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Chemical Bonds (chemical + bond)
Selected AbstractsExploring the Actinide,Actinide Bond: Theoretical Studies of the Chemical Bond in Ac2, Th2, Pa2, and U2CHEMINFORM, Issue 12 2007Bjoern O. Ross Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Topological Analysis of Electron Densities: Is the Presence of an Atomic Interaction Line in an Equilibrium Geometry a Sufficient Condition for the Existence of a Chemical Bond?CHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2004Arne Haaland Prof. Abstract The structure, energetics, and electron density in the inclusion complex of He in adamantane, C10H16, have been studied by density functional theory calculations at the B3LYP6-311++G(2p,2d) level. Topological analysis of the electron density shows that the He atom is connected to the four tertiary tC atoms in the cage by atomic interaction lines with (3,,1) critical points. The calculated dissociation energy of the complex He@adamantane(g)=adamantane(g) + He(g) of ,E=,645 kJ,mol,1 nevertheless shows that the He,tC interactions are antibonding. [source] ChemInform Abstract: Phase Stabilities of Monoclinic Oxoborates LaB3O6 and GdB3O6 in C121 and I12/a1 Phase,Energetics and Chemical Bonds Derived from First-Principles Calculations.CHEMINFORM, Issue 18 2008Jun Yang Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Analytic Models of Domain-Averaged Fermi Holes: A New Tool for the Study of the Nature of Chemical BondsCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2008Robert Ponec Prof. Abstract Simple analytical models are introduced that significantly enhance the ability to understand and rationalise the nature of bonding interactions depicted by domain-averaged Fermi hole (DAFH) analysis. The examples presented show that besides shedding new light on the role of electron-sharing in ordinary two-centre two-electron (2c,2e) chemical bonds that are well represented by the classical Lewis model, the proposed approach also provides interesting new insights into the nature of bonding interactions that go beyond the traditional Lewis paradigm. This is, for example, the case of 3c,2e multicentre bonding, but a straightforward extension of the approach also reveals for direct metal,metal bonding the existence of a completely new type of bonding interaction that involves the mutual exchange of electrons between the lone pairs on adjacent metal atoms. [source] The charge capacitance of the chemical bond: Application to bonds containing metalsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2009Asbjørn Holt Abstract The charge capacitance of metal containing complexes are studied. For molecules with multiple bonding between the metal atoms it is found that the charge capacitance is correlated to the maximum bond order, natural bond order, and to some extent the effective bond order. Furthermore the charge capacitance of some methylidene metal dihydride complexes are studied. These molecules have agostic interactions of varying strength, and it is concluded that this strength is very well reflected in the charge capacitances of the systems. In accordance with the definition of agostic interactions it is therefor concluded that the charge capacitance holds information about the strength of covalent interactions. The effect therefore on the agostic interactions upon substitution of one of the hydrogen atoms with fluorine in the methylidene metal complexes is studied, and found to reduce the agostic interactions. It is also demonstrated that there is an agostic interaction in an ArCrCrAr complex. The distance dependence of the charge capacitance is also discussed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Bond length features of linear carbon chains of finite to infinite size: Visual interpretation from Pauling bond ordersINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2003Zexing Cao Abstract Schemes for Kekulé structure counting of linear carbon chains are suggested. Mathematical formulas, which calculate the Pauling bond order P(k, N) of a chemical bond numbered by k, are given for the carbon chain with N carbon atoms. By use of the least-squares fitting of a linearity, relationships between Pauling bond orders and bond lengths are obtained, and such correlation of the Pauling bond order,bond length can be qualitatively extended to the excited states. The relative magnitudes of Pauling bond orders in unsaturated carbon chains dominate C,C bond lengths a well as the bond length feature with the chain size increasing. