaker donors, namely 5-fluoroindole and water. A simple NMR method is proposed for the simultaneous measurement of the strengths of hydrogen bonds between an acceptor and a donor or water. Important implications of these results for enzymatic/chemical reactions involving fluorine, for chemical and physical properties, and for ligand/protein (19) F NMR screening are analyzed through experiments and theoretical simulations. PMID:27112430

  4. The competition between hydrogen bonding and chemical change in carbohydrate nanoparticles

    NASA Astrophysics Data System (ADS)

    Jetzki, Martin; Signorell, Ruth

    2002-11-01

    Broad band Fourier transform infrared spectroscopy was used to study the competition between hydrogen bonding and conformational and chemical changes in molecular nanoparticles. Particles of small carbohydrates (dihydroxyacetone, glyceraldehyde, fructose, and glucose) with sizes between 20 and 100 nm served as model systems. The variation of the preparation conditions allowed to control the chemical composition of the particles. For dihydroxyacetone, the particle generation in an electrospray is accompanied by the formation of intermolecular hemiketals. In contrast, particles generated directly from the vapor phase in a collisional cooling cell consist exclusively of the monomeric form. Quantum chemical simulations demonstrate that conformational changes upon particle formation can be understood to a good approximation on a molecular level. In contrast to that, the characteristic band shape observed for all carbohydrates studied is determined by the interaction of the whole ensemble of OH-oscillators. These contributions could be described within a continuum model.

  5. Enhancement of chemical stability and crystallinity in porphyrin-containing covalent organic frameworks by intramolecular hydrogen bonds.

    PubMed

    Kandambeth, Sharath; Shinde, Digambar Balaji; Panda, Manas K; Lukose, Binit; Heine, Thomas; Banerjee, Rahul

    2013-12-01

    A strong bond: A strategy based on intramolecular hydrogen-binding interactions in 2D covalent organic frameworks (COFs) is shown to improve the crystallinity, porosity, and chemical stability of the material. The concept is validated by removing the hydrogen-bonding interaction in the methoxy analog which showed a lower stability and crystallinity. PMID:24127339

  6. Correlation of Chemical Bond Directions and Functional Group Orientations in Solids by Two-Dimensional NMR

    NASA Astrophysics Data System (ADS)

    Weliky, D. P.; Dabbagh, G.; Tycko, R.

    We describe a new two-dimensional NMR technique for structural studies of polycrystalline and noncrystalline solids. The technique is a variant of 2D exchange spectroscopy applicable to organic molecules, macromolecules, or molecular complexes that are doubly 13C-labeled at a specific carboncarbon bond and singly 13C labeled at a specific functional group. A Carr-Purcell sequence is used to obtain dipolar spectra in the t1 dimension. Spectra in the t2 dimension are determined primarily by the chemical-shift anisotropy. With spin diffusion among the labeled sites between the t1 and t2 periods, the resulting 2D spectra reveal correlations between the direction of the labeled bond and the orientation of the labeled functional group. Experimental spectra of two polycrystalline model compounds, dimethyl succinate and diammonium succinate, are presented and compared with simulations to illustrate the structural information contained in the 2D spectra.

  7. Nitrogen implantation effects on the chemical bonding and hardness of boron and boron nitride coatings

    SciTech Connect

    Anders, S; Felter, T; Hayes, J; Jankowski, A F; Patterson, R; Poker, D; Stamler, T

    1999-02-08

    Boron nitride (BN) coatings are deposited by the reactive sputtering of fully dense, boron (B) targets utilizing an argon-nitrogen (Ar-N{sub 2}) reactive gas mixture. Near-edge x-ray absorption fine structure analysis reveals features of chemical bonding in the B 1s photoabsorption spectrum. Hardness is measured at the film surface using nanoindentation. The BN coatings prepared at low, sputter gas pressure with substrate heating are found to have bonding characteristic of a defected hexagonal phase. The coatings are subjected to post-deposition nitrogen (N{sup +} and N{sub 2}{sup +}) implantation at different energies and current densities. The changes in film hardness attributed to the implantation can be correlated to changes observed in the B 1s NEXAFS spectra.

  8. Electronic structure and chemical bonding of amorphous chromium carbide thin films.

    PubMed

    Magnuson, Martin; Andersson, Matilda; Lu, Jun; Hultman, Lars; Jansson, Ulf

    2012-06-01

    The microstructure, electronic structure and chemical bonding of chromium carbide thin films with different carbon contents have been investigated with high-resolution transmission electron microscopy, electron energy loss spectroscopy and soft x-ray absorption-emission spectroscopies. Most of the films can be described as amorphous nanocomposites with non-crystalline CrC(x) in an amorphous carbon matrix. At high carbon contents, graphene-like structures are formed in the amorphous carbon matrix. At 47 at.% carbon content, randomly oriented nanocrystallites are formed creating a complex microstructure of three components. The soft x-ray absorption-emission study shows additional peak structures exhibiting non-octahedral coordination and bonding. PMID:22553115

  9. LOBSTER: A tool to extract chemical bonding from plane-wave based DFT.

    PubMed

    Maintz, Stefan; Deringer, Volker L; Tchougréeff, Andrei L; Dronskowski, Richard

    2016-04-30

    The computer program LOBSTER (Local Orbital Basis Suite Towards Electronic-Structure Reconstruction) enables chemical-bonding analysis based on periodic plane-wave (PAW) density-functional theory (DFT) output and is applicable to a wide range of first-principles simulations in solid-state and materials chemistry. LOBSTER incorporates analytic projection routines described previously in this very journal [J. Comput. Chem. 2013, 34, 2557] and offers improved functionality. It calculates, among others, atom-projected densities of states (pDOS), projected crystal orbital Hamilton population (pCOHP) curves, and the recently introduced bond-weighted distribution function (BWDF). The software is offered free-of-charge for non-commercial research. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc. PMID:26914535

  10. Determination of double bond position in conjugated dienes by chemical ionization mass spectrometry with isobutane

    SciTech Connect

    Doolittle, R.E.; Tumlinson, J.H.; Proveaux, A.

    1985-07-01

    The chemical ionization (CI) mass spectra of a series of functionalized conjugated dienes, including aldehydes, alcohols, formates, acetates, and hydrocarbons were investigated to determine whether fragmentations occur that are characteristic of the position of the conjugated system within the hydrocarbon chain. CI with isobutane as ionizing gas produces structure-specific fragment ions with m/z ratios that can be used to locate the positions of the double bonds in most of the cases studied. When the conjugated system is proximal to the functional group or conjugated with the functional group, other fragmentation processes take precedence. These patterns of fragmentations constitute a very useful analytical tool for the location of conjugated double bonds in a variety of natural products. 34 references, 3 tables, 3 figures.

  11. Electronic properties and chemical bondings of Csbnd Hsbnd Nsbnd O compounds

    NASA Astrophysics Data System (ADS)

    Qin, Han; Liu, Qi-Jun; Chai, Rui-Qing; Liu, Fu-Sheng; Liu, Zheng-Tang

    2016-07-01

    We have performed the density functional theory (DFT) calculations within generalized gradient approximation (GGA) plus TS scheme to calculate the structural and electronic properties of Csbnd Hsbnd Nsbnd O compounds. Due to the insufficient description of intermolecular interactions within the GGA calculations, the GGA plus TS dispersion correction have been used to optimize the structural parameters. The calculated results based on the GGA + TS calculations are in agreement with the experimental data, indicating that the introduction of TS dispersion correction can effectively obtain the structural properties of Csbnd Hsbnd Nsbnd O compounds. The electronic properties of nineteen Csbnd Hsbnd Nsbnd O compounds including density of states, Mulliken charges, bond populations and band structures have been obtained and analyzed. According to these calculated data, the electron distributions and charge transfers have been investigated, which are conducive to clarify the chemical bonding characters and further help to future research of understanding the initial chemistry within the detonating energetic materials.

  12. An Effective Hamiltonian Molecular Orbital-Valence Bond (MOVB) Approach for Chemical Reactions Applied to the Nucleophilic Substitution Reaction of Hydrosulfide Ion and Chloromethane.

    PubMed

    Song, Lingchun; Mo, Yirong; Gao, Jiali

    2009-01-01

    An effective Hamiltonian mixed molecular orbital and valence bond (EH-MOVB) method is described to obtain an accurate potential energy surface for chemical reactions. Building upon previous results on the construction of diabatic and adiabatic potential surfaces using ab initio MOVB theory, we introduce a diabatic-coupling scaling factor to uniformly scale the ab initio off-diagonal matrix element H(12) such that the computed energy of reaction from the EH-MOVB method is in agreement with the target value. The scaling factor is very close to unity, resulting in minimal alteration of the potential energy surface of the original MOVB model. Furthermore, the relative energy between the reactant and product diabatic states in the EH-MOVB method can be improved to match the experimental energy of reaction. A key ingredient in the EH-MOVB theory is that the off-diagonal matrix elements are functions of all degrees of freedom of the system and the overlap matrix is explicitly evaluated. The EH-MOVB method has been applied to the nucleophilic substitution reaction between hydrosulfide and chloromethane to illustrate the methodology and the results were matched to reproduce the results from ab initio valence bond self-consistent valence bond (VBSCF) calculations. The diabatic coupling (the off-diagonal matrix element in the generalized secular equation) has small variations along the minimum energy reaction path in the EH-MOVB model, whereas it shows a maximum value at the transition state and has nearly zero values in the regions of the ion-dipole complexes from VBSCF calculations. The difference in the diabatic coupling stabilization is attributed to the large overlap integral in the computationally efficient MOVB method. PMID:20047006

  13. An Effective Hamiltonian Molecular Orbital-Valence Bond (MOVB) Approach for Chemical Reactions Applied to the Nucleophilic Substitution Reaction of Hydrosulfide Ion and Chloromethane

    PubMed Central

    Song, Lingchun; Mo, Yirong; Gao, Jiali

    2009-01-01

    An effective Hamiltonian mixed molecular orbital and valence bond (EH-MOVB) method is described to obtain an accurate potential energy surface for chemical reactions. Building upon previous results on the construction of diabatic and adiabatic potential surfaces using ab initio MOVB theory, we introduce a diabatic-coupling scaling factor to uniformly scale the ab initio off-diagonal matrix element H12 such that the computed energy of reaction from the EH-MOVB method is in agreement with the target value. The scaling factor is very close to unity, resulting in minimal alteration of the potential energy surface of the original MOVB model. Furthermore, the relative energy between the reactant and product diabatic states in the EH-MOVB method can be improved to match the experimental energy of reaction. A key ingredient in the EH-MOVB theory is that the off-diagonal matrix elements are functions of all degrees of freedom of the system and the overlap matrix is explicitly evaluated. The EH-MOVB method has been applied to the nucleophilic substitution reaction between hydrosulfide and chloromethane to illustrate the methodology and the results were matched to reproduce the results from ab initio valence bond self-consistent valence bond (VBSCF) calculations. The diabatic coupling (the off-diagonal matrix element in the generalized secular equation) has small variations along the minimum energy reaction path in the EH-MOVB model, whereas it shows a maximum value at the transition state and has nearly zero values in the regions of the ion-dipole complexes from VBSCF calculations. The difference in the diabatic coupling stabilization is attributed to the large overlap integral in the computationally efficient MOVB method. PMID:20047006

  14. Bergman Clusters, Multiple Bonds, and Defect Planes: Synthetic Outcomes of Chemical Frustration in Ternary Intermetallic Systems

    NASA Astrophysics Data System (ADS)

    Hadler, Amelia Beth

    Intermetallics crystallize in a variety of complex structures, many of which show unusual bonding or intriguing properties. Understanding what factors drive this structural chemistry would be a valuable step towards designing new intermetallics with specific structures or properties. One pathway towards understanding and predicting the structures of complex intermetallics is chemical frustration, a design tool which harnesses competition between incompatible bonding or packing modes to induce complexity in ternary intermetallic systems. The research outlined in this thesis focuses on developing chemical frustration through exploratory synthesis in ternary systems designed to induce frustration between the tetrahedral close packing of many intermetallics and the simple cubic packing seen for ionic salts or elemental metals. Syntheses in three systems yielded six new ternary intermetallics, four of which crystallize in novel structure types. Three were discovered in the Ca-Cu-Cd system: Ca5Cu2Cd and Ca2Cu 2Cd9, which adopt ternary variants of binary structures, and Ca10Cu2Cd27, which crystallizes in a new structure built from Bergman clusters. All three structures can be traced to electronic packing frustration induced by the similar electronegativities but different metallic radii of Cu and Cd. The Gd-Fe-C system yielded the new carbometalate Gd13Fe 10C13 and an oxycarbide derivative. These phases crystallize in structures built from Gd tricapped trigonal prisms interpenetrated by an Fe-C network. Theoretical analyses reveal that Fe-Fe and Fe-C multiple bonding is found throughout this network. A theoretical investigation of similar carbides uncovers additional metal-metal, metal-carbon, and carbon-carbon multiple bonding. This unusual bonding stabilizes the carbides by satisfying preferred electron counts for their transition metal sites. One new phase, Mg4.5Pd5Ge1.5, was found in the Mg-Pd-Ge system. Its structure is closely related to the CsCl-type structure of

  15. Mechanical properties and chemical bonding characteristics of Cr7C3 type multicomponent carbides

    NASA Astrophysics Data System (ADS)

    Xiao, B.; Feng, J.; Zhou, C. T.; Jiang, Y. H.; Zhou, R.

    2011-01-01

    The first principles calculations based on density functional theory are performed to investigate the mechanical properties and chemical bonding features of several Cr7C3 type multicomponent carbides (Fe16Cr12C12, Fe12Cr12W4C12, Fe12Cr12Mo4C12, Fe12Cr12W4C8B4, and Fe12Cr12Mo4C8B4). The full set elastic constants are calculated using stress-strain method. The Voigt-Reuss-Hill approximation is used to evaluate the mechanical moduli. The mechanical anisotropy is characterized by calculating several different anisotropic indexes and factors, such as universal anisotropic index (AU), shear anisotropic factors (A1, A2, and A3), and percent anisotropy (AB and AG). The surface constructions of bulk and Young's moduli are illustrated to indicate the mechanical anisotropy. The obtained results indicate that the presence of B could deteriorate the mechanical moduli of (Fe,Cr)7C3 phase in Cr cast iron slightly. On the other hand, the chemical stability of (Fe,Cr)7C3 carbides is improved due to chemical bonding modifications through doping with B. These conclusions are in agreement with experimental results.

  16. Studies of interdiffusion, chemical bonding, and intermolecular interactions in fiber-matrix adhesion

    SciTech Connect

    Chou, Chiate.

    1990-01-01

    A study of the key factors involved in adhesion was conducted to determine a quantitative relation between the underlying physicochemical mechanisms of adhesion and the adhesive performance at the fiber-matrix interface. Aramid fiber was modified by attaching pendent chains to its surface to change the nature of its interaction with matrix materials. The relative importance of the three fundamental factors of adhesion (interdiffusion, intermolecular interactions, and chemical bonding) was studied by evaluating the fiber-matrix adhesive performance of these modified fiber-matrix systems.

  17. Chemical bonding in equiatomic cerium intermetallics - The case of CeMgSn, CePdSn, and CeMgPb

    NASA Astrophysics Data System (ADS)

    Matar, Samir F.; Pöttgen, Rainer

    2015-10-01

    The electronic and magnetic structures and the properties of chemical bonding in isopointal CeMgSn and CePdSn (both phases belong to the family of TiNiSi related intermetallics, space group Pnma) and CeMgPb belonging to the family of CeScSi intermetallics, space group I4/mmm, have been investigated within the density functional theory (DFT). The charge analyses indicate negatively charged tin and lead leading to assign the compounds as stannides and plumbides, as also illustrated by the mapping of the electron localization function ELF. Calculations within spin-degenerate non-magnetic spin-polarized ferro- (SP-F) and SP-antiferromagnetic configurations led to assign a major role of Ce 4f states in the onset of ordered moments within SP-AF ground states from energy differences. Chemical bonding analyses from crystal orbital overlap populations revealed the strongest interactions for Ce-Sn in CeMgSn, Ce-Pb in CeMgPb, and Ce-Pd in CePdSn.

  18. Location and chemical bond of radionuclides in neutron-irradiated nuclear graphite

    NASA Astrophysics Data System (ADS)

    Vulpius, D.; Baginski, K.; Fischer, C.; Thomauske, B.

