Search

For a period of several years the philosopher of science Hasok Chang has promoted various inter-related views including pluralism, pragmatism, and an associated view of natural kinds. He has also argued for what he calls the persistence of everyday terms in the scientific view. Chang claims that terms like phlogiston were never truly abandoned but became transformed into different concepts that remain useful. On the other hand, Chang argues that some scientific terms such as acidity have suffered a form of "rupture", especially in the case of the modern Lewis definition of acids. Chang also complains that the degree of acidity of a Lewis acid cannot be measured using a pH meter and seems to regard this as a serious problem. The present paper examines some of these views, especially what Chang claims to be a rupture in the definition of acidity. It is suggested that there has been no such rupture but a genuine generalization, on moving from the Brønsted-Lowry theory to the Lewis theory of acidity. It will be shown how the quantification and measurement of Lewis acidity can easily be realized through the use of equilibrium theory and the use of stability constants. [ABSTRACT FROM AUTHOR]

With the rapid development of semiconductors, the number of materials needed to be polished sharply increases. The material properties vary significantly, posing challenges to chemical mechanical polishing (CMP). Accordingly, the study aimed to classify the material removal mechanism. Based on the CMP and atomic force microscopy results, the six representative metals can be preliminarily classified into two groups, presumably due to different material removal modes. From the tribology perspective, the first group of Cu, Co, and Ni may mainly rely on the mechanical plowing effect. After adding H2O2, corrosion can be first enhanced and then suppressed, affecting the surface mechanical strength. Consequently, the material removal rate (MRR) and the surface roughness increase and decrease. By comparison, the second group of Ta, Ru, and Ti may primarily depend on the chemical bonding effect. Adding H2O2 can promote oxidation, increasing interfacial chemical bonds. Therefore, the MRR increases, and the surface roughness decreases and levels off. In addition, CMP can be regulated by tuning the synergistic effect of oxidation, complexation, and dissolution for mechanical plowing, while tuning the synergistic effect of oxidation and ionic strength for chemical bonding. The findings provide mechanistic insight into the material removal mechanism in CMP. [ABSTRACT FROM AUTHOR]

The quantum theory of atoms in molecules (QTAIM) uses physics to define an atom and its contribution to observable properties in a given system. It does so using the electron density and its flow in a magnetic field, the current density. These are the two fields that Schrödinger said should be used to explain and understand the properties of matter. It is the purpose of this paper to show how QTAIM bridges the conceptual gulf that separates the observations of chemistry from the realm of physics and do so in a manner that is both rigorous and conceptually simple. Since QTAIM employs real measurable fields, it enables one to present the findings of complex quantum mechanical calculations in a pictorial manner that isolates the essential physics. The time has arrived for a sea change in our attempts to predict and classify the observations of chemistry, time to replace the use of simplified and arbitrary models with the full predictive power of physics, as applied to an atom in a molecule. [ABSTRACT FROM AUTHOR]

Students' misconceptions before or after formal instruction have become a major concern among researchers in science education because they influence how students learn new scientific knowledge, play an essential role in subsequent learning and become a hindrance in acquiring the correct body of knowledge. In this paper some students' misconceptions on chemical bonding reported in the literature were investigated and presented. With this aim, a detailed literature review of chemical bonding was carried out and the collected data was presented from past to day historically. On the basis of the results some suggestions for teaching were made. [ABSTRACT FROM AUTHOR]

Keywords: Experimental Technique; Chemical Bonding; Model Diffusion; Molecular Modeling; International Workshop EN Experimental Technique Chemical Bonding Model Diffusion Molecular Modeling International Workshop 1 1 1 10/20/23 20230902 NES 230902 An erratum to this article is available at http://dx.doi.org/10.1007/s00894-005-0283-4. The international workshop "Modeling & Design of Molecular Materials", held 16-20 September 2004 in Wroclaw Experimental Technique, Chemical Bonding, Model Diffusion, Molecular Modeling, International Workshop. [Extracted from the article]

