Model catalytic cycle

Ab initio quantum-mechanical calculations have also been carried out to estimate theoretically the relative energies of different transition states and intermediates. To keep the calculations at a manageable level the model catalytic cycle shown in Fig. 7.4 has been considered. According to this calculation, conversion of 7.14 to 7.15 is found to be the rate-determining step. This is consistent with the empirical kinetic results. Isomerization of 7.15 to 7.16 is found to be thermodynamically highly favorable. The reductive elimination step, that is, 7.16 to 7.7, is found to have a very low free energy of activation and be thermoneutral, that is, thermodynamically neither strongly favorable nor unfavorable. 

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

Scheme 9.1 Three depictions of the same model catalytic cycle. In theory, they are mathematically equivalent. In practice, they are designed to show different aspects of their chemistry.

Scheme 9.3 k-representation and -representation of a model catalytic cycle (in s" and kcal mol" ), and the simplification process to obtain the one-vertex, one-edge simple cycle, equivalent to finding the determining intermediate and transition state. 

Scheme 9.14 Model catalytic cycle of order 3 in the k- and -representations.

Fig. 8. A model for the catalytic cycle of hydrodenase based on several lines of evidence (see text).

Mechanistic studies, i.e. model studies of the elementary steps of the catalytic cycle, are currently under way [112]. 

A kinetic model describing the HRP-catalyzed oxidation of PCP by H202 should account for the effects of the concentrations of HRP, PCP, and H202 on the reaction rate. To derive such an equation, a reaction mechanism involving saturation kinetics is proposed. Based on the reaction scheme described in Section 17.3.1, which implies that the catalytic cycle is irreversible, the three distinct reactions steps (Equations 17.2 to 17.4) are modified to include the formation of Michaelis-Menten complexes ... 

The catalytic cycle of function 3 is able to turn over, when flowing the activated form of HC (CrIl yO alcohol, aldehyde, etc.) according to the model of Figure 5.2... 

It has been chosen, for presenting the three-function model, to start from the true DeNOx catalytic cycle corresponding to function 3, which leads to the N—N bonding and N2 release from the catalyst. Subsequently, it appears that two other functions are necessary to assist function 3. 

Figure 2-9. Reaction scheme for the complete catalytic cycle in glutathione peroxidase (left). Numbers represent calculated reaction barriers using the active-site model. The detailed potential energy diagram for the first elementary reaction, (E-SeH) + H2O2 - (E-SeOH) + H2O, calculated using both the active-site (dashed line) and ONIOM model (grey line) is shown to the right (Adapted from Prabhakar et al. [28, 65], Reprinted with permission. Copyright 2005, 2006 American Chemical Society.)...

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

Scheme 2.25. Proposed catalytic cycle and stereochemical model.

Currently, the density functional theory (DFT) method has become the method of choice for the study of reaction mechanism with transition-metals involved. Gradient corrected DFT methods are of particular value for the computational modeling of catalytic cycles. They have been demonstrated in numerous applications for several elementary processes, to be able to provide quantitative information of high accuracy concerning structural and energetic properties of the involved key species and also to be capable of treating large model systems.30... 

A tetracoordinated complex (20)4 was actually isolated. Complex 20 in the presence of ethylene forms the coordinated complex 21, as can be seen from H NMR. Complex 21 is a model of the intermediate for the additional reaction to form C6 dienes. The model catalyst had been shown to be a codimerization catalyst under more severe conditions (high temperature), although the rate of reaction was very slow compared to the practical systems. These studies are extremely useful in demonstrating the basic steps of the codimerization reactions taking place on the Ni atom. The catalytic cycle based on these model complexes as visualized by Tolman is summarized in Scheme 7. A more complete scheme taking into consideration by-product formation can be found in Tolman (40). 

Catalytic cycles resembling that of Tolman can be applied using 24 as reaction center. Tolman s model shows that the coordination number on the Ni atom changes between 4 and 5 (see Scheme 7) during the following catalytic reaction ... 

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