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991.
Christoph Schädle Dr. Andreas Fischbach Dr. Eberhardt Herdtweck Prof. Dr. Karl W. Törnroos Prof. Dr. Reiner Anwander 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(48):16334-16341
Yttrocene‐carboxylate complex [Cp*2Y(OOCArMe)] (Cp*=C5Me5, ArMe=C6H2Me3‐2,4,6) was synthesized as a spectroscopically versatile model system for investigating the reactivity of alkylaluminum hydrides towards rare‐earth‐metal carboxylates. Equimolar reactions with bis‐neosilylaluminum hydride and dimethylaluminum hydride gave adduct complexes of the general formula [Cp*2Y(μ‐OOCArMe)(μ‐H)AlR2] (R=CH2SiMe3, Me). The use of an excess of the respective aluminum hydride led to the formation of product mixtures, from which the yttrium‐aluminum‐hydride complex [{Cp*2Y(μ‐H)AlMe2(μ‐H)AlMe2(μ‐CH3)}2] could be isolated, which features a 12‐membered‐ring structure. The adduct complexes [Cp*2Y(μ‐OOCArMe)(μ‐H)AlR2] display identical 1J(Y,H) coupling constants of 24.5 Hz for the bridging hydrido ligands and similar 89Y NMR shifts of δ=?88.1 ppm (R=CH2SiMe3) and δ=?86.3 ppm (R=Me) in the 89Y DEPT45 NMR experiments. 相似文献
992.
Venkataramana Imandi Sooraj Kunnikuruvan Dr. Nisanth N. Nair 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(15):4724-4731
The complete reaction mechanism and kinetics of the Wacker oxidation of ethene in water under low [Cl?], [PdII], and [CuII] conditions are investigated in this work by using ab initio molecular dynamics. These extensive simulations shed light on the molecular details of the associated individual steps, along two different reaction routes, starting from a series of ligand‐exchange processes in the catalyst precursor PdCl42? to the final aldehyde‐formation step and the reduction of PdII. Herein, we report that hydroxylpalladation is not the rate‐determining step and is, in fact, in equilibrium. The newly proposed rate‐determining step involves isomerization and follows the hydroxypalladation step. The mechanism proposed herein is shown to be in excellent agreement with the experimentally observed rate law and rate. Moreover, this mechanism is in consensus with the observed kinetic isotope effects. This report further confirms the outer‐sphere (anti) hydroxypalladation mechanism. Our calculations also ratify that the final product formation proceeds through a reductive elimination, assisted by solvent molecules, rather than through β‐hydride elimination. 相似文献
993.
Dr. Niels J. M. Pijnenburg Dr. Yves H. M. Cabon Prof. Dr. Gerard van Koten Prof. Dr. Robertus J. M. Klein Gebbink 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(15):4858-4868
This paper describes a mechanistic study of the SCS‐pincer PdII‐catalyzed auto‐tandem reaction consisting of the stannylation of cinnamyl chloride with hexamethylditin, followed by an electrophilic allylic substitution of the primary tandem‐reaction product with 4‐nitrobenzaldehyde to yield homoallylic alcohols as the secondary tandem products. As it turned out, the anticipated stannylation product, cinnamyl trimethylstannane, is not a substrate for the second part of the tandem reaction. These studies have provided insight in the catalytic behavior of SCS‐pincer PdII complexes in the auto‐tandem reaction and on the formation and possible involvement of Pd0 species during prolonged reaction times. This has led to optimized reaction conditions in which the overall tandem reaction proceeds through SCS‐pincer PdII‐mediated catalysis, that is, true auto‐tandem catalysis. Accordingly, this study has provided the appropriate reaction conditions that allow the pincer catalysts to be recycled and reused. 相似文献
994.
Jannike Solsvik Hugo A. Jakobsen 《Journal of Dispersion Science and Technology》2014,35(11):1626-1642
In the churn-turbulent bubbly flow regime with highly nonuniform bubble size distributions, bubble breakage and coalescence are important processes because they govern the bubble size distribution and consequently directly affect the interfacial mass, momentum, and heat transfer fluxes through the renewal bubble surfaces. At present, accurate prediction of bubble size distributions of dispersed gas–liquid flows by use of the population balance (PB) equation is a difficult task. The modeling of bubble breakup and coalescence rates is very complex and is based on the knowledge of collision and breakup frequencies, breakage daughter size distributions, and probability of coalescence. In this work, we focus on the coalescence phenomenon. The coalescence models are still on an empirical level and the mechanisms are not fully understood. This motivates the analysis of the suitability of the coalescence closures for the prediction of experimental data obtained from coalescence dominated gas–liquid flows. For this task, a cross-sectional averaged combined multifluid-PB model is adopted. Based on different theories for the coalescence efficiency, the simulation results show a similar trend in the prediction of the experimental data. Good prediction of the Sauter mean diameter is achieved although the shape of the bubble size distribution is not completely reproduced. The second aim of this work is to review the PB framework. Here, focus is placed on the coalescence term and the combined multifluid-PB model based on kinetic theory approach. 相似文献
995.