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 144,149, 2003 [source] HF,CC model for atoms and molecules,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2002E. Clementi Abstract The Hartree,Fock,Clementi,Corongiu method (HF,CC) is revisited, aiming at an unified formulation for post-HF energy computations in atomic and molecular systems. For atomic systems new parameterizations of the HF,CC functional are proposed for the computation of atoms. The previous HF,CC molecular functional (Clementi, E.; Corongiu, G. Theochem 2001, 543, 39), revisited and recalibrated with a new optimization of the parameters, is tested with a sample of 131 molecules, including radicals, H-bond, and van der Waals systems. The atomization energy is decomposed into "HF classic" energy (the sum of the HF nuclear electron, HF kinetic, and HF Coulomb energies), "HF exchange" energy, and correlation energy; the latter is computed with a scaling functional with atomic, covalent, ionic, and van der Waals contributions. For the sample of 131 molecules, the computed HF,CC atomization energies have an average standard deviation of 1.89 kcal/mol. The atomic and molecular components of the correlation energy are decomposed into nuclear electron, kinetic, Coulomb, and exchange contributions; these decompositions characterize the HF,CC model and are used to explain the origin of the chemical bond. Computations on van der Waals systems show the validity of the HF,CC method also for long-range weak interactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] FTIR, 1H-NMR spectra, and thermal characterization of water-based polyurethane/acrylic hybridsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2008O. R. Pardini Abstract Polyurethane (PU) polymer was synthesized following a prepolymer mixing process, by polyaddition of isophorone diisocyanate (IPDI), poly(propylene glycol) (PPG), 2-hydroxyethyl methacrylate (HEMA), and 2,2-bis(hydroxymethyl)propionic acid (DMPA). The PU anionomer having 2-ethoxymethacrylate terminal groups was dispersed in water by prior neutralization of carboxylic acid groups of DMPA with triethylamine (TEA), chain extended with hydrazine (HZM) in water and a dispersion polymerization with methyl methacrylate/n -butyl acrylate/acrylic acid mixture was performed. The above polymerization reactions lead to the formation of PU/acrylic hybrids having a chemical bond between PU and acrylic moieties. Acrylic content was varied from 0 to 50 wt % and samples were purified to eliminate oligomers and impurities before characterization. The FTIR and 1H-NMR spectra of these purified hybrid samples were obtained and bands and peaks assignments were discussed. Thermal properties (DSC and TGA) were also discussed. Breaking hydrogen bonds is the main reason for changes in properties with increasing acrylic content. Particle size data of dispersions is also presented and discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Electron pairing and chemical bonds: Pair localization in ELF domains from the analysis of domain averaged Fermi holesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2007Robert Ponec Abstract This article reports the application of a recently proposed formalism of domain averaged Fermi holes to the problem of the localization of electron pairs in electron localization function (ELF) domains and its possible implications for the electron pair model of chemical bond. The main focus was on the systems, such as H2O or N2, in which the "unphysical" population of ELF domains makes the parallel between these domains and chemical bond questionable. On the basis of the results of the Fermi-hole analysis, we propose that the above problems could be due to the fact that in some cases the boundaries of the ELF domains need not be determined precisely enough. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2006 [source] Electron pairing and chemical bonds.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2005Electron fluctuation, pair localization in ELF domains Abstract This article reports the numerical comparison of the quantities characterizing the extent of electron fluctuation and pair localization in the domains determined by the direct minimization of electron fluctuation with the domains resulting from the partitioning of the molecules based on the topological analysis of the so-called electron localization function (ELF). Such a comparison demonstrates that the ELF partitioning can be regarded as a feasible alternative to computationally much more demanding direct optimization of minimum fluctuation domains. This opened the possibility of the systematic scrutiny of the electron pair model of the chemical bond, and as it was demonstrated, the previous pessimistic claims about the applicability of this model are not completely justified. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1205,1213, 2005 [source] Magnetic properties of NdMn2,xFexGe2 (x = 0.3, 0.4 and 1.0) compoundsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003. Gondek Abstract Magnetic properties of the NdMn2,xFexGe2 (x = 0.3, 0.4 and 1.0) are investigated by AC magnetic measurements and neutron diffraction. In ambient pressure in compound with x = 0.3 below 200 K the noncollinear magnetic structure is observed. Above this temperature the magnetic moments transform to the collinear one. For x = 0.4 at 1.5 K the Nd magnetic moments form collinear ferromagnetic structure, while Mn moments noncollinear. With increase of the temperature at T = 50 K Nd moment disappears while Mn moments form two different magnetic structures antiferro- (AF) and ferromagnetic (F). The change between AF and F phases is connected with the change of the lattice constants. For x = 1.0 the Mn moments form collinear antiferromagnetic structure stable in the temperature range between 1.5 K and TN = 203 K. For x = 0.3 the external pressure up to 1.5 GPa causes the change of the critical temperature only, while for x = 0.4 induces the