Página 1 dos resultados de 428 itens digitais encontrados em 2.976 segundos

Conformational study of erythritol and threitol in the gas state by density functional theory calculations

Jesus, António J. Lopes; Tomé, Luciana I. N.; Rosado, Mário Túlio S.; Leitão, M. Luísa P.; Redinha, José S.
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Artigo de Revista Científica Formato: aplication/PDF
ENG
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Density functional theory calculations using the B3LYP functional and the 6-311++G(d,p) basis set were carried out on the isolated molecules of erythritol and L-threitol. For the meso isomer, a relatively large number of conformers have to be considered to describe the gas state structure. The lowest energy conformer is characterized by the establishment of a strong intramolecular H-bond between the two terminal hydroxyl groups, giving rise to a seven-membered ring and two additional weaker H-bonds between vicinal OH groups. In the case of L-threitol, two conformers are predominant in the gas state, and both are stabilized by the formation of a cyclic system of four intramolecular hydrogen bonds involving all OH groups. The conformational stability in both diastereomers is discussed in terms of the electronic energy and of the Gibbs energy. The weighted mean enthalpy of both diastereomers in the gas state at 298.15 K was obtained from the thermodynamic data and Boltzmann populations of the low-energy conformers.; http://www.sciencedirect.com/science/article/B6TFF-4F31PT7-3/1/ae9dae05894f1bf2f4865221eb1b29bb

Energetics and stability of vacancies in carbon nanotubes

Sousa, José Eduardo Padilha de; Amorim, Rodrigo Garcia; Rocha, Alexandre Reily; Silva, Antonio Jose Roque da; Fazzio, Adalberto
Fonte: PERGAMON-ELSEVIER SCIENCE LTD Publicador: PERGAMON-ELSEVIER SCIENCE LTD
Tipo: Artigo de Revista Científica
ENG
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In this work we present ab initio calculations of the formation energies and stability of different types of multi-vacancies in carbon nanotubes. We demonstrate that, as in the case of graphene, the reconstruction of the defects has drastic effects on the energetics of the tubes. In particular, the formation of pentagons eliminates the dangling bonds thus lowering the formation energy. This competition leads to vacancies having an even number of carbon atoms removed to be more stable. Finally the appearance of magic numbers indicating more stable defects can be represented by a model for the formation energies that is based on the number of dangling bonds of the unreconstructed system, the pentagons and the relaxation of the final form of the defect formed after the relaxation. (C) 2011 Elsevier Ltd. All rights reserved.

Caracterização da sílica amorfa extraída da casca de arroz obtida por pré-hidrólise ácida e calcinação, e sua aplicação em borracha de estireno-butadieno (SBR); Characterization of amorphous rice husk silica obtained by acidic prehydrolysis and calcination, and its application in styrene-butadiene rubber (SBR)

Budemberg, Eduardo Roque
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 14/06/2013 PT
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O crescimento dos países em desenvolvimento aumentará em várias vezes o consumo de energia, materiais e alimentos. Para suprir essa demanda é necessário o aproveitamento racional dos resíduos gerados por processos industriais, agrícolas e lixo urbano, transformando-os em energia economicamente competitiva com as outras formas de energia, e em materiais com propriedades otimizadas. O objetivo do presente trabalho foi comparar o comportamento da sílica da casca de arroz obtida por pré-hidrólise ácida seguida de calcinação controlada (Sil-PH) com sílicas de casca de arroz comerciais obtidas simplesmente pela queima da casca (Sil-B e Sil-C) e com a sílica comercial obtida da areia quartzítica (Sil-Z), como carga de reforço em borracha de estireno-butadieno (SBR), que é a mais usada na indústria de fabricação de pneus. Estudos foram conduzidos em SBR 1502, preparados conforme norma ASTM 3191-2010. Foram adicionadas à borracha as sílicas em concentrações de 0, 10, 20 e 40 ppcb. Foram conduzidos testes físicos, químicos, mecânicos, reométricos, dinâmico-mecânicos e análise térmica, bem como ensaios de inchamento dos corpos de prova em toluol e análise microestrutural das sílicas e dos compostos por microscopia eletrônica de varredura...

