Electron transfer kinetics and mechanisms

If isolated reaction centers from Rb. sphaeroides or Rp. viridis are excited with a subpicosecond flash, the transfer of an electron from P to BPhL occurs with a time constant of 3 to 4 ps [72,131,132,147,148]. The kinetics can be measured by following the bleaching of the BPh s absorption bands at 545 and 760 (or 800 nm for the latter in Rp. viridis) and the appearance of broad absorption bands due to BPh and P at 760 and 1250 nm (1325 nm in Rp. viridis). Prior to the reduction of the BPh, P can be detected by its broad absorption bands in the visible and near-IR regions of the spectrum, and by its stimulated emission (fluorescence) at 920 or 1000 nm. The stimulated emission from P decays with kinetics that match the formation of BPh . 

The electron transfer reaction between P and BPh is slightly/aster at 80 K than it is at 295 K, indicating that it does not require a thermal activation energy [131]. This agrees with earlier observations that charge separation in the reaction center occurs with a high quantum yield at temperatures as low as 1 K [149]. 

Measurements with flashes lasting 10 to 40 ps have suggested that a transient P BChl state precedes the formation of P BPh [74,150,151]. However, the evidence for this conclusion has been criticized [77,152], and recent studies with higher time resolution do not support it [131,132,133,148]. Because BChiL is located al- 

Even if the P BChlL charge-transfer state is not formed as a distinct intermediate, it probably mixes quantum mechanically with P and with P BPhL. This mixing could facilitate electron tunneling from P to the BPh by the process known as superexchange [131,153], Mixing of the excited states of BChlL with those of P could also play an important role in the reaction [130]. 

Extracting the nonheme Fe from the reaction center slows electron transfer from Qa to Qb by about a factor of 2 [168], a remarkably modest effect in view of the Fe s location between the two quinones (Fig. 4). Electron transfer from EPFl to 

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Much has been written about solid metal electrodes, which have now largely displaced liquid mercury. Those most often used as redox ( inert ) electrodes for studying electron transfer kinetics and mechanism, and determining thermodynamic parameters are platinum, gold, and silver. However, it should be remembered that their inertness is relative at certain values of applied potential bonds are formed between the metal and oxygen or hydrogen in aqueous and some non-aqueous solutions. Platinum also exhibits catalytic properties. 

The study of fundamental electron transfer processes at nanoelectrodes has also been extended to the field of bioelectrochemistry, notably in the elucidation of enzyme electron transfer kinetics and mechanism via protein film voltammetry. This typically involves immobilizing a film of redox active enzymes onto an electrode such that electronic contact is achieved between the enzyme active site and the underlying surface, enabling voltammetry to... 

The simple but rather useful molecular mechanics calculations were applied to both electron transfer kinetics and reduction potentials for a wide range of hexamine cobalt(III/II) complexes with primary, secondary, tertiary, and macrobicyclic amine ligands [237]. The redox potentials of the Co +/2+ couples varied from... 

Voltammetry is a common electroanalytical technique for characterization of enzyme-modified electrodes. In cyclic voltammetry (CV), a potential window is scaimed in the forward and reverse directions while the resulting current is measured. This technique is useful for determining the reduction potential of the enzyme or coenzyme and for determining the overpotential for the system, which, in turn, corresponds to efficiency. Using this technique, detailed information about the catalytic cycle of the system can be determined including electron transfer kinetics, reaction mechanisms, current densities, and reduction potentials [6,7]. 

Kinetics and mechanism of the outer sphere electron transfer reactions between complex ions. E. D. German, Rev. Inorg. Chem., 1983, 5,123-184 (132). 

Butler-Volmer kinetics and mechanism of electron transfer, 587... 

Redox systems, organic, with multiple electrophores, electron storage and transfer in, 28, 1 Reduction.of C—X and X—X bonds (X = 0, S), kinetics and mechanism of the dissociative, 36, 85... 

Strong conformational changes may accompany electron transfer. This issue has been discussed in Section 1.5 and illustrated by an experimental example in Section 1.5.5, in the case where conformational change and electron transfer are concerted and the second electron transfer becomes easier than the first. Conformational changes do not necessarily cause the second electron transfer to be easier than the first. In all cases, their influence on the kinetics and mechanisms of electrochemical reactions should be analyzed. 

In a recent upsurge of studies on electron transfer kinetics, importance was placed on the outer shell solvent continuum, and the solvent was replaced by an effective model potential or a continuum medium with an effective dielectric constant. Studies in which the electronic and molecular structure of the solvent molecules are explicitly considered are still very rare. No further modem quantum mechanical studies were made to advance the original molecular and quantum mechanical approach of Gurney on electron and proton (ion) transfer reactions at an electrode. 

Molecular modeling treatments of electron transfer kinetics for reactions involving bond breaking were developed much earlier than the continuum theories originated by Weiss in 1951. Gurney in 193l published a landmark paper (the foundation of quantum electrochemistry) on a molecular and quantum mechanical model of proton and electron transfer... 

Reviews of kinetics and mechanisms deal with long-range electron transfer and with the uptake and release of dioxygen species, especially uptake from organic peroxides. " " The photochemistry of iron porphyrin complexes has been documented." ... 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

B. Working Mechanism and Electron-Transfer Kinetics 1. Working Mechanism... 

The coordination chemist may be interested in the electrosynthesis of compounds, the generation and detection of unstable species in unusual oxidation states and the study of their mechanisms of decay or their spectroscopic properties, or in simply obtaining thermodynamic data. These, and related topics such as using electrogenerated metallo intermediates to catalyze the transformation of inert molecules, modifying the properties of an electrode surface by adsorbing or otherwise binding a coordination compound to it, or fundamental aspects of electron-transfer kinetics, are readily studied by the application of modem electrochemical techniques. 

Before discussing particular carbon electrode materials, we should define the qualities on which a choice of material will be based. These are the criteria that matter the most to the user, and the importance of each will vary with the application. For example, a carbon electrode to be used for detecting eluents from a liquid chromatograph should have a low background current and long stability, whereas an electrode used for studying redox mechanisms should usually exhibit fast electron transfer kinetics. The criteria relevant to carbon electrodes are conveniently classified into four types. 

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