change of the magnetic properties from ferro- to antiferromagnetic. For x = 1.0 the Néel temperature increases with increasing external pressure. The above results indicate that in the NdMn2,xFexGe2 system, the increase in the iron content is followed by a change in chemical bond that influences the magnetic properties. [source] A single law for the activation energies of self-diffusion of various cubic metals, intermetallic compounds, ionic crystals and oxidesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2007Hans Siethoff Abstract In previous publications the author put forward a relation which, for face-centred cubic metals and intermetallic compounds with B2 and L12 structures, connected the activation energy of self-diffusion with lattice constant and shear modulus. It is one aim of the present study to show that this formalism can be extended to intermetallic compounds with C1, D03 and C15 crystal structures. Since the covalently bonded cubic semiconductors and ceramics obey a different law, the question concerning the influence of the chemical bond was additionally investigated. Therefore ionic crystals and oxides with B1, B2 and C1 structures were analysed. It is demonstrated that these materials obey the same law as the metals and intermetallic compounds, for the B1 structure, however, the prefactor of the common rule is different. To be able to evaluate such differences, the proposed relation had to be more quantitatively derived than it was done before. Some cubic transition metals do not fit in the general picture. The deviations are traced back to the binding properties of the electronic d-bands. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of pressure on the lengths of chemical bondsACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2003I. David Brown An expression to describe the force that a chemical bond exerts on its terminal atoms is proposed, and is used to derive expressions for the bond force constant and bond compressibility. The unknown parameter in this model, the effective charge on the atoms that form the bond, is determined by comparing the derived force constants with those obtained spectroscopically. The resultant bond compressibilities are shown to generally agree well with those determined from high-pressure structure determinations and from the bulk moduli of high-symmetry structures. Bond valences can be corrected for pressure by recognizing that the bond-valence parameter, R0, changes with pressure according to the equation [source] Bond strengths between composite resin and auto cure glass ionomer cement using the co-cure techniqueAUSTRALIAN DENTAL JOURNAL, Issue 2 2006GM Knight Abstract Background: The clinical technique for sandwich restorations prescribes etching initially set auto cure glass ionomer cement (GIC) prior to placing a layer of resin bond to develop a weak mechanical bond between composite resin and GIC. Co-curing a resin modified glass ionomer cement (RMGIC) bond and composite resin to GIC may create a chemical bond and improve the bond strengths between these two materials. Methods: A total of 48 specimens were prepared, 12 in each of four categories. Capsulated GIC was placed into a mould and allowed to set for four minutes, etched for five seconds followed by placement of a resin bond and photo cured for five seconds over which a composite resin was puddled onto the resin bond and photo cured for 10 seconds. Capsulated GIC was placed into a mould and allowed to set for four minutes after which a sample of RMGIC (Riva LC) was prepared using twice the liquid powder ratio and painted over the surface of the set GIC using a micro brush. An increment of composite resin was added over the RMGIC and both materials were photo co-cured for 10 seconds. Capsulated GIC was placed into a mould and RMGIC (Riva LC) that had been prepared using twice the liquid powder was brushed over the GIC (prior to initial set) followed by the placement of a layer of composite resin and photo co-cured for 10 seconds. Capsulated GIC was placed into a mould and RMGIC (Fuji II LC) that had been prepared using twice the liquid powder was brushed over the GIC (prior to initial set) followed by the placement of a layer of composite resin and photo co-cured for 10 seconds. Shear testing of each of the samples was carried out and specimens were examined to determine the nature of the fracture. Selected samples were prepared for SEM investigation to observe the interfaces between the GIC and composite resin. Results: There were significantly lower bond strengths (P < 0.05) amongst samples that had been etched and bonded (2.42MPa) compared to the other samples that had been co-cure bonded with RMGIC (6.48,7.05MPa). There were no significant differences amongst the bond strengths of the samples co-cure bonded with RMGIC. Specimens prepared by the ,etch and bond' technique failed adhesively and co-cured specimens failed cohesively within the GIC. SEM investigation showed chemical bonds between RMGIC bond and GIC and composite resin. Conclusions: The co-cured RMGIC bonding system eliminates several placement steps and produces a significantly stronger chemical bond between GIC and composite resin than the ,etch and bond' technique. RMGIC bond and composite resin