    2013-07-01

    The locations and the chemical forms (chemical bonds) of radionuclides in neutron-irradiated nuclear graphite have been determined in order to develop principal strategies for the management of graphitic nuclear waste. Due to the relatively low concentration of radionuclides in neutron-irradiated nuclear graphite (<1 ppm) direct spectroscopic methods are not applicable to investigate chemical structures. Therefore, methods by analogy have been applied. Such methods are investigations of the chemically detectable precursors of radionuclides in neutron-irradiated nuclear graphite and subjection of irradiated graphite to different chemical reactions followed by measurements of the radionuclide-containing reaction products by sensitive radiochemical methods. The paper discusses the applicability of these methods. The radionuclides investigated in this study can be divided into three parts: tritium, radiocarbon and metallic activation and fission products. Tritium can be bound in neutron-irradiated nuclear graphite as strongly adsorbed tritiated water (HTO), in oxygen-containing functional groups (e.g. C-OT) and as hydrocarbons (C-T). Radiocarbon is covalently bound with the graphite structure. The activity can be described by a homogeneously distributed part and a heterogeneously distributed part (enriched on surfaces or in hotspots). Metallic radionuclides can be bound as ions or covalent metal-carbon compounds. The distribution of all these radionuclides is mainly dependent on the distribution of their inactive precursors.

  19. Enhancing the Scratch Resistance by Introducing Chemical Bonding in Highly Stretchable and Transparent Electrodes.

    PubMed

    Guo, Chuan Fei; Chen, Yan; Tang, Lu; Wang, Feng; Ren, Zhifeng

    2016-01-13

    Stretchable transparent electrodes are key elements in flexible electronics and e-skins. However, existing stretchable transparent electrodes, including graphene sheets, carbon nanotube, and metal nanowire networks, weakly adheres to the substrate by van der Waals forces. Such electrodes suffer from poor scratch-resistance or poor durability, and this issue has been one of the biggest problems for their applications in industry. Here we show that, by introducing a Au-S bond between a Au nanomesh (AuNM) and the underlying elastomeric substrate, the AuNM strongly adheres to the substrate and can withstand scratches of a pressure of several megapascals. We find that the strong chemical bond, on the other hand, leads to a stiffening effect and localized rupture of the AuNM upon stretching; thus the stretchability is poor. A prestraining process is applied to suppress the localized rupture and has successfully improved the stretchability: electrical resistance of the prestrained AuNM exhibits modest change by one-time stretching to 160%, or repeated stretching to 50% for 25 000 cycles. This conductor is an ideal platform for robust stretchable photoelectronics. The idea of introducing a covalent bond to improve the scratch-resistance may also be applied to other systems including Ag nanowire films, carbon nanotube films, graphene, and so forth. PMID:26674364

  20. Chemical bonding and electronic structures at magnesium/copper phthalocyanine interfaces

    NASA Astrophysics Data System (ADS)

    Tang, J. X.; Lee, C. S.; Lee, S. T.

    2006-03-01

    Chemistry, electronic structure and electrical behavior at the interfaces between copper phthalocyanine (CuPc) and Mg with a reverse formation sequence were investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and current-voltage ( I- V) measurements. A chemical reaction occurs between CuPc and Mg irrespective of the deposition sequence. Despite having different reaction zone thicknesses, both the CuPc-on-Mg and the Mg-on-CuPc interfaces exhibit chemistry-induced gap states and identical carrier injection barriers, which are confirmed by the symmetric electrical behavior obtained from I- V characteristics of devices with a structure of Mg/CuPc/Mg. These findings contrast with those expected from physisorptive noble metal-CuPc interfaces and suggest that strong local chemical bonding is a primary factor determining molecular level alignment at reactive metal-CuPc interfaces.

  1. Chemical Bonding, Interfaces and Defects in Hafnium Oxide/Germanium Oxynitride Gate Stacks on Ge (100)

    SciTech Connect

    Oshima, Yasuhiro; Sun, Yun; Kuzum, Duygu; Sugawara, Takuya; Saraswat, Krishna C.; Pianetta, Piero; McIntyre, Paul C.; /Stanford U., Materials Sci. Dept.

    2008-10-31

    Correlations among interface properties and chemical bonding characteristics in HfO{sub 2}/GeO{sub x}N{sub y}/Ge MIS stacks were investigated using in-situ remote nitridation of the Ge (100) surface prior to HfO{sub 2} atomic layer deposition (ALD). Ultra thin ({approx}1.1 nm), thermally stable and aqueous etch-resistant GeO{sub x}N{sub y} interfaces layers that exhibited Ge core level photoelectron spectra (PES) similar to stoichiometric Ge{sub 3}N{sub 4} were synthesized. To evaluate GeO{sub x}N{sub y}/Ge interface defects, the density of interface states (D{sub it}) was extracted by the conductance method across the band gap. Forming gas annealed (FGA) samples exhibited substantially lower D{sub it} ({approx} 1 x 10{sup 12} cm{sup -2} eV{sup -1}) than did high vacuum annealed (HVA) and inert gas anneal (IGA) samples ({approx} 1x 10{sup 13} cm{sup -2} eV{sup -1}). Germanium core level photoelectron spectra from similar FGA-treated samples detected out-diffusion of germanium oxide to the HfO{sub 2} film surface and apparent modification of chemical bonding at the GeO{sub x}N{sub y}/Ge interface, which is related to the reduced D{sub it}.

  2. Experimental evidence of chemical components in the bonding of helium and neon with neutral molecules.

    PubMed

    Cappelletti, David; Bartocci, Alessio; Grandinetti, Felice; Falcinelli, Stefano; Belpassi, Leonardo; Tarantelli, Francesco; Pirani, Fernando

    2015-04-13

    The complexes of helium and neon with gaseous neutral molecules are generally perceived to be van der Waals adducts held together by physical (non-covalent) forces, owing to the combination of size (exchange) repulsion with dispersion/induction attraction. Molecular beam experiments confirm that this is the case for He-CF4 , Ne-CF4 adducts, but revealed that the interaction of He and Ne with CCl4 features an appreciable contribution of chemical components that arise from the anisotropy of the electron density of CCl4 that enhances a charge transfer from Ng (Ng=He, Ne). These findings furnish a novel assay of the bonding capabilities of helium and neon, and invite to revisit the neutral complexes of these elements as systems of chemical relevance. The CCl4 -Ng are also peculiar examples of halogen bonds, a group of interactions of major current concern. Finally, this investigation is a prelude to the development of semi-empirical models for force fields aimed to the unified description of static and dynamical properties of systems of comparable or higher complexity. PMID:25755007

  3. Direct computation of general chemical energy differences: Application to ionization potentials, excitation, and bond energies

    SciTech Connect

    Beste, Ariana; Harrison, Robert J; Yanai, Takeshi

    2006-01-01

    Chemists are mainly interested in energy differences. In contrast, most quantum chemical methods yield the total energy which is a large number compared to the difference and has therefore to be computed to a higher relative precision than would be necessary for the difference alone. Hence, it is desirable to compute energy differences directly, thereby avoiding the precision problem. Whenever it is possible to find a parameter which transforms smoothly from an initial to a final state, the energy difference can be obtained by integrating the energy derivative with respect to that parameter (c.f., thermodynamic integration or adiabatic connection methods). If the dependence on the parameter is predominantly linear, accurate results can be obtained by single-point integration. In density functional theory (DFT) and Hartree-Fock, we applied the formalism to ionization potentials, excitation energies, and chemical bond breaking. Example calculations for ionization potentials and excitation energies showed that accurate results could be obtained with a linear estimate. For breaking bonds, we introduce a non-geometrical parameter which gradually turns the interaction between two fragments of a molecule on. The interaction changes the potentials used to determine the orbitals as well as constraining the orbitals to be orthogonal.

  4. Two functions of the density matrix and their relation to the chemical bond

    NASA Astrophysics Data System (ADS)

    Schmider, Hartmut L.; Becke, Axel D.

    2002-02-01

    We examine and compare two previously introduced functions of the one-particle density matrix that are suitable to represent its off-diagonal structure in a condensed form and that have illustrative connections to the nature of the chemical bond. One of them, the Localized-Orbital Locator (LOL) [J. Molec. Struct. (THEOCHEM) 527, 51 (2000)], is based only on the noninteracting kinetic-energy density τ and the charge density ρ at a point, and gives an intuitive measure of the relative speed of electrons in its vicinity. Alternatively, LOL focuses on regions that are dominated by single localized orbitals. The other one, the Parity Function P [J. Chem. Phys. 105, 11134 (1996)], is a section through the Wigner phase-space function at zero momentum, and contains information about the phase of the interference of atomiclike orbital contributions from bound centers. In this paper, we discuss the way in which these functions condense information in the density matrix, and illustrate on a variety of examples of unusual chemical bonds how they can help to understand the nature of "covalence."

  5. Effect of surface moisture on chemically bonded phosphor for thermographic phosphor thermometry

    NASA Astrophysics Data System (ADS)

    Cai, Tao; Kim, Dong; Kim, Mirae; Liu, Ying Zheng; Kim, Kyung Chun

    2016-09-01

    This study examined the effect of surface moisture on the calibration lifetime in chemically bonded phosphor paint preparation. Mg4FGeO6:Mn was used as a sensor material, which was excited by a pulsed UV LED. A high-speed camera with a frequency of 8000 Hz was used to conduct phosphor thermometry. Five samples with different degrees of surface moisture were selected during the preparation process, and each sample was calibrated 40 times at room temperature. A conventional post-processing method was used to acquire the phosphorescent lifetime for different samples with a 4  ×  4-pixel interrogation window. The measurement error and paint uniformity were also studied. The results showed that there was no obvious phosphorescence boundary between the wet parts and dry parts of phosphor paint. The lifetime increased by about 0.0345% per hour during the preparation process, showing the degree of surface moisture had almost no influence on the lifetime measurement. The lifetime changed only after annealing treatment. There was also no effect on the measurement error and uniformity. These results provide a reference for developing a real-time measurement method using thermographic phosphor thermometry. This study also provides a feasible basis for chemically bonded phosphor thermometry applications in humid and low-temperature environments.

  6. Stabilization of hazardous ash waste with newberyite-rich chemically bonded magnesium phosphate ceramic

    SciTech Connect

    Wagh, A.S.; Singh, D.; Jeong, S.Y.

    1995-11-01

    A novel newberyite-rich magnesium-phosphate ceramic, intended for the stabilization of the US Department of Energy`s low-level mixed-waste streams, has been developed by an acid-base reaction between magnesium oxide and a phosphoric acid solution. The reaction slurry, formed at room temperature, sets rapidly and forms a lightweight hard ceramic with low open porosity and a high compression strength of {approx} 6,200 psi. It is a composite of stable mineral phases of newberyite, luenebergite, and residual Mg oxide. Using this matrix, the authors developed superior waste forms for a surrogate ash waste stream. The final waste form is a low-permeability structural-quality ceramic, in which hazardous contaminants are chemically fixed and physically encapsulated. The compression strength of the waste form is an order of magnitude higher than the land disposal requirement, even at high waste loading. The high compression strength is attributed to stronger bonds in the waste form that result from participation of ash waste in the setting reactions. Long-term leaching studies show that the waste form is stable in an aqueous environment. The chemically bonded phosphate ceramic approach in this study may be a simple, inexpensive, and efficient method for fabricating high-performance waste forms either for stabilizing waste streams or for developing value-added construction materials from high-volume benign waste streams.

  7. Direct computation of general chemical energy differences: Application to ionization potentials, excitation, and bond energies

    NASA Astrophysics Data System (ADS)

    Beste, A.; Harrison, R. J.; Yanai, T.

    2006-08-01

    Chemists are mainly interested in energy differences. In contrast, most quantum chemical methods yield the total energy which is a large number compared to the difference and has therefore to be computed to a higher relative precision than would be necessary for the difference alone. Hence, it is desirable to compute energy differences directly, thereby avoiding the precision problem. Whenever it is possible to find a parameter which transforms smoothly from an initial to a final state, the energy difference can be obtained by integrating the energy derivative with respect to that parameter (cf. thermodynamic integration or adiabatic connection methods). If the dependence on the parameter is predominantly linear, accurate results can be obtained by single-point integration. In density functional theory and Hartree-Fock, we applied the formalism to ionization potentials, excitation energies, and chemical bond breaking. Example calculations for ionization potentials and excitation energies showed that accurate results could be obtained with a linear estimate. For breaking bonds, we introduce a nongeometrical parameter which gradually turns the interaction between two fragments of a molecule on. The interaction changes the potentials used to determine the orbitals as well as the constraint on the orbitals to be orthogonal.

  8. Electronic structure and anisotropic chemical bonding in TiNF from ab initio study

    SciTech Connect

    Matar, Samir F.

    2012-01-15

    Accounting for disorder in anatase titanium nitride fluoride TiNF is done through atoms re-distributions based on geometry optimizations using ultra soft pseudo potentials within density functional theory DFT. The fully geometry relaxed structures are found to keep the body centering of anatase (I4{sub 1}/amd No. 141). The new structural setups are identified with space groups I-4m2 No. 119 and Imm2 No. 44 which obey the 'group to subgroup' relationships with respect to anatase. In the ground state Imm2 structure identified from energy differences, TiNF is found semi-conducting with similar density of states features to anatase TiO{sub 2} and a chemical bonding differentiated between covalent like Ti-N versus ionic like Ti-F. Inter-anion N-F bonding is also identified. - Graphical Abstract: The geometry optimized ground state anatase derived TiNF structure with arrangement of open faceted TiN3F3 distorted octahedra. The insert shows the arrangement of octahedra in anatase TiO{sub 2}. Highlights: Black-Right-Pointing-Pointer Original approach of TiNF structure for addressing the electronic band structure. Black-Right-Pointing-Pointer Based on anatase, two different ordering scheme models with geometry optimization. Black-Right-Pointing-Pointer New structures obeying the group{yields}subgroup relationships with Imm2 ground state from energy. Black-Right-Pointing-Pointer In the ground state TiNF is found semi-conducting with similar density of states to anatase TiO{sub 2}. Black-Right-Pointing-Pointer Chemical bonding differentiated between covalent like Ti-N and ionic Ti-F.

  9. Undergraduate chemistry students' conceptions of atomic structure, molecular structure and chemical bonding

    NASA Astrophysics Data System (ADS)

    Campbell, Erin Roberts

    The process of chemical education should facilitate students' construction of meaningful conceptual structures about the concepts and processes of chemistry. It is evident, however, that students at all levels possess concepts that are inconsistent with currently accepted scientific views. The purpose of this study was to examine undergraduate chemistry students' conceptions of atomic structure, chemical bonding and molecular structure. A diagnostic instrument to evaluate students' conceptions of atomic and molecular structure was developed by the researcher. The instrument incorporated multiple-choice items and reasoned explanations based upon relevant literature and a categorical summarization of student responses (Treagust, 1988, 1995). A covalent bonding and molecular structure diagnostic instrument developed by Peterson and Treagust (1989) was also employed. The ex post facto portion of the study examined the conceptual understanding of undergraduate chemistry students using descriptive statistics to summarize the results obtained from the diagnostic instruments. In addition to the descriptive portion of the study, a total score for each student was calculated based on the combination of correct and incorrect choices made for each item. A comparison of scores obtained on the diagnostic instruments by the upper and lower classes of undergraduate students was made using a t-Test. This study also examined an axiomatic assumption that an understanding of atomic structure is important in understanding bonding and molecular structure. A Pearson Correlation Coefficient, ṟ, was calculated to provide a measure of the strength of this association. Additionally, this study gathered information regarding expectations of undergraduate chemistry students' understanding held by the chemical community. Two questionnaires were developed with items based upon the propositional knowledge statements used in the development of the diagnostic instruments. Subgroups of items from

  10. Application of chemical structure and bonding of actinide oxide materials for forensic science

    SciTech Connect

    Wilkerson, Marianne Perry

    2010-01-01

    We are interested in applying our understanding of actinide chemical structure and bonding to broaden the suite of analytical tools available for nuclear forensic analyses. Uranium- and plutonium-oxide systems form under a variety of conditions, and these chemical species exhibit some of the most complex behavior of metal oxide systems known. No less intriguing is the ability of AnO{sub 2} (An: U, Pu) to form non-stoichiometric species described as AnO{sub 2+x}. Environmental studies have shown the value of utilizing the chemical signatures of these actinide oxide materials to understand transport following release into the environment. Chemical speciation of actinide-oxide samples may also provide clues as to the age, source, or process history of the material. The scientific challenge is to identify, measure and understand those aspects of speciation of actinide analytes that carry information about material origin and history most relevant to forensics. Here, we will describe our efforts in material synthesis and analytical methods development that we will use to provide the fundamental science to characterize actinide oxide molecular structures for forensic science. Structural properties and initial results to measure structural variability of uranium oxide samples using synchrotron-based X-ray Absorption Fine Structure will be discussed.