Robin Hendry has presented an account of two equally valid ways of understanding the nature of chemical bonding, consisting of what the terms the structural and the energetic views respectively. In response, Weisberg has issued a "challenge to the structural view", thus implying that the energetic view is the more correct of the two conceptions. In doing so Weisberg identifies the delocalization of electrons as the one robust feature that underlies the increasingly accurate quantum mechanical calculations starting with the Heitler-London method and moving on to such approaches as the valence bond and molecular orbital theories of chemical bonding. The present article provides a critical evaluation of Weisberg's article and concludes that he fails to characterize the nature of chemical bonding in several respects. I claim that Hendry's structural and energetic views remain as equally viable ways of understanding chemical bonding. Whereas the structural view is more appropriate for chemists, the energetic view is preferable to physicists. Neither view is more correct unless one subscribes to the naively reductionist view of considering that the more physical energetic view is the more correct one. [ABSTRACT FROM AUTHOR]

Better models to interpret physical properties as well as use of new preparative techniques have allowed in recent years the development of the predictive character of solid state chemistry. Various examples of this evolution have been given in this paper. It has been shown for instance that new materials (sometimes metastable ones) can be prepared, either by ion exchange or by chemical (or electrochemical) intercalation-deintercalation methods. The structural requirements of such reactions will be discussed. Another example is provided by the way one selects fast ionic conductors for energy storage. Various factors like amorphous nature, covalency, bond competition, presence of vacancies, influence of cationic lone pairs are discussed in the case of alkali ion or fluorine ion electrolytes. Attention is also paid to the stabilization of unusual electronic configurations, particularly for high oxidation states of transition elements, on the base of relevant structural and bonding considerations. In the field of magnetic properties, the possibility of obtaining new ferrimagnetic or ferromagnetic materials is discussed, as well as the ability to modify 1D-or 2D-behaviour of chain-or layer-type materials. Examples are also given showing how the performance of non-linear materials can be modified by playing with the factors determining structural distortions. Optical properties of materials are also strongly influenced by structural and bonding properties. It will be shown for instance how thanks to the relevant choice of a host, the lattice as well as the chemical composition of the material may lead to pure monochromatic 4 f UV emission in a Eu-doped material. Another example illustrates the possibility of obtaining a strong green emission in a[Ce, Tb]doped phosphate by choosing a formulation leading to best possible over-lapping of the Ce emission spectrum under appropriate UV illumination and the D → F excitation spectrum of Tb [ABSTRACT FROM AUTHOR]

Two-dimensional MoS2 film can grow on oxide substrates including Al2O3 and SiO2. However, it cannot grow usually on non-oxide substrates such as a bare Si wafer using chemical vapor deposition. To address this issue, we prepared as-synthesized and transferred MoS2 (AS-MoS2 and TR-MoS2) films on SiO2/Si substrates and studied the effect of the SiO2 layer on the atomic and electronic structure of the MoS2 films using spherical aberration-corrected scanning transition electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The interlayer distance between MoS2 layers film showed a change at the AS-MoS2/SiO2 interface, which is attributed to the formation of S–O chemical bonding at the interface, whereas the TR-MoS2/SiO2 interface showed only van der Waals interactions. Through STEM and EELS studies, we confirmed that there exists a bonding state in addition to the van der Waals force, which is the dominant interaction between MoS2 and SiO2. The formation of S–O bonding at the AS-MoS2/SiO2 interface layer suggests that the sulfur atoms at the termination layer in the MoS2 films are bonded to the oxygen atoms of the SiO2 layer during chemical vapor deposition. Our results indicate that the S–O bonding feature promotes the growth of MoS2 thin films on oxide growth templates. [ABSTRACT FROM AUTHOR]

First-principles investigation of elastic, electronic, and optical properties of orthorhombic Na3AlF6 has been carried out by DFT using plane-wave pseudo-potentials within the LDA and GGA. Calculated lattice parameters agree well with experimental results. From calculated elastic constants, Na3AlF6 is a mechanically stable anisotropic and behaves in a ductile manner. Electronic structure analysis indicates that Na3AlF6 behaves as an insulator with a direct band gap of 6.065 eV in LDA and 5.868-5.949 eV in GGA. DOS, population analysis, and charge densities difference indicate that Al-F bonds are mainly ionic as well as partially covalent due to the hybridization of F-2p and Al-3s (3p) states. Moreover, the imaginary part of calculated dielectric function ε2(ω) shows three prominent peaks due to the inter band transitions F 2p states→Na 3s states. From calculated ε (ω), other optical properties such as reflectivity and refractive index are also obtained up to the photon energy range of 40 eV. [ABSTRACT FROM AUTHOR]