may be co-cured to GIC either before or after initial set has occurred. [source] Ligand Association/Dissociation Paths and Ill-Defined Coordination NumbersCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2010Antonio Ruiz-Martínez Abstract The continuous shape measures approach provides a means for handling the common situation in which a metal atom presents an ill-defined coordination number. Those cases are characterized by the presence of secondary interactions to Lewis bases at distances significantly longer than those expected for a chemical bond. Systematic ways of analyzing ligand association/dissociation pathways that describe such structures and their application to a variety of specific cases are presented. The concepts and methodology presented here apply to molecules and extended solids as well and provide, when needed, a more flexible and precise stereochemical description of the metal coordination sphere than that of an integer coordination number and the associated polyhedral shape. El mètode de les mesures contínues de forma ens permet tractar la situació trobada freqüentment, en què un àtom metàl,lic presenta un nombre de coordinació no ben definit, caracteritzat per la presència d,interaccions secundàries amb bases de Lewis a distàncies significativament més llargues que les que correspondrien a un enllaç químic. Aquí es presenten formes sistemàtiques d,analitzar els camins d,associació/dissociació de lligands que descriuen aquestes estructures, així com llur aplicació a diverses famílies de compostos. Els conceptes i la metodologia presentats en aquest article s'apliquen tant a molècules com a sòlids infinits i ens proporcionen una descripció estereoquímica més flexible i alhora precisa de les esferes de coordinació que no pas un nombre de coordinació enter i la forma polièdrica associada. [source] Die chemische Bindung , Lernhindernisse und mögliche LernhilfenCHEMKON - CHEMIE KONKRET, FORUM FUER UNTERRICHT UND DIDAKTIK, Issue 1 2003Christiane S. Reiners Prof. Dr. Abstract THE CHEMICAL BOND There is an often-quoted parable about a group of blind men who are taken to examine an elephant. Each comes in contact with a different part of the elephant and, therefore, draws a different conclusion about its nature: the man examining the trunk thinks, it is some kind of snake, the one touching its leg thinks it is a tree, and so on. The lesson of this story is, of course, that we can have many different views of the same reality, depending on where we are standing and what data we use for our conclusions. The chemical bond is the chemist's "elephant". Unable to see this construct that they use to explain the aggregation of atoms into molecules, the chemists interpret its existence in terms of their own views of chemistry, To each of these chemists the chemical bond has different descriptors, although they may all be investigating the same molecule. Thus, the chemist concerned with quantum mechanics regards the chemical bond as an interaction of wave functions, the more classical physical chemist sees a balancing of energies and charges among the component atoms, and the organic or inorganic chemist interested in the three-dimensional structure of the molecule interprets the bonds in terms of the geometry of the overlapping orbitals. [1, S. 190] [source] Disproving a Silicon Analog of an Alkyne with the Aid of Topological Analyses of the Electronic Structure and Ab Initio Molecular Dynamics CalculationsCHEMPHYSCHEM, Issue 9 2005Carlo A. Pignedoli Dr. Abstract A silicon compound has recently been synthesized that was claimed to exhibit the first realization of a silicon,silicon triple bond. We debate this classification on the basis of a thorough investigation of the nature of the chemical bond, using the rigorous topological analysis of the electron density as developed in Bader's atoms-in-molecules theory, that of the electron localization function and the related orbital-independent definitions of the bond order. Our results refer both to the ground-state geometry and to nonequilibrium configurations, which are accessed by the system in a room-temperature ab initio molecular dynamics simulation. We also use the reciprocal compliance force constant as an independent chemical descriptor. All the above procedures are in agreement and do not support the classification of the silicon,silicon central bond as triple. The characterization which consistently emerges from the present study is one in which two electron pairs participate in the bonding and the other pair belongs mainly to nonbonding regions. [source] Synthesis and characterization of potassium magnesium sulphate hexahydrate crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 4 2006M. Dhandapani Abstract Potassium magnesium sulphate hexahydrate (picromerite) was synthesized and single crystals were obtained from saturated aqueous solution by slow evaporation method at room temperature. The crystals were bright, colourless and transparent having well defined external faces. The grown crystals were characterized through Fourier Transform Infrared (FTIR) spectral studies and thermal analysis. The FTIR data were used to assign the characteristic vibrational frequencies of the various chemical bonds in the compound. The