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Bond Energy (bond + energy)
Selected AbstractsDirect Estimate of the Strength of Conjugation and Hyperconjugation by the Energy Decomposition Analysis MethodCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2006Israel Fernández Dr. Abstract The intrinsic strength of , interactions in conjugated and hyperconjugated molecules has been calculated using density functional theory by energy decomposition analysis (EDA) of the interaction energy between the conjugating fragments. The results of the EDA of the trans -polyenes H2CCH(HCCH)nCHCH2 (n=1,3) show that the strength of , conjugation for each CC moiety is higher than in trans -1,3-butadiene. The absolute values for the conjugation between SiSi , bonds are around two-thirds of the conjugation between CC bonds but the relative contributions of ,E, to ,Eorb in the all-silicon systems are higher than in the carbon compounds. The , conjugation between CC and CO or CNH bonds in H2CCHC(H)O and H2CCHC(H)NH is comparable to the strength of the conjugation between CC bonds. The , conjugation in H2CCHC(R)O decreases when R=Me, OH, and NH2 while it increases when R=halogen. The hyperconjugation in ethane is around a quarter as strong as the , conjugation in ethyne. Very strong hyperconjugation is found in the central CC bonds in cubylcubane and tetrahedranyltetrahedrane. The hyperconjugation in substituted ethanes X3CCY3 (X,Y=Me, SiH3, F, Cl) is stronger than in the parent compound particularly when X,Y=SiH3 and Cl. The hyperconjugation in donor,acceptor-substituted ethanes may be very strong; the largest ,E, value was calculated for (SiH3)3CCCl3 in which the hyperconjugation is stronger than the conjugation in ethene. The breakdown of the hyperconjugation in X3CCY3 shows that donation of the donor-substituted moiety to the acceptor group is as expected the most important contribution but the reverse interaction is not negligible. The relative strengths of the , interactions between two CC double bonds, one CC double bond and CH3 or CMe3 substituents, and between two CH3 or CMe3 groups, which are separated by one CC single bond, are in a ratio of 4:2:1. Very strong hyperconjugation is found in HCCC(SiH3)3 and HCCCCl3. The extra stabilization of alkenes and alkynes with central multiple bonds over their terminal isomers coming from hyperconjugation is bigger than the total energy difference between the isomeric species. The hyperconjugation in MeC(R)O is half as strong as the conjugation in H2CCHC(R)O and shows the same trend for different substituents R. Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of , interactions and electrostatic forces may compensate for the hyperconjugative effect. [source] Decomposition Cascades of Dicoordinate Copper(I) ChalcogenidesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 33 2007Heiko Jacobsen Abstract Cu,E, Si,E, and Cu,P bond energies of R3PCuESiR3 and CuESiR3 complexes (E = O, S, Se) have been investigated using PBE density-functional calculations, and including empirical corrections for dispersive interactions (DFT-D). The bond energies have been used to investigate likely pathways of molecular decomposition. The energy profile for thermal decomposition is to a large degree independent of the nature of the phosphane ligands and silyl groups. Oxides, sulfides, and selenides have qualitatively the same thermal decomposition profile. Thermal decomposition is not likely to produce copper chalcogenide units CuE, but elemental copper Cu instead. Consideration of intermolecular van der Waals attraction suggests that the linear geometry of system tBu3PCuOSiPh3 as found in the crystal is most likely due to crystal packing and intermolecular forces.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Hetero-,-systems from 2 + 2 cycloreversion, part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH, O, SiH2, PH, S),HETEROATOM CHEMISTRY, Issue 7 2007Leonid E. Gusel'nikov Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3-diheterocyclobutanes, cyclo -(CH2CH2CH2X), and cyclo -(CH2XCH2X), where X = NH, O, SiH2, PH, S, was undertaken by calculating closed-shell singlet molecules at three levels of theory: MP4/6-311G(d)//MP2/6-31G(d)+ZPE, MP4/6-311G(d,p)//MP2/6-31G (d,p)+ZPE, and B3LYP/6-311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3-diazetidine and 1,3-dioxtane. Strain energies of the four-membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one-bond homolysis that results in the formation of the corresponding 1,4-diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH3CH2,CH2XH, CH3CH2,XCH3, and HXCH2,XCH3. