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81.
The reaction of ring opening of epoxides under the action of a halogenic acid has been investigated using as model the system C2H4O + HF → CH2OHCH2F. The physical conditions chosen for the model more directly correspond to the gas phase reaction and under these conditions the preferred mechanism leads to the formation of a halohydrine having the same configuration at the C atom as the reagent. Parallel investigations have been performed on other mechanisms which postulate the preliminary formation of the conjugate acid of the oxirane (C2H4OH+) and proceed via the well known mechanismsA 1 orA 2. In this case the best mechanism corresponds to the so-called “borderlineA 2” mechanism. This last type of mechanism probably is the dominant one in protic solutions, but by coupling the present calculations with experimental conductometric measurements in anhydrous aprotic media one could consider the first concerted mechanism as a possible candidate also for the reaction in “inert” media. A qualitative analysis of the transition state indicates, in addition, that the propension for the retention path, is probably emphasized by the use of HF as reactant, and that with other acids, like HC1, or even by assuming the presence of dimers like HF·HF, the inversion path could be preferred. The investigations have been done by determining the geometry of the transition state and the reaction coordinate withab initio SCF STO-3G calculations on the whole nuclear configuration space (21 dimensions). These calculations have been supplemented by a few CI calculations on the same basis set and by a few SCF calculations with a larger basis set. 相似文献
82.
R. Lucas C. Gregoire C. Ngô J. Poitou E. Tomasi A. Gobbi A. Olmi H. Sann G. Wirth J.V. Kratz U. Lynen 《Nuclear Physics A》1984,413(3):516-534
Some results on the charge equilibration process for the 205Pb (1550 MeV) + 74Ge and 86Kr (550 MeV) + 197Au reactions obtained by on-line measurements are discussed and compared with the previous results of the 132Xe (890 MeV) +197Au reaction investigated using radiochemical techniques. In all cases global and isobaric charge widths were measured as a function of the excitation energy. It is shown that the corrections associated with both the evaporation process and the experimental mass or charge resolution are most important in obtaining correct primary distributions. The results confirm a preferential neutron exchange at the beginning of the reaction, as was suggested by the Brosa and Gross model. Further attempts to analyse the data with existing dynamical models have not succeeded in their present stage of development in discriminating between the assumptions of statistical versus quantal fluctuations as observed in the study of the Kr + Mo system. 相似文献
83.
Rosanna Bonaccorsi Roberto Cammi Jacopo Tomasi 《International journal of quantum chemistry》1986,29(3):373-378
Counterpoise corrections for the basis set superposition error to the components of the bimolecular interaction energy are defined for three methods of decomposition. The results for the case of the NH3 + BH3 interaction are presented and discussed. 相似文献
84.
In a recent article [R. Cammi, S. Corni, B. Mennucci, and J. Tomasi, J. Chem. Phys. 122, 104513 (2005)], we demonstrated that the state-specific (SS) and the linear-response (LR) approaches, two different ways to calculate solute excitation energies in the framework of quantum-mechanical continuum models of solvation, give different excitation energy expressions. In particular, they differ in the terms related to the electronic response of the solvent. In the present work, we further investigate this difference by comparing the excitation energy expressions of SS and LR with those obtained through a simple model for solute-solvent systems that bypasses one of the basic assumptions of continuum solvation models, i.e., the use of a single Hartree product of a solute and a solvent wave function to describe the total solute-solvent wave function. In particular, we consider the total solute-solvent wave function as a linear combination of the four products of two solute states and two solvent electronic states. To maximize the comparability with quantum-mechanical continuum model the resulting excitation energy expression is recast in terms of response functions of the solvent and quantities proper for the solvated molecule. The comparison of the presented expressions with the LR and SS ones enlightens the physical meaning of the terms included or neglected by these approaches and shows that SS agrees with the results of the four-level model, while LR includes a term classified as dispersion in previous treatments and neglects another related to electrostatic. A discussion on the possible origin of the LR flaw is finally given. 相似文献
85.
We report on the observation of nonlocalized modes or necklace states of light waves in disordered systems in the Anderson localized regime. The samples consist of positional-disordered binary multilayer systems. Anderson localized modes manifest themselves as narrow high-transmission peaks in the transmission spectrum, whereas the average of the logarithm of the transmission coefficient decreases linearly with thickness. Optical necklace states are observed as modes with a characteristic multiresonance time response and relatively fast decay time. 相似文献
86.
A method is presented which utilizes the calculation of the molecular electrostatic potential or the electric field at a discrete number of preselected points to evaluate the environmental effects of a solvent on the properties of a molecular system. No limitations are imposed on the composition and dimension of the solute, on the goodness of the corresponding wavefunction, or on the shape of the cavity in the dielectric. Several levels of approximation, which evidence the effect of self-polarization of the system of surface charges, the influence of the tails of the solute charge distribution going beyond the limits of the cavity, and the effect of the polarization of the solute, are examined and discussed. 相似文献
87.
The electrostatic potentials arising from ab initio MO LCAO GTO SCF wave functions for adenine, thymine and cytosine are given and discussed.Well defined characteristic regions of immediate chemical significance are found. The analysis of such results aims at comparing different protonation sites in the same molecule as well as in different ones. Differences among the proton affinities of the nitrogen atoms (pyridine-like, amine and imine) are evidenced, as well as the distinction between oxygen and nitrogen atoms.
Zusammenfassung Die mit Hilfe von ab initio MO LCAO GTO SCF Wellenfunktionen berechneten elektrostatischen Potentiale von Adenin, Thymin und Cytosin werden angegeben und diskutiert.Man findet genau bestimmte charakteristische Zonen von klarer chemischer Bedeutung. Durch die Analyse solcher Resultate können verschiedene Protonierungsplätze sowohl in ein und demselben Molekül als auch in verschiedenen Molekülen verglichen werden. Deutlich zeigen sich Unterschiede der Protonenaffinitäten der unterschiedlich chemisch gebundenen Stickstoffatome (pyridinartiger, Amin- und Iminstickstoff) und der Unterschied zwischen Stickstoff- und Sauerstoffatomen.
Résumé Les potentiels electrostatiques moléculaires calculés à partir de fonctions d'onde ab initio MO SCF LCAO GTO sont donnés et discutés pour l'adénine, la thymine et la cytosine.Des régions bien définies sont obtenues dont la signification chimique apparait clairement. L'analyse de l'ensemble des résultats permet la comparaison des differents sites de protonation dans une même molécule ainsi que dans differentes molécules, ainsi que la distinction entre oxygene et azote d'une part, azotes de differents types d'autre part.相似文献
88.
Sgrignani Jacopo Novati Beatrice Colombo Giorgio Grazioso Giovanni 《Journal of computer-aided molecular design》2015,29(5):441-450
Journal of Computer-Aided Molecular Design - AmpC β-lactamase is a hydrolytic enzyme conferring resistance to β-lactam antibiotics in multiple Gram-negative bacteria. Therefore,... 相似文献
89.
90.