compound crystallizes in monoclinic lattice with the space group P21/c. The thermogravimetry (TG) indicates the removal of only two water molecules around 100 °C. A suitable decomposition pattern was formulated based on the percentage weight losses observed in TG of the compound. The results of differential thermal analysis (DTA) conform to the results of TGA. Differential scanning calorimetry (DSC) analysis carried out at high temperature suggests that the occurrence of two phase transitions in the crystal between 140 and 180 °C. When the crystal was cooled below the room temperature up to ,170 °C, no thermal anomaly was observed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A Series of Lithium Rare Earth Polyphosphates [LiLn(PO3)4] (Ln = La, Eu, Gd) and Their Structural, Optical, and Electronic PropertiesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2007Jing Zhu Abstract The structural, optical, and electronic properties of a series of lithium rare earth polyphosphates [LiLn(PO3)4] [Ln = La (1), Eu (2), Gd (3)] have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectroscopic measurements, as well as calculations of energy-band structures, density of states, and optical response functions by density functional methods. These LiLn(PO3)4 systems are monoclinic with space group C2/c and Z = 4. Their unit-cell parameters decrease as the ionic radius of Ln3+ decreases (La3+ > Eu3+ > Gd3+). Both (PO4)3, zig-zag chains and infinite chains formed by the alternate connection of LnO8 polyhedra and LiO4 tetrahedra run parallel to the b -axis in the structure. The energy-band structures, density of states, the chemical bonds, and optical properties have been investigated by density functional methods for some of the title compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Synthesis of SiCNO Nanowires Through Heat-Treatment of Polymer-Functionalized Single-Walled Carbon Nanotubes,ADVANCED MATERIALS, Issue 13 2004R.-G. Duan SiCNO nanowires (see Figure) have been successfully synthesized by heat-treatment of polymer-functionalized single-walled carbon nanotubes (SWNTs). The tangled networks of SWNT ropes associate tightly and uniformly with the polymer precursor, most likely via chemical bonds between the polymer and the SWNT surface. The SWNTs act as a template to confine the reaction with the polymer in the synthesis of the nanowires. [source] Theory of chemical bonds in metalloenzymes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010Abstract Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken-symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the d,-p, bond in high-valent (HV) Mn(X)O systems and the d,-p,-d, bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ,Mn(IV)O, and ,MnOMn,; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese-oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] The role of the Frontier orbitals in acid,base chemistry of organic amines probed by ab initio and chemometric techniquesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2010Felipe A. La Porta Abstract The Frontier effective-for-reaction molecular orbital (FERMO) concept emerges as a powerful and innovative implement to investigate the role of molecular orbitals (MOs) applied in the description of breakage and formation of chemical bonds. In this work, theoretical calculations were carried out for conjugated acids of 18 amines and their acid,base behavior was analyzed using MO energies. We observed that highest occupied MO (HOMO) energies are inadequate to describe the acid,base behavior of these compounds. By using the FERMO concept, the reactions that are driven by HOMO, and those that are not, can be better explained, independent of the calculation method used, as independent of the calculation method used, both HF and Kohn,Sham methodologies lead to the same FERMO. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Theory of chemical bonds in metalloenzymes XIII: Singlet and triplet diradical mechanisms of hydroxylations with iron-oxo species and P450 are revisitedINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2009Kizashi Yamaguchi Abstract Electronic structures of the Compound I (CpdI) in P450 are investigated on the basis of spin coupling forms of iron-oxo (Fe(IV)O) cores and radical ligand (,L) groups to generalize previous singlet and triplet diradical (TD) mechanisms for oxygenations of alkanes with Fe( IV)O. Orbital interaction schemes for four lower-lying spin configurations of CpdI with HC bond of substrate are examined to elucidate how magnetic coupling modes correlate with radical reaction pathways for hydroxylation reactions on the basis of the broken symmetry (BS) molecular orbital (MO) model. The configuration correlation diagrams for the four configurations model are depicted on the basis of the isoelectronic analogy among O, O2, and Fe( IV)O, in addition to Coulomb exchange energy on the iron site, which determines its local spin configuration. Important role of ligand spin (,L) of CpdI for regulation of hydroxylation mechanisms is clarified with the aid of the spin coupling forms. Transition states for one quartet and three doublet configurations under the BS MO approximation are examined on the basis of potential curve crossings along