Double activation model

Subsequently, the Feng group developed an enantioselective cyanosilylation of ketones by a catalytic double-activation catalyst system composed of chiral (J ,J )-salen 16-triethylaluminium complex and N-oxide 17 (Scheme 19.10). High catalytic turnovers (200 for aromatic ketones, 1000 for aliphatic ones) with high enantioselectivity (up to 94% enantiomeric excess for aromatic ketones, up to 90% enantiomeric excess for aliphatic ones) were achieved under mild reaction conditions. Based on the control experiments, a double-activation model was suggested (Scheme 19.10). The chiral aluminium complex performed as a Lewis acid to activate the ketone, whereas the N-oxide acted as a Lewis base to activate trimethylsilyl cyanide and form an isocyanide species. The activated nucleophile and ketone attracted and approached each other, and so the transition state was formed. The intramolecular transfer of cyanide to the carbonyl group gives the product cyanohydrin O-TMS ether. 

Various pc electrode models have been tested.827 Using the independent diffuse layer electrode model74,262 the value of E n = -0.88 V (SCE) can be simulated for Cd + Pb alloys with 63% Pb if bulk and surface compositions coincide. However, large deviations of calculated and experimental C,E curves are observed at a 0. Better correspondence between experimental and calculated C,E curves was obtained with the common diffuse-layer electrode model,262 if the Pb percentage in the solid phase is taken as 20%. However, the calculated C, at a Ois noticeably lower than the experimental one. It has been concluded that Pb is the surface-active component in Cd + Pb alloys, but there are noticeable deviations from electrical double-layer models for composite electrodes.827... 

A very simple model for reaction between two ions in solution is shown in Fig. 8.1. This model is referred as double sphere model. ZA and ZB indicate the number of positive or negative charges on the ions. Initially the ions are at infinite distance from each other and in activated state they are considered to be intact and they are dAB distance apart. 

It is important to stress that the activity coefficients (and the concentrations) in equation 16.18 refer to the species close to the surface of the electrode, and so can be very different from the values in the bulk solution. This is portrayed in figure 16.6, which displays the Stern model of the double layer [332], One (positive) layer is formed by the charges at the surface of the electrode the other layer, called the outer Helmholtz plane (OHP), is created by the solvated ions with negative charge. Beyond the OHP, the concentration of anions decreases until it reaches the bulk value. Although more sophisticated double-layer models have been proposed [332], it is apparent from figure 16.6 that the local environment of the species that are close to the electrode is distinct from that in the bulk solution. Therefore, the activity coefficients are also different. As these quantities are not... 

The active Irunsport of chloride has also been demonstrated across ihe wall of the IVog stomach, rat ileum, dog ileum, and the human ileum. Experiments have produced a double exchange model in which bicarbonate secretion and chloride absorption are linked hy an isoelectric mechanism to hydrogen ion secretion and sodium absorption across the human ileum. In 1972. a group of researchers proposed a similar model of coupled transport... 

The object of the present work is to show that the double activation energy or EE model is a suitable approximation to the decomposition rate equation for propane, n-butane, and n-hexane and, by extension, other paraffins and isoparaffins. Ethane is a somewhat special case and is excluded for the present. 

So the advances during the last 50 years have been mainly in our qualitative understanding of the double layer. Given the complexity of the interface, this is no small achievement. However, quantitative results are clearly desirable. Most probably they will come from further advances in simulation techniques and computing power, while the contribution of proper theory will diminish. This trend toward more numerical works may be regrettable, but is unavoidable. In any case, there are many open questions in double-layer modeling, and it will stay an active field of research for years to come. 

Let us consider the double-layer model circuit as shown in Fig. 3.4. This circuit can be modified based on Randles circuit [2], a prevalent circuit in electrochemistry [7]. It consists of an active electrolyte resistance Rg in series with the parallel combination of the double-layer capacitance Cj and an impedance of a faradaic reaction. The faradaic reaction consists of an active charge transfer resistance R and Warburg resistance Rw- Hence, the electrical equivalent circuit can be modified as shown in Fig. 3.5. 

Milgrom and Weber (1982) describes a variant of the English Auction where the price is posted electronically. All bidders are active at price zero. The price is raised continuously, and a bidder who wishes to remain active at the current price must depress a button. When she releases the button, she is dropped out of the auction. No bidders who has dropped out can become active again. After any bidder withdraws, all remaining bidders know the price at which she drops out. When there is only one bidder left in the room, the auction ends. McAfee (1992) proposes an oral double auction work in a similar fashion, but with multiple buyers and sellers. In the following, we use the oral double auction model to characterize the basic functions of exchange coordination carried out by a market intermediary (see Figure 3.4). 

An important result of the double-barrier model is that the transfer coefficient a in the final kinetic equation is lower than that of either individual transfer coefficient or 2- This results in a higher value of the Tafel slope than in traditional single-barrier kinetic equations. The apparent reaction order with respect to the active component whose concentration appears in the kinetic equation is also modified. 

The double-layer model by Arabczyk et suggested that 0.48% (mass fraction) of K2O is sufficient to cover iron surface by monolayer potassium. Industrial catalysts commonly contain 0.5%-l% (mass fraction) of K2O, with remanent potassium forming a tousy compound with alumina, silica or calcium in catalysts. Because at least 10% of potassium in the compounds is unchangeable, the content of 0.55% (mass fraction) is sufficient. This conclusion is obtained from the experimental results by Kowalczyk. It was found in their experiment that in the double-promoted catalysts, those with content of potassium in the range of 0.5%-0.7% (mass fraction) resulted in highest activity. 

For the catalyst system WCU-CsHbAICIs-CzHsOH, Calderon et al. (3, 22, 46) also proposed a kinetic scheme in which one metal atom, as the active center, is involved. According to this scheme, which was applied by Calderon to both acyclic and cyclic alkenes, the product molecules do not leave the complex in pairs. Rather, after each transalkylidenation step an exchange step occurs, in which one coordinated double bond is exchanged for the double bond of an incoming molecule. In this model the decomposition of the complex that is formed in the transalkylidenation step is specified, whereas in the models discussed earlier it is assumed that the decom-plexation steps, or the desorption steps, are kinetically not significant. 

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