Electron transfer mechanisms principle

Most PET fluorescent sensors for cations are based on the principle displayed in Figure 10.7, but other photoinduced electron transfer mechanisms can take place with transition metal ions (Fabbrizzi et al., 1996 Bergonzi et al., 1998). In fact, 3d metals exhibit redox activity and electron transfer can occur from the fluorophore... 

It is well known that the flotation of sulphides is an electrochemical process, and the adsorption of collectors on the surface of mineral results from the electrons transfer between the mineral surface and the oxidation-reduction composition in the pulp. According to the electrochemical principles and the semiconductor energy band theories, we know that this kind of electron transfer process is decided by electronic structure of the mineral surface and oxidation-reduction activity of the reagent. In this chapter, the flotation mechanism and electron transferring mechanism between a mineral and a reagent will be discussed in the light of the quantum chemistry calculation and the density fimction theory (DFT) as tools. 

Note that different absorption frequencies correspond to different electron transfer mechanisms with the participation of radiation. In principle, this difference can be used to distinguish between various possible mechanisms. A comparison of the efficiency of various electron tunneling processes with the participation of radiation has been carried out in ref. 31. 

Systems 13 and 14 represent prototypes of molecular switches of luminescence which are operated through a redox input the system consists of a luminescent unit (the light bulb of the everyday life) and of a metal-containing redox unit (the true switch) [8], The on/off situation is achieved when one of the two stable oxidation states of the metal quenches the nearby excited fluorophore and the other does not. In principle, redox switches of luminescence based on an electron transfer mechanism can be obtained by properly assembling a photoactive fragment and a metal-centered redox couple, possibly hosted by a cyclic framework. A further example based on the Cu VCu redox couple will be discussed in Section 5.4. 

Sensitization and interfacial electron transfer mechanisms have been described by Gerischer [4-6]. A basic assumption is that electron transfer, like light absorption, occurs under the restriction of the Franck-Condon principle. The time-scale for interfacial electron transfer is much shorter than that for nuclear motion. This means that the energy terms for electron transfer are different from the thermodynamic formal reduction potentials described above. Gerischer considered the appropriate energy levels and derived a distribution of energy levels when the sensi-... 

In principle, phenoxyl radicals can react with other molecules also by a hydrogen-abstraction mechanism. The net result of such reactions may be equivalent to that of the electron transfer processes discussed above. It is likely that in aqueous solutions such reactions are much slower than the electron transfer reactions, as indicated by the fact that most reactions between phenoxyl radicals and other phenols are much slower with the neutral phenols than with the phenolate ions. It is possible that even reactions with neutral phenols in aqueous solutions involve an electron transfer mechanism. On the other hand, reactions in organic solvents may well take place by hydrogen abstraction, as discussed before . These reactions take place with much lower rate constants than the electron transfer reactions the most rapid hydrogen abstraction by a phenoxyl radical is probably five orders of magnitude slower than the diffusion-controlled limit and most of them are orders of magnitude slower than that. 

In principle, if the X values for a set of like reactions are similar to one another, and W r) is small or constant, a plot of AG versus AG° will be linear and have a slope of 0.5. As noted above, it is rarely possible to measure AG values directly. An alternative option for plotting data using a linear relationships is to use Equation 1.15 and the definition of the equilibrium constant, K = A exp( AG° // 7), in which A is a constant. If the equilibrium constants for a series of reactions can be measured or calculated, one can plot In A versus In AT (Equation 1.15). A linear result with a slope of 0.5 is indicative of a common outer-sphere electron transfer mechanism. 

To improve MES in the future, more detailed information about electron transfer mechanisms from electrodes to microorganisms is needed. The opposite electron transfer direction - from microorganisms to extracellular substrates, such as minerals or electrodes - has been investigated in detail over last years. It can be assumed that similar principles are involved in MES. Particularly the electron transfer mechanisms of... 

Thus, the inner and the outer sphere electron-transfer mechanisms cannot be distinguished on the basis of kinetic laws. Activation enthalpies of electron transfer are lower for the outer-sphere electron transfers than for the inner-sphere electron transfers. Both mechanisms have, in principle, negative entropies of activation. These general data show that it is not easy to distinguish between these two mechanisms. 

This chapter introduces the principles and applications of MFCs, with emphases on the nature of electricity-producing bacteria, anodic electron transfer mechanisms, power generation of MFCs and efficiency of electrode materials. Different types of MFCs and other microbial-electrochemical conversion devices are also discussed. 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

As regards intimate mechanism, electron transfer reactions of metal complexes are of two basic types. These have become known as outer-sphere and inner-sphere (see Chapter 4, Volume 2). In principle, an outer-sphere process occurs with substitution-inert reactants whose coordination shells remain intact in... 

The first step in the polymerization is the electron transfer from sodium to dichlorosilane and the formation of the corresponding radical anion. The latter upon elimination of the chloride anion is transformed to the silyl radical. To fit the chain growth mechanism, the reactivities of the macromolecular radicals must be higher than the reactivities of the monomeric radicals. The latter after electron transfer and elimination of chloride anion could be transformed to the reactive silylenes. Thus, in principle, two or more mechanisms of chain growth are possible ... 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

In principle, outer sphere mechanism involves electron transfer from reductant to oxidant when there is no change in the number or nature of the groups (coordination shells or spheres) attached to each of them. For example,... 

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