reaction pathways. The four transition structures and corresponding radical intermediates for methane and trimethyl methane with CpI are located by the BS hybrid Kohn,Sham density functional theory (DFT) (B3LYP) method to confirm the orbital interaction schemes. Spin density populations obtained by the BS B3LYP calculations are found to be consistent with the theoretical predictions based on the four configurations model. The configuration and state correlation diagrams by BS B3LYP before and after spin projection are also consistent with the BS MO interaction schemes, which provide local SD and TD mechanisms of hydroxylation with CpdI. The present BS MO-theoretical framework is useful for systematic understanding of a lot of recent BS hybrid DFT computational results for hydroxylation reactions with CpdI and configuration correlation diagrams reported by several groups. Implications of the present theoretical and computational results are discussed in relation to several experimental characteristics of hydroxylation reactions with iron-oxo species and P450. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Theory of chemical bonds in metalloenzymes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2008Abstract A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low-lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 (cpd1) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)O and a doublet ligand radical. This excited state lies 20.8 kcal mol,1 above the ground spin state, which is a non-negligible energy level as compared with the activation energy barrier of ,E# = 26.6 kcal mol,1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: 3O(3p) + CH4. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (3O-model) and insertion (1O-model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Theory of chemical bonds in metalloenzymes III: Full geometry optimization and vibration analysis of ferredoxin-type [2Fe,2S] clusterINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Mitsuo Shoji Abstract The nature of chemical bonds in a ferredoxin-type [2Fe,2S] cluster has been investigated on the basis of natural orbitals and several bond indices developed in Parts I and II of this study. The broken-symmetry hybrid density functional theory (BS-HDFT) with spin projection approach has been applied to elucidate the natural orbitals and occupation numbers for a model compound [Fe2S2(SCH3)4] (1), which is used to calculate the indices. The molecular structure, vibration frequencies, electronic structures, and magnetic properties in both oxidized and reduced forms of 1 have been calculated and compared with the experimental values. The optimized molecular structures after approximate spin projection have been in good agreement with experimental data. The structure changes upon one-electron reduction have been slight (<0.1 Å) and only limited around one side of the Fe atom. Raman and infrared (IR) spectra have been calculated, and their vibration modes have been assigned using the bridging 34S isotope substitution. Their magnetic properties have been examined in terms of spin Hamiltonians that contain exchange interactions and double exchange interactions. The BS-HDFT methods have provided the magnetic parameters; i.e., effective exchange integral (J) values and valence delocalization (B) values, which agree with the experimental results. It is found that large charge transfer (CT) from the bridging sulfur to the iron atoms has led to the strong antiferromagnetic interactions between iron atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Group functions approach based on the combination of strictly local geminals and molecular orbitalsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2006A. M. Tokmachev Abstract Thegroup functions technique is a natural way to introduce local description into quantum chemistry. It can also be a basis for construction of numerically effective computational schemes having almost linear growth of computational costs with that of the size of the system. Previously, we constructed a family of computationally efficient semiempirical methods based on the variationally determined strictly local geminals (SLGs). It was implemented with four popular parameterization schemes (MINDO/3, MNDO, AM1, and PM3). Because of construction details, its applicability was restricted only to compounds with well-defined two-electron two-center chemical bonds and lone pairs. We generalize the previous treatment to make the electronic structure calculations possible for a wider class of compounds without loss of computational efficiency. The proposed scheme (SLG/SCF) is based on the general group function approach combining different descriptions for different electron groups: essentially local two-electron ones are described by geminals, while those with other numbers of electrons are described in the one-electron approximation. We implement the RHF, UHF, and ROHF approaches for the groups with delocalized electrons. This approach is tested for a series of radicals and molecules with extended ,-electron systems. It is shown that the SLG/SCF-based methods describe the experimental data not worse than the corresponding SCF procedures and provide a good starting point for calculations