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排序方式: 共有157条查询结果,搜索用时 15 毫秒
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2.
Friedrich J Dolg M Gansäuer A Geich-Gimbel D Lauterbach T 《Journal of the American Chemical Society》2005,127(19):7071-7077
The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive. 相似文献
3.
Water concentrations in dichloroacetic acid in the range of 0<C <1 % (C = percent by weight) can be determined directly by photometry at 1425 nm. The absorbance A at this maximum is described by the function A=1.267×C0.93 (cell path d=5 cm, correlation coefficient r=0.997). The variation coefficient for water concentrations of ≈0.06% was found to be V=6.5%. 相似文献
4.
Luise Oemisch Kai-Uwe Goss Satoshi Endo 《Analytical and bioanalytical chemistry》2013,405(8):2567-2574
Experimental determination of oil-water partition coefficients often poses difficulties associated with emulsion formation. The aim of this work was to find an appropriate technique for determination of oil–water partition coefficients of polar, nonvolatile compounds. Two different methods were tested. The first method used a “silicone membrane equilibrator.” For the second method, solid-phase microextraction (SPME) fibers with a polyacrylate (PA) coating were used as a passive sampler. With both methods, oil–water partition coefficients for 14 compounds with polar functional groups were determined at 37 °C with good repeatability (standard deviation 0.11 log units or lower). The partition coefficients determined with the silicone membrane equilibrator method ranged from 0.50 to 3.49 log units. The oil–water partition coefficients obtained with the PA-SPME passive sampling approach were significantly higher than those obtained with the silicone membrane equilibrator method for nine of 14 compounds. The differences were up to 0.39 log units (i.e., a factor of 2.5). Additional experiments suggested that this difference occurred because the sorption properties of the PA fibers used were influenced by the surrounding phase, e.g., through swelling of the polymer phase. Therefore, the SPME passive sampling method using PA fibers seems to be less reliable, whereas the silicone membrane equilibrator method was found to be a convenient technique for the determination of oil–water partitioning. 相似文献
5.
Prof. Dr. A. Stephen K. Hashmi Tobias Lauterbach Dipl.‐Chem. Pascal Nösel Dr. Mie Højer Vilhelmsen Dr. Matthias Rudolph Dr. Frank Rominger 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(3):1058-1065
A series of dinuclear gold σ,π‐propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π‐complex of gold with a gold acetylide. The air‐stable and storable catalysts can be isolated as silver‐free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw‐away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis. 相似文献
6.
Gansäuer A Lauterbach T Geich-Gimbel D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(20):4983-4990
A catalyzed synthesis of cyclopropanes and cyclobutanes via radical chemistry is described. The method that generally proceeds in high yields uses epoxides as radical precursors and titanocene(III) complexes as the electron transfer catalysts (see scheme). The key to the success of the transformation is constituted by the chemoselectivity of radical reduction. Electrophilic enol radicals generated through cyclization are reduced rapidly whereas their precursors, nucleophilic alkyl radicals, remain unaffected. 相似文献
7.
Stanislaus Maier-Paape Reiner Lauterbach 《Transactions of the American Mathematical Society》2000,352(7):2937-2991
Recently it has been observed, that perturbations of symmetric ODE's can lead to highly nontrivial dynamics. In this paper we want to establish a similar result for certain nonlinear partial differential systems. Our results are applied to equations which are motivated from chemical reactions. In fact we show that the theory applies to the Brusselator on a sphere. To be more precise, we consider solutions of a semi-linear parabolic equation on the 2-sphere. When this equation has an axisymmetric equilibrium , the group orbit of (under rotations) gives a whole (invariant) manifold of equilibria. Under generic conditions we have that, after perturbing our equation by a (small) -equivariant perturbation, persists as an invariant manifold . However, the flow on is in general no longer trivial. Indeed, we find slow dynamics on and, in the case (the tetrahedral subgroup of ), we observe heteroclinic cycles. In the application to chemical systems we would expect intermittent behaviour. However, for the Brusselator equations this phenomenon is not stable. In order to see it in a physically relevant situation we need to introduce further terms to get a higher codimension bifurcation.