Hydrogen bonds between pyrazine and HX linear acids (X = F, NC, CN and CCH): a theoretical study

Silva,João Bosco P. da; Silva Junior,Mário R.; Ramos,Mozart N.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/2005 EN
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Theoretical studies using the B3LYP and MP2 levels of calculation, with the 6-31++G** basis set, have been used to characterize hydrogen bonded complexes between pyrazine and HX linear acids with X = F, NC, CN and CCH. The molecular properties of these complexes and, the structural, electronic and vibrational changes that occur in the isolated species due to H-bond formation have been calculated and then, compared with those previously obtained in T-shaped complexes, involving simple unsaturated hydrocarbons as proton acceptors. Our calculations have shown that these changes in Pyz…HX complexes are much more pronounced than those observed in the pi…HX complexes, as well as their binding energies. BSSE corrections to the energy of binding-H estimated by the B3LYP results are approximately three times smaller than those obtained through MP2 results. Our calculations have also shown that stronger hydrogen bonds in the complexes Pyz…HX are associated with smaller H-bond length values, larger intermolecular charge-transfer and greater downward displacement to the stretching frequency H-X . The H-bond strength for these complexes follows the order: X = F > X = NC > X = CN > X = CCH. As expected, the H-X stretching intensity was enhanced substantially after the H-bond formation. Finally...

Understanding β-hairpin formation

Dinner, Aaron R.; Lazaridis, Themis; Karplus, Martin
Fonte: The National Academy of Sciences Publicador: The National Academy of Sciences
Tipo: Artigo de Revista Científica
Publicado em 03/08/1999 EN
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The kinetics of formation of protein structural motifs (e.g., α-helices and β-hairpins) can provide information about the early events in protein folding. A recent study has used fluorescence measurements to monitor the folding thermodynamics and kinetics of a 16-residue β-hairpin. In the present paper, we obtain the free energy surface and conformations involved in the folding of an atomistic model for the β-hairpin from multicanonical Monte Carlo simulations. The results suggest that folding proceeds by a collapse that is downhill in free energy, followed by rearrangement to form a structure with part of the hydrophobic cluster; the hairpin hydrogen bonds propagate outwards in both directions from the partial cluster. Such a folding mechanism differs from the published interpretation of the experimental results, which is based on a helix–coil-type phenomenological model.

Energy considerations show that low-barrier hydrogen bonds do not offer a catalytic advantage over ordinary hydrogen bonds

Warshel, Arieh; Papazyan, Arno
Fonte: The National Academy of Sciences of the USA Publicador: The National Academy of Sciences of the USA
Tipo: Artigo de Revista Científica
Publicado em 26/11/1996 EN
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Low-barrier hydrogen bonds have recently been proposed as a major factor in enzyme catalysis. Here we evaluate the feasibility of transition state (TS) stabilization by low-barrier hydrogen bonds in enzymes. Our analysis focuses on the facts that (i) a low-barrier hydrogen bond is less stable than a regular hydrogen bond in water, (ii) TSs are more stable in the enzyme active sites than in water, and (iii) a nonpolar active site would destabilize the TS relative to its energy in water. Combining these points and other experimental and theoretical facts in a physically consistent framework shows that a low-barrier hydrogen bond cannot stabilize the TS more than an ordinary hydrogen bond. The reason for the large catalytic effect of active site hydrogen bonds is that their formation entails a lower reorganization energy than their solution counterparts, due to the preorganized enzyme environment.

The speed limit for protein folding measured by triplet–triplet energy transfer

Bieri, Oliver; Wirz, Jakob; Hellrung, Bruno; Schutkowski, Mike; Drewello, Mario; Kiefhaber, Thomas
Fonte: The National Academy of Sciences Publicador: The National Academy of Sciences
Tipo: Artigo de Revista Científica
Publicado em 17/08/1999 EN
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A direct measure of intramolecular chain diffusion is obtained by the determination of triplet–triplet energy-transfer rates between a donor and an acceptor chromophore attached at defined points on a polypeptide chain. Single exponential kinetics of contact formation are observed on the nanosecond time scale for polypeptides in which donor and acceptor are linked by repeating units of glycine and serine residues. The rates depend on the number of peptide bonds (N) separating donor and acceptor and show a maximum for the shortest peptides (N = 3) with a time constant (τ = 1/k) of 20 ns. This sets an upper limit for the speed of formation of the first side-chain contacts during protein folding.

