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991.
The cycloaddition of ethylene to butadiene has been studied by theab initio LCAO-SCF-MO method of Roothaan using STO-3G and 7s-3p basis sets. The potential energy hypersurface of the supersystem formed by the reactants has been calculated in order to determine the reaction path. It was found that, during the approach of the partners, the planes of the molecules form an angle around 70 °. The activated complex has a geometry which prefigures the half-chair conformation of cyclohexene and exhibits no biradical character. Our theoretical results are in reasonable agreement with the corresponding experimental ones. 相似文献
992.
J. Goulon E. Georges C. Goulon-Ginet Y. Chauvin D. Commereuc H. Dexpert E. Freund 《Chemical physics》1984,83(3):357-366
A preliminary EXAFS study is reported of precursors of homogeneous hydrogenation catalysts obtained by addition of AlEt3 to nickel or cobalt octoate solutions in benzene. For a given nickel octoate solution studied before reduction, monodentate coordination of the nickel cations to at least four carboxylic anions was established and accurate Ni...O1, Ni...C1 distances (i.e. 2.06±0.01 Å, 3.01±0.02 Å) were determined. The spectra of the reduced solutions are however clearly dominated by intense Ni*...Ni or Co*...Co signals at distances comparable to the relevant distances of the bulk metal, and thus suggest the presence of rather disordered, amorphous-like metal clusters. Strong evidence is also produced in favour of additional metal...carbon bonds which were often assumed to play an important role in the catalytic activity of these or related solutions. 相似文献
993.
All-valence electron calculations on the chiroptic properties of oligopeptides rapidly become intractable, as the size of the molecule increases. A Frozen Core procedure is here proposed, which is inspired by the PPP method. It takes explicitly into account only the (pseudo) π electrons and oxygen n electrons of the amide moieties, as well as the π electrons of eventual long-wavelength absorbing sidegroups. Local n-π interaction is taken into account by an adjustable parameter. Other parameters are calibrated on the isolated amide and carbonyl chromophores. The method then follows the usual SCF-CI procedure. Rotatory strengths and f-values for the transitions are computed without further approximations, as previously described. Comparisons with results from exciton calculations and experimental data on polypeptides show the computed quantities to have a consistent and satisfactory order of magnitude. 相似文献
994.
Georges Pasquet Dominique Boucherot William R. Pilgrim Brian Wright 《Tetrahedron letters》1980,21(10):931-934
The antibiotic negamycin and its diastereomer have both been synthesized in racemic form. The latter has been found to possess antibacterial activity, but at a lower level than negamycin itself. 相似文献
995.
The aim of this Letter is to show that the Poincare-Dulac theorem for holomorphic finite-dimensional representation, is valid for any nilpotent Lie algebrag. We reduce the classification problem of representations with a semisimple linear part satisfying the Poincaré condition to an algebraic problem. We develop a complete computation in a particular case. 相似文献
996.
997.
Georges Skandalis 《Journal of Functional Analysis》1984,56(3):337-347
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