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1.
Résumé Une étude théorique ab initio SCF-LCAO-MO de la méthylène imine et de l'anion vinyle a été effectuée. Les barrières d'énergie calculées pour l'inversion de l'azote et pour la rotation autour de la liaison C=N sont respectivement de 26–28 kcal/mole et de 57 kcal/mole.La barrière d'inversion du carbanion calculée pour l'anion vinyle est de 39 kcal/mole et l'isomérisation des imines se fait donc par inversion de l'azote. Les anions vinyliques ont une configuration stable.L'origine physique de ces barrières d'inversion est étudiée et caractérisée par les variations des termes énergétiques, de la composition de l'OM du doublet libre et des populations électroniques.La comparaison des processus d'inversion du carbanion, de l'azote et de l'oxonium dans la série CH2=CH, CH2=NH et CH2=OH a permis de mettre en évidence l'évolution des termes énergétiques, de la hauteur de la barrière et des transferts de population.
Précédent article de cette série: cf. Réf. [1].
Laboratoire associé au C.N.R.S. 相似文献
Theoretical conformational analysisIV. An ab initio SCF-LCAO-MO study of methylenimine and of vinyl anion
An ab initio SCF-LCAO-MO study of methylenimine and vinyl anion has been performed. The calculated barriers to nitrogen inversion and to rotation about the C=N bond are equal to 26–28 kcal/mole and 57 kcal/mole respectively.The calculated barrier to carbanion inversion in the vinyl anion amounts to 39 kcal/mole. Thus isomerization of imines occurs by a nitrogen inversion process and vinyl anions are configurationally stable.The physical origin of these inversion barriers is studied and characterized by the changes in the energy terms, in lone pair MO composition, and in electron populations.Comparison of the carbanion, nitrogen and oxonium inversion processes in the isoelectronic series CH2=CH, CH2=NH and CH2=OH permits to delineate general trends in the energy terms, barrier height and population transfers.
Zusammenfassung Für Methylenimin und das Vinylanion wird eine ab initio SCF-LCAO-MO-Studie durchgeführt. Die berechneten Barrieren für die Stíckstoffinversion und die Rotation um die C=N-Bindung betragen 26–28 kcal/Mol bzw. 57 kcal/Mol.Die berechnete Barriere für die Inversion des Carbanions im Vinylanion beträgt 39 kcal/Mol. Daher wird die Isomerisation von Iminen durch einen Stickstoffinversionsprozeß hervorgerufen, und die Konfigurationen von Vinyl-Anionen sind stabil.Der physikalische Ursprung dieser Inversionsbarrieren wird studiert und durch Änderungen in den Energietermen, in der Zusammensetzung von Ione pair-MO's und in den Besetzungszahlen der Elektronen charakterisiert.Ein Vergleich der Inversionsprozesse des Carbanions, des Stickstoffs und des Oxonium-Ions in der isoelektronischen Reihe CH2=CH–, CH2=NH and CH2=OH+ ermöglicht es, allgemeine Züge in den Energietermen, in den Höhen der Barrieren und den Besetzungsübergängen abzuleiten.
Précédent article de cette série: cf. Réf. [1].
Laboratoire associé au C.N.R.S. 相似文献
2.
Image segmentation is required to be studied in detail some particular features (areas of interest) of a digital image. It forms an important and exigent part of image processing and requires an exhaustive and robust search technique for its implementation. In the present work we have studied the working of MRLDE, a newly proposed variant of differential evolution combined with Otsu method, a well known image segmentation method for bi-level thresholding. The proposed variant, termed as Otsu+MRLDE, is tested on a set of 10 images and the results are compared with Otsu method and some other well known metaheuristics. 相似文献
3.
West NP Cergol KM Xue M Randall EJ Britton WJ Payne RJ 《Chemical communications (Cambridge, England)》2011,47(18):5166-5168
The first targeted inhibitors of an essential M. tuberculosis cell wall lipase, Rv3802c, are described. Lead compounds exhibited nanomolar inhibition of the enzyme, and encouraging antibacterial activity against M. tuberculosis in vitro, supporting Rv3802c as a novel TB drug target. 相似文献
4.
