Rate constant electron transfer processes

The rates of electron-transfer processes in the oxidation of polycyclic aromatic hydrocarbons have been considered by Peover and White, who estimated from cyclic voltammograms the apparent standard rate constant for 9,10-diphenylanthracene radical cation formation to be of the order 1 cm... 

Equation 9.11 implies that the rates of electron transfer processes should decrease exponentially with distance between the electron donor and acceptor. This prediction is supported by the experimental evidence that we discussed in Section 8.11, where we showed that, when the temperature and Gibbs energy of activation are held constant, the rate constant of electron transfer is proportional to e where r is the edge-to-edge distance between electron donor and acceptor and /i is a constant with a value that depends on the medium through which the electron must travel from donor to acceptor. It follows that tunneling is an essential mechanistic feature of the electron transfer processes between proteins, such as those associated with oxidative phosphorylation. 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

Nitroblue tetrazolium (NBT, 3,3 -(3,3,-dimethoxy-l,l,-biphenyl-4,4 -diyl)bis-2-(4-nitrophe-nyl)-5-phenyl-2H-tetrazolium dichloride) is reduced by superoxide to formazan as a final product, which can be measured spectrophotometrically [73]. Although the rate constant for NBT reduction by superoxide is moderately high 5.88+0.12x 104 1 mol 1 s 1 [74], the formation of formazan is not a simple one-electron transfer process, and the final product is formed as a result of disproportionation of intermediate free radicals. Similar to cytochrome c, NBT is easily reduced by the other reductants that confines its application for superoxide detection. Moreover, similar to epinephrine, NBT free radical is apparently... 

A general difficulty encountered in kinetic studies of outer-sphere electron-transfer processes concerns the separation of the precursor formation constant (K) and the electron-transfer rate constant (kKT) in the reactions outlined above. In the majority of cases, precursor formation is a diffusion controlled step, followed by rate-determining electron transfer. In the presence of an excess of Red, the rate expression is given by... 

According to the collision theory, the rate constant (kET) of the electron-transfer process is given by ... 

Using the dyad shown in Figure 6.26, it has been possible to investigate the driving-force dependence of the rate constants for the electron-transfer processes. 

Similar to homogeneous electron-transfer processes, one can consider the observed electrochemical rate constant, k, , to be related to the electrochemical free energy of reorganization for the elementary electron-transfer step, AG, by... 

There is currently much interest in electron transfer processes in metal complexes and biological material (1-16, 35). Experimental data for electron transfer rates over long distances in proteins are scarce, however, and the semi-metheme-rythrin disproportionation system appears to be a rare authentic example of slow electron transfer over distances of about 2.8 nm. Iron site and conformational changes may also attend this process and the tunneling distances from iron-coordinated histidine edges to similar positions in the adjacent irons may be reduced from the 3.0 nm value. The first-order rate constant is some 5-8 orders of magnitude smaller than those for electron transfer involving some heme proteins for which reaction distances of 1.5-2.0 nm appear established (35). 

Like the standard rate constant, k°, the exchange current, io, characterizes the rate of the electron transfer process inside a redox couple. 

The time course of the charge-separated intermediate I can be measured in a flash photolysis experiment that monitors the (I — A) transient absorbance difference at a ground state/triplet state isosbestic point (e.g., 432 nm for Mg, and 435 nm for Zn). We have observed this intermediate for the [M, Fe] hybrids with M = Mg, Zn representative kinetic progress curves are shown in Fig. 3 [7a]. In a kinetic scheme that includes Eqs. (1) and (2) as the only electron-transfer processes, when the I A step is slow (kb < kp) the intermediate builds up (exponentially) during the lifetime of A and exponentially disappears with rate constant kb (Fig. 4A). This behavior is not observed for the hybrids, where the I - A process is more rapid than A - I, with kb > kp. In this case, I appears exponentially at early times with rate-constant kb and is expected to disappear completely in synchrony with A in an exponential fall with rate-constant kp (Fig. 4B). 

Our data show that in all hybrids, the thermal electron transfer process (Eq (2)) is remarkably insensitive to temperature, suggesting that ET proceeds by quantum mechanical tunnelling to quite high temperatures. Figure 8 shows the temperature dependence of k, the rate constant for the thermal Fe (CN )P-> (MP) electron transfer for M = Mg and Zn. For [(ZnP), Fe (CN )P], kb decreases by less than a factor of three from 300 K to 100 K ... 

This scheme includes ET rate constants only for the d - d electron-transfer processes, in which the system conformation is conserved, and conformational and ET steps only occur sequentially. Intuitively, it might be expected that the kinetic scheme must include ET that is synchronous with a conformational change in the medium coordinate. However, we showed [10a] that it is not necessary to include the diagonal processes (e.g., A Ig) when considering stable substates. 

Emission quenching is also observed with mononucleotides. In that case the quenching efficiency decreases from GMP (guanosine 5 monophosphate) to AMP (adenosine 5 monophosphate) i.e. it also follows the redox potentials of the bases, as G is more easily oxidisable than A, although the oxidation potential valura reported in the literature are rather different from one author to the other [101-104], Moreover the quenching rate constant by GMP in a Kries of different TAP and HAT complexes plotted versus the reduction potential of the excited state (Fig. 12) [95] is consistent with an electron transfer process. Indeed, as will be demonstrated in Sect. 4.3.1, these quenchings (by the mono-and polynucleotides) originate from such processes. 

Substituent effects on the electron-transfer processes between pyrrolidinofullerenes and tetrakis(dimethylamino)ethylene (TDAE) were studied in both the ground state and excited triplet state. ° Equilibrium constants and rate constants for forward and backward electron-transfer processes in the ground state, in addition to rate constants of the forward electron transfer in the excited triplet state were measured. 

Rate constants in excess of 10 M s are determined by pulse-radiolysis methods [4, 5]. High-energy irradiation of a solution containing the substrate and an excess of the aromatic species, generates the aromatic radical-anion. The decay of this by electron transfer to the substrate is followed using uv-spectroscopy and affords a rate constant for the second-order process. Slow rates of electron transfer are determined by adding the substrate to a solution of the aromatic radical-anion and following the reaction by conventional methods. 

Cyclopropyl carbanions are capable of maintaining their configuration whereas the CT-radical has been shown to reach inversion equlibrium with a rate constant of lO" s". ITie cyclopropyl bromide 13, and the corresponding iodide, are reduced in a single two-electron polarographic wave and the S +)-isomer yields the R(-)-hydrocarbon with 26% enantiomeric excess [67, 68]. Such a substantial retention of configuration during reduction of the carbon-bromine bond indicates a very fast second electron transfer process. Results from reduction of the cyclopropyl bro-... 

Of course, in inner sphere bridged electron transfer reactions, one of the steps in the mechanistic sequence is the substitution of one of the complexes into the coordination shell of the second. Thus, if the electron transfer process that follows is fast enough, the substitutional step may become rate determining. I am not sure that there is any clear cut evidence that this is the case for any systems actually examined. I did cite a case of a bridged electron transfer reaction that proceeded with a rate constant of 109—i.e.,... 

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