  11. Effect of ultraviolet light irradiation and sandblasting treatment on bond strengths between polyamide and chemical-cured resin.

    PubMed

    Asakawa, Yuya; Takahashi, Hidekazu; Iwasaki, Naohiko; Kobayashi, Masahiro

    2014-01-01

    The aim of this study was to evaluate the effects of ultraviolet light (UV) irradiation and sandblasting treatment on the shear bond strength between polyamide and chemical-cured resin. Three types of commercial polyamides were treated using UV irradiation, sandblasting treatment, and a combining sandblasting and UV irradiation. The shear bond strength was measured and analyzed using the Kruskal-Wallis test (α=0.05). Comparing shear bond strengths without surface treatment, from 4.1 to 5.7 MPa, the UV irradiation significantly increased the shear bond strengths except for Valplast, whose shear bond strengths ranged from 5.2 to 9.3 MPa. The sandblasting treatment also significantly increased the shear bond strengths (8.0 to 11.4 MPa). The combining sandblasting and UV irradiation significantly increased the shear bond strengths (15.2 to 18.3 MPa) comparing without surface treatment. This combined treatment was considered the most effective at improving the shear bond strength between polyamide and chemical-cured resin. PMID:25087663

  12. Shake for Sigma, Pray for Pi: Classroom Orbital Overlap Analogies

    ERIC Educational Resources Information Center

    Dicks, Andrew P.

    2011-01-01

    An introductory organic classroom demonstration is discussed where analogies are made between common societal hand contact and covalent bond formation. A handshake signifies creation of a [sigma] bond ("head-on" orbital overlap), whereas the action of praying illustrates "sideways" overlap and generation of a [pi] bond. The nature of orbital and…

  13. {open_quotes}Neglect of diatomic differential overlap{close_quotes} in nonempirical quantum chemical orbital theories. V. A calculus of error concerning the justification of the neglect of diatomic differential overlap (NDDO) approximation

    SciTech Connect

    Neymeyr, K.

    1995-03-05

    Several types of approximations have been used for the justification of the Neglect of Diatomic Differential Overlap (NDDO) in Part IV but control of the introduced error remains insufficient. Analytic formulas describing the induced error for all types of approximations are given. Numerically lower bounds for these errors can be derived from the discussion on diatomic molecules. Far-reaching consequences on the applicability of NDDO will be discussed. 7 refs., 6 tabs.

  14. The variational subspace valence bond method

    SciTech Connect

    Fletcher, Graham D.

    2015-04-07

    The variational subspace valence bond (VSVB) method based on overlapping orbitals is introduced. VSVB provides variational support against collapse for the optimization of overlapping linear combinations of atomic orbitals (OLCAOs) using modified orbital expansions, without recourse to orthogonalization. OLCAO have the advantage of being naturally localized, chemically intuitive (to individually model bonds and lone pairs, for example), and transferrable between different molecular systems. Such features are exploited to avoid key computational bottlenecks. Since the OLCAO can be doubly occupied, VSVB can access very large problems, and calculations on systems with several hundred atoms are presented.

  15. Theoretical prediction of hydrogen-bond basicity pKBHX using quantum chemical topology descriptors.

    PubMed

    Green, Anthony J; Popelier, Paul L A

    2014-02-24

    Hydrogen bonding plays an important role in the interaction of biological molecules and their local environment. Hydrogen-bond strengths have been described in terms of basicities by several different scales. The pKBHX scale has been developed with the interests of medicinal chemists in mind. The scale uses equilibrium constants of acid···base complexes to describe basicity and is therefore linked to Gibbs free energy. Site specific data for polyfunctional bases are also available. The pKBHX scale applies to all hydrogen-bond donors (HBDs) where the HBD functional group is either OH, NH, or NH+. It has been found that pKBHX can be described in terms of a descriptor defined by quantum chemical topology, ΔE(H), which is the change in atomic energy of the hydrogen atom upon complexation. Essentially the computed energy of the HBD hydrogen atom correlates with a set of 41 HBAs for five common HBDs, water (r2=0.96), methanol (r2=0.95), 4-fluorophenol (r2=0.91), serine (r2=0.93), and methylamine (r2=0.97). The connection between experiment and computation was strengthened with the finding that there is no relationship between ΔE(H) and pKBHX when hydrogen fluoride was used as the HBD. Using the methanol model, pKBHX predictions were made for an external set of bases yielding r2=0.90. Furthermore, the basicities of polyfunctional bases correlate with ΔE(H), giving r2=0.93. This model is promising for the future of computation in fragment-based drug design. Not only has a model been established that links computation to experiment, but the model may also be extrapolated to predict external experimental pKBHX values. PMID:24460383

  16. Plutonium and transplutonium element trioxides: molecular structures, chemical bonding, and isomers.

    PubMed

    Zaitsevskii, Andréi

    2015-10-14

    Ground-state equilibrium geometries, energetics, and vibrational frequencies of AnO3 molecules, An = Pu through Cf, and their isomers are calculated using an accurate small-core pseudopotential model and the two-component relativistic density functional theory. The qualitative features of chemical bonding in these molecules are discussed in terms of oxidation states and bond orders. The actinide oxidation state (VI) is reached only in the plutonium trioxide molecule, whereas heavier actinide atoms in T-shaped trioxide molecules should be considered as pentavalent. At least at low temperatures, PuO3 and, to a lesser degree, AmO3 and BkO3 molecules should be stable both with respect to the isomerization into oxoperoxides or oxosuperoxides and the decay into dioxides and molecular oxygen. These trioxides can form dimers with significant (above 250 kJ mol(-1)) dissociation energies; the oxidation states of actinide atoms in the lowest-energy configurations of these dimers coincide with those in the corresponding monomers. The ability to reach high oxidation states in oxygen compounds gradually decreases from Pu onwards, with the only exception being the unexpectedly stable Bk(v)O3. PMID:26343514

  17. Chemical Bonding In Amorphous Si Coated-carbon Nanotube As Anodes For Li ion Batteries: A XANES Study

    SciTech Connect

    Zhou, Jigang; Hu, Yongfeng; Li, Xiaolin; Wang, Chong M.; Zuin, Lucia

    2014-03-11

    The chemical bonding nature and its evolution upon electrochemical cycling in amorphous Si coated-carbon nanotube (Si-CNT) anode has been investigated using comprehensive X-ray absorption spectroscopy (XANES) at Si L- and K-edges along with C and O K-edges. The Si nanolayer on CNT is found to be anchored to CNT via Si-O-C bonding. This bond weakens upon electrochemical cycling accompanied with generation of Li2CO3 on the surface of Si-CNT. Those findings are crucial in designing further improved Si-C composite anode for lithium ion battery.

  18. Carol Anne Bond v the United States of America: how a woman scorned threatened the Chemical Weapons Convention.

    PubMed

    Muldoon, Anna; Kornblet, Sarah; Katz, Rebecca

    2011-09-01

    The case of Carol Anne Bond v the United States of America stemmed from a domestic dispute when Ms. Bond attempted to retaliate against her best friend by attacking her with chemical agents. What has emerged is a much greater issue--a test of standing on whether a private citizen can challenge the Tenth Amendment. Instead of being prosecuted in state court for assault, Ms. Bond was charged and tried in district court under a federal criminal statute passed as part of implementation of the Chemical Weapons Convention (CWC). Ms. Bond's argument rests on the claim that the statute exceeded the federal government's enumerated powers in criminalizing her behavior and violated the Constitution, while the government contends legislation implementing treaty obligations is well within its purview. This question remains unanswered because there is dispute among the lower courts as to whether Ms. Bond, as a citizen, even has the right to challenge an amendment guaranteeing states rights when a state is not a party to the action. The Supreme Court heard the case on February 22, 2011, and, if it decides to grant Ms. Bond standing to challenge her conviction, the case will be returned to the lower courts. Should the court decide Ms. Bond has the standing to challenge her conviction and further questions the constitutionality of the law, it would be a significant blow to implementation of the CWC in the U.S. and the effort of the federal government to ensure we are meeting our international obligations. PMID:21819227

  19. Chemically bonded phosphate ceramics for low-level mixed waste stabilization

    SciTech Connect

    Singh, D.; Wagh, A.S.; Cunnane, J.C.; Mayberry, J.L.

    1994-12-31

    Novel chemically bonded phosphate ceramics (CBPCs) are being developed and fabricated for low-temperature stabilization and solidification of mixed waste streams which are amenable to conventional high-temperature stabilization processes due to presence of volatiles such as heavy metal chloride and fluorides and/or pyrophorics in the wastes. Phosphates of Mg, Mg-Na and Zr are being developed as candidate matrix materials. In this paper, we present the fabrication procedures of phosphate waste forms using surrogates compositions of three typical mixed wastes streams -- ash, cement sludges, and salts. The performance of the final waste forms such as compression strength, leachability of the contaminants, durability in aqueous environment were conducted. In addition, parameteric studies have been conducted to establish the optimal waste loading in a particular binder system. Based on the results, we present potential applications in the treatment of various mixed waste streams.

  20. Collision-free spatial hash functions for structural analysis of billion-vertex chemical bond networks

    NASA Astrophysics Data System (ADS)

    Zhang, Cheng; Bansal, Bhupesh; Branicio, Paulo S.; Kalia, Rajiv K.; Nakano, Aiichiro; Sharma, Ashish; Vashishta, Priya

    2006-09-01

    State-of-the-art molecular dynamics (MD) simulations generate massive datasets involving billion-vertex chemical bond networks, which makes data mining based on graph algorithms such as K-ring analysis a challenge. This paper proposes an algorithm to improve the efficiency of ring analysis of large graphs, exploiting properties of K-rings and spatial correlations of vertices in the graph. The algorithm uses dual-tree expansion (DTE) and spatial hash-function tagging (SHAFT) to optimize computation and memory access. Numerical tests show nearly perfect linear scaling of the algorithm. Also a parallel implementation of the DTE + SHAFT algorithm achieves high scalability. The algorithm has been successfully employed to analyze large MD simulations involving up to 500 million atoms.

  1. Novel Chemically-Bonded Phosphate Ceramic Borehole Sealants (Ceramicretes) for Arctic Environments

    SciTech Connect

    Shirish Patil; Godwin A. Chukwu; Gang Chen; Santanu Khataniar

    2008-12-31

    Novel chemically bonded phosphate ceramic borehole sealant, i.e. Ceramicrete, has many advantages over conventionally used permafrost cement at Alaska North Slope (ANS). However, in normal field practices when Ceramicrete is mixed with water in blenders, it has a chance of being contaminated with leftover Portland cement. In order to identify the effect of Portland cement contamination, recent tests have been conducted at BJ services in Tomball, TX as well as at the University of Alaska Fairbanks with Ceramicrete formulations proposed by the Argonne National Laboratory. The tests conducted at BJ Services with proposed Ceramicrete formulations and Portland cement contamination have shown significant drawbacks which has caused these formulations to be rejected. However, the newly developed Ceramicrete formulation at the University of Alaska Fairbanks has shown positive results with Portland cement contamination as well as without Portland cement contamination for its effective use in oil well cementing operations at ANS.

  2. Dynamics of chemical bonding mapped by energy-resolved 4D electron microscopy.

    PubMed

    Carbone, Fabrizio; Kwon, Oh-Hoon; Zewail, Ahmed H

    2009-07-10

    Chemical bonding dynamics are fundamental to the understanding of properties and behavior of materials and molecules. Here, we demonstrate the potential of time-resolved, femtosecond electron energy loss spectroscopy (EELS) for mapping electronic structural changes in the course of nuclear motions. For graphite, it is found that changes of milli-electron volts in the energy range of up to 50 electron volts reveal the compression and expansion of layers on the subpicometer scale (for surface and bulk atoms). These nonequilibrium structural features are correlated with the direction of change from sp2 [two-dimensional (2D) graphene] to sp3 (3D-diamond) electronic hybridization, and the results are compared with theoretical charge-density calculations. The reported femtosecond time resolution of four-dimensional (4D) electron microscopy represents an advance of 10 orders of magnitude over that of conventional EELS methods. PMID:19589997

  3. Method for Producing Chemically Bonded Phosphate Ceramics and for Stabilizing Contaminants Encapsulated therein Utilizing Reducing Agents

    SciTech Connect

    Singh, Dileep; Wagh, Arun S.; Jeong, Seung-Young

    1999-05-05

    Known phosphate ceramic formulations are improved and the ability to produce iron-based phosphate ceramic systems is enabled by the addition of an oxidizing or reducing step during the acid-base reactions that form the phosphate ceramic products. The additives allow control of the rate of the acid-base reactions and concomitant heat generation. In an alternate embodiment, waste containing metal anions is stabilized in phosphate ceramic products by the addition of a reducing agent to the phosphate ceramic mixture. The reduced metal ions are more stable and/or reactive with the phosphate ions, resulting in the formation of insoluble metal species within the phosphate ceramic matrix, such that the resulting chemically bonded phosphate ceramic product has greater leach resistance.

  4. Electronic structure, chemical bonding, and geometry of pure and Sr-doped CaCO3.

    PubMed

    Stashans, Arvids; Chamba, Gaston; Pinto, Henry

    2008-02-01

    The electronic structure, chemical bonding, geometry, and effects produced by Sr-doping in CaCO(3) have been studied on the basis of density-functional theory using the VASP simulation package and molecular-orbital theory utilizing the CLUSTERD computer code. Two calcium carbonate structures which occur naturally in anhydrous crystalline forms, calcite and aragonite, were considered in the present investigation. The obtained diagrams of density of states show similar patterns for both materials. The spatial structures are computed and analyzed in comparison to the available experimental data. The electronic properties and atomic displacements because of the trace element Sr-incorporation are discussed in a comparative manner for the two crystalline structures. PMID:17654648

  5. Method for producing chemically bonded phosphate ceramics and for stabilizing contaminants encapsulated therein utilizing reducing agents

    DOEpatents

    Singh, Dileep; Wagh, Arun S.; Jeong, Seung-Young

    2000-01-01

    Known phosphate ceramic formulations are improved and the ability to produce iron-based phosphate ceramic systems is enabled by the addition of an oxidizing or reducing step during the acid-base reactions that form the phosphate ceramic products. The additives allow control of the rate of the acid-base reactions and concomitant heat generation. In an alternate embodiment, waste containing metal anions are stabilized in phosphate ceramic products by the addition of a reducing agent to the phosphate ceramic mixture. The reduced metal ions are more stable and/or reactive with the phosphate ions, resulting in the formation of insoluble metal species within the phosphate ceramic matrix, such that the resulting chemically bonded phosphate ceramic product has greater leach resistance.

  6. Demonstration of packaging of Fernald Silo I waste in chemically bonded phosphate ceramic.

    SciTech Connect

    Wagh, A. S.

    1999-01-27

    This paper summarizes our experience in bench-scale packaging of Fernald Silo I waste in chemically bonded phosphate ceramics. The waste was received from the Fernald Environmental Management Project (FEMP), and its treatability was studied in our laboratory. This waste contained As{sup 5+}, Ba, Cr{sup 6+}, Ni, Pb, Se{sup 4+}, and Zn as the hazardous contaminants. In addition, the total specific activity of all the radioactive isotopes in the waste was 3.85 {micro}Ci/g, of which that of radium alone was 0.477 {micro}Ci/g. This indicated that radon (a daughter product of the radium) in the waste could present a serious handling problem during this study. For this reason, the waste was handled and stored in a flowing-air glovebox. We made waste form samples with an actual waste loading of 66.05 wt.% and subjected them to the Environmental Protection Agency (EPA) Toxicity Characteristic Leaching Procedure (TCLP). The results showed excellent stabilization of all contaminants. Actual levels detected in the leachate were well below the EPA's most stringent Universal Treatment Standards and in almost all cases were one order of magnitude below this limit. Radioactivity in the leachate was also very low. Alpha activity was 25 {+-} 2.5 pCi/mL, while beta activity was 9.81 {+-} 0.98 pCi/mL. This very low activity was attributed to the efficient stabilization of radium as insoluble radium phosphate in the waste form, thus prohibiting its leaching. This study indicates that the chemically bonded phosphate ceramic process may be a very suitable way to package Silo I waste for transportation and storage or disposal.

