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Search Results: 1 - 10 of 8609 matches for " Chemical Bond "
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The Genesis of the Quantum Theory of the Chemical Bond  [PDF]
Salvatore Esposito, Adele Naddeo
Advances in Historical Studies (AHS) , 2014, DOI: 10.4236/ahs.2014.35020
Abstract: An historical overview is given of the relevant steps that allowed the genesis of the quantum theory of the chemical bond, starting from the appearance of the new quantum mechanics and following later developments till approximately 1931. General ideas and some important details are discussed concerning molecular spectroscopy, as well as quantum computations for simple molecular systems performed within perturbative and variational approaches, for which the Born- Oppenheimer method provided a quantitative theory accounting for rotational, vibrational and electronic states. The novel concepts introduced by the Heitler-London theory, complemented by those underlying the method of the molecular orbitals, are critically analyzed along with some of their relevant applications. Further improvements in the understanding of the nature of the chemical bond are also considered, including the ideas of one-electron and three-electron bonds introduced by Pauling, as well as the generalizations of the Heitler-London theory firstly performed by Majorana, which allowed the presence of ionic structures into homopolar compounds and provided the theoretical proof of the stability of the helium molecular ion. The study of intermolecular interactions, as developed by London, is finally examined.
A rule-based algorithm for automatic bond type perception
Zhang Qian,Zhang Wei,Li Youyong,Wang Junmei
Journal of Cheminformatics , 2012, DOI: 10.1186/1758-2946-4-26
Abstract: Assigning bond orders is a necessary and essential step for characterizing a chemical structure correctly in force field based simulations. Several methods have been developed to do this. They all have advantages but with limitations too. Here, an automatic algorithm for assigning chemical connectivity and bond order regardless of hydrogen for organic molecules is provided, and only three dimensional coordinates and element identities are needed for our algorithm. The algorithm uses hard rules, length rules and conjugation rules to fix the structures. The hard rules determine bond orders based on the basic chemical rules; the length rules determine bond order by the length between two atoms based on a set of predefined values for different bond types; the conjugation rules determine bond orders by using the length information derived from the previous rule, the bond angles and some small structural patterns. The algorithm is extensively evaluated in three datasets, and achieves good accuracy of predictions for all the datasets. Finally, the limitation and future improvement of the algorithm are discussed.
Experimental Investigations on the Influence of Adhesive Oxides on the Metal-Ceramic Bond
Susanne Enghardt,Gert Richter,Edgar Richter,Bernd Reitemeier,Michael H. Walter
Metals , 2015, DOI: 10.3390/met5010119
Abstract: The objective of this study was to test the influence of selected base metals, which act as oxide formers, on the metal-ceramic bond of dental veneer systems. Using ion implantation techniques, ions of Al, In and Cu were introduced into near-surface layers of a noble metal alloy containing no base metals. A noble metal alloy with base metals added for oxide formation was used as a reference. Both alloys were coated with a low-temperature fusing dental ceramic. Specimens without ion implantation or with Al 2O 3 air abrasion were used as controls. The test procedures comprised the Schwickerath shear bond strength test (ISO 9693-1), profile height (surface roughness) measurements (ISO 4287; ISO 4288; ISO 25178), scanning electron microscopy (SEM) imaging, auger electron spectroscopy (AES) and energy dispersive X-ray analysis (EDX). Ion implantation resulted in no increase in bond strength. The highest shear bond strengths were achieved after oxidation in air and air abrasion with Al 2O 3 (41.5 MPa and 47.8 MPa respectively). There was a positive correlation between shear bond strength and profile height. After air abrasion, a pronounced structuring of the surface occurred compared to ion implantation. The established concentration shifts in alloy and ceramic could be reproduced. However, their positive effects on shear bond strength were not confirmed. The mechanical bond appears to be of greater importance for metal-ceramic bonding.