Exploring the folding free energy surface of a three-helix bundle protein

Guo, Zhuyan; Brooks, Charles L.; Boczko, Erik M.
Fonte: The National Academy of Sciences of the USA Publicador: The National Academy of Sciences of the USA
Tipo: Artigo de Revista Científica
Publicado em 16/09/1997 EN
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The multidimensional free energy surface for a small fast folding helical protein is explored based on first-principle calculations. The model represents the 46-residue segment from fragment B of staphylococcal protein A. The relationship between collapse and tertiary structure formation, and the order of collapse and secondary structure formation, are investigated. We find that the initial collapse process gives rise to a transition state with about 30% of the native tertiary structure and 50–70% of the native helix content. We also observe two distinct distributions of native helix in this collapsed state (Rg ≈ 12 Å), one with about 20% of the native helical hydrogen bonds, the other with near 70%. The former corresponds to a local minimum. The barrier from this metastable state to the native state is about 2 kBT. In the latter case, folding is essentially a downhill process involving topological assembly. In addition, the order of formation of secondary structure among the three helices is examined. We observe cooperative formation of the secondary structure in helix I and helix II. Secondary structure in helix III starts to form following the formation of certain secondary structure in both helix I and helix II. Comparisons of our results with those from theory and experiment are made.

Energetic analysis of an antigen/antibody interface: alanine scanning mutagenesis and double mutant cycles on the HyHEL-10/lysozyme interaction.

Pons, J.; Rajpal, A.; Kirsch, J. F.
Fonte: Cold Spring Harbor Laboratory Press Publicador: Cold Spring Harbor Laboratory Press
Tipo: Artigo de Revista Científica
Publicado em /05/1999 EN
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Alanine scanning mutagenesis of the HyHEL-10 paratope of the HyHEL-10/HEWL complex demonstrates that the energetically important side chains (hot spots) of both partners are in contact. A plot of deltadeltaG(HyHEL-10_mutant) vs. deltadeltaG(HEWL_mutant) for the five of six interacting side-chain hydrogen bonds is linear (Slope = 1). Only 3 of the 13 residues in the HEWL epitope contribute >4 kcal/mol to the free energy of formation of the complex when replaced by alanine, but 6 of the 12 HyHEL-10 paratope amino acids do. Double mutant cycle analysis of the single crystallographically identified salt bridge, D32H/K97, shows that there is a significant energetic penalty when either partner is replaced with a neutral side-chain amino acid, but the D32(H)N/K97M complex is as stable as the WT. The role of the disproportionately high number of Tyr residues in the CDR was evaluated by comparing the deltadeltaG values of the Tyr --> Phe vs. the corresponding Tyr --> Ala mutations. The nonpolar contacts in the light chain contribute only about one-half of the total deltadeltaG observed for the Tyr --> Ala mutation, while they are significantly more important in the heavy chain. Replacement of the N31L/K96 hydrogen bond with a salt bridge, N31D(L)/K96...

The interaction of xenon difluoride with Si(100)

Holt, Judson Robert, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 281 p.; 16697922 bytes; 16697674 bytes; application/pdf; application/pdf
ENG
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The interaction of low energy XeF2 with Si(100)2x1 has been investigated by studying both the surface-bound and gas-phase products of the reaction. Helium atom diffraction, beam-surface scattering and thermal desorption measurements are the major techniques used to probe the surface-bound products of the reaction. It is found that XeF2 dissociatively chemisorbs with high probability solely on the Si dangling bonds up to a coverage of about one monolayer (ML). Molecular fluorine has previously been observed to react similarly, saturating the dangling bonds at 1 ML coverage. The thermal desorption products and kinetics from the fluorinated layer produced by XeF2 exposure are identical to that produced by F2 exposure. The interactions of XeF2 and F2 are also strikingly similar with respect to the order of the Si surface up to about 1 ML coverage. The surface order is monitored by He diffraction detected by a line-of-sight, differentially pumped mass spectrometer. In both systems, the diffracted He beams exhibit a sharp decrease in intensity due to the disorder produced by the fluorination of random surface unit cells as the coverage increases from zero to about 0.3 ML. The intensity then increases until the fluorine overlayer has fully recovered its 2x 1 periodicity at about 1 ML. This recovery corresponds to the decoration of each Si dangling bond with a fluorine atom. A critical observation of this study is that despite the large exothermicity of the dissociative chemisorption of XeF2 or F2...