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C,H and X,H bond energies in CH3XH calculated at G2 level of theory. Except 1,3-disiletane, in which ring-opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4-diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring-opening enthalpies were negative, the least exothermicity was calculated for , CH2SiH2CH2CH2. The only exception was 1,3-disiletane, which being diradical, CH2SiH2CH2SiH2, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3-disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704,720, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20377 [source] Intramolecular hydrogen bond in 3-imino-propenylamine isomers: AIM and NBO studiesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2010H. Raissi Abstract The molecular structure and intramolecular hydrogen bond energy of 18 conformers of 3-imino-propenyl-amine were investigated at MP2 and B3LYP levels of theory using the standard 6-311++G** basis set. The atom in molecules or AIM theory of Bader, which is based on the topological properties of the electron density (,), was used additionally and the natural bond orbital (NBO) analysis was also carried out. Furthermore calculations for all possible conformations of 3-imino-propenyl-amin in water solution were also carried out at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the imine,amine conformers of this compound are more stable than the other conformers. B3LYP method predicts the IMA-1 as global minimum. This stability is mainly due to the formation of a strong NH···N intramolecular hydrogen bond, which is assisted by ,-electrons resonance, and this ,-electrons are established by NH2 functional group. Hydrogen bond energies for all conformers of 3-imino-propenyl-amine were obtained from the related rotamers methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Gas-phase theoretical prediction of the metal affinity of copper(I) ion for DNA and RNA basesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 3 2003Nino Russo Abstract The most stable tautomeric forms of free DNA and RNA bases were considered as substrates for the interaction of Cu+ ion. Several suitable attachment sites were selected that involved mono- and bi-coordination of the cation. B3LYP/6,311 + G(2df,2p) bond energies showed that copper ion has the major affinity for guanine and cytosine bases. The proposed values of Cu+ ion affinity are 59.9, 60.0, 80.2, 88.0 and 69.0 kcal mol,1 for uracil, thymine, cytosine, guanine and adenine, respectively. The preference for the mono- or bi-coordination depends on the particular tautomer for each base. Copyright © 2003 John Wiley & Sons, Ltd. [source] Subgap states, doping and defect formation energies in amorphous oxide semiconductor a-InGaZnO4 studied by density functional theoryPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2010Toshio Kamiya Abstract Amorphous In-Ga-Zn-O (a-IGZO) is expected for channel layers in thin-film transistors (TFTs). It is known that a-IGZO is sensitive to an O/H-containing atmosphere; therefore, it is important to clarify the roles of oxygen and hydrogen in a-IGZO. This paper provides atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT). It was confirmed that oxygen deficiencies having small formation energies (2,3.6,eV) form either deep fully-occupied localized states near the valence band maximum or donor states, which depend on their local structures. All the hydrogen doping form OH bond and work as a donor. The stable OH bonds have small formation energy of ,0.45,eV and consist of three metal cations coordinated to the O ion. The bond energy of GaO is calculated to be ,2.0,eV, which is the largest among the chemical bonds in a-IGZO (1.7,eV for InO and 1.5,eV for ZnO). This result supports the idea that the incorporation of Ga stabilizes a-IGZO TFTs. [source] Synthesis and Characterisation of ,3 -Octahedral [Ni36Pd8(CO)48]6, and [Ni35Pt9(CO)48]6, Clusters Displaying Unexpected Surface Segregation of Pt Atoms and Molecular and/or Crystal Substitutional Ni/Pd and Ni/Pt DisorderCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2004Cristina Femoni Dr. Abstract The synthesis and structure, as well as the chemical and electrochemical characterisation of two new ,3 -octahedral bimetallic clusters with the general [Ni44,xMx(CO)48]6, (M = Pd, x = 8; M = Pt, x = 9) formula is reported. The [Ni35Pt9(CO)48]6, cluster was obtained in reasonable yields (56,% based on Pt) by reaction of [Ni6(CO)12]2, with 1.1 equivalents of PtII complexes, in ethyl acetate or THF as the solvent. The [Ni36Pd8(CO)48]6, cluster was obtained from the related reaction