  7. Effect of cross-linking with riboflavin and ultraviolet A on the chemical bonds and ultrastructure of human sclera

    NASA Astrophysics Data System (ADS)

    Jung, Gyeong-Bok; Lee, Hui-Jae; Kim, Ji-Hye; Lim, Jin Ik; Choi, Samjin; Jin, Kyung-Hyun; Park, Hun-Kuk

    2011-12-01

    This study examined the effect of the cross-linking with riboflavin-ultraviolet A (UVA) irradiation on the chemical bonds and ultrastructural changes of human sclera tissues using Raman spectroscopy and atomic force microscopy (AFM). Raman spectroscopy of the normal and cross-linked human sclera tissue revealed different types of the riboflavin-UVA and collagen interactions, which could be identified from their unique peaks, intensity, and shape. Raman spectroscopy can prove to be a powerful tool for examining the chemical bond of collagenous tissues at the molecular level. After riboflavin-UVA treatment, unlike a regular parallel arrangement of normal collagen fibrils, the AFM image revealed interlocking arrangements of collagen fibrils. The observed changes in the surface topography of the collagen fibrils, as well as in their chemical bonds in the sclera tissue, support the formation of interfibrilar cross-links in sclera tissues.

  8. Chemical bonding in hard and elastic amorphous carbon-nitride films

    NASA Astrophysics Data System (ADS)

    Gammon, Wesley Jason

    In this study, the chemical bonding in hard and elastic amorphous carbon nitride (a-CNx) films is investigated with x-ray photoelectron spectroscopy (XPS) and 15N, 13C, and 1H nuclear magnetic resonance (NMR) spectroscopy. The films were deposited by DC Magnetron sputtering in a pure nitrogen discharge on Si(001) substrates at 300--400°C. Nanoindentation measurements reveal an elastic modulus of ˜50 GPa and a hardness of ˜5 GPa, thus confirming our films are highly elastic but resist plastic deformation. Our 13C NMR study demonstrates the absence of sp 3-bonded carbon in this material. Collectively, our N(1s) XPS, 13C NMR, and 15N NMR data suggest a film-bonding model that has an aromatic carbon structure with sp2-hybridized nitrogen incorporated in heterocyclic rings. We demonstrate that the nitrogen bonding is predominantly in configurations similar to those in pyridine and pyrrole. In addition, the data indicate that the a-CNx films prepared for this study have low hydrogen content, but are hydrophilic. Specifically, results from 15N and 13C cross polarization (CP) and 1H magic angle spinning (MAS) NMR experiments suggest that nitrogen sites are susceptible to protonation from water absorbed during sample preparation for the NMR experiments. The sensitivity of the surface of a-CNx to water absorption may impact tribological applications for this material. In accord with our XPS and NMR spectroscopic studies on a-CN x films, we propose a film-structure model consisting of buckled graphitic planes that are cross-linked together by sp2 hybridized carbons. The curvature and cross-linking is attributed to a type of compound defect, which is formed by placing a pentagon next to single-atom vacancy in a graphite layer. Our proposed film structure is called the pentagon-with-vacancy-defect (5VD) model. Using Hartree-Fock calculations, we show that the 5VD, film-structure model is compatible with our XPS, NMR, and nanoindentation measurements and with previous

  9. Stabilization of lead-rich low-level mixed wastes in chemically bonded phosphate ceramic.

    SciTech Connect

    Jeong, S.-Y.

    1999-08-10

    A chemically bonded magnesium potassium phosphate ceramic has been developed by an acid-base reaction at room temperature, for use in stabilizing U.S. Department of Energy low-level mixed waste streams that include hazardous metals and low-level radioactive elements. Using this ceramic, we solidified, in monolithic waste forms, low-level mixed waste streams containing various levels of PbCl{sub 2} and PbCO{sub 3}. These final waste forms were evaluated for their land disposal suitability. The results showed low open porosity (1.48-4.61 vol.%); hence, low permeability, and higher compression strengths (4310-6734 psi) that were one order of magnitude above that required. The level of lead in the leachate following the Toxicity Characteristic Leaching Procedure test was reduced from 50,000 to <0.1 ppm. Leachability indexes from the long-term leaching test (ANS 16.1 test) were between 11.9 and 13.6. This excellent lead retention is due to its chemical fixation as insoluble lead phosphate and to physical encapsulation by the phosphate matrix.

  10. CHEMICAL ACTIVATION OF MOLECULES BY METALS: EXPERIMENTAL STUDIES OF ELECTRON DISTRIBUTIONS AND BONDING

    SciTech Connect

    LICHTENBERGER, DENNIS L.

    2002-03-26

    This research program is directed at obtaining detailed experimental information on the electronic interactions between metals and organic molecules. These interactions provide low energy pathways for many important chemical and catalytic processes. A major feature of the program is the continued development and application of our special high-resolution valence photoelectron spectroscopy (UPS), and high-precision X-ray core photoelectron spectroscopy (XPS) instrumentation for study of organometallic molecules in the gas phase. The study involves a systematic approach towards understanding the interactions and activation of bound carbonyls, C-H bonds, methylenes, vinylidenes, acetylides, alkenes, alkynes, carbenes, carbynes, alkylidenes, alkylidynes, and others with various monometal, dimetal, and cluster metal species. Supporting ligands include -aryls, alkoxides, oxides, and phosphines. We are expanding our studies of both early and late transition metal species and electron-rich and electron-poor environments in order to more completely understand the electronic factors that serve to stabilize particular organic fragments and intermediates on metals. Additional new directions for this program are being taken in ultra-high vacuum surface UPS, XPS, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) experiments on both physisorbed and chemisorbed organometallic thin films. The combination of these methods provides additional electronic structure information on surface-molecule and molecule-molecule interactions. A very important general result emerging from this program is the identification of a close relationship between the ionization energies of the species and the thermodynamics of the chemical and catalytic reactions of these systems.

  11. Overlap in Bibliographic Databases.

    ERIC Educational Resources Information Center

    Hood, William W.; Wilson, Concepcion S.

    2003-01-01

    Examines the topic of Fuzzy Set Theory to determine the overlap of coverage in bibliographic databases. Highlights include examples of comparisons of database coverage; frequency distribution of the degree of overlap; records with maximum overlap; records unique to one database; intra-database duplicates; and overlap in the top ten databases.…

  12. Chemical composition, crystal structure, and their relationships with the intrinsic properties of spinel-type crystals based on bond valences.

    PubMed

    Liu, Xiao; Wang, Hao; Lavina, Barbara; Tu, Bingtian; Wang, Weimin; Fu, Zhengyi

    2014-06-16

    Spinel-type crystals may possess complex and versatile chemical composition and crystal structure, which leads to difficulty in constructing relationships among the chemical composition, crystal structure, and intrinsic properties. In this work, we develop new empirical methods based on bond valences to estimate the intrinsic properties, namely, compressibility and thermal expansion of complex spinel-type crystals. The composition-weighted average of bond force constants in tetrahedral and octahedral coordination polyhedra is derived as a function of the composition-weighted average of bond valences, which can be calculated according to the experimental chemical composition and crystal structural parameters. We discuss the coupled effects of tetrahedral and octahedral frameworks on the aforementioned intrinsic properties. The bulk modulus could be quantitatively calculated from the composition-weighted average of bond force constants in tetrahedral and octahedral coordination polyhedra. In contrast, a quantitative estimation of the thermal expansion coefficient could be obtained from the composition-weighted average of bond force constants in octahedral coordination polyhedra. These empirical methods have been validated by the results obtained for a new complex quaternary spinel-type oxynitride Mg0.268Al2.577O3.733N0.267 as well as MgAl2O4 and Al2.85O3.45N0.55 from the literature. Further, these empirical methods have the potential to be extensively applied in other types of complex crystals. PMID:24871452

  13. Understanding Boron through Size-Selected Clusters: Structure, Chemical Bonding, and Fluxionality

    SciTech Connect

    Sergeeva, Alina P.; Popov, Ivan A.; Piazza, Zachary A.; Li, Wei-Li; Romanescu, Constantin; Wang, Lai S.; Boldyrev, Alexander I.

    2014-04-15

    Conspectus Boron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar structures, stabilized by localized two-center–two-electron (2c–2e) σ bonds on the periphery and delocalized multicenter–two-electron (nc–2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron’s electron deficiency and leads to fluxional behavior, which has been observed in B13+ and B19–. A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structures of boron clusters is evident in the striking similarities of the π-systems of planar boron clusters to those of polycyclic aromatic hydrocarbons, such as benzene, naphthalene, coronene, anthracene, or phenanthrene. Chemical bonding models developed for boron clusters not only allowed the rationalization of the stability of boron clusters but also lead to the design of novel metal-centered boron wheels with a record-setting planar coordination number of 10. The unprecedented highly coordinated borometallic molecular wheels provide insights into the interactions between transition metals and boron and expand the frontier of boron chemistry. Another interesting feature discovered through cluster studies is boron transmutation. Even though it is well-known that B–, formed by adding one electron to boron, is isoelectronic to carbon, cluster studies have considerably expanded the possibilities of new structures and new materials using the B

  14. Understanding boron through size-selected clusters: structure, chemical bonding, and fluxionality.

    PubMed

    Sergeeva, Alina P; Popov, Ivan A; Piazza, Zachary A; Li, Wei-Li; Romanescu, Constantin; Wang, Lai-Sheng; Boldyrev, Alexander I

    2014-04-15

    Boron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar structures, stabilized by localized two-center-two-electron (2c-2e) σ bonds on the periphery and delocalized multicenter-two-electron (nc-2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron's electron deficiency and leads to fluxional behavior, which has been observed in B13(+) and B19(-). A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structures of boron clusters is evident in the striking similarities of the π-systems of planar boron clusters to those of polycyclic aromatic hydrocarbons, such as benzene, naphthalene, coronene, anthracene, or phenanthrene. Chemical bonding models developed for boron clusters not only allowed the rationalization of the stability of boron clusters but also lead to the design of novel metal-centered boron wheels with a record-setting planar coordination number of 10. The unprecedented highly coordinated borometallic molecular wheels provide insights into the interactions between transition metals and boron and expand the frontier of boron chemistry. Another interesting feature discovered through cluster studies is boron transmutation. Even though it is well-known that B(-), formed by adding one electron to boron, is isoelectronic to carbon, cluster studies have considerably expanded the possibilities of new structures and new materials using the B(-)/C analogy. It is

  15. Chemical adhesion rather than mechanical retention enhances resin bond durability of a dental glass-ceramic with leucite crystallites.

    PubMed

    Meng, X F; Yoshida, K; Gu, N

    2010-08-01

    This study aims to evaluate the effect of chemical adhesion by a silane coupler and mechanical retention by hydrofluoric acid (HFA) etching on the bond durability of resin to a dental glass ceramic with leucite crystallites. Half of the ceramic plates were etched with 4.8% HFA (HFA group) for 60 s, and the other half were not treated (NoHFA group). The scale of their surface roughness and rough area was measured by a 3D laser scanning microscope. These plates then received one of the following two bond procedures to form four bond test groups: HFA/cement, NoHFA/cement, HFA/silane/cement and NoHFA/silane/cement. The associated micro-shear bond strength and bond failure modes were tested after 0 and 30 000 thermal water bath cycles. Four different silane/cement systems (Monobond S/Variolink II, GC Ceramic Primer/Linkmax HV, Clearfil Ceramic Primer/Clearfil Esthetic Cement and Porcelain Liner M/SuperBond C&B) were used. The data for each silane/cement system were analyzed by three-way ANOVA. HFA treatment significantly increased the surface R(a) and R(y) values and the rough area of the ceramic plates compared with NoHFA treatment. After 30 000 thermal water bath cycles, the bond strength of all the test groups except the HFA/Linkmax HV group was significantly reduced, while the HFA/Linkmax HV group showed only adhesive interface failure. The other HFA/cement groups and all NoHFA/cement groups lost bond strength completely, and all NoHFA/silane/cement groups with chemical adhesion had significantly higher bond strength and more ceramic cohesive failures than the respective HFA/cement groups with mechanical retention. The result of the HFA/silane/cement groups with both chemical adhesion and mechanical retention revealed that HFA treatment could enhance the bond durability of resin/silanized glass ceramics, which might result from the increase of the chemical adhesion area on the ceramic rough surface and subsequently reduced degradation speed of the silane coupler

  16. Study of the structure and chemical bonding of crystalline Ge4Sb2Te7 using first principle calculations

    NASA Astrophysics Data System (ADS)

    Singh, Janpreet; Singh, Satvinder; Singh, Gurinder; Kaura, Aman; Tripathi, S. K.

    2016-05-01

    The atomic arrangements and chemical bonding of stable Ge4Sb2Te7 (GeTe rich), a phase-change material, have been investigated by means of ab initio total energy calculations. To study the atomic arrangement, GeTe block is considered into -TeSbTeSbTe- block and -Te-Te- layer in the stacking I and II respectively. The stacking I is energetically more stable than the stacking II. The reason for more stability of the stacking I has been explained. The chemical bonding has been studied with the electronic charge density distribution around the atomic bonds. The quantity of electronic charge loosed or gained by atoms has been calculated using the Bader charge analysis. The metallic character has been studied using band structures calculations. The band gap for the stacking I and II is 0.463 and 0.219 eV respectively.

  17. Enhancing Students' Understanding of the Concept of "Chemical Bonding" by Using Activities Provided on an Interactive Website

    ERIC Educational Resources Information Center

    Frailich, Marcel; Kesner, Miri; Hofstein, Avi

    2009-01-01

    This study investigated the effectiveness of a web-based learning environment in enhancing 10th grade high-school students' understanding of the concept of "chemical bonding". Two groups participated in this study: an experimental group (N = 161) and a comparison one (N = 93). The teachers in the experimental group were asked to implement four…

  18. Learning about Atoms, Molecules, and Chemical Bonds: A Case Study of Multiple-Model Use in Grade 11 Chemistry.

    ERIC Educational Resources Information Center

    Harrison, Allan G.; Treagust, David F.

    2000-01-01

    Reports in detail on a year-long case study of multiple-model use at grade 11. Suggests that students who socially negotiated the shared and unshared attributes of common analogical models for atoms, molecules, and chemical bonds used these models more consistently in their explanations. (Author/CCM)

  19. A Review of Chemical Bonding Studies: Needs, Aims, Methods of Exploring Students' Conceptions, General Knowledge Claims and Students' Alternative Conceptions

    ERIC Educational Resources Information Center

    Unal, Suat; Calik, Muammer; Ayas, Alipasa; Coll, Richard K.

    2006-01-01

    The present paper presents a detailed thematic review of chemical bonding studies. To achieve this, a matrix is developed to summarize and present the findings by focusing on insights derived from the related studies. The matrix incorporates the following themes: needs, aims, methods of exploring students' conceptions, general knowledge claims,…

  20. Effect of RF power and annealing on chemical bonding and morphology of a-CNx thin films as humidity sensor

    NASA Astrophysics Data System (ADS)

    Aziz, N. F. H.; Ritikos, R.; Kamal, S. A. A.; Hussain, N. S. Mohamed; Awang, R.

    2013-11-01

    Amorphous carbon nitride (a-CNx) thin films were deposited using radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) technique. A set of a-CNx thin films were prepared using pure methane (CH4) gas diluted with nitrogen (N2) gas. The rf power was varied at 50, 60, 70, 80, 90 and 100 W. These films were then annealed at 400 °C in a quartz tube furnace in argon (Ar) gas. The effects of rf power and thermal annealing on the chemical bonding and morphology of these samples were studied. Surface profilometer was used to measure film thickness. Fourier transform infra-red spectroscopy (FTIR) and Field emission scanning electron microscopy (FESEM) measurements were used to determine their chemical bonding and morphology respectively. The deposition rate of the films increased constantly with increasing rf power up to 80W, before decreasing with further increase in rf power. Fourier transform infra-red spectroscopy (FTIR) studies showed a systematic change in the spectra and revealed three main peaks included C-N, C=N, C=C and C≡N triple bond. C=N and C≡N bonds decreased with increased C-N bonds after thermal annealing process. The FESEM images showed that the structure is porous for as-deposited and covered by granule-like grain structure after thermal annealing process was done. The resistance of the a-CNx thin film changed from 23.765 kΩ to 5.845 kΩ in the relative humidity range of 5 to 92 % and the film shows a good response and repeatability as a humidity sensing materials. This work showed that rf power and thermal annealing has significant effects on the chemical bonding and surface morphology of the a-CNx films and but yield films which are potential candidate as humidity sensor device.