On bond graph modelling of thermo-chemical processes
Monica Roman,Eugen Bobasu,Eugen Iancu,Dorin Sendrescu
Acta Montanistica Slovaca , 2010,
Abstract: The paper presents an approach of the Bond Graph modelling applied to thermo-chemical processes. The proposedwork focused on combustion process kinetics with respect to reactant and reactor input data. The model provides informationon the time variation of the heat of reaction, reaction products concentration, and reactants concentration / accumulation, basedon global mass and energy balance of the process. The basic reaction between solid carbon and oxygen was considered to modelthe combustion solid fuel. The model can be used as base for the development of multi-component combustion reactions with enhancedthermal transfer.
The odd and contradictory relationship between chemists and theoretical chemistry [A estranha e contraditória rela o entre os químicos e a química teórica.]
André Gustavo H. Barbosa
Revista Virtual de Química , 2009,
Abstract: Through a brief historical analysis we show how the chemistry language had evolved independentlyof other sciences until the end of the nineteenth century. At that time chemical models were entirelyincompatible with those from physics. From the 1910 decade on, after the beginning of the physicistselucidation of some elements of atomic structure, chemists started using ideas from atomic physics in theirmodels for the chemical bonding. But only with the advent of quantum mechanics in the decade of 1920 itbecame possible, in principle, to base an entire chemical model on the results of electronic structure theoreticalconsiderations. However, the chemist problems were more complicated than what the physicists were able todeal with. For this reason, simplified models were formulated. The chemist choice of which model to use is onlyguided by availability, easiness to use and lack of need for extensive additional study. The consequence of suchprocedure is the proliferation of baseless concepts and ideas which eventually become present in manychemistry textbooks. A few examples are discussed where it is demonstrated that the utilization of wavefunctions that fully reflect the fact that the electrons are indistinguishable, can provide qualitatively correct anduseful models. We conclude establishing the real purpose of theoretical chemistry and proposing ways toimprove the chemistry teaching.
How the Even-Odd Rule, by Defining Electrons Pairs and Charge Positions, Can Be Used as a Substitute to the Langmuir-Octet Rule in Understanding Interconnections between Atoms in Ions and Molecules  [PDF]
Geoffroy Auvert
Open Journal of Physical Chemistry (OJPC) , 2015, DOI: 10.4236/ojpc.2015.52004
Abstract: In the course of time, numerous rules were proposed to predict how atoms connect through covalent bonds. Based on the classification of elements in the periodic table, the rule of eight was first proposed to draw formulas of organic compounds. The later named octet rule exhibited shortcomings when applied to inorganic compounds. Another rule, the rule of two, using covalent bonds between atoms, was proposed as an attempt to unify description of organic and inorganic molecules. This rule unfortunately never managed to expand the field of application of the octet rule to inorganic compounds. In order to conciliate organic and inorganic compounds, the recently put forward even-odd and the isoelectronicity rules suggest the creation of one group of compounds with pairs of electrons. These rules compass the rule of two for covalent bonds as well as the octet rule for organic compounds and suggest transforming bonds of multi-bonded compounds in order to unify representations of both groups of compounds. The aim of the present paper is fourfold: to extend the rule of two to every atom shells; to replace the well-known octet rule by the even-odd rule; to apply the isoelectronicity rule to each atom and to reduce the influence range of the charge of an atom in a compound. According to both rules, the drawing of one atom with its single-covalent bonds is described with electron pairs and charge positions. To illustrate the rules, they are applied to 3D configurations of clusters.