with PdII salts in THF, and was isolated only in low yields (5,10,% based on Pd), mainly because of insufficient differential solubility of its salts. The unit cell of the [NBu4]6[Ni35Pt9(CO)48] salt contains a substitutionally Ni,Pt disordered [Ni24(Ni14,xPtx)Pt6(CO)48]6, (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. The unit cell of the corresponding [Ni36Pd8(CO)48]6, salt contains two independent [Ni30(Ni8,xPdx)Pd6(CO)48]6, (x = 2) hexaanions. The two display similar substitutional Ni,Pd disorder, which probably arises only from crystal disorder. The structure of [Ni36Pd8(CO)48]6, establishes the first similarity between the chemistry of Ni-Pd and Ni-Pt carbonyl clusters. A comparison of the chemical and electrochemical properties of [Ni35Pt9(CO)48]6, with those of the related [Ni38Pt6(CO)48]6, cluster shows that surface colouring of the latter with Pt atoms decreases redox as well as protonation propensity of the cluster. In contrast, substitution of all internal Pt and two surface Ni with Pd atoms preserves the protonation behaviour and is only detrimental with respect to its redox aptitude. A qualitative rationalisation of the different surface-site selectivity of Pt and Pd, based on distinctive interplays of MM and MCO bond energies, is suggested. [source] 9-Fluoro-18-hydroxy-[3.3]metacyclophane: Synthesis and Estimation of a C,F···H,O Hydrogen BondEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2004Hiroyuki Takemura Abstract A cyclophane composed of fluorobenzene and phenol units was synthesized in order to observe the C,F···H,O hydrogen bond. In the crystal structure, 20% of the molecule clearly shows the intramolecular hydrogen bond and the other 80% is free from hydrogen bonding. On the other hand, a distinct low-field shift of the phenolic OH proton was observed in the 1H NMR spectrum compared to that of the F-free analog. Furthermore, O,H···F through-space coupling was observed. From the results of the crystallographic analysis, IR, and NMR spectra, the C,F···H,O hydrogen bond energy of this system was estimated to be 0.84,3.7 kJ·mol,1. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Increase in the fracture toughness and bond energy of clay by a root exudateEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 5 2008B. Zhang Summary Root exudates help drive the formation of the rhizosphere by binding soil particles, but the underlying physical mechanisms have not been quantified. This was addressed by measuring the impact of a major component of root exudates, polygalacturonic acid (PGA), on the interparticle bond energy and fracture toughness of clay. Pure kaolinite was mixed with 0, 1.2, 2.4, 4.9 or 12.2 g PGA kg,1 to form test specimens. Half of the specimens were washed repeatedly to remove unbound PGA and evaluate the persistence of the effects, similar to weathering in natural soils. Fracture toughness, KIC, increased exponentially with added PGA, with washing increasing this trend. In unwashed specimens KIC ranged from 54.3 ± 2.5 kPa m,1/2 for 0 g PGA kg,1 to 86.9 ± 4.7 kPa m,1/2 for 12.2 g PGA kg,1. Washing increased KIC to 61.3 ± 1.2 kPa m,1/2 for 0 g PGA kg,1 and 132.1 ± 4.9 kPa m,1/2 for 12.2 g PGA kg,1. The apparent bond energy, ,, of the fracture surface increased from 5.9 ± 0.6 J m,2 for 0 g kg,1 to 12.0 ± 1.1 J m,2 for 12.2 g kg,1 PGA in the unwashed specimens. The washed specimens had , of 13.0 ± 1.9 J m,2 for 0 g kg,1 and 21.3 ± 2.6 J m,2 for 12.2 g PGA kg,1. Thus PGA, a major component of root exudates, has a large impact on the fracture toughness and bond energy of clay, and is likely to be a major determinant in the formation of the rhizosphere. This quantification of the thermodynamics of fracture will be useful for modelling rhizosphere formation and stability. [source] Thermochemistry for enthalpies and reaction paths of nitrous acid isomersINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2007Rubik Asatryan Recent studies show that nitrous acid, HONO, a significant precursor of the hydroxyl radical in the atmosphere, is formed during the photolysis of nitrogen dioxide in soils. The term nitrous acid is largely used interchangeably in the atmospheric literature, and the analytical methods employed do not often distinguish between the HONO structure (nitrous acid) and HNO2 (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO2 isomer, which has not been determined experimentally, and to evaluate its thermal and atmospheric stability relative to HONO. The thermochemistry of these isomers is also needed for reference and internal consistency in the calculation of larger nitrite and nitryl systems. We