  1. Nanoscale metals and semiconductors for the storage of solar energy in chemical bonds

    NASA Astrophysics Data System (ADS)

    Manthiram, Karthish

    The transduction of electrical energy into chemical bonds represents one potential strategy for storing energy derived from intermittent sources such as solar and wind. Driving the electrochemical reduction of carbon dioxide using light requires (1) developing light absorbers which convert photons into electron-hole pairs and (2) catalysts which utilize these electrons and holes to reduce carbon dioxide and oxidize water, respectively. For both the light absorbers and catalysts, the use of nanoscale particles is advantageous, as charge transport length scales are minimized in the case of nanoscale light absorbers and catalytic surface-area-to-volume ratio is maximized for nanoscale catalysts. In many cases, although semiconductors and metals in the form of thin films and foils are increasingly well-characterized as photoabsorbers and electrocatalysts for carbon dioxide reduction, respectively, the properties of their nanoscale counterparts remain poorly understood. This dissertation explores the nature of the light absorption mode of non-stoichiometric semiconductors which are utilized as light absorbers and the development of catalysts with enhanced stability, activity, and selectivity for carbon dioxide reduction. Chapter 1 provides an overview of the state of development of methods of transducing the energy of photons into chemical bonds. Chapters 2 and 3 investigate the development of stable, active, and selective catalysts for the electrochemical reduction of carbon dioxide. Chapter 2 examines how copper nanoparticles have enhanced activities and selectivities for methanation compared to copper foils. Chapter 3 focuses on the development of strategies to stabilize high-surface-area catalysts to prevent surface area loss during electrochemical carbon dioxide reduction. Chapters 4 and 5 entail a fundamental understanding of the light absorption mode of nanoscale photoabsorbers used in both photoelectrochemical cells and in photovoltaics. Chapter 4 focuses on the

  2. Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films

    SciTech Connect

    Ramana, C. V. Rubio, E. J.; Barraza, C. D.; Miranda Gallardo, A.; McPeak, Samantha; Kotru, Sushma; Grant, J. T.

    2014-01-28

    Gallium oxide (Ga{sub 2}O{sub 3}) thin films were made by sputter deposition employing a Ga{sub 2}O{sub 3} ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (T{sub s}), from 25 to 600 °C. The effect of T{sub s} on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p{sub 3/2} and Ga 2p{sub 1/2} peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga{sup 3+}) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ∼ 531 eV, which is also characteristic of Ga-O bonds in the Ga{sub 2}O{sub 3} phase. The granular morphology of the nanocrystalline Ga{sub 2}O{sub 3} films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing T{sub s}. The SE analyses indicate that the index of refraction (n) of Ga{sub 2}O{sub 3} films increases with increasing T{sub s} due to improved structural quality and packing density of the films. The n(λ) of all the Ga{sub 2}O{sub 3} films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (∼200 Ω-cm) for amorphous Ga{sub 2}O{sub 3} films grown at T{sub s} = RT-300 °C and decreased to ∼1 Ω-cm for nanocrystalline Ga{sub 2}O{sub 3} films grown at T{sub s} ≥ 500–600 °C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga{sub 2}O{sub 3} films is derived.

  3. Chemical reactivity of C-F bonds attached to graphene with diamines depending on their nature and location.

    PubMed

    Li, Baoyin; He, Taijun; Wang, Zaoming; Cheng, Zheng; Liu, Yang; Chen, Teng; Lai, Wenchuan; Wang, Xu; Liu, Xiangyang

    2016-06-29

    The attachment of fluorine to graphene is a facile means to activate the carbon bonds for subsequent covalent bonding to other molecules for the preparation of desired graphene derivatives. Therefore, an insight into the chemical reactivity of fluorinated graphene (FG) is very essential to enable precise control of the composition and structure of the final products. In this study, FG has been treated with various mass amounts of poly(oxypropylene)diamine (PEA) ranging from starvation to saturation to explore the dependence of a substitution reaction of diamines on the nature and location (attached onto the basal planes or along defects or edges) of C-F bonds. X-ray photoelectron spectroscopy directly tracked the atomic percentage of fluorine present and the carbon 1s bonding state, showing that the grafting ratio of diamines gradually increases with increased diamine mass ratio. The varying of the types and orientation of C-F bonds characterized by polarized attenuated total reflectance Fourier transform infrared spectroscopy indicates that "covalent" C-F bonds are more sensitive to the substitution reaction of diamines than ''semi-ionic'' C-F bonds, and the C-F bonds attached onto basal planes more preferably participate in the functionalization reaction of diamines than that of C-F bonded on non-coplanar regions (edges or defects). The one-dimensional expansion along the graphene c-axis shown by wide angle X-ray diffraction provides further evidence on the preferred functionalization reaction of C-F attached on the basal planes, resulting in a change of the average intersheet distance by various magnitudes. PMID:27302862

  4. Development of a Fundamental Understanding of Chemical Bonding and Electronic Structure in Spinel Compounds

    SciTech Connect

    Sickafus, K.E.; Wills, J.M.; Chen, S.-P.; Terry, J.H., Jr.; Hartmann, T.; Sheldon, R.I.

    1999-05-14

    This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos national Laboratory (LANL). Hundreds of ceramic compounds possess the spinel crystal structure and exhibit a remarkable variety of properties, ranging from compounds that are electrical insulators to compounds that are superconducting, or from compounds with ferri- and antiferromagnetic behavior to materials with colossal magnetoresistive characteristics. The unique crystal structure of spinel compounds is in many ways responsible for the widely varying physical properties of spinels. The objective of this project is to investigate the nature of chemical bonding, point defects, and electronic structure in compounds with the spinel crystal structure. Our goal is to understand and predict the stability of the spinel structure as a function of chemical composition, stoichiometry, and cation disorder. The consequences of cation disorder in spinel materials can be profound . The ferromagnetic characteristics of magnesioferrite, for instance, are entirely attributable to disorder on the cation sublattices. Our studies provide insight into the mechanisms of point defect formation and cation disorder and their effects on the electronic band structure and crystal structure of spinel-structure materials. our ultimate objective is to develop a more substantive knowledge of the spinel crystal structure and to promote new and novel uses for spinel compounds. The technical approach to achieve our goals is to combine first-principles calculations with experimental measurements. The structural and electronic properties of spinel samples were experimentally determined primarily with X-ray and neutron scattering, optical and X-ray absorption, and electron energy-loss spectroscopy. Total energy electronic structure calculations were performed to determine structural stability, band structure, density of states, and electron distribution. We also used shell

  5. Development of a Fundamental Understanding of Chemical Bonding and Electronic Structure in Spinel Compounds

    SciTech Connect

    Sickafus, K.E.; Wills, J.M.; Chen, S.-P.; Terry, J.H., Jr.; Hartmann, T.; Sheldon, R.I.

    1999-06-03

    This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Hundreds of ceramic compounds possess the spinel crystal structure and exhibit a remarkable variety of properties, ranging from compounds that are electrical insulators to compounds that are superconducting, or from compounds with ferri- and antiferromagnetic behavior to materials with colossal magnetoresistive characteristics. The unique crystal structure of spinel compounds is in many ways responsible for the widely varying physical properties of spinels. The objective of this project is to investigate the nature of chemical bonding, point defects, and electronic structure in compounds with the spinel crystal structure. Our goal is to understand and predict the stability of the spinel structure as a function of chemical composition, stoichiometry, and cation disorder. The consequences of cation disorder in spinel materials can be profound . The ferromagnetic characteristics of magnesioferrite, for instance, are entirely attributable to disorder on the cation sublattices. Our studies provide insight into the mechanisms of point defect formation and cation disorder and their effects on the electronic band structure and crystal structure of spinel-structure materials. Our ultimate objective is to develop a more substantive knowledge of the spinel crystal structure and to promote new and novel uses for spinel compounds. The technical approach to achieve our goals is to combine first-principles calculations with experimental measurements. The structural and electronic properties of spinel samples were experimentally determined primarily with X-ray and neutron scattering, optical and X-ray absorption, and electron energy-loss spectroscopy. Total energy electronic structure calculations were performed to determine structural stability, band structure, density of states, and electron distribution. We also used shell

  6. Stabilization of Rocky Flats Pu-contaminated ash within chemically bonded phosphate ceramics

    NASA Astrophysics Data System (ADS)

    Wagh, A. S.; Strain, R.; Jeong, S. Y.; Reed, D.; Krause, T.; Singh, D.

    A feasibility study was conducted on the use of chemically bonded phosphate ceramics for stabilization of combustion residue of high transuranic (TRU) wastes. Using a matrix of magnesium potassium phosphate formed by the room-temperature reaction of MgO and KH 2PO 4 solution, we made waste forms that contained 5 wt% Pu to satisfy the requirements of the Waste Isolation Pilot Plant. The waste forms were ceramics whose compression strength was twice that of conventional cement grout and whose connected porosity was ≈50% that of cement grout. Both surrogate and actual waste forms displayed high leaching resistance for both hazardous metals and Pu. Hydrogen generation resulting from the radiolytic decomposition of water and organic compounds present in the waste form did not appear to be a significant issue. Pu was present as PuO 2 that was physically microencapsulated in the matrix. In the process, pyrophoricity was removed and leaching resistance was enhanced. The high leaching resistance was due to the very low solubility of PuO 2 coupled with superior microencapsulation. As a result, the waste forms satisfied the current Safeguard Termination Limit requirement for storage of TRU combustion residues.

  7. Principles of Chemical Bonding and Band Gap Engineering in Hybrid Organic–Inorganic Halide Perovskites

    PubMed Central

    2015-01-01

    The performance of solar cells based on hybrid halide perovskites has seen an unparalleled rate of progress, while our understanding of the underlying physical chemistry of these materials trails behind. Superficially, CH3NH3PbI3 is similar to other thin-film photovoltaic materials: a semiconductor with an optical band gap in the optimal region of the electromagnetic spectrum. Microscopically, the material is more unconventional. Progress in our understanding of the local and long-range chemical bonding of hybrid perovskites is discussed here, drawing from a series of computational studies involving electronic structure, molecular dynamics, and Monte Carlo simulation techniques. The orientational freedom of the dipolar methylammonium ion gives rise to temperature-dependent dielectric screening and the possibility for the formation of polar (ferroelectric) domains. The ability to independently substitute on the A, B, and X lattice sites provides the means to tune the optoelectronic properties. Finally, ten critical challenges and opportunities for physical chemists are highlighted. PMID:25838846

  8. Central C-C Bonding Increases Optical and Chemical Stability of NIR Fluorophores

    PubMed Central

    Hyun, Hoon; Owens, Eric A.; Narayana, Lakshminarayana; Wada, Hideyuki; Gravier, Julien; Bao, Kai; Frangioni, John V.; Choi, Hak Soo; Henary, Maged

    2014-01-01

    Functional near-infrared (NIR) fluorophores have played a major role in the recent advances in bioimaging. However, the optical and physicochemical stabilities of NIR fluorophores in the biological and physiological environment are still a challenge. Especially, the ether linkage on the meso carbon of heptamethine core is fragile when exposed to serum proteins or other amine-rich biomolecules. To solve such a structural limitation, a rigid carbon-carbon bond was installed onto the framework of ether-linked NIR fluorophores through the Suzuki coupling. The robust fluorophores replaced as ZW800-1C and ZW800-3C displayed enhanced optical and chemical stability in various solvents and a 100% warm serum environment (> 99%, 24 h). The biodistribution and clearance of C-C coupled ZW800 compounds were almost identical to the previously developed oxygen-substituted ZW800 compounds. When conjugated with a small molecule ligand, ZW800-1C maintained the identical stable form in warm serum (>98%, 24 h), while ZW800-1A hydrolyzed quickly after 4 h incubation (34%, 24 h). PMID:25530846

  9. Metalorganic chemical vapor deposition of few-layer sp2 bonded boron nitride films

    NASA Astrophysics Data System (ADS)

    Paduano, Qing; Snure, Michael; Weyburne, David; Kiefer, Arnold; Siegel, Gene; Hu, Jianjun

    2016-09-01

    A systematic study of the growth of atomically smooth few-layer sp2 bonded BN on 50 mm sapphire substrates by metalorganic chemical vapor deposition (MOCVD) using Triethylboron (TEB) and NH3 as precursors is described. Based on the experimental results obtained using Raman spectroscopy, atomic force microscopy (AFM), X-ray reflectance measurements and transmission electron microscopy, we explored the growth parameter space and identified three different growth modes: random three-dimensional (3D) growth, a self-terminating few-layer growth mode, and a very slow layer-by-layer mode. The growth mode depends on the temperature, pressure, V/III ratio, and surface nitridation conditions, as follows: 3D island growth is dominant in the low V/III range and is characterized by a decreasing growth rate with increasing deposition temperature. When the V/III ratio is increased this 3D island growth mode transitions to a self-terminating few-layer growth mode. An additional transition from self-terminating growth to 3D growth occurs when the growth pressure is increased. Very slow layer by layer growth is found at high temperature and low pressure. Finally, substrate surface nitridation promotes self-terminating growth that results in atomically smooth films.

  10. Chemical bonding analysis for solid-state systems using intrinsic oriented quasiatomic minimal-basis-set orbitals

    SciTech Connect

    Yao, Y. X.; Wang, C. Z.; Ho, K. M.

    2010-06-16

    A chemical bonding scheme is presented for the analysis of solid-state systems. The scheme is based on the intrinsic oriented quasiatomic minimal-basis-set orbitals (IO-QUAMBOs) previously developed by Ivanic and Ruedenberg for molecular systems. In the solid-state scheme, IO-QUAMBOs are generated by a unitary transformation of the quasiatomic orbitals located at each site of the system with the criteria of maximizing the sum of the fourth power of interatomic orbital bond order. Possible bonding and antibonding characters are indicated by the single particle matrix elements, and can be further examined by the projected density of states. We demonstrate the method by applications to graphene and (6,0) zigzag carbon nanotube. The oriented-orbital scheme automatically describes the system in terms of sp{sup 2} hybridization. The effect of curvature on the electronic structure of the zigzag carbon nanotube is also manifested in the deformation of the intrinsic oriented orbitals as well as a breaking of symmetry leading to nonzero single particle density matrix elements. In an additional study, the analysis is performed on the Al{sub 3}V compound. The main covalent bonding characters are identified in a straightforward way without resorting to the symmetry analysis. Our method provides a general way for chemical bonding analysis of ab initio electronic structure calculations with any type of basis sets.

  11. Cutting a chemical bond with demon's scissors: Mode- and bond-selective reactivity of methane on metal surfaces

    NASA Astrophysics Data System (ADS)

    Lozano, A.; Shen, X. J.; Moiraghi, R.; Dong, W.; Busnengo, H. F.

    2015-10-01

    In this paper we discuss several aspects of methane reactive sticking on Pt(111) in the light of supersonic molecular beam experiments (including state-resolved measurements) and quasi-classical trajectory calculations based on an accurate reaction specific reactive force field constructed from Density Functional Theory (DFT) data. With the aim of understanding the origin of the full bond selectivity recently achieved experimentally and to predict how selectivity depends on the collision conditions, we discuss in detail, the role of initial translational and (mode-specific) vibrational energy of CH4 and all its deuterated isotopomers, as well as surface temperature effects. Last but not least, the systematic and detailed theoretical analysis presented here serves as an illustration of the possibilities and usefulness of accurate reaction specific reactive force fields built from DFT data. This approach allows investigating dynamical aspects of the interaction of polyatomic molecules on surfaces through quasi-classical trajectory calculations accounting for the full dimensionality of the system (including both molecular and surface degrees of freedom): something that a few years ago was just a dream for the gas-surface dynamics community.

  12. Coordination compounds of tetravalent silicon, germanium and tin: the structure, chemical bonding and intermolecular interactions in them

    NASA Astrophysics Data System (ADS)

    Korlyukov, A. A.

