Kinetic competition

Failure to give a product because of diffusion away of a reactant may give rise to kinetic competition between two processes reaction with activation energy E and diffusion with activation energy Ej- This competition can easily be handled using assumed first-order kinetics (for correlated pairs of reactants) and considering the fraction, F, of the available reaction sites which lead to products within infinite time compared to the fraction, — F, which give no reaction—presumably by diffusion away of a reactant. This treatment leads to the expression... 

An increase in the fraction of the four-electron reduction pathway at more reducing potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The first is based on the kinetic competition between the release of H2O2 from the ferric-hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible) reduction to a ferrous-hydroperoxo species, which undergoes rapid 0-0 bond heterolysis (18.13b). Because H2O2 and particularly HO2 are more basic ligands... 

Reduction of either the exo or endo isomer of 2-phenyl-2-norbornanol with trifluoroacetic acid and triethylsilane, triphenylsilane, or phenylsilane in dichloro-methane gives endo-2-phenylnorbomane quantitatively (Eq. 24).164 The stereospecific formation of only the endo-hydrocarbon can be understood on the basis that only exo approach by organosilicon hydride toward the 2-phenylnorbornyl cation intermediate is kinetically competitive for product formation.164... 

The polymerization mechanism for the dual-side catalysts is totally different from the C2-symmetric complexes. Due to their geometry, the dual-side complexes show different stereoselectivities for monomer coordination and insertion. It was shown that the introduction of the stereoerror formation by the 5-substituted asymmetric catalysts originates predominately from the kinetic competition between chain back-skip and monomer coordination at the aspecific side of the catalyst [9],... 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

Although this reaction occurs too rapidly for direct observation, the relative rate constants for pairs of phosphines were determined by standard kinetic competition techniques. The rate constant ratios for P(CeH4R)3 relative to PPh3 correlate well with the Hammett constant 3a, giving p = -0.70. 

A theoretical study at a HF/3-21G level of stationary structures in view of modeling the kinetic and thermodynamic controls by solvent effects was carried out by Andres and coworkers [294], The reaction mechanism for the addition of azide anion to methyl 2,3-dideaoxy-2,3-epimino-oeL-eiythrofuranoside, methyl 2,3-anhydro-a-L-ciythrofuranoside and methyl 2,3-anhydro-P-L-eiythrofuranoside were investigated. The reaction mechanism presents alternative pathways (with two saddle points of index 1) which act in a kinetically competitive way. The results indicate that the inclusion of solvent effects changes the order of stability of products and saddle points. From the structural point of view, the solvent affects the energy of the saddles but not their geometric parameters. Other stationary points geometries are also stable. 

These expressions are designed for cyclic voltammetry. The expressions appropriate for potential step chronoamperometry or impedance measurements, for example, are obtained by replacing IZT/Fv by the measurement time, tm, and the inverse of the pulsation, 1/co, respectively. Thus, fast and slow become Af and Ah I and -C 1, respectively. The outcome of the kinetic competition between electron transfer and diffusion is treated in detail in Section 1.4.3 for the case of cyclic voltammetry, including its convolutive version and a brief comparison with other electrochemical techniques. 

In the electrochemical case, using, for example, cyclic voltammetry, one way of driving the potential toward more negative values is to increase the scan rate. This is true whether the linearization procedure or the convolution approach is followed. In the first case, equation (3.4) shows that the activation free energy at the peak, AG, is a decreasing function of the scan rate as a result of the kinetic competition between electron transfer and diffusion. The larger the scan rate, the faster the diffusion and thus the faster the electron transfer has to be in order to compete. This implies a smaller value AG, which is achieved by a shift of the peak potential toward more negative values. 

Under these conditions, as sketched on the left-hand side of Figure 4.16, the linear diffusion layer has become very thin, on the same order as the constrained diffusion layer. The response amounts therefore to the steady-state response of an assembly of nas independent disk microelectrodes. The shape of the S-wave and the location of the half-wave potential is a function of the last term in the denominator on the right-hand side of equation (4.18). The parameter that governs the kinetic competition between electron transfer and constrained diffusion is therefore... 

The fates of the radical ion pairs produced upon electron transfer depends on the nature of their production. As already mentioned, the Bp DMA" com formed from irradiation of the ground-state CT complex. Bp - DMA, is suggested by Mataga and co-workers [24] to decay only by febet, on a timescale of 85 ps. Diffusional separation to solvent separated radical ion pairs or proton transfer within Bp -DMA com are not kinetically competitive. The triplet CRIP Bp -I- DMA" ip has two decay pathways that occur on the picosecond timescale. The first process is proton transfer, fept, to generate a triplet radical pair, BpH-l- DMA ] (Scheme 2.3). In acetonitrile, this occurs with a rate constant of fept of 1.3 x 10 s [43]. The second process leading to the decay of the CRIP is diffusional separation to the SSRIP, kips, which occurs with a rate constant of 5 x 10 s (Scheme 2.3) [43]. Thus the efficiency of the... 