review, evaluate, and compare the thermochemical properties of several small nitric oxide and hydrogen nitrogen oxide molecules. The enthalpies of HONO and HNO2 are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closed- and open-shell reference molecules. Three high-level composite methods G3, CBS-QB3, and CBS-APNO are used for the computation of enthalpy. The enthalpy of formation, ,Hof(298 K), for HONO is determined as ,18.90 ± 0.05 kcal mol,1 (,79.08 ± 0.2 kJ mol,1) and as ,10.90 ± 0.05 kcal mol,1 (,45.61 ± 0.2 kJ mol,1) for nitryl hydride (HNO2), which is significantly higher than values used in recent NOx combustion mechanisms. H-NO2 is the weakest bond in isonitrous acid; but HNO2 will isomerize to HONO with a similar barrier to the HONO bond energy; thus, it also serves as a source of OH in atmospheric chemistry. Kinetics of the isomerization is determined; a potential energy diagram of H/N/O2 system is presented, and an analysis of the triplet surface is initiated. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 378,398, 2007 [source] Intramolecular hydrogen bond in 3-imino-propenylamine isomers: AIM and NBO studiesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2010H. Raissi Abstract The molecular structure and intramolecular hydrogen bond energy of 18 conformers of 3-imino-propenyl-amine were investigated at MP2 and B3LYP levels of theory using the standard 6-311++G** basis set. The atom in molecules or AIM theory of Bader, which is based on the topological properties of the electron density (,), was used additionally and the natural bond orbital (NBO) analysis was also carried out. Furthermore calculations for all possible conformations of 3-imino-propenyl-amin in water solution were also carried out at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the imine,amine conformers of this compound are more stable than the other conformers. B3LYP method predicts the IMA-1 as global minimum. This stability is mainly due to the formation of a strong NH···N intramolecular hydrogen bond, which is assisted by ,-electrons resonance, and this ,-electrons are established by NH2 functional group. Hydrogen bond energies for all conformers of 3-imino-propenyl-amine were obtained from the related rotamers methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Enhanced photodegradation of bisphenol A in the presence of ,-cyclodextrin under UV lightJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 5 2006Guanghui Wang Abstract Enhancement of the photodegradation of bisphenol A (BPA) by ,-cyclodextrin (,-CD) was investigated under a 30 W UV disinfection lamp (,max = 254 nm). The photodegradation rate of BPA in aqueous solutions with ,-CD was faster than that in aqueous solutions without ,-CD; for example, after 50 min of UV irradiation, ,-CD had increased the photodegradation efficiency by about 46.5% for 10 mg dm,3 BPA. The photodegradation of 2.5,20 mg dm,3 BPA in aqueous solutions was found to follow pseudo-first-order kinetics. The first-order rate constant showed a 12-fold increase in the presence of ,-CD. Factors such as ,-CD concentration, pH, BPA initial concentration and organic solvent influencing the photodegradation of BPA were studied and are described in detail. Variations in the pH and electrical conductivity of solutions were observed during the photodegradation process. The enhancement of photodegradation of BPA results mainly from the lower bond energy between some atoms in the BPA molecule after inclusion interaction with ,-CD. Copyright © 2006 Society of Chemical Industry [source] Nanomechanics of single keratin fibres: A Raman study of the ,-helix ,,-sheet transition and the effect of waterJOURNAL OF RAMAN SPECTROSCOPY, Issue 5 2007Raphaël Paquin Abstract The use of micro-Raman spectroscopy, through chemical-bond, nano-scale probes, allows the changes in conformations (,-helix ,,-sheet), chain orientation, breakage of disulfide bonds (20%) and the increase of intra- and inter-chain distances during the application of stress to be distinguished. The combination of micro-Raman spectroscopy and a Universal Fibre Tester allows a quantitative measurement of the extension of chemical bonds in the peptide chain during loading. The nano-structural transformations of keratin during strain of human hair in a dry environment (40,60% relative humidity) and saturated with water have been studied. Water permits the sliding of the chains and decreases the bond energy of the hair. Spectral analyses and 2D correlation are two coherent and independent methods to follow the structural nano-mechanical (Raman) and micro-mechanical (strain/stress) analyses, and