The nuclear magnetic shielding of 29Si, 31P, 33S and 35Cl in SiH4, PH3, SH2 and ClH has been computed. The inner shells of the heavy atom are not introduced in the calculation for the molecule. Their effect on the valence shell is represented by a pseudo-potential. The agreement with available theoretical and experimental values is satisfactory. 相似文献
5.
Maurel F Aubard J Millie P Dognon JP Rajzmann M Guglielmetti R Samat A 《The journal of physical chemistry. A》2006,110(14):4759-4771
Ab initio and semiempirical quantum mechanical calculations were performed to study the electronic spectra of spiroxazine photochromic compounds as well as the corresponding photoisomers. Ground-state geometries were optimized based on density functional theory (DFT). Excitation energies of the different forms were calculated using the time-dependent density functional theory (TD-DFT) method. Semiempirical calculations including configuration interactions were performed to detail the mechanism of ring opening in excited states. On the basis of the obtained potential energy profile, a complete mechanism of photocoloration able to clarify some experimental findings is provided. A correlation of the experimental quantum yield of photocoloration with the calculated properties as a function of substituent effects is proposed. 相似文献
6.
An ab initio calculation of dichroic vibrational band intensities has been performed for the n-π1 transition of the carbonyl group in acetaldehyde (as a model compound) in a rigid asymmetric conformation. Results show a 10% contribution of vibrational effects adding to the electronic contributions. Different signs are effectively obtained according to the vibrational bands, which is promising for further applications. Concerning the method itself, results are analogous to those obtained for oscillator strengths. 相似文献
7.
Daniel J. Schell Jody Farmer Millie Newman James D. McMillan 《Applied biochemistry and biotechnology》2003,105(1-3):69-85
Corn stover is a domestic feedstock that has potential to produce significant quantities of fuel ethanol and other bioenergy
and biobased products. However, comprehensive yield and carbon mass balance information and validated kinetic models for dilute-sulfuric
acid (H2SO4) pretreatment of corn stover have not been available. This has hindered the estimation of process economics and also limited
the ability to perform technoeconomic modeling to guide research. To better characterize pretreatment and assess its kinetics,
we pretreated corn stover in a continuous 1 t/d reactor. Corn stover was pretreated at 20% (w/w) solids concentration over
a range of conditions encompassing residence times of 3–12 min, temperatures of 165–195°C, and H2SO4 concentrations of 0.5–1.4% (w/w). Xylan conversion yield and carbon mass balance data were collected at each run condition.
Performance results were used to estimate kinetic model parameters assuming biphasic hemicellulose hydrolysis and a hydrolysis
mechanism incorporating formation of intermediate xylo-oligomers. In addition, some of the pretreated solids were tested in
a simultaneous saccharification and fermentation (SSF) process to measure the reactivity of their cellulose component to enzymatic
digestion by cellulase enzymes. Monomeric xylose yields of 69–71% and total xylose yields (monomers and oligomers) of 70–77%
were achieved with performance level depending on pretreatment severity. Cellulose conversion yields in SSF of 80–87% were
obtained for some of the most digestible pretreated solids. 相似文献
8.
A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si-C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms. 相似文献
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10.
A detailed study of the intrinsic consistency of the semiempirical method of P. Claverie namely, the effects of the basis set and intramolecular correlation on the multipole distributions of molecular subunits and the influence of the electronic population of each atom in the molecular subunit on its van der Waals radius, is performed on some van der Waals dimers. The validity, limits of this model and the appropriate way to use it is established. In particular, the dependence of the geometry and the interaction energy on the basis set chosen and the intramolecular correlation shows that the multipole distribution involved in the calculation of the electrostatic and polarization terms must be derived from a correlated wave function within an extended basis set. Associated to non local methods for finding stationary points, the method of P. Claverie reproduce reliably the intermolecular geometrical parameters observed for the equilibrium structures and the transition states of the dimer and trimer of acetylene. In addition, a study of the equilibrium structures of the ethylene dimer is presented, the aim of being to clarify the considerable uncertainty in their number and their geometry. 相似文献