    2015-04-01

    The review is devoted to analysis and generalization of the results of (i) quantum chemical studies on the structure, chemical bonding and intermolecular interactions in coordination compounds of tetravalent silicon, germanium and tin in crystals, in solutions and in the gas phase and (ii) experimental investigations of the electron density distribution in these systems. The bibliography includes 147 references. In memoriam of Corresponding Member of the Russian Academy of Sciences M Yu Antipin (1951 - 2013), Academician of the Russian Academy of Sciences M G Voronkov (1921 - 2014) and Dr. S P Knyazev, Lomonosov Moscow University of Fine Chemical Technology (1949 - 2012).

  13. From Common Sense Concepts to Scientifically Conditioned Concepts of Chemical Bonding: An Historical and Textbook Approach Designed to Address Learning and Teaching Issues at the Secondary School Level

    ERIC Educational Resources Information Center

    Croft, Michael; de Berg, Kevin

    2014-01-01

    This paper selects six key alternative conceptions identified in the literature on student understandings of chemical bonding and illustrates how a historical analysis and a textbook analysis can inform these conceptions and lead to recommendations for improving the teaching and learning of chemical bonding at the secondary school level. The…

  14. Effect of coverage density and structure of chemically bonded silica stationary phases on the separation of compounds with various properties.

    PubMed

    Buszewski, Bogusław; Krupczyńska, Katarzyna; Rychlicki, Gerhard; Lobiński, Ryszard

    2006-04-01

    The chemical character, geometry, and architecture of chemically formed surface layers determine interactions between stationary phase, analyte, and mobile phase, and therefore the retention mechanisms (partitioning, adsorption, ion exchange, steric exclusion) of separated analytes. These interactions also depend on the structure and chemical character of the solutes and the composition of the mobile phase. High-molecular-weight fullerenes (C60 and C70) and water-soluble selenium-containing peptides (833 and 2607 Da) were used for the evaluation of laboratory-prepared octadecyl stationary phases with high and low coverage density before and after end-capping. The aim of this work was to study differences in surface coverage density and homogeneity and conformational changes of chemically bonded moieties and the influence of these parameters on the separation of mixtures of selenopeptides and fullerenes with significantly different molecular masses. A topographical model of the chemically modified stationary surface is presented. PMID:16830495

  15. Exploring the chemical bonding, infrared and UV-vis absorption spectra of OH radicals adsorption on the smallest fullerene.

    PubMed

    Li, Xiaojun; Ren, Hongjiang; Yang, Xiaohui; Song, Jing

    2015-06-01

    In the present work, the density-functional theory calculations were performed on C20 hydroxylated fullerene. B3LYP functionals with 6-31G(d,p) basis set were utilized to gain insight into the bonding characters and intramolecular interactions of hydroxyl groups adsorbed on the cage. Interestingly, we observed that the C20 cage has the bonding patterns with spherical orbitals configuration [1S(2)1P(6)1D(10)1F(2)], and the adsorbed hydroxyl groups significantly affect the chemical bonding of the cage surface. Analysis of vertical electron affinities and vertical ionization potentials indicates that the polyhydroxylated derivative with eight hydroxyl groups is more stable than others. The intramolecular interaction of these derivatives considered here reveals that the more the hydroxyl groups in derivatives, the stronger the interaction in stabilizing structures. On the basis of theoretical studies, the hydroxyl groups largely enhance the infrared intensities, especially for the polyhydroxylated derivatives. PMID:25766372

  16. A Simple Visualization of Double Bond Properties: Chemical Reactivity and UV Fluorescence

    ERIC Educational Resources Information Center

    Grayson, Scott M.

    2012-01-01

    A simple, easily visualized thin-layer chromatography (TLC) staining experiment is presented that highlights the difference in reactivity between aromatic double bonds and nonaromatic double bonds. Although the stability of aromatic systems is a major theme in organic chemistry, the concept is rarely reinforced "visually" in the undergraduate…

  17. Characteristics of hydrogen bond revealed from water clusters

    NASA Astrophysics Data System (ADS)

    Song, Yan; Chen, Hongshan; Zhang, Cairong; Zhang, Yan; Yin, Yuehong

    2014-09-01

    The hydrogen bond network is responsible for the exceptional physical and chemical properties of water, however, the description of hydrogen bond remains a challenge for the studies of condensed water. The investigation of structural and binding properties of water clusters provides a key for understanding the H-bonds in bulk water. In this paper, a new set of geometric parameters are defined to describe the extent of the overlap between the bonding orbital of the donor OH and the nonbonding orbital of the lone-pair of the acceptor molecule. This orbital overlap plays a dominant role for the strength of H-bonds. The dependences of the binding energy of the water dimer on these parameters are studied. The results show that these parameters properly describe the H-bond strength. The ring, book, cage and prism isomers of water hexamer form 6, 7, 8 and 9 H-bonds, and the strength of the bonding in these isomers changes markedly. The internally-solvated and the all-surface structures of (H2O) n for n = 17, 19 and 21 are nearly isoenergetic. The internally-solvated isomers form fewer but stronger H-bonds. The hydrogen bonding in the above clusters are investigated in detail. The geometric parameters can well describe the characters of the H-bonds, and they correlate well with the H-bond strength. For the structures forming stronger H-bonds, the H-bond lengths are shorter, the angle parameters are closer to the optimum values, and their rms deviations are smaller. The H-bonds emanating from DDAA and DDA molecules as H-donor are relatively weak. The vibrational spectra of (H2O) n ( n = 17, 19 and 21) are studied as well. The stretching vibration of the intramolecular OH bond is sensitive to its bonding environment. The H-bond strength judged from the geometric parameters is in good agreement with the bonding strength judged from the stretching frequencies.

  18. X-ray photoelectron spectra structure and chemical bond nature in NpO2

    NASA Astrophysics Data System (ADS)

    Teterin, Yu. A.; Teterin, A. Yu.; Ivanov, K. E.; Ryzhkov, M. V.; Maslakov, K. I.; Kalmykov, St. N.; Petrov, V. G.; Enina, D. A.

    2014-01-01

    Quantitative analysis was done of the x-ray photoelectron spectra structure in the binding energy (BE) range of 0 to ˜35 eV for neptunium dioxide (NpO2) valence electrons. The BEs and structure of the core electronic shells (˜35-1250 eV) as well as the relativistic discrete variation calculation results for the finite fragment of the NpO2 lattice and the data of other authors were taken into account. The experimental data show that the many-body effects and the multiplet splitting contribute to the spectral structure much less than the effects of formation of the outer (0-˜15 eV) and the inner (˜15-˜35 eV) valence molecular orbitals (OVMO and IVMO, respectively). The filled Np 5f electronic states were shown to form in the NpO2 valence band. The Np 6p electrons participate in formation of both the IVMO and the OVMO (bands). The filled Np 6p3/2 and the O 2s electronic shells were found to take the maximum part in the IVMO formation. The MO composition and the sequence order in the BE range 0-˜35 eV in NpO2 were established. The experimental and theoretical data allowed a quantitative MO scheme for NpO2, which is fundamental for both understanding the chemical bond nature in neptunium dioxide and the interpretation of other x-ray spectra of NpO2.

  19. Iron-phosphate-based chemically bonded phosphate ceramics for mixed waste stabilization

    SciTech Connect

    Wagh, A.S.; Jeong, S.Y.; Singh, D.

    1997-01-01

    In an effort to develop chemically bonded phosphate ceramics for mixed waste stabilization, a collaborative project to develop iron-phosphate based ceramics has been initiated between Argonne National Laboratory and the V. G. Khlopin Radium Institute in St. Petersburg, Russia. The starter powders are oxides of iron that are generated as inexpensive byproduct materials in the iron and steel industry. They contain iron oxides as a mixture of magnetite (Fe{sub 3}O{sub 4}) and haematite (Fe{sub 2}O{sub 3}). In this initial phase of this project, both of these compounds were investigated independently. Each was reacted with phosphoric acid solution to form iron phosphate ceramics. In the case of magnetite, the reaction was rapid. Adding ash as the waste component containing hazardous contaminants resulted in a dense and hard ceramic rich in glassy phase. On the other hand, the reaction of phosphoric acid solution with a mixture of haematite and ash waste contaminated with cesium and americium was too slow. Samples had to be molded under pressure. They were cured for 2-3 weeks and then hardened by heating at 350{degrees}C for 3 h. The resulting ceramics in both cases were subjected to physical tests for measurement of density, open porosity, compression strength, phase analyses using X-ray diffraction and differential thermal analysis, and leaching tests using toxicity characteristic leaching procedure (TCLP) and ANS 16.1 with 7 days of leaching. Using the preliminary information obtained from these tests, we evaluated these materials for stabilization of Department of Energy`s mixed waste streams.

  20. Influence of chemical bonds on the lifetime of the molecular-field-split 2p levels in H{sub 2}S

    SciTech Connect

    Bueno, Andre Machado; Brito, Arnaldo Naves de; Fink, Reinhold F.; Baessler, Margit; Bjoerneholm, Olle; Burmeister, Florian; Feifel, Raimund; Miron, Catalin; Sorensen, Stacey L.; Wang Honghong; Svensson, Svante

    2003-02-01

    Different lifetime broadenings in molecular-field-split 2p core levels in H{sub 2}S are predicted theoretically and are identified in an experimental investigation of the S 2p Auger electron spectrum. The measurements were performed for the transition to the vibrationally resolved X{sup 1}A{sub 1} ground state of H{sub 2}S{sup 2+}. The lifetimes of the 3e{sub 1/2} and 5e{sub 1/2} levels of the 2p ionized molecule are found to be 64 and 74 meV, respectively. This unambiguous determination of the lifetime difference of 10{+-}1 meV is only possible as the 4e{sub 1/2}{yields}X{sup 1}A{sub 1}(2b{sub 1}{sup -2}) decay channel that overlaps the 5e{sub 1/2}{yields}X{sup 1}A{sub 1}(2b{sub 1}{sup -2}) channel is practically suppressed in Auger decay in H{sub 2}S. The lifetime difference is confirmed by ab initio calculations. A theoretical analysis shows that it results from the mutual orientation of the core hole in the intermediate states and the valence electron density in the sulfur 3p orbitals. Both are strongly influenced by the chemical bond. Thus the observed effect is the direct result of a fundamental property of molecular electronic structure.

  1. Chemical Bonding in Si52- and NaSi5- via Photoelectron Spectroscopy and Ab Initio Calculations

    SciTech Connect

    Yu, Dmitry; Boldyrev, Zubarev; Boldyrev, Alexander I.; Li, Xi; Cui, Lifeng; Wang, Lai S.

    2005-12-22

    Photoelectron spectroscopy and ab initio calculations are used to investigate the electronic structure and chemical bonding of Si5 - and Si5 2- in NaSi5 -. Photoelectron spectra of Si5 - and NaSi5 - are obtained at several photon energies and are compared with theoretical calculations at four different levels of theory, TD-B3LYP, R(U)OVGF, UCCSD(T), and EOM-CCSD(T), all with 6-311+G(2df) basis sets. Excellent agreement is observed between experiment and theory, confirming the obtained ground-state structures for Si5 - and Si5 2-, which are both found to be trigonal bipyramid with D3h symmetry at several levels of theory. Chemical bonding in Si5, Si5 -, and Si5 2- is analyzed using NPA, molecular orbitals, ELF, and NICS indices. The bonding in Si5 2- is compared with that in the isoelectronic and isostructural B5H5 2- species, but they are found to differ due to the involvement of electron densities, which are supposed to be lone pairs in the skeletal bonding in Si5 2-.

  2. Anisotropy of Chemical Bonds in Collagen Molecules Studied by X-ray Absorption Near-Edge Structure (XANES) Spectroscopy

    PubMed Central

    Lam, Raymond S.K.; Metzler, Rebecca A.; Gilbert, Pupa U.P.A.; Beniash, Elia

    2012-01-01

    Collagen type I fibrils are the major building blocks of connective tissues. Collagen fibrils are anisotropic supra-molecular structures, and their orientation can be revealed by polarized light microscopy and vibrational microspectroscopy. We hypothesized that the anisotropy of chemical bonds in the collagen molecules, and hence their orientation, might also be detected by X-ray photoemission electron spectromicroscopy (X-PEEM) and X-ray absorption near-edge structure (XANES) spectroscopy, which use linearly polarized synchrotron light. To test this hypothesis, we analyzed sections of rat-tail tendon, composed of parallel arrays of collagen fibrils. The results clearly indicate that XANES-PEEM is sensitive to collagen fibril orientation and, more specifically, to the orientations of carbonyl and amide bonds in collagen molecules. These data suggest that XANES-PEEM is a promising technique for characterizing the chemical composition and structural organization at the nanoscale of collagen-based connective tissues, including tendons, cartilage, and bone. PMID:22148847

  3. The Load and Time Dependence of Chemical Bonding-Induced Frictional Ageing of Silica at the Nanoscale

    NASA Astrophysics Data System (ADS)

    Tian, K.; Gosvami, N. N.; Goldsby, D. L.; Carpick, R. W.

    2015-12-01

    Rate and state friction (RSF) laws are empirical relationships that describe the frictional behavior of rocks and other materials in experiments, and reproduce a variety of observed natural behavior when employed in earthquake models. A pervasive observation from rock friction experiments is the linear increase of static friction with the log of contact time, or 'ageing'. Ageing is usually attributed to an increase in real area of contact associated with asperity creep. However, recent atomic force microscopy (AFM) experiments demonstrate that ageing of nanoscale silica-silica contacts is due to progressive formation of interfacial chemical bonds in the absence of plastic deformation, in a manner consistent with the multi-contact ageing behavior of rocks [Li et al., 2011]. To further investigate chemical bonding-induced ageing, we explored the influence of normal load (and thus contact normal stress) and contact time on ageing. Experiments that mimic slide-hold-slide rock friction experiments were conducted in the AFM for contact loads and hold times ranging from 23 to 393 nN and 0.1 to 100 s, respectively, all in humid air (~50% RH) at room temperature. Experiments were conducted by sequentially sliding the AFM tip on the sample at a velocity V of 0.5 μm/s, setting V to zero and holding the tip stationary for a given time, and finally resuming sliding at 0.5 μm/s to yield a peak value of friction followed by a drop to the sliding friction value. Chemical bonding-induced ageing, as measured by the peak friction minus the sliding friction, increases approximately linearly with the product of normal load and the log of the hold time. Theoretical studies of the roles of reaction energy barriers in nanoscale ageing indicate that frictional ageing depends on the total number of reaction sites and the hold time [Liu & Szlufarska, 2012]. We combine chemical kinetics analyses with contact mechanics models to explain our results, and develop a new approach for curve

  4. Ab initio study of chemical bond interactions between covalently functionalized carbon nanotubes via amide, ester and anhydride linkages

    NASA Astrophysics Data System (ADS)

    Ben Doudou, Bessem; Chen, Jun; Vivet, Alexandre; Poilâne, Christophe

    2016-03-01

    In this paper, we have investigated the chemical bond interactions between covalently functionalized zigzag (5,0) and (8,0) SWCNT-SWCNT via various covalent linkages. Side-to-side junctions connected via amide, ester and anhydride linkages were particularly studied. The geometries and energy of the forming reaction were investigated using first-principles density functional theory. Furthermore, the band structures and the total density of states (DOS) of the junctions have also been analyzed. Our results show that several promising structures could be obtained by using chemical connection strategy and particularly the junctions formed by coupling amino functionalized SWCNT and carboxylic acid functionalized SWCNT was more favorable.

  5. Spectral Correspondence to the Evolution of Chemical Bond and Valence Band in Oxidation

    NASA Astrophysics Data System (ADS)

    Sun, Chang Q.; Zhang, Sam; Hing, Peter; Wei, Jun; Xie, Hong; Wee, Andrew T. S.

    Spectroscopy covering energies around Fermi level of a metal is of particular interest as it does give direct information of bond forming and its consequence on the energy states (DOS). However, the origin of some outstanding spectral features from metal surface with chemisorbed oxygen has not yet been well established. It is shown in this letter that a bond-to-band model for oxidation enables the nature of a sum of spectral features to be consistently defined. All the spectral changes in the STS, XPS, UPS, photoemission spectroscopy (PES) and the thermal desorption spectroscopy (TDS) are classified by four DOS features around Fermi level and three bonding processes. Such a definition leads not only to new knowledge of the nature of O-metal interaction but also to a direct understanding of the bond-and-band forming dynamics.