Estimation method for the thermochemical properties of polycyclic aromatic molecules

Yu, Joanna
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 384 p.; 17728388 bytes; 17745329 bytes; application/pdf; application/pdf
ENG
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Polycyclic aromatic molecules, including polycyclic aromatic hydrocarbons (PAHs) have attracted considerable attention in the past few decades. They are formed during the incomplete combustion of hydrocarbon fuels and are precursors of soot. Some PAHs are known carcinogens, and control of their emissions is an important issue. These molecules are found in many materials, including coal, fuel oils, lubricants, and carbon black. They are also implicated in the formation of fullerenes, one of the most. chemically versatile class of molecules known. Clearly, models that provide predictive capability for their formation and growth are highly desirable. Thlermochemical properties of the species in the model are often the most important parameter, particularly for high temperature processes such as the formation of PAH and other aromatic molecules. Thermodynamic consistency requires that reverse rate constants be calculated from the forward rate constants and from the equilibrium constants. The later are obtained from the thermochemical properties of reactants and products. The predictive ability of current kinetic models is significantly limited by the scarcity of available thermochemical data.; (cont.) In this work we present the development of a Bond-Centered Group Additivity method for the estimation of the thermochemical properties of polycyclic aromatic molecules...

The mechanics of mechanotransduction : analyses of cell perturbation

Karcher, Hélène
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 2 v. (218 leaves)
ENG
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Cells sense mechanical stimuli and respond by changing their phenotype, e.g. shape, gene expression, motility. This process, termed mechanotransduction, was investigated using computational and theoretical approaches, as well as comparisons with experiments. As a first step, a three-dimensional viscoelastic finite element model was developed to simulate cell micromanipulation by magnetocytometry. The model provided a robust tool for analysis of detailed strain/stress fields induced within a single cell or cell monolayer produced by forcing one tethered microbead. On the assumption of structural homogeneity, stress and strain patterns were highly localized, suggesting that the effects of magnetocytometry are confined to a region extending less than 10tm from the bead. Modification of the model to represent experimental focal adhesion attachments supported a non-uniform force transmission to basal surface focal adhesion sites. Proteins in identified zones of high stresses in the cell are candidate mechanosensors and their molecular response to force was hence investigated, A generic model of protein extension under external forcing was created inspired by Kramers theory for reaction rate kinetics in liquids.; (cont.) The protein was hypothesized to have two distinct conformational states: a relaxed state...

A computer simulation and molecular-thermodynamic framework to model the micellization of ionic branched surfactants in aqueous solution

Lin, Shangchao
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 132 leaves
ENG
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Surfactants, or surface active agents, are chemicals exhibiting amphiphilic behavior toward a solvent. This amphiphilic character leads to increased activity at interfaces and to self-assembly into micellar aggregates beyond a threshold surfactant concentration, referred to as the critical micelle concentration (CMC), in bulk solutions. As a result of these unique attributes, surfactants are used in many pharmaceutical, industrial, and environmental applications, including biological separations, fat metabolism during digestion, drug delivery, and water purification. Selection of the appropriate surfactant for a given application is often motivated by the need to control bulk solution micellization properties, such as the CMC and the micelle shape and size. The ability to make molecular-level predictions of these surfactant properties would allow formulators in industry to speed up the design and optimization of new surfactant formulations. In this thesis, a combined computer simulation/molecular-thermodynamic (CS-MT) modeling approach was developed and utilized to study the micellization behavior of ionic branched surfactants, which are a class of surfactants of great industrial relevance in applications such as detergency, emulsification...