confirm the validity of the experimental results, tools and principles used, as well as the agreement with the structural model of keratin fibres described by Chapman and Hearle. Copyright © 2006 John Wiley & Sons, Ltd. [source] Surface Structure of Converter Slag Stabilized by HeatingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2007Naomitsu Tsuyuki Converter slag contains free lime (CaO) and unstable iron oxides (FeO, FeOOH) that may lead to expansive self-destruction. A typical industry practice for converter slag has been stabilization by steam curing and autoclaving; however, the stabilization can only reach the surface, and not the inside, of slag particles. A new method is proposed in this study to stabilize the converter slag by heating at a low temperature. After magnetic separation, specimens of converter slag were subjected to heating for 2 h at a temperature of 500°C, resulting in a decrease of free lime content irrespective of the particle size. This effect was attributed to the formation of Ca2Fe9O13 and complicated apatite groups owing to the heating. The iron oxides in the converter slag were analyzed by X-ray photoelectron spectra. It was found that after heating, the unstable FeO (wustite) content decreased and an oxidized ,-Fe2O3 (hematite) increased. This led to the prevention of the iron-induced expansion. The rate of heat liberation by the free lime in converter slag was smaller than that of the reagent CaO. This suggests that the presumed free lime is in a different form based on the Ca bond energy in the surface of slag particles. [source] Charge-Shift Bonding,A Class of Electron-Pair Bonds That Emerges from Valence Bond Theory and Is Supported by the Electron Localization Function ApproachCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2005Sason Shaik Prof. Abstract This paper deals with a central paradigm of chemistry, the electron-pair bond. Valence bond (VB) theory and electron-localization function (ELF) calculations of 21 single bonds demonstrate that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the fluctuation of the electron pair density plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RECS, and in ELF by a depleted basin population with large variances (fluctuations). CS bonding is shown to be a fundamental mechanism that is necessary to satisfy the equilibrium condition, namely the virial ratio of the kinetic and potential energy contributions to the bond energy. The paper defines the atomic propensity and territory for CS bonding: Atoms (fragments) that are prone to CS bonding are compact electronegative and/or lone-pair-rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and involves: a) homopolar bonds of heteroatoms with zero static ionicity, b) heteropolar , and , bonds of the electronegative and/or electron-pair-rich elements among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc), c) all hypercoordinate molecules. Several experimental manifestations of charge-shift bonding are discussed, such as depleted bonding density, the rarity of ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reactions as compared to hydrogen-transfers. [source] Novel Superalkali Superhalogen Compounds (Li3)+(SH), (SH=LiF2, BeF3, and BF4) with Aromaticity: New Electrides and AlkalidesCHEMPHYSCHEM, Issue 5 2006Fang-Fang Wang Dr. Abstract Optimized structures, with all real frequencies, of superalkali superhalides (Li3)+(SH), (SH=LiF2, BeF3, and BF4), are obtained, for the first time, at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. These superalkali superhalides possess three characteristics that are significantly different from normal alkali halides. 1) They have a variety of structures, which come from five bonding mode types: edge,face, edge,edge, face,face, face,edge, and staggered face,edge. We find that the bonding mode type closely correlates with the Li3SH bond energy. 2) The valence electrons on the Li3 ring are pushed out by the (SH), anion, and become excess electrons, conferring alkalide or electride characteristics on these Li3SH species, depending on the bonding mode type. 3) The highest occupied molecular orbital of each Li3SH species is a doubly occupied delocalized , bonding orbital on the Li3 ring, which indicates its aromaticity. It is noticeable that the maximum negative nucleus-independent chemical shift value (about ,10 ppm) moves out from the center of the Li3 ring, owing to repulsion by the SH, anion. We find that these superalkali superhalides are not only complicated "supermolecules", but are also a new type of alkalide or electride, with aromaticity. [source] | ||||||||||