Cluster kinetic studies

The mechanism of the first half-reaction has been studied by a combination of reductive titrations with CO and sodium dithionite and pre-steady-state kinetic studies by rapid freeze quench EPR spectroscopy (FQ-EPR) and stopped-flow kinetics 159). These combined studies have led to the following mechanism. The resting enzyme is assumed to have a metal-bound hydroxide nucleophile. Evidence for this species is based on the similarities between the pH dependence of the EPR spectrum of Cluster C and the for the for CO, deter-... 

Kinetic studies of the hydride cluster [W3S4H3(dmpe)3] with acids in a non-coordinating solvent, i.e., dichloromethane, under the pseudo-first-order condition of acid excess, show a completely different mechanism with three kineti-cally distinguishable steps associated to the successive formal substitution of the coordinated hydrides by the anion of the acid, i.e., Ch in HCl [37]. The first two kinetic steps show a second-order dependence with the acid concentration. 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

Picosecond time regime kinetic studies of proton transfer are coming into vogue (28, 29, 30), particularly for intramolecular processes that can be very fast. Bound to play an increasingly important role in the elucidation of proton transfers are the gas phase ion-solvent cluster techniques that reveal dramatically the role played by solvent molecules in these reactions (M, 32). 

Ruthenium(III) catalyses the oxidative decarboxylation of butanoic and 2-methylpropanoic acid in aqueous sulfuric acid. ° Studies of alkaline earth (Ba, Sr) metal alkoxides in amide ethanolysis and of alkali metal alkoxide clusters as highly effective transesterification catalysts were covered earlier. Kinetic studies of the ethanolysis of 5-nitroquinol-8-yl benzoate (228) in the presence of lithium, sodium, or potassium ethoxide revealed that the highest catalytic activity is observed with Na+.iio... 

Washington, J. and Kubiak, C.P. (1998) Infrared spectroelectrochemical and electrochemical kinetics studies of the reaction of nickel cluster radicals [Ni3(p2-dppm)3(P3-L)(P3-I)] (L = CNR,... 

A very interesting and complex protonation mechanism has been snggested for the hydride cluster [W3S4H3(dmpe)3]PF6 in CH2CI2 solutions. In the presence of an excess of HCl, a careful kinetic study of the process in eq. (10.4) by the stopped-flow technique [9] has revealed three kinetically distinguishable steps very fast, fast, and slow, with rate constants A 1, ki, and k3. The kinetic order in the initial hydride cluster in the slow step has been measured as 1. At the same time, rate constants k and A 2 have corresponded to a second-order dependence on acid concentration, while the third step has shown a zero kinetic order on HCl. The rate constants have been determined as A i =2.41 x 10 M-2/s, k2 = 1.03 X 10 M /s, A 3 = 4 X 10 s . Note that the protonation process becomes simple at lower concentrations of HCl. Under these conditions it shows a single step with a first kinetic order on the acid. 

Actually, the kinetics study of the redox potential of transient clusters (Section 20.3.2) has shown that beyond the critical nuclearity, they receive electrons without delay from an electron donor already present. The critical nuclearity depends on the donor potential and then the autocatalytic growth does not stop until the metal ions or the electron donor are not exhausted (Fig. 8c). An extreme case of the size development occurs, despite the presence of the polymer, when the nucleation induced by radiolytic reduction is followed by a chemical reduction. The donor D does not create new nuclei but allows the supercritical clusters to develop. This process may be used to select the cluster final size by the choice of the radiolytic/chemical reduction ratio. But it also occurs spontaneously any time when even a mild reducing agent is present during the radiolytic synthesis. The specificity of this method is to combine the ion reduction successively ... 

Actually, the kinetic study of the cluster redox potential by pulse radiolysis [31] (Section 20.3.2) somewhat mimics the process of the black-and-white photographic development, except that clusters are free in the solution (not fixed on AgBr crystals), and that they are produced by ionizing radiation (as in radiography and not by visible photons but the last choice had been incompatible with the time-resolved optical detection in the visible. Beyond the critical nuclearity, they receive electrons without delay from the developer already present (actually, the photographic development is achieved in a delayed step). 

This account will deal in turn with structural features of the one-iron, two-iron, four-iron and three-iron proteins, and also with the most important topic of interconversion between the three-iron and four-iron clusters. Model compounds will be described briefly. Finally, kinetic studies and the role of iron-sulfur proteins in complex enzymes will be discussed. 

Ray Kapral came to Toronto from the United States in 1969. His research interests center on theories of rate processes both in systems close to equilibrium, where the goal is the development of a microscopic theory of condensed phase reaction rates,89 and in systems far from chemical equilibrium, where descriptions of the complex spatial and temporal reactive dynamics that these systems exhibit have been developed.90 He and his collaborators have carried out research on the dynamics of phase transitions and critical phenomena, the dynamics of colloidal suspensions, the kinetic theory of chemical reactions in liquids, nonequilibrium statistical mechanics of liquids and mode coupling theory, mechanisms for the onset of chaos in nonlinear dynamical systems, the stochastic theory of chemical rate processes, studies of pattern formation in chemically reacting systems, and the development of molecular dynamics simulation methods for activated chemical rate processes. His recent research activities center on the theory of quantum and classical rate processes in the condensed phase91 and in clusters, and studies of chemical waves and patterns in reacting systems at both the macroscopic and mesoscopic levels. 

So far the discussion on synthetic clusters illustrates the accumulating information that a variety of nitrogenase substrates can be transformed by simple hydronation reactions at reduced synthetic Fe-S-based clusters. The next level of detail must address the mechanisms of these transformations. Already we have indicated several cases where kinetic studies have been performed. The major problem with the approaches taken so far, looking directly at substrate transformation, is that they can lead to erroneous conclusions. This is because in this approach the experimenter relies on the kinetics to define the number of species essential to accomplish the transformation. For example, the order with respect to hydrons has been established in several of the catalyic systems discussed and invariably found to be one. It is tempting to jump to the conclusion that only one hydron is necessary to activate the cluster. However, studies on the hydronation of Fe-S clusters show that the kinetics of simple hydronation reactions is much more complicated. 

A kinetic study of the hydroformylation of soybean oil was undertaken by Kandanarachchi [25] the pressure was varied between 40 and 110 bar, and the conversion rate increased with the pressure. The activation energy was calculated both for a rhodium system with (PhO)3P and with (Ph)3P showing that the phosphine species has a lower activation energy. Also, the temperature effect was studied, and it was found that the reaction rate increased until 100°C. Above that, the high temperature apparently inhibited the reaction due to phosphido-bridged clusters which are favored at higher temperatures. 

The theoretical treatment of cluster kinetics borrows most of its concepts and techniques from studies of smaller and larger systems. Some of the methods used for such smaller and larger systems are more useful than others for application to cluster kinetics and dynamics, however. This chapter is a review of specific approaches that have found fruitful use in theoretical and computational studies of cluster dynamics to date. The review includes some discussion of methodology it also discusses examples of what has been learned from the various approaches, and it compares theory to experiment. A special emphasis is on microsolvated reactions—that is, reactions where one or a few solvent molecules are clustered onto gas-phase reactants and hence typically onto the transition state as well. 

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