8.
F. W. N. de Boer H. J. Wollersheim H. Emling H. Grein E. Grosse W. Spreng G. Eckert Th. W. Elze K. Stelzer Ch. Lauterbach 《Zeitschrift für Physik A Hadrons and Nuclei》1986,325(4):457-466
In a systematic study of nucleon transfer reactions accompanied by Coulomb excitation we have bombarded152Sm,160Gd and232Th with206, 208pb beams at incident energies close to the Coulomb barrier. Particle-gamma coincidence techniques were used to identify excited states of reaction products populated through inelastic scattering and in nucleon transfer reactions. One-neutron stripping and pick-up reactions on152Sm were observed leading to known states of the rotation alignedi 13/2-bands in153Sm and151Sm. In the160Gd+206, 208Pb systems no significant population of low lying states of product nuclei was found in the nucleon-transfer channels. Large cross sections were observed for one- and two-neutron pick-up from232Th at an incident energy of 6.4 MeV/u. Around the grazing angle they are of the same order of magnitude as the cross section measured for inelastic scattering. The results are analyzed in the framework of semiclassical models. 相似文献
9.
In numerical simulations with the finite element method the dependency on the mesh – and for time-dependent problems on the time discretization – arises. Adaptive refinements in space (and time) based on goal-oriented error estimation [1] become more and more popular for finite element analyses to balance computational effort and accuracy of the solution. The introduction of a goal quantity of interest defines a dual problem which has to be solved to estimate the error with respect to it. Often such procedures are based on a space-time Galerkin framework for instationary problems [2]. Discretization results in systems of equations in which the unknowns are nodal values. Contrary, in current finite element implementations for path-dependent problems some quantities storing information about the path-dependence are located at the integration points of the finite elements [3], e.g. plastic strains etc. In this contribution we propose an approach – similar to [4] for sensitivity analysis – for the approximation of the dual problem which mainly maintains the structure of current finite element implementations for path-dependent problems. Here, the dual problem is introduced after discretization. A numerical example illustrates the approach. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
Lars Lauterbach Hongxin Wang Marius Horch Leland B. Gee Yoshitaka Yoda Yoshihito Tanaka Ingo Zebger Oliver Lenz Stephen P. Cramer 《Chemical science》2015,6(2):1055-1060
Hydrogenases are complex metalloenzymes that catalyze the reversible splitting of molecular hydrogen into protons and electrons essentially without overpotential. The NAD+-reducing soluble hydrogenase (SH) from Ralstonia eutropha is capable of H2 conversion even in the presence of usually toxic dioxygen. The molecular details of the underlying reactions are largely unknown, mainly because of limited knowledge of the structure and function of the various metal cofactors present in the enzyme. Here, all iron-containing cofactors of the SH were investigated by 57Fe specific nuclear resonance vibrational spectroscopy (NRVS). Our data provide experimental evidence for one [2Fe2S] center and four [4Fe4S] clusters, which is consistent with the amino acid sequence composition. Only the [2Fe2S] cluster and one of the four [4Fe4S] clusters were reduced upon incubation of the SH with NADH. This finding explains the discrepancy between the large number of FeS clusters and the small amount of FeS cluster-related signals as detected by electron paramagnetic resonance spectroscopic analysis of several NAD+-reducing hydrogenases. For the first time, Fe–CO and Fe–CN modes derived from the [NiFe] active site could be distinguished by NRVS through selective 13C labeling of the CO ligand. This strategy also revealed the molecular coordinates that dominate the individual Fe–CO modes. The present approach explores the complex vibrational signature of the Fe–S clusters and the hydrogenase active site, thereby showing that NRVS represents a powerful tool for the elucidation of complex biocatalysts containing multiple cofactors. 相似文献