NMR studies of the exchange reactions of CH3CN.BX3 with excess CH3CN /|nJoseph Fogelman. -- 260 St. Catharines, Ont. : [s. n.],

Fogelman, Joseph.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
ENG
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Exchange reactions between molecular complexes and excess acid or base are well known and have been extensively surveyed in the literature(l). Since the exchange mechanism will, in some way involve the breaking of the labile donor-acceptor bond, it follows that a discussion of the factors relating to bonding in molecular complexes will be relevant. In general, a strong Lewis base and a strong Lewis acid form a stable adduct provided that certain stereochemical requirements are met. A strong Lewis base has the following characteristics (1),(2) (i) high electron density at the donor site. (ii) a non-bonded electron pair which has a low ionization potential (iii) electron donating substituents at the donor atom site. (iv) facile approach of the site of the Lewis base to the acceptor site as dictated by the steric hindrance of the substituents. Examples of typical Lewis bases are ethers, nitriles, ketones, alcohols, amines and phosphines. For a strong Lewis acid, the following properties are important:( i) low electron density at the acceptor site. (ii) electron withdrawing substituents. (iii) substituents which do not interfere with the close approach of the Lewis base. (iv) availability of a vacant orbital capable of accepting the lone electron pair of the donor atom. Examples of Lewis acids are the group III and IV halides such (M=B...

Molecular Interaction of (Ethanol)2 - Water Heterotrimers; The Journal of Physical Chemistry A

Mejia, Sol M.; Espinal, Juan F.; Restrepo, Albeiro
Fonte: Pontifícia Universidade Javeriana Publicador: Pontifícia Universidade Javeriana
Formato: 8250–8256
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Vol. 111, No. 33; The potential energy surface of the (ethanol)2−water heterotrimers for the trans and gauche conformers of ethanol was studied using density functional theory. The same approximation was used for characterizing representative clusters of (ethanol)3, (methanol)3, and (methanol)2−water. Trimerization energies and enthalpies as well as the analysis of geometric parameters suggest that the structures with a cyclic pattern in the three hydrogen bonds of the type O−H---O (primary hydrogen bonds), where all molecules are proton donor−acceptor at the same time, are more stable than those with just two primary hydrogen bonds. Additionally, we propose the formation of “secondary hydrogen bonds” between hydrogen atoms of the methyl group of ethanol and the oxygen atom of water or other ethanol molecule (C−H---O), which were found to be weaker than the primary hydrogen bonds.

Characterization and requirements for Cu-Cu bonds for three-dimensional integrated circuits; Characterization and requirements for copper-copper bonds for 3D IC

Tadepalli, Rajappa, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 206 p.
ENG
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57.4975%
Three-dimensional integrated circuit (3D IC) technology enables heterogeneous integration of devices fabricated from different technologies, and reduces global RC delay by increasing the device density per unit chip area. Wafer-level Cu-Cu thermocompression bonding provides an attractive route to 3D IC fabrication, with Cu serving as both the electrical and mechanical interconnection between adjacent device layers. While the bonding process is currently employed for such applications, the lack of quantitative understanding of the bond quality and reliability has made developing robust processes extremely challenging. The current work addresses this problem through the development and implementation of bond toughness measurement techniques that investigate the effects of thin film patterning, surface chemistry and process parameters on the Cu-Cu bond quality under a range of loading conditions. The four-point bend test was used to quantify Cu-Cu bond toughness, Gc, under mixed-mode loading and to develop an optimized process flow that enabled the creation of high- toughness bonds (> 5 J/m2) at a bonding temperature of 300 oC. Mixed-mode loading induces significant plastic energy dissipation in ductile layers, resulting in an overestimation of the true adhesive strength of the interface.; (cont.) The chevron test method has been developed to allow bond toughness measurements under mode I loading...

Molecular-thermodynamic theories of micellization of multicomponent surfactant mixtures and of pH-sensitive surfactants

Goldsipe, Arthur Clayton
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 264 p.; 1538384 bytes; 1581044 bytes; application/pdf; application/pdf
ENG
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This thesis focuses on two research areas that are particularly relevant to the practical application of surfactant science: (1) the micellization of multicomponent surfactant mixtures, and (2) the micellization of pH-sensitive (or amphoteric) surfactants. Surfactant formulations of practical utility typically consist of many surfactant components. In many practical applications, pH-sensitive surfactants are added as a secondary surfactant because they enhance performance properties, including solubility, foaming, and mildness to the skin or to the eyes. In addition, pH-sensitive surfactants may be used eectively in novel applications where pH variations can be utilized to control self-assembly, including controlled drug release, targeted gene delivery, and the fabrication of nanoscale materials for optics, electronics, and sensors. First, a molecular-thermodynamic (MT) theory was developed to account for counterion binding to mixed micelles composed of ionic-nonionic and ionic-zwitterionic binary surfactant mixtures. The theory successfully predicted the degree of counterion binding ([beta]) of monovalent and multivalent ions to mixed micelles as a function of the micelle composition ([alpha]).; (cont.) The theory was also found to be consistent with the concept of critical counterion binding. An inection in the [beta vs. [alpha] curve was correlated to a micelle shape transition. Second...