This paper investigates the event-triggered sliding mode control (SMC) problem for singular systems with disturbance. Firstly, an event-triggered sliding mode control law is designed to guarantee the reachability of sliding surface. Different from the related methods, in order to deal with the difficulty caused by event-triggered SMC strategy, a novel Lemma is proposed in this paper. Secondly, the admissibility of sliding motion is presented, which is used to solve the controller gain. Then, a positive lower bound of the inter execution time can be guaranteed and the Zeno behavior is avoided. Finally, two simulation examples are presented to show the effectiveness of derived theoretical results. 相似文献
996.
This paper is concerned with the self-triggered filtering problem for a class of Markovian jumping nonlinear stochastic systems. The event-triggered mechanism (ETM) is employed between the sensor and the filter to reduce unnecessary measurement transmission. Governed by the ETM, the measurement is transmitted to the filter as long as a predefined condition is satisfied. The purpose of the addressed problem is to synthesize a filter such that the dynamics of the filtering error is bounded in probability (BIP). A sufficient condition is first given to ensure the boundedness in probability of the filtering error dynamics, and the characterization of the desired filter gains is then realized by means of the feasibility of certain matrix inequalities. Furthermore, a self-triggered mechanism is designed to guarantee the filtering error dynamics to be BSP with excluded Zeno phenomenon. In the end, numerical simulation is carried out to illustrate the usefulness of the proposed self-triggered filtering algorithm. 相似文献
997.
Detailed simulation of primary atomization mechanisms in Diesel jet sprays (isolated identification of liquid jet tip effects) 总被引:2,自引:0,他引:2
In this study, the atomization characteristics of Diesel jet front tip have been investigated to elucidate the physical mechanisms by detailed numerical simulation. The computations are carried out with the finest grid resolutions ever that can resolve the final droplet generation by surface tension. The numerical methods are based on level-set interface tracking. The methods were validated by test cases and the grid resolution survey shows that the resolutions for the present study are sufficient. The present flow setup excludes nozzle disturbances to investigate how the disturbances from the liquid jet front would lead to atomization where the liquid jet impacts against the quiescent gas. The liquid jet front becomes an umbrella-like shape. From the front umbrella tip edge, ligament breakup first occurs. Ligament breakup is strongly correlated with the gas motion in the vicinity. The gas region behind the front is highly disturbed by atomization. By the gas recirculation motion here, air and some droplets are entrained and mixed. Also, the disturbances are fed back to the front umbrella by this motion and become synchronized with the breakup. Droplet pinch-off is mainly in the short-wave mode, but some ligaments are elongated by local gas stretch to finally have a long-wave mode shape, namely a mode shift occurs. The above findings of liquid jet front umbrella formation, atomization at the umbrella edge, mixing and atomization loop in the recirculation flow region and droplet generation mode give an insight to the modeling of droplet generation in actual sprays. 相似文献
998.
Enrique A. Castro Marcela Gazitúa José G. Santos 《Journal of Physical Organic Chemistry》2011,24(6):466-473
The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl), O‐(4‐chlorophenyl) (4‐nitrophenyl), O‐(4‐chlorophenyl) S‐phenyl, and O‐(4‐methylphenyl) S‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (kobs) are found. For some of the reactions, plots of kobs vs. free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T±) and the other anionic (T?). In some cases, there is a kinetically significant proton transfer from T± to an amine to yield T?. Values of the rate micro constants k1 (amine attack to form T±), k?1 (its back step), k2 (nucleofuge expulsion from T±), and k3 (proton transfer from T± to the amine) are determined for some reactions. The Brønsted plots for k1 are linear with slopes β1 = 0.2–0.4 in accordance with the slope values found when T± formation is the rate‐determining step. The sensitivity of log k1 and log k?1 to the pKa of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k2 and k3 steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
999.
1000.
Electrolysis is a potential candidate for a quick method of wastewater cleansing. However, it is necessary to know what compounds might be formed from bioorganic matter. We want to know if there are toxic intermediates and if it is possible to influence the product formation by the variation in initial conditions. In the present study, we use Car–Parrinello molecular dynamics to simulate the fastest reaction steps under such circumstances. We investigate the behavior of amino acids and peptides under anodic conditions. Such highly reactive situations lead to chemical reactions within picoseconds, and we can model the reaction mechanisms in full detail. The role of the electric current is to discharge charged species and, hence, to produce radicals from ions. This leads to ultra-fast radical reactions in a bulk environment, which can also be seen as redox reactions as the oxidation states change. In the case of amino acids, the educts can be zwitterionic, so we also observe complex acid–base chemistry. Hence, we obtain the full spectrum of condensed-phase chemistry. 相似文献