The highly precise spin-polarized density functional approach is utilized to study the structural properties, chemical bonding, mechanical and optoelectronic properties of the cubic YMO (M = Ti, V and Nb) pyrochlores. The calculated structural parameters and geometries are consistent with experimental results. Electronic charge density explains that the nature of the bond between Y and M is covalent, whereas Ti-O, V-O and Nb-O bonds are ionic. Mechanical properties of these compounds demonstrate that YNbO and YVO are ductile, whereas YTiO is brittle in nature. The electronic band structures confirm the semiconducting nature for all the three materials with band gaps 2.8 eV, 0.2 eV, and 0.4 eV for YTiO, YVO and YNbO, respectively. Optical properties are calculated for the optically active compounds, which show that YTiO and YVO are highly dielectric materials and are optically active in the visible and infrared regions of the electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

The ground and low-lying electronic states of ReB were studied at the CASPT2//CASSCF level (multiconfigurational second-order perturbation theory) and quadruple-ζ ANO-RCC basis sets. Spectroscopic constants, potential energy curves, wavefunctions, and Mulliken population analysis are given. The ground state of ReB is of X Σ symmetry ( R = 1.817 Å, ω = .909 cm, and μ = 2.87 D), giving rise to a Ω = 0 ground state after including spin-orbit coupling. [ABSTRACT FROM AUTHOR]

The aim of this study was to determine the effect of jigsaw cooperative learning and computer animation techniques on academic achievements of first year university students attending classes in which the unit of chemical bonding is taught within the general chemistry course and these students' learning of the particulate nature of matter of this unit. The sample of this study consisted of 115 first-year science education students who attended the classes in which the unit of chemical bonding was taught in a university faculty of education during the 2009-2010 academic year. The data collection instruments used were the Test of Scientific Reasoning, the Purdue Spatial Visualization Test: Rotations, the Chemical Bonding Academic Achievement Test, and the Particulate Nature of Matter Test in Chemical Bonding (CbPNMT). The study was carried out in three different groups. One of the groups was randomly assigned to the jigsaw group, the second was assigned to the animation group (AG), and the third was assigned to the control group, in which the traditional teaching method was applied. The data obtained with the instruments were evaluated using descriptive statistics, one-way ANOVA, and MANCOVA. The results indicate that the teaching of chemical bonding via the animation and jigsaw techniques was more effective than the traditional teaching method in increasing academic achievement. In addition, according to findings from the CbPNMT, the students from the AG were more successful in terms of correct understanding of the particulate nature of matter. [ABSTRACT FROM AUTHOR]

By means of first-principles calculations, we have investigated the effects of rare earth elements (REEs) on the structures and mechanical properties of magnesium. The lattice parameters, elastic constants, bulk moduli, shear moduli, Young's moduli and anisotropic parameter of these solid solutions have been calculated and analyzed. The nearest-neighbor distance between Mg and the REEs is also analyzed to explore the correlation with the bulk moduli. The results show that the 4 f-electrons and atomic radii play an important role in the strengthening process. The anomalies of the lattice parameters and mechanical properties at Eu and Yb are due to the half-filled and full-filled 4 f-electron orbital states. Finally, the increase of directional bonding character near the alloying elements may account for the anisotropy and brittleness of these magnesium alloys. [ABSTRACT FROM AUTHOR]