The Even-Odd and the Isoelectronicity Rules Applied to Single Covalent Bonds in Ionic, Double-Face-Centered Cubic and Diamond-Like Crystals  [PDF]
Geoffroy Auvert, Marine Auvert
Open Journal of Physical Chemistry (OJPC) , 2016, DOI: 10.4236/ojpc.2016.62002
Abstract: Although atom configuration in crystals is precisely known thanks to imaging techniques, there is no experimental way to know the exact location of bonds or charges. Many different representations have been proposed, yet no theory to unify conceptions. The present paper describes methods to derive bonds and charge location in double-face-centered cubic crystals with 4 and 6 atoms per unit cell using two novel rules introduced in earlier works: the even-odd and the isoelectronicity rules. Both of these rules were previously applied to ions, molecules and some solids, and the even-odd rule was also tested on two covalent crystal structures: centered-cubic and single-face-centered cubic crystals. In the present study, the diamond-like structure was subjected to the isoelectronicity rule in order to derive Zinc-blende structures. Rock-salt-like crystals were derived from each other using both rules. These structures represent together more than 230 different crystals. Findings for these structures are threefold: both rules describe a very sure method to obtain valid single covalent-bonded structures; single covalent structures can be used in every case instead of the classical ionic model; covalent bonds and charges positions do not have any relation with the valence number given in the periodic table.
Bonding Characteristics of TiC and TiN  [PDF]
Kuiying Chen, Sami Kamran
Modeling and Numerical Simulation of Material Science (MNSMS) , 2013, DOI: 10.4236/mnsms.2013.31002
Abstract:

Using ab initio density functional theory calculations, the electron localization function (ELF) of typical transition metal carbide TiC and nitride TiN were computed and analyzed to reveal their nature of the chemical bonds. The ELF approach was initially validated through typical examples of covalent-bonding Diamond (C) and ionic-bonding sodium chloride NaCl. Our results clearly demonstrate the dominantly ionic bonding characteristics of TiC and TiN. It is also suggested that the high mechanical hardness of TiC and TiN can be explained without evoking strong covalence.

Synthesis, Spectroscopic Investigations, Quantum Chemical Studies (Ab-initio & DFT) and Antimicrobial Activities of 3-(3-Chloro-4,5-dimethoxy-phenyl)-1-(4, 5-dimethoxy-2-methyl-Phenyl) prop-2-en-1-one  [PDF]
Urmila H. Patel, Sahaj A. Gandhi, Vijay M. Barot, Mitesh C. Patel
Crystal Structure Theory and Applications (CSTA) , 2013, DOI: 10.4236/csta.2013.24023
Abstract:

The chalcones (1,3-diaryl-2-propenones) and their derivatives are important intermediates in organic synthesis and have widespread applications in medicinal industry. The title choloro chalcone derivative, 3-(3-chloro-4,5-dimethoxyphenyl)-1-(4,5-dimethoxy-2-methyl phenyl) prop-2-en-1-one, has been synthesized. It is characterized by FTIR, 1H NMR, 13C NMR and single crystal X-ray diffraction. Title compound crystallizes in monoclinic space group C2/c with a = 23.540(11) ?, b = 9.738(4) ?, c = 17.305(7) ?, β = 106.37 (3)°, V = 3806(3) ?3 and Z = 8. The mean plane of the two substituted benzene rings are twisted by 66.29 (12)° with respect to each other. Ab-initio and density functional Theory (DFT) calculations have been carried out for the title molecule using RHF/6-311G and B3LYP/6-311G basis set respectively. The calculated

The nature of the chemical bond
Nascimento, Marco Antonio Chaer;
Journal of the Brazilian Chemical Society , 2008, DOI: 10.1590/S0103-50532008000200007
Abstract: in this paper we briefly review the basic requirements that must be satisfied by any wave function representing many-electron systems. following that, we examine the conditions under which the classical concepts of molecular structure, chemical structure and chemical bond can be translated into a quantum-mechanical language. essential to this aim is the utilization of an independent particle model (ipm) for a many-electron system. in spite of the great popularity of the hartree-fock (hf) model only valence-bond (vb) type wave functions with optimized, singly occupied and non necessarily orthogonal atomic-like orbitals, can provide a quantum-mechanical translation of the classical concepts of chemical structure and chemical bond, although the hf model can still be useful for translating the concept of molecular structure. finally, a quantum-dynamical-type of analysis allows us to conclude that, from the quantum mechanical point of view, the chemical bond is a consequence of interference effects. from the energetic point of view, the interference effect responsible for the bond formation manifests itself as a reduction of the kinetic energy of the electrons as the bond is formed.
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