of polyatomic molecular systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Toward a BSSE-free description of strongly interacting systemsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2002G.J. Halász Abstract The so-called "chemical Hamiltonian approach" (CHA) gives perfect a priori BSSE-free description of weak intermolecular interactions, but has been found inappropriate for describing strong interactions taking place within a molecule. Here, we propose a simple modification of the CHA/F BSSE-free SCF method, which retains all the good properties of the CHA/F method for the intermolecular case but can be used also to describe covalent and ionic interactions. This is vital for calculating whole potential surfaces of chemical reactions in a consistent manner, which was found impossible by using the a posteriori counterpoise correction method. Model calculations are presented for covalent and ionic chemical bonds and for a rare gas,proton system. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Symplectic molecular dynamics integration using normal mode analysisINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2001anka Jane Abstract The split integration symplectic method (SISM) for molecular dynamics (MD) integration using normal mode analysis based on a factorization of the Liouville propagator is presented. This approach is quite distinct from others that use fractional-step methods, owing to the analytical treatment of high-frequency motions. The method involves splitting the total Hamiltonian of the system into a harmonic part and the remaining part. Then the Hamilton equations are solved using a second-order generalized leapfrog integration scheme in which the purely harmonic Hamiltonian (which represents the main contribution of the chemical bonds and angles) is treated analytically, i.e., independent of the step size of integration, by a normal mode analysis that is carried out only once, at the beginning of calculation. The whole integration step combines analytical evolution of the harmonic part of the Hamiltonian with a correction arising from the remaining part. The proposed algorithm requires only one force evaluation per integration step. The algorithm was tested on a simple system of linear chain molecules. Results demonstrate the method makes possible the integration of the MD equations over larger time steps without loss of stability while being economical in computer time. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 84: 2,12, 2001 [source] Superficial modification in recycled PET by plasma etching for food packagingJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2010S. A. Cruz Abstract An oxygen plasma treatment has been used to improve the adhesion of amorphous hydrogenated carbon (a-C:H) films onto surfaces of recycled poly(ethylene terephthalate) (PET). Modifications produced by the oxygen plasma on the PET surface in chemical bonds and morphology were investigated by X-ray photoelectron spectroscopy and atomic force microscopy, respectively. Contact angle measurements were used to study the changes in the surface wettability. Adhesion of the a-C:H film onto the PET surface was investigated by the tape test method. It was observed that the improvement in film adhesion is in good correlation with the increase in surface roughness, due to plasma etching, and with the appearance of oxygen-related functional groups at the surface. The results of this study indicate that a-C:H-coated recycled PET can be used in food packaging. The a-C:H film could be used as a functional barrier to reduce or prevent migration of contaminants from the polymer to the package content. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] The influence of in situ modification of silica on filler network and dynamic mechanical properties of silica-filled solution styrene,butadiene rubberJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2008You-Ping Wu Abstract The influence of in situ modification of silica with bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) on filler network in silica filled solution SBR compound was investigated. In situ modification greatly increased the bound rubber content. TEM observation of silica gel showed that bridging and interlocking of absorbed chains on the surface of silica particles formed the filler network. Rubber processing analyzer (RPA) was used to characterize the filler network and interaction between silica and rubber by strain and temperature sweeps. In situ modification improved the dispersion of silica, and in the meantime, the chemical bonds were formed between silica and rubber, which conferred the stability of silica dispersion during the processing. Compared to the compound without in situ modification, the compound with in situ modification of silica exhibited higher tan , at low strains and lower tan , at high strains, which can be explained in terms of filler network in the compounds. After in situ modification, DMTA results showed silica-filled SSBR vulcanizate exhibited higher tan , in the temperature range of ,30 to 10°C, and RPA results showed that it had lower tan , at 60°C when the strain was more than 3%. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] | |||||||||||||||||||||||||||||||