The possible formation of an alloyed or a core-shell cluster depends on the kinetic competition between, on one hand, the irreversible release of the metal ions displaced by the excess ions of the more noble metal after electron transfer and, on the other hand, the radiation-induced reduction of both metal ions, which depends on the dose rate (Table 5). The pulse radiolysis study of a mixed system [66] (Fig. 7) suggested that a very fast and total reduction by the means of a powerful and sudden irradiation delivered for instance by an electron beam (EB) should prevent the intermetal electron transfer and produce alloyed clusters. Indeed, such a decisive effect of the dose rate has been demonstrated [102]. However, the competition imposed by the metal displacement is more or less serious, because, depending on the couple of metals, the process may not occur [53], or, on the contrary, may last only hours, minutes, or even seconds [102]. 

DSSCs efficiencies up to 10.4%8 have been reported for devices employing nanocrystalline Ti02 films. Several studies have addressed the use of alternative metal oxides including SnCU,9 10 ZnO,11,12 and Nb205.13 The performance of dye-sensitized solar cells can be understood in view of the kinetic competition among the various redox processes involved in the conversion of light into electricity. Ultrafast electron injection (k2) has been observed in the femtosecond-picosecond (10 l5-... 

There are several examples of fast decomposition reactions of the a-adducts derived from 5-membered rings. These reactions can be viewed as resulting from effective kinetic competition of reaction paths other than return to the reactants. In all ascertained cases the products of decomposition result from ring opening, which presumably occurs subsequent to a-adduct formation. Thus 2-nitrothiophene reacts with aliphatic secondary amines to yield bis-(4-dialkylamino-l-nitrobuta-l,3-dienyl) disulfides 156. This compound is suggested to be the end product of a sequence originating from 153, whose formation is not as yet established, according to Scheme 10.187... 

Ring fluorination of pyridine and its benzo derivatives is suggested to occur through nucleophilic attack of fluoride ion on an initial pyridine fluorine complex (87TL255, 91BCJ1081, 93AHC(58)29l). In electron-deficient pyridines and their benzo derivatives, fluorination on the pyridine ring is kinetically competitive with annular and side chain fluorination. 

Specific deuterium labeling at C(3) was applied as well in the chiral spirocyclic system 7 all three possible one-center epimerization events are kinetically competitive at 198.9 °C161. 

The experimental and theoretical work published by the early 1970s viewed the stereomutations of cyclopropanes as kinetically competitive one-center and two-center stereomutations some details, especially regarding relative rate constants for one-center epimerizations which defined relative rotational propensities, remained unclear, but all agreed that neither the Smith mechanism (one-center only) nor any two-center-only formulation for stereomutations could be sufficient. Thus when kinetic studies275 278 on the isomerizations shown by chiral samples of l-phenyl-2-d-cyclopropane and 1,2-d2-cyclo-propane purported to show that, actually, two-center stereomutations were kinetically dominant, many were stimulated to fresh speculations and accommodations. Theoretical work at times hinted that the parent hydrocarbon might be an exceptional case and might... 

Where there is direct overlap with the valence band edge, the electron transfer process may be so facile as to give rise to the Hofer-Moest reaction (.2), in which the intermediate alkyl radical is itself oxidized (while it is still adsorbed to the electrode surface) to give a carbonium ion. The reaction of this carbonium ion with the aqueous electrolyte would then yield water-soluble products such as methanol, in keeping with our observation that anodic gas evolution is suppressed under these conditions. In acidic solutions, where the Kolbe reaction is energetically allowed, its kinetic competition with the other reactions on SrTiC>3 thus depends on the absence of defect surface states which are present in some electrode crystals and not in others. 

Because of its cyclic nature, this process presents analogies with molecular catalysis it may be considered as physical catalysis operating a change in location, a translocation, on the substrate, like chemical catalysis operates a transformation into products. The carrier is the transport catalyst which strongly increases the rate of passage of the substrate with respect to free diffusion and shows enzyme-like features (saturation kinetics, competition and inhibition phenomena, etc.). The active species is the carrier-substrate supermolecule. The transport of substrate Sj may be coupled to the flow of a second species S2 in the same (symport) or opposite antiport) direction. 

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