  6. Promoting Understanding of Chemical Bonding and Spontaneity through Student Explanation and Integration of Ideas.

    ERIC Educational Resources Information Center

    Teichert, Melonie A.; Stacy, Angelica M.

    2002-01-01

    Explores the effectiveness of intervention discussion sections for a college general chemistry course designed to apply research on student preconceptions, knowledge integration, and student explanations. Describes the testing of two interventions on bond energy and spontaneity. (Contains 44 references.) (DDR)

  7. Influence of the chemical bond on the K emission spectrum of oxygen and fluorine.

    NASA Technical Reports Server (NTRS)

    Koster, A. S.

    1971-01-01

    The K emission spectrum of oxygen and fluorine from a number of simple oxides and fluorides is divided into three to six sub-peaks. The spectra of many of these oxides and fluorides resemble one another owing to their basically ionic bonding. Certain sub-peaks, however, are ascribed to cross-over transitions and partially covalent energy levels. The different fluorine spectrum of Teflon is due to the hybrid nature of its covalent bonds.

  8. Stabilization of contaminated soil and wastewater with chemically bonded phosphate ceramics

    SciTech Connect

    Wagh, A.S.; Jeong, S.Y.; Singh, D.

    1997-01-01

    At Argonne National Laboratory, we have developed chemically Bonded phosphate ceramic (CBPC) technology to stabilize the U.S. Department of Energy`s problem mixed waste streams, for which no other stabilization technology is suitable. In this technology, solid waste is mixed with MgO and reacted with aqueous solutions of phosphoric acid or acid phosphates at room temperature to form a slurry that sets in {approx}2 h into a hard and dense ceramic waste form. Initial studies involved stabilizing the surrogate waste streams and then testing the waste forms for leaching of contaminants. After achieving satisfactory performance of the waste forms, we next incorporated actual waste streams at bench scale and produced waste forms that were then tested with the Toxicity Characteristic Leaching Procedure (TCLP). This presentation deals with stabilization of soil contaminated with Cd, Cr, Pb, Ag, Ba, and Hg, and of low-level radioactive wastewater. To enhance the contaminant levels in the soil, we further spiked the soil with additional amounts of Cd, Cr, Pb, and Hg. Both the soil and the wastewater were incorporated in the same waste form by stabilizing them with the CBPC process. The waste forms had a total waste loading of {approx}77 wt.% and were dense with an open porosity of 2.7 vol.% and a density of 2.17 g/cm{sup 3}. Compression strength was 4910 psi. The TCLP results showed excellent immobilization of all the RCRA metals, and radioactive contaminant levels were below the detection limit of 0.2 pCi/mL. Long-term leaching studies using the ANS 16.1 procedure showed that the retention of contaminants is excellent and comparable to or better than most of other stabilization processes. These results demonstrate that the CBPC process is a very superior process for treatment of low level mixed wastes; we therefore conclude that the CBPC process is well suited to the treatment of low-level mixed waste streams with high waste loading.

  9. CHEMICALLY BONDED CEMENTS FROM BOILER ASH AND SLUDGE WASTES. PHASE I REPORT AUGUST 1997 - JULY 1998

    SciTech Connect

    SUGAMA,T.; YAGER,K.A.

    2002-08-05

    In exploring methods to recycle boiler ash (BA) and waste water treatment sludge (WWTS), by-products generated from Keyspan's power plants, into commercially viable materials, we synthesized chemically bonded cements (CBC) offering the following three specific characteristics; (1) immobilization of hazardous heavy metals, such as Pb, Ni, and V, (2) rapid hardening and setting properties, and (3) development of high mechanical strength. The CBCs were prepared through an acid-base reaction between these by-products acting as the solid base reactants and the sodium polyphosphate solution as the cement-forming acid reactant, followed by a hydrating reaction. Furthermore, two additives, the calcium aluminate cements (CAC) and the calcium silicate cements (CSC) were incorporated into the CBC systems to improve their properties. Using a CBC formulation consisting of 53.8 wt% WWTS, 23.1 wt% CSC, and 23.1 wt% [40 wt% -(-NaPO{sub 3}-)-{sub n}]{sub 2} the Toxicity Characteristics Leaching Procedure (TCLP) tests showed that the concentrations of Pb, Ni, and V metals leached out from the specimens were minimal. This formulation originally contained {approx} 28800 mg/kg of Pb, {approx} 6300 mg/kg of Ni, and {approx} 11130 mg/kg of V; the amounts leaching into the acid extraction fluid were only 0.15 mg/L of Pb, 0.15 mg/L of Ni, and 4.63 mgiL of V. On the other hand, CBC specimens derived from a formulation consisting of 42 wt% BA, 18 wt% CAC and 40 wt% [40 wt% -(-NaPO{sub 3}-)-{sub n}] displayed an excellent compressive strength of 10.8 MPa at an early curing age of 2 hours after mixing at room temperature. The reason for its rapid hardening was due to a high exothermic energy evolved by the acid-base reaction. Furthermore, when these specimens were immersed for 28 days in water at 25 C, and exposed for 20 hours to steam at 80 C, a very high compressive strength of 3.32 MPa developed. Two physico-chemical factors played an important role in improving the mechanical strength of

  10. CuAl{sub 2} revisited: Composition, crystal structure, chemical bonding, compressibility and Raman spectroscopy

    SciTech Connect

    Grin, Yuri . E-mail: grin@cpfs.mpg.de; Wagner, Frank R.; Armbruester, Marc; Kohout, Miroslav; Leithe-Jasper, Andreas; Schwarz, Ulrich; Wedig, Ulrich; Georg von Schnering, Hans

    2006-06-15

    The structure of CuAl{sub 2} is usually described as a framework of base condensed tetragonal antiprisms [CuAl{sub 8/4}]. The appropriate symmetry governed periodic nodal surface (PNS) divides the space of the structure into two labyrinths. All atoms are located in one labyrinth, whereas the second labyrinth seems to be 'empty'. The bonding of the CuAl{sub 2} structure was analyzed by the electron localization function (ELF), crystal orbital Hamiltonian population (COHP) analysis and Raman spectroscopy. From the ELF representation it is seen, that the 'empty' labyrinth is in fact the place of important covalent interactions. ELF, COHP in combination with high-pressure X-ray diffraction and Raman spectroscopy show that the CuAl{sub 2} structure is described best as a network built of interpenetrating graphite-like nets of three-bonded aluminum atoms with the copper atoms inside the tetragonal-antiprismatic cavities. - Graphical abstract: Atomic interactions in the crystal structure of the intermetallic compound CuAl{sub 2}: Three-bonded aluminum atoms form interpenetrating graphite-like nets. The copper atoms are located in the channels of aluminum network by means of three-center bonds. The bonding model is in agreement with the result of polarized Raman spectroscopy and high-pressure X-ray powder diffraction.

  11. Plasmon-driven dimerization via S-S chemical bond in an aqueous environment

    PubMed Central

    Cui, Lin; Wang, Peijie; Chen, Xiaowei; Fang, Yurui; Zhang, Zhenglong; Sun, Mengtao

    2014-01-01

    The surface-enhanced Raman scattering (SERS) spectra of thioanisole are experimentally investigated in an electrochemical environment in this study. Two Raman peaks, which depend strongly not only on electric potential but also on the local surface plasmon resonances (LSPR), have been observed. Theoretical calculations reveal that thioanisole is first dissociated from thiophenol via the S-CH3 bond; plasmons then drive the dimerisation of thiophenol via the S-S bond, which is strongly potential dependent. One Raman peak corresponds to the S-S vibrational mode of the thiophenol dimer, and the other corresponds to the asymmetric C-C stretching modes of the benzenyl of the thiophenol dimer. The potential-dependent two Raman modes is the potential-dependent dimerisation dynamics of thiophenol via the S-S bond. Our experimental findings provide insight into the structural elucidation of adsorbed molecules and molecular surface reaction dynamics. PMID:25427897

  12. A periodic energy decomposition analysis method for the investigation of chemical bonding in extended systems

    SciTech Connect

    Raupach, Marc; Tonner, Ralf

    2015-05-21

    The development and first applications of a new periodic energy decomposition analysis (pEDA) scheme for extended systems based on the Kohn-Sham approach to density functional theory are described. The pEDA decomposes the bonding energy between two fragments (e.g., the adsorption energy of a molecule on a surface) into several well-defined terms: preparation, electrostatic, Pauli repulsion, and orbital relaxation energies. This is complemented by consideration of dispersion interactions via a pairwise scheme. One major extension toward a previous implementation [Philipsen and Baerends, J. Phys. Chem. B 110, 12470 (2006)] lies in the separate discussion of electrostatic and Pauli and the addition of a dispersion term. The pEDA presented here for an implementation based on atomic orbitals can handle restricted and unrestricted fragments for 0D to 3D systems considering periodic boundary conditions with and without the determination of fragment occupations. For the latter case, reciprocal space sampling is enabled. The new method gives comparable results to established schemes for molecular systems and shows good convergence with respect to the basis set (TZ2P), the integration accuracy, and k-space sampling. Four typical bonding scenarios for surface-adsorbate complexes were chosen to highlight the performance of the method representing insulating (CO on MgO(001)), metallic (H{sub 2} on M(001), M = Pd, Cu), and semiconducting (CO and C{sub 2}H{sub 2} on Si(001)) substrates. These examples cover diverse substrates as well as bonding scenarios ranging from weakly interacting to covalent (shared electron and donor acceptor) bonding. The results presented lend confidence that the pEDA will be a powerful tool for the analysis of surface-adsorbate bonding in the future, enabling the transfer of concepts like ionic and covalent bonding, donor-acceptor interaction, steric repulsion, and others to extended systems.

  13. Interfacial chemical bonding state and band alignment of CaF{sub 2}/hydrogen-terminated diamond heterojunction

    SciTech Connect

    Liu, J. W.; Liao, M. Y.; Cheng, S. H.; Imura, M.; Koide, Y.

    2013-03-28

    CaF{sub 2} films are deposited on hydrogen-terminated diamond (H-diamond) by a radio-frequency sputter-deposition technique at room temperature. Interfacial chemical bonding state and band alignment of CaF{sub 2}/H-diamond heterojunction are investigated by X-ray photoelectron spectroscopy. It is confirmed that there are only C-Ca bonds at the CaF{sub 2}/H-diamond heterointerface. Valence and conductance band offsets of the CaF{sub 2}/H-diamond heterojunciton are determined to be 3.7 {+-} 0.2 and 0.3 {+-} 0.2 eV, respectively. It shows a type I straddling band configuration. The large valence band offset suggests advantage of the CaF{sub 2}/H-diamond heterojunciton for the development of high power and high frequency field effect transistors.

  14. Chemisorption of Si on Al(111) surfaces: A local-chemical-bond analysis from Auger transition density of states

    NASA Astrophysics Data System (ADS)

    Muñoz, M. C.; Sacedón, J. L.; Soria, F.; Martinez, V.

    1986-07-01

    Auger and electron loss spectroscopies have been used to study the local chemical bond between Si and Al, in the first stages of growth of Si deposited at room temperature on Al(111) surfaces. Si follows a layer-by-layer mechanism up to 2 monolayers with the formation of an Al(111)-3 × 3-Si structure at about 0.44 monolayers. A detailed analysis of the L 2,3VV Auger spectra for this structure allows to interpret the Si and Al Auger transition density of states (TDOS) in terms of the actual p-like partial DOS centered on the Si and Al sites. The experimental results indicate a strong SiAl interaction with the formation of a p-type local covalent bond between the Si and Al surface atoms.

  15. Chemical bond structure and MOSFET device damages of electron beam cured siloxane spin-on-dielectric films

    NASA Astrophysics Data System (ADS)

    Pyo, Sung Gyu

    2014-11-01

    This paper presents the properties of electron beam cured SOG (spin-on-glass) films in terms of the chemical bond structure and accompanying device damages. It is observed that e-beam cured SOG films do not show the Si-CH bonds in the FT-IR spectra but still have the carbons in the film which are thermally stable. As for the device damage, the experimental result shows that the threshold voltage of nMOSFET decreases by positive charging of gate oxide after e-beam curing process and it shows antenna ratio independence. Also, the variation of threshold voltage is largely affected by the cathode voltage, which determines the electron energy and interlayer dielectric thickness. [Figure not available: see fulltext.

  16. CHEMICALLY BONDED CEMENTS FROM BOILER ASH AND SLUDGE WASTES. PHASE II REPORT, SEPT.1998-JULY 1999.

    SciTech Connect

    SUGAMA,T.YAGER,K.A.BLANKENHORN,D.

    1999-08-01

    Based upon the previous Phase I research program aimed at looking for ways of recycling the KeySpan-generated wastes, such as waste water treatment sludge (WWTS) and bottom ash (BA), into the potentially useful cementitious materials called chemically bonded cement (CBC) materials, the emphasis of this Phase II program done at Brookhaven National Laboratory, in a period of September 1998 through July 1999, was directed towards the two major subjects: One was to assess the technical feasibility of WWTS-based CBC material for use as Pb-exchange adsorbent (PEA) which remediates Pb-contaminated soils in the field; and the other was related to the establishment of the optimum-packaging storage system of dry BA-based CBC components that make it a promising matrix material for the steam-cured concrete products containing sand and coarse aggregate. To achieve the goal of the first subject, a small-scale field demonstration test was carried out. Using the PEA material consisting of 30 wt% WWTS, 13 wt% Type I cement and 57 wt% water, the PES slurry was prepared using a rotary shear concrete mixer, and then poured on the Pb-contaminated soil. The PEA-to-soil ratio by weight was a factor of 2.0. The placed PEA slurry was blended with soil using hand mixing tools such as claws and shovels. The wettability of soils with the PEA was very good, thereby facilitating the soil-PEA mix procedures. A very promising result was obtained from this field test; in fact, the mount of Pb leached out from the 25-day-aged PEA-treated soil specimen was only 0.74 mg/l, meeting the requirement for EPA safe regulation of < 5 mg/l. In contrast, a large amount (26.4 mg/l) of Pb was detected from the untreated soil of the same age. Thus, this finding demonstrated that the WWTS-based CBC has a potential for use as PEA material. Regarding the second subject, the dry-packed storage system consisting of 68.7 wt% BA, 13.0 wt% calcium aluminate cement (CAC), 13.0 wt% Type I portland cement and 5.3 wt

  17. Chemical bond as a test of density-gradient expansions for kinetic and exchange energies

    NASA Astrophysics Data System (ADS)

    Perdew, John P.; Levy, Mel; Painter, G. S.; Wei, Siqing; Lagowski, Jolanta B.

    1988-01-01

    Errors in kinetic and exchange contributions to the molecular bonding energy are assessed for approximate density functionals by reference to near-exact Hartree-Fock values. From the molecular calculations of Allan et al.and of Lee and Ghosh, it is demonstrated that the density-gradient expansion does not accurately describe the noninteracting kinetic contribution to the bonding energy, even when this expansion is carried to fourth order and applied in its spin-density-functional form to accurate Hartree-Fock densities. In a related study, it is demonstrated that the overbinding of molecules such as N2 and F2, which occurs in the local-spin-density (LSD) approximation for the exchange-correlation energy, is not attributable to errors in the self-consistent LSD densities. Contrary to expectations based upon the Gunnarsson-Jones nodality argument, it is found that the LSD approximation for the exchange energy can seriously overbind a molecule even when bonding does not create additional nodes in the occupied valence orbitals. LSD and exact values for the exchange contribution to the bonding energy are displayed and discussed for several molecules.

  18. Chemical bond as a test of density-gradient expansions for kinetic and exchange energies

    SciTech Connect

    Perdew, J.P.; Levy, M.; Painter, G.S.; Wei, S.; Lagowski, J.B.

    1988-01-15

    Errors in kinetic and exchange contributions to the molecular bonding energy are assessed for approximate density functionals by reference to near-exact Hartree-Fock values. From the molecular calculations of Allan et al. and of Lee and Ghosh, it is demonstrated that the density-gradient expansion does not accurately describe the noninteracting kinetic contribution to the bonding energy, even when this expansion is carried to fourth order and applied in its spin-density-functional form to accurate Hartree-Fock densities. In a related study, it is demonstrated that the overbinding of molecules such as N/sub 2/ and F/sub 2/, which occurs in the local-spin-density (LSD) approximation for the exchange-correlation energy, is not attributable to errors in the self-consistent LSD densities. Contrary to expectations based upon the Gunnarsson-Jones nodality argument, it is found that the LSD approximation for the exchange energy can seriously overbind a molecule even when bonding does not create additional nodes in the occupied valence orbitals. LSD and exact values for the exchange contribution to the bonding energy are displayed and discussed for several molecules.

