Rates return electron transfer

Mode of Action of Dicyclohexylcarbodiimide (DCCD) When DCCD is added to a suspension of tightly coupled, actively respiring mitochondria, the rate of electron transfer (measured by 02 consumption) and the rate of ATP production dramatically decrease. If a solution of 2,4-dinitro-phenol is now added to the preparation, 02 consumption returns to normal but ATP production remains inhibited. 

The energy gap between the ion-pair and ground state can also control the rate of electron return, k,e and in effect influence ip. As noted earlier, the rate of electron transfer increases with an increase in the energy gap between an initial and final state until kel reaches a maximum value, after which the rate begins to decrease again (the inverted region). This prediction can be used to control the... 

Returning to intermolecular electron transfer (outer-sphere electron transfer), we assume that the electronic coupling term V12 in the encounter pair (A -D) is sufficient to ensure a high probability of electron transfer at the crossing points, but much smaller than AG (0) = 2/4. The rate of electron transfer is then given by Eyring s transition state theory (Equation 5.6). 

Let us now return to the original observation of this section, namely that diffusion rate is related to scan rate and competes with the reverse rate of electron transfer. 

We now return to the thermal electron transfer reaction in eq 20, in which the rate-limiting activation process has been shown to proceed from the electron donor acceptor complex (23), i.e.,... 

FIGURE 4.15. a Cyclic voltammetric response of a monolayer catalytic coating for the reaction scheme in Figure 4.10 with a slow P/Q electron transfer. Catalysis kinetic parameter kr°/ /DAFv/TIT — 5. Same electrode electron transfer MHL law as in Figure 1.18. Dotted line Nemstian limiting case. Solid lines From left to right, F(>k 1/sjD Fv/ lZT = 1, 0.1, 0.01. b Convoluted current, c Derivation of the catalytic rate constant (return curve have been omitted, d Derivation of the kinetic law. 

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

In this case, the cyclic voltammetric response is essentially similar to the preceding case, with the difference that, given the irreversibility of the electron transfer, the return peak is missing. Thus, if kf is low, the response is that of a simple irreversible electron transfer. As k increases, the greater the potential scan rate the higher the peak current (compared to simple irreversible electron transfer). This continues up to a maximum value at which the response assumes a S-like shape. 

Reversible electron transfer within Mg and Zn-substituted hemoglobin hybrids is initiated by flash photoproduction of the long-lived triplet state ( MP). According to Scheme I, the triplet return to the ground state involves two pathways, intrinsic triplet decay (with rate constant kp) and electron transfer quenching (with rate constant k,). 

These examples illustrate a broad range of interesting behaviors for surface-confined, redox-active dendrimers. However, let us return to our original questions How does the generation of a dendrimer influence the rate and redox potential for heterogeneous electron transfers Now we can ask this question with regard to surface-confined dendrimers. 

The system then returns to its initial state by back electron transfer from Fe(II)P to (ZnP)+. The decay of the TZnP triplet state is enhanced only when the p subunit contains Fe(III)P. When the j3 subunit contains Fe(II)P triplet, decay is unaffected. The rate constant of electron transfer, t, for [a(Zn)/J(Fe)] hybrid hemoglobin was found to be 102s 1 at room temperature [90]. The same value of kt was obtained for [a(Fe)/ (Zn)] hybrid hemoglobin [70]. 

A simpler procedure has been implemented in applications to electron transfer in collisions of ions with metal surfaces.[35] Returning to Eq.(31), the second line can be interpreted as describing the rate of change of coherence of p- and s-regions over time. This is not likely to change much from its initial value for short collision times, so that the equation can be replaced on the average by a phenomenological rate equation,... 

This shows that for an irreversible process, the peak potential is shifted towards more negative (reduction reaction) or more positive (oxidation reaction) potentials by about 0.03 V per decade of increase in the scan rate. For a totally irreversible reaction, no return peak is observed due to the fact that the kinetics are so slow that the opposite reaction cannot occur. The activation energy, overcome by application of a potential, is so high that it is not possible to apply such a potential under experimental conditions. However, the absence of a return peak does not necessarily imply slow electron transfer, but can also be due to a fast following chemical reaction. 

The effect of the FC term on ICT and MLCT-based chemosensors appears when the electron transfer rate constant is generalized within the context of nonradiative decay theory [191-193], MLCT excited states are produced directly upon excitation whereas ICT states are produced by a surface crossing from an initially prepared localized excited state (see Fig. 9). Return of the system from the charge transfer excited state to ground state has the overall form of an electron transfer recombination problem that is described by the inverted Marcus curve of Fig. 13. As described by the FC term of Eq. (5), the rate constant for... 

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