The structure, energy, and electronic states of vacancies in Ge nanocrystals

Bayus, Kenneth; Paz, O.; Beckman, S. P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/08/2010
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The atomic structure, energy of formation, and electronic states of vacancies in H-passivated Ge nanocrystals are studied by density functional theory (DFT) methods. The competition between quantum self-purification and the free surface relaxations is investigated. The free surfaces of crystals smaller than 2 nm distort the Jahn-Teller relaxation and enhance the reconstruction bonds. This increases the energy splitting of the quantum states and reduces the energy of formation to as low as 1 eV per defect in the smallest nanocrystals. In crystals larger than 2 nm the observed symmetry of the Jahn-Teller distortion matches the symmetry expected for bulk Ge crystals. Near the nanocrystal's surface the vacancy is found to have an energy of formation no larger than 0.5 to 1.4 eV per defect, but a vacancy more than 0.7 nm inside the surface has an energy of formation that is the same as in bulk Ge. No evidence of the self-purification effect is observed; the dominant effect is the free surface relaxations, which allow for the enhanced reconstruction. From the evidence in this paper, it is predicted that for moderate sized Ge nanocrystals a vacancy inside the crystal will behave bulk-like and not interact strongly with the surface, except when it is within 0.7 nm of the surface.; Comment: In Press at Phys. Rev. B

The Nature of the Chemical Bond. VI. The Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures

Pauling, Linus; Sherman, J.
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /08/1933
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In the first part of this paper there is given a set of bond‐energy values for single, double, and triple bonds between atoms, obtained from thermochemical data, such that the total energy of formation from separate atoms of a molecule containing given bonds is equal to the sum of the energies for those bonds. In the derivation of these values data were used only for molecules for which it is probable that one electronic structure, corresponding to one distribution of valence bonds, represents the normal state to a satisfactory degree of approximation. For other molecules more than one electronic structure of this type contributes essentially to the normal state, the energy of formation of the molecule then being larger than that for any one of the contributing structures. On comparing the energies of formation given by thermochemical data with the values calculated for various structures, it is verified that this difference is always positive or zero (to within the limits of error involved). The difference in energy is interpreted as the resonance energy of the molecule among several electronic structures, and its existence in a given case provides strong evidence that more than one structure is contributing to the normal state of the molecule...

Heats of formation of alkali metal and alkaline earth metal oxides and hydroxides: Surprisingly demanding targets for high-level ab initio procedures

Sullivan, Michael; Iron, Mark; Redfern, Paul; Martin, Jan; Curtiss, Larry; Radom, Leo
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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High-level ab initio calculations, including variants of the Wn and G3 procedures, have been used to determine the structures and heats of formation of the alkali metal and alkaline earth metal oxides and hydroxides (M2O, MOH: M = Li, Na, and K; MO, M(OH)2: M = Be, Mg, and Ca). Our best structures were obtained at the CCSD(T)(riv,rv)/aug'-cc-pWCVQZ level and are in uniformly close agreement with available experimental data, with a mean absolute deviation from experimental metal-oxygen bond lengths of just 0.007 Å.Structures obtained with CCSD(T)/cc-pWCVQZ, B3-LYP/cc-pVTZ, B3-LYP/6-31G(2df,p), and MP2(full)/6-311+G(3df,2p) are also in good agreement with experiment. Zero-point vibrational energies and enthalpy temperature corrections are found to be relatively insensitive to the various procedures employed. However, the heats of formation for these molecules are challenging targets for high-level ab initio procedures. In the Wn-type procedures, it is found that expanding the correlation space on the metal atoms from the normal relaxed valence (rv) (or frozen-core) specification to relaxed inner valence (riv) requires the use of newly developed core-valence basis sets (cc-pWCVnZ) in the extrapolation calculations to obtain reliable results. Our best calculated heats of formation (ΔHf 298) come from a procedure designated W2C//ACQ...