The present study characterizes changes in the electronic structure of reactants during chemical reactions based on the combined charge and energy decomposition scheme, ETS-NOCV (extended transition state-natural orbitals for chemical valence). Decomposition of the activation barrier, Δ E, into stabilizing (orbital interaction, Δ E, and electrostatic, Δ E) and destabilizing (Pauli repulsion, Δ E, and geometry distortion energy, Δ E) factors is discussed in detail for the following reactions: ( I) hydrogen cyanide to hydrogen isocyanide, HCN → CNH isomerization; ( II) Diels-Alder cycloaddition of ethene to 1,3-butadiene; and two catalytic processes, i.e., ( III) insertion of ethylene into the metal-alkyl bond using half-titanocene with phenyl-phenoxy ligand catalyst; and ( IV) B-H bond activation catalyzed by an Ir-containing catalyst. Various reference states for fragments were applied in ETS-NOCV analysis. We found that NOCV-based deformation densities (Δ ρ) and the corresponding energies Δ E(i) obtained from the ETS-NOCV scheme provide a very useful picture, both qualitatively and quantitatively, of electronic density reorganization along the considered reaction pathways. Decomposition of the barrier ΔE into stabilizing and destabilizing contributions allowed us to conclude that the main factor responsible for the existence of positive values of Δ E for all processes ( I, II, III and IV) is Pauli interaction, which is the origin of steric repulsion. In addition, in the case of reactions II, III and IV, a significant degree of structural deformation of the reactants, as measured by the geometry distortion energy, plays an important role. Depending on the reaction type, stabilization of the transition state (relatively to the reactants) originating either from the orbital interaction term or from electrostatic attraction can be of vital importance. Finally, use of the ETS-NOCV method to describe catalytic reactions allows extraction of information on the role of catalysts in determination of Δ E. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Memory-quality silicon nitride has been deposited using plasma-enhanced chemical vapor deposition (PECVD). Film composition was varied by controlling the nitrogen concentration of the reactant gases. The effects of the source and content of the nitriding agent on the physical properties of the film were studied using ellipsometry and ultraviolet (UV), fourier transform infrared (FTIR) and Auger electron spectroscopy. Refractive index of the films varied from 1.77 to 1.95 corresponding to Si/N ratios of 0.75 to 1.03. Ultraviolet spectroscopy yielded band edge values of 4.9 to 2.2 eV depending on the Si/N ratio. Window size, endurance and retention performance is comparable to that reported for both atmospheric- and low-pressure chemical vapor deposited films. A strong correlation between the Si-H bond concentration and the memory performance was observed. Although some excess silicon in the film is needed for memory operation in a metal-nitride-oxide-silicon (MNOS) structure, excessive amounts result in low breakdown fields, small memory windows and poor retention characteristics. [ABSTRACT FROM AUTHOR]

When going beyond the Hartree-Fock level to correlated methods, one observes a significant reduction in the delocalization index. This is commonly interpreted as a weakening of electron sharing due to electron correlation, although this is rather counter-intuitive to the concomitant energy lowering. In this study, we use an analytical valence bond model and full CI calculations to show that this reduction in the delocalization index actually goes hand in hand with increased covalent contributions at the expense of ionic contributions. This suggests that we should be careful in formulating interpretations of these results in (de)localization indices.Variation of the localization Δ(ΩA, ΩA) and delocalization index Δ(ΩA, ΩB) as a function of the parameter ω. By adjusting this parameter ω from π4 to 0, we can gradually change the underlying wave function from a Hartree-Fock to a Heitler-London description. [ABSTRACT FROM AUTHOR]

The Hellmann-Feynman theorem has, with a few exceptions, not been exploited to the degree that it merits. This is due, at least in part, to a widespread failure to recognize that its greatest value may be conceptual rather than numerical, i.e., in achieving insight into molecular properties and behavior. In this brief overview, we shall discuss three examples of significant concepts that have come out of the Hellmann-Feynman theorem: (1) The forces exerted upon the nuclei in molecules are entirely Coulombic in nature. (2) The total energies of atoms and molecules can be expressed rigorously in terms of just the electrostatic potentials at their nuclei that are produced by the electrons and other nuclei. (3) Dispersion forces are due to the attractions of nuclei to their own polarized electronic densities. To summarize, energy and force analyses should not be viewed as competitive but rather as complementary. [ABSTRACT FROM AUTHOR]