  19. Conformational analysis and intramolecular hydrogen bonding of cis-3-aminoindan-1-ol: a quantum chemical study.

    PubMed

    Kheffache, Djaffar; Guemmour, Hind; Dekhira, Azzedine; Benaboura, Ahmed; Ouamerali, Ourida

    2013-11-01

    In the present work, we carried out a conformational analysis of cis-3-aminoindan-1-ol and evaluated the role of the intramolecular hydrogen bond in the stabilization of various conformers using quantum mechanical DFT (B3LYP) and MP2 methods. On the basis of relative energies, we have found nine conformational minima, which can interchange through the ring-puckering and the internal rotation of the OH and NH2 groups on the five-membered ring. The intramolecular hydrogen bonds such as OH∙∙∙∙π, NH∙∙∙∙π, NH∙∙∙∙OH and HN∙∙∙∙HO are expected to be of critical importance for the conformational stabilities. The intramolecular interactions of the minima have been analyzed by calculation of electron density (ρ) and Laplacian (ρ) at the bond critical points (BCPs) using atoms-in-molecule (AIM) theory. The existence or absence of OH∙∙∙∙π and NH∙∙∙∙π in cis-3-aminoindan-1-ol remains unclear since the geometrical investigation has not been confirmed by topological criteria. The results of theoretical calculations demonstrate that this compound exists predominantly in one ring-puckering form stabilized by strong hydrogen bond HN∙∙∙∙HO Interaction. PMID:24026578

  20. Microwave and Quantum Chemical Study of Intramolecular Hydrogen Bonding in 2-Propenylhydrazine (H2C═CHCH2NHNH2).

    PubMed

    Møllendal, Harald; Samdal, Svein; Guillemin, Jean-Claude

    2016-01-28

    The microwave spectrum of 2-propenylhydrazine (H2C═CHCH2NHNH2) was studied in the 12-61 and 72-123 GHz spectral regions. A variety of intramolecular hydrogen bonds between one or more of the hydrogen atoms of the hydrazino group and the π-electrons are possible for this compound. Assignments of the spectra of four conformers, all of which are stabilized with intramolecular hydrogen bonds are reported. One hydrogen bond exists in two of these conformers, whereas the π-electrons are shared by two hydrogen atoms in the two other rotamers. Vibrationally excited-state spectra were assigned for three of the four conformers. The internal hydrogen bonds are weak, probably in the 3-6 kJ/mol range. A total of about 4400 transitions were assigned for these four forms. The microwave work was guided by quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory. These calculations indicated that as many as 18 conformers may exist for 2-propenylhydrazine and 11 of these have either one or two intramolecular hydrogen bonds. The four conformers detected in this work are among the rotamers with the lowest CCSD electronic energies. The CCSD method predicts rotational constants that are very close to the experimental rotational constants. The B3LYP calculations yielded quartic centrifugal distortion constants that deviated considerably from their experimental counterparts in most cases. The calculation of vibration-rotation constants and sextic centrifugal distortion constants by the B3LYP method were generally found to be in poor agreement with the corresponding experimental constants. PMID:26696467

  1. The interplay between interface structure, energy level alignment and chemical bonding strength at organic-metal interfaces.

    PubMed

    Willenbockel, M; Lüftner, D; Stadtmüller, B; Koller, G; Kumpf, C; Soubatch, S; Puschnig, P; Ramsey, M G; Tautz, F S

    2015-01-21

    What do energy level alignments at metal-organic interfaces reveal about the metal-molecule bonding strength? Is it permissible to take vertical adsorption heights as indicators of bonding strengths? In this paper we analyse 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on the three canonical low index Ag surfaces to provide exemplary answers to these questions. Specifically, we employ angular resolved photoemission spectroscopy for a systematic study of the energy level alignments of the two uppermost frontier states in ordered monolayer phases of PTCDA. Data are analysed using the orbital tomography approach. This allows the unambiguous identification of the orbital character of these states, and also the discrimination between inequivalent species. Combining this experimental information with DFT calculations and the generic Newns-Anderson chemisorption model, we analyse the alignments of highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) with respect to the vacuum levels of bare and molecule-covered surfaces. This reveals clear differences between the two frontier states. In particular, on all surfaces the LUMO is subject to considerable bond stabilization through the interaction between the molecular π-electron system and the metal, as a consequence of which it also becomes occupied. Moreover, we observe a larger bond stabilization for the more open surfaces. Most importantly, our analysis shows that both the orbital binding energies of the LUMO and the overall adsorption heights of the molecule are linked to the strength of the chemical interaction between the molecular π-electron system and the metal, in the sense that stronger bonding leads to shorter adsorption heights and larger orbital binding energies. PMID:25475998

  2. Identification of the Chemical Bonding Prompting Adhesion of a-C:H Thin Films on Ferrous Alloy Intermediated by a SiCx:H Buffer Layer.

    PubMed

    Cemin, F; Bim, L T; Leidens, L M; Morales, M; Baumvol, I J R; Alvarez, F; Figueroa, C A

    2015-07-29

    Amorphous carbon (a-C) and several related materials (DLCs) may have ultralow friction coefficients that can be used for saving-energy applications. However, poor chemical bonding of a-C/DLC films on metallic alloys is expected, due to the stability of carbon-carbon bonds. Silicon-based intermediate layers are employed to enhance the adherence of a-C:H films on ferrous alloys, although the role of such buffer layers is not yet fully understood in chemical terms. The chemical bonding of a-C:H thin films on ferrous alloy intermediated by a nanometric SiCx:H buffer layer was analyzed by X-ray photoelectron spectroscopy (XPS). The chemical profile was inspected by glow discharge optical emission spectroscopy (GDOES), and the chemical structure was evaluated by Raman and Fourier transform infrared spectroscopy techniques. The nature of adhesion is discussed by analyzing the chemical bonding at the interfaces of the a-C:H/SiCx:H/ferrous alloy sandwich structure. The adhesion phenomenon is ascribed to specifically chemical bonding character at the buffer layer. Whereas carbon-carbon (C-C) and carbon-silicon (C-Si) bonds are formed at the outermost interface, the innermost interface is constituted mainly by silicon-iron (Si-Fe) bonds. The oxygen presence degrades the adhesion up to totally delaminate the a-C:H thin films. The SiCx:H deposition temperature determines the type of chemical bonding and the amount of oxygen contained in the buffer layer. PMID:26135943

  3. Electronic Structure and Bonding in Co-Based Single and Mixed Valence Oxides: A Quantum Chemical Perspective.

    PubMed

    Singh, Vijay; Major, Dan Thomas

    2016-04-01

    The mixed valence cobalt oxide, Co3O4, is a potential candidate as a photovoltaic (PV) material, which also exhibits intriguing chemical and catalytic properties. Here, we present a comparative study of the electronic, magnetic, and chemical bonding properties of mixed valence Co3O4 (i.e., Co(2+/3+)) with the related single valence CoO (i.e., Co(2+)) and Co2O3 (i.e., Co(3+)) oxides using density functional theory (DFT). We have employed a range of theoretical methods, including pure DFT, DFT+U, and a range-separated exchange-correlation functional (HSE06). We compare the electronic structure and band gap of the oxide materials, with available photoemission spectroscopy and optical band gaps. Our calculations suggest that the bonding between Co(3+) and O(2-) ions in Co2O3 and Co3O4 and Co(2+) and O(2-) ions in CoO and Co3O4 are rather different. We find that Co2O3 and Co3O4 are weakly correlated materials, whereas CoO is a strongly correlated material. Furthermore, our computed one-electron energy level diagrams reveal that strong Co-O antibonding states are present at the top of the valence band for all the cobalt oxides, hinting at a defect tolerant capacity in these materials. These results, which give a detailed picture of the chemical bonding in related single and mixed valence cobalt oxides, may serve as a guide to enhance the PV or photoelectrochemical activity of Co3O4, by reducing its internal defect states or changing its electronic structure by doping or alloying with suitable elements. PMID:27010797

  4. Theory of chemical bonds in metalloenzymes - Manganese oxides clusters in the oxygen evolution center -

    NASA Astrophysics Data System (ADS)

    Yamaguchi, K.; Shoji, M.; Saito, T.; Isobe, H.; Yamada, S.; Nishihara, S.; Kawakami, T.; Kitagawa, Y.; Yamanaka, S.; Okumura, M.

    2012-12-01

    In early 1980 we have initiated broken-symmetry (BS) MO theoretical calculations of transition-metal oxo species M = O (M = Ti,V,Cr,Mn,Fe,Ni,Cu) to elucidate the nature of dσ-pσ and dπ-pπ bonds. It has been concluded that high-valent M = O species such as [Mn(IV) = O]2+ and [Fe(IV) = O]2+ exhibit electrophilic property in a sharp contrast with nucleophilic character of low-valent M = O bonds: [M(II)O2-]0, and closed-shell dπ-pπ bonds of high-valent M = O species often suffer the triplet-instability, giving rise to open-shell (BS) configurations with significant metal-diradical (MDR) character: •M-O•: note that these bonds are therefore regarded as typical examples of strongly correlated electron systems. Because of the MDR character, 1,4-metal diradical mechanism was indeed preferable to four-centered mechanism in the case of addition reaction of naked Mn(IV) = O to ethylene. Recently the manganese-oxo species have been receiving renewed interest in relation to catalytic cycle of oxygen evolution from water molecules in the photosynthesis II (PSII) system. Accumulated experimental results indicate that this process is catalyzed with four manganese oxide clusters coordinated with calcium ion (CaMn4O4). Past decade we have performed BS MO theoretical investigations of manganese oxide clusters related to CaMn4O4. These calculations have elucidated that high-valent Mn(X) = O (X = IV,V) bonds exhibit intermediate MDR character (y=40-60%) in the case of total low-spin (LS) configuration but the MDR character decreases with coordination of Ca2+ and water molecules. While the MDR character of the Mn-oxo bonds becomes very high at the high-spin (HS) configuration. Our computational results enabled us to propose two possible mechanisms on the theoretical ground: (A) electrophilic (EP) mechanism and (B) radical coupling (RC) mechanism. The theoretical results indicate that the EP mechanism is preferable for the low-spin (LS) state in polar media like in the protein

  5. Insights into the Nature of the Chemical Bonding in Thiophene-2-thiol from X-ray Absorption Spectroscopy.

    PubMed

    Cotelesage, Julien J H; Pushie, M Jake; Vogt, Linda; Barney, Monica; Nissan, Andrew; Pickering, Ingrid J; George, Graham N

    2016-09-01

    Thiophenes are the simplest aromatic sulfur-containing compounds; they are widespread in fossil fuels and a variety of natural products, and they have vital roles in determining characteristic aromas that are important in food chemistry. We used a combination of sulfur K-edge X-ray absorption spectroscopy and density functional theory to investigate the chemical bonding in the novel sulfur-containing heterocycle thiophene-2-thiol. We show that solutions of thiophene-2-thiol contain significant quantities of the thione tautomer, which may be the energetically preferred 5H-thiophene-2-thione or the more accessible 3H-thiophene-2-thione. PMID:27508425

  6. Synthesis, chemical bonding and physical properties of RERhB{sub 4} (RE=Y, Dy-Lu)

    SciTech Connect

    Veremchuk, I.; Mori, T.; Prots, Yu.; Schnelle, W.; Leithe-Jasper, A.; Kohout, M.; Grin, Yu.

    2008-08-15

    The compounds of rare-earth metals with rhodium and boron RERhB{sub 4} (RE=Y, Dy-Lu) crystallize with the orthorhombic structure type YCrB{sub 4} (space group Pbam, Pearson symbol oP24). The crystal structures of the compounds with RE=Y, Er, Tm and Yb were refined by using single-crystal diffraction data. Analysis of chemical bonding for YRhB{sub 4} and YbRhB{sub 4} was performed by electron localizability indicator and by calculation of quantum chemical charges (quantum theory of atoms in molecules). Boron and rhodium form the 3-D polyanion containing planar nets of three-bonded boron atoms interconnected by rhodium along [001]. The interaction of the RE species with the rhodium-boron polyanion is predominantly ionic. Magnetic susceptibility data of TmRhB{sub 4} and YbRhB{sub 4} showed that the RE species are in 4f{sup 12} (Tm) and 4f{sup 13} (Yb) electronic states, respectively. In the low-temperature region, the specific heat revealed a Schottky anomaly for TmRhB{sub 4} while an antiferromagnetic transition is observed at 3.5 K for YbRhB{sub 4}. X-ray absorption measurement at the Yb L{sub III} edge for YbRhB{sub 4} reveals the 4f{sup 13} state of ytterbium. - Graphical abstract: The compounds of rare-earth metals with rhodium and boron RERhB{sub 4} (RE=Y, Dy-Lu) crystallize with the orthorhombic structure type YCrB{sub 4}. Analysis of chemical bonding for YRhB{sub 4} and YbRhB{sub 4} was performed by electron localizability indicator and by calculation of quantum chemical charges (quantum theory of atoms in molecules). Boron and rhodium form the 3-D polyanion containing planar nets of three-bonded boron atoms interconnected by rhodium along [001]. The interaction of the RE species with the rhodium-boron polyanion is predominantly ionic.

  7. Ab initio study of electron-ion structure factors in binary liquids with different types of chemical bonding

    SciTech Connect

    Klevets, Ivan; Bryk, Taras

    2014-12-07

    Electron-ion structure factors, calculated in ab initio molecular dynamics simulations, are reported for several binary liquids with different kinds of chemical bonding: metallic liquid alloy Bi–Pb, molten salt RbF, and liquid water. We derive analytical expressions for the long-wavelength asymptotes of the partial electron-ion structure factors of binary systems and show that the analytical results are in good agreement with the ab initio simulation data. The long-wavelength behaviour of the total charge structure factors for the three binary liquids is discussed.

  8. High pressure stability analysis and chemical bonding of Ti1-xZrxN alloy: A first principle study

    NASA Astrophysics Data System (ADS)

    Chauhan, Mamta; Gupta, Dinesh C.

    2016-05-01

    First-principles pseudo-potential calculations have been performed to analyze the stability of Ti1-xZrxN alloy under high pressures. The first order phase transition from B1 to B2 phase has been observed in this alloy at high pressure. The variation of lattice parameter with the change in concentration of Zr atom in Ti1-xZrxN is also reported in both the phases. The calculations for density of states have been performed to understand the alloying effects on chemical bonding of Ti-Zr-N alloy.

  9. Non-empirical analysis of unusual chemical bonds. Part III*. [LiBH(NH 2) 2] +

    NASA Astrophysics Data System (ADS)

    Barone, Vincenzo; Lelj, Francesco; Pastore, Annalisa; Luigi Cristnziano, Pier; Russo, Nino

    Ab initio computations have been performed on the compound [LiBH(NH 2) 2] +in order to achieve a better understanding of the peculiar characteristics of the metal-to-ligand bond. Both bridged and classical syn and anti structures have been investigated and their geometries optimized with respect to all internal coordinates. The bridged structures is found to be more stable than syn form by 13 kcal mol -1, compared to 26.8 kcal mol -1 in the case of the isoelectronic allylithium. Furthermore the anti classical form is more stable than the syn form, while the converse is true for allylithium. A simple bonding scheme allows the interpretation of these results.

  10. Phase stability, chemical bonding and mechanical properties of titanium nitrides: a first-principles study.

    PubMed

    Yu, Shuyin; Zeng, Qingfeng; Oganov, Artem R; Frapper, Gilles; Zhang, Litong

    2015-05-01

    We have performed first-principles evolutionary searches for stable Ti-N compounds and have found, in addition to the well-known rock-salt TiN, new ground states Ti3N2, Ti4N3, Ti6N5 at atmospheric pressure, and Ti2N and TiN2 at higher pressures. The latter nitrogen-rich structure contains encapsulated N2 dumbbells with a N-N distance of 1.348 Å at 60 GPa. TiN2 is predicted to be mechanically stable and quenchable. Our calculations on the mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness) are in excellent agreement with the available experimental data. Further analyses of the electronic density of states, crystal orbital Hamilton population and the electron localization function reveal that the hardness is enhanced by strengthening directional covalent bonds and disappearance of Ti-Ti metallic bonding. PMID:25869225