Concurrent electron transfer

This reaction also produces Ph Ph and [Fe(CO)2(> -C5H5)]2, indicating the occurrence of a concurrent electron-transfer process . 

The field of protonic CTC has been opened up by Matsunaga and his school,and has been reviewed by Morokuma. This author points to the close linkage existing in such adducts between proton and concurrent electron transfer one does not occur without the other. The proton complex is said to involve an electron transfer from, e.g., an amine to the... 

The recognition that corrosion occurs by a mechanism involving two concurrent electron transfer processes at the same surface has implications elsewhere in chemistry. Perhaps this is a mechanism which should be considered for other chemical reactions. 

As with any other physical methods, the CIDNP method is not universal and not immune to misinterpretation. It has certain drawbacks The polarization is weak and hardly detected in reactions involving extremely short-lived radicals and, if so, the polarization disappears quickly. It is often difficult to attribute the polarization to products of the main conversion, rather than the side or reverse conversions. The latter threat is most serious for the reactions with participation of ion-radicals—the formation of end products often proceeds concurrently with the restoration of the initial neutral molecules, due to a reverse electron transfer as in Scheme 4.29. 

An extremely interesting feature of these mechanisms is the fact that superoxide and the alkene radical cation are both formed in the reduction (Fig. 20) and also in the Frei oxidation (Fig. 19). In the Frei photo-oxidation, however, they are formed concurrently in a tight ion pair and collapse to product more rapidly than their diffusive separation. In the reduction (Fig. 20), the formation of the radical cation and superoxide occur in independent spatially separated events allowing the unimpeded diffusion of superoxide which precludes back-electron transfer (BET) and formation of oxidized products. The nongeminate formation of these two reactive species provides the time necessary for the radical cation to abstract a hydrogen atom from the solvent on its way to the reduced product. 

Figure 14.1 Schematic representation depicting the importance of electron transfer mediators as well as the concurrence of microbial and abiotic processes for reductive transformations of organic pollutants. Adapted from Schwarzen-bach et al. (1997).

Mo042 is not protonated. The same would be true of nitrogenous ligands of a protein that might be coordinated with the bound molybdenum. On the other hand, reduction to Mo(IV) would tend to favor protonation of ligands such as the His 442 imidazole seen in Fig. 24-3A. Concurrently with the electron transfer from molybdenum to N2 these protons could be transferred to the N2 molecule (Eq. 24-10). The fact that strictly ds-dideuteroethylene is formed from acetylene in the presence of 2H20 is in accord with this idea. 

Under oxygen in the absence of water, toluene will transfer an electron to the positive hole, concurrently with electron transfer from the conduction band to oxygen, to give a toluene radical cation. On the other hand, in the presence of water, both toluene and water will transfer an electron to the positive holes. The resulting toluene radical cation may subsequently lose a proton affording a benzyl radical, which will be oxidized with oxygen or the superoxide anion to benzyl alcohol and benzaldehyde, as proposed for the reactions of Fenton s reagent with toluene (57). 

Given that triplet-triplet energy transfer proceeds via a Dexter (electron exchange) mechanism, it is not surprising that electron transfer can also occur via upper triplet states. Two-color experiments with anthracene in acetonitrile in the presence of ethylbromoacetate, a dissociative electron acceptor, showed that excitation to an upper triplet state led to depletion of the T-T absorption and concurrent production of the anthracene cation radical as a result of electron transfer (Scheme 1) [52]. 

No activation (energy) barrier separates the donor and the acceptor from the ET products (and vice versa). The electron transfer in Scheme 18 is not a kinetic process, but is dependent on the thermodynamics, whereby electron redistribution is concurrent with complex formation. Accordingly, the rate-limiting activation barrier is simply given by the sum of the energy gain from complex formation and the driving force for electron transfer, i.e. ... 

Under a nitrogen atmosphere, cobalt carbonyl probably experiences disproportionation by base (Scheme 5) to give Co(CO)4. One of the disproportionation by-products is the Co(II) ion which gives a blue color in aqueous NaOH due to the presence of small amounts of the Co(OH)42-ion. The subsequent reaction of the cobalt tetracarbonyl anion with 14 is probably a displacement (path a), giving the cr-allyl complex 16. However, the possibility of an electron-transfer pathway (path b), either as an alternative to, or concurrent with, the displacement pathway cannot be dismissed at this time. 

This paper describes a new seeding process for electroless metallization of polyimides and other electroactive polymers. Polyimide films can be reduced electrochemically at an electrode surface or by contact with an appropriate reducing agent in an electrolyte solution. In the latter case, only the outer surface of the film undergoes reduction. Once the polyimide surface is reduced it then can mediate electron transfer to metal ions or metal complexes in solution causing metal to be deposited at the surface with concurrent reoxidation of the polyimide. 

The field of protonic charge-transfer complexes was introduced by Mat-sunaga and co-workers [5]. Morokuma [30] cites the close exchange in such adducts between proton and electron transfer one occurs concurrently with the other, as with proton complexes of some amines. In some cases electron transfer is catalyzed by the presence of a proton and thus enhanced above its thermally controlled rate [31]. 

A similar case of concurrence of one-electron transfer and nucleophilic addition is observed in the thermal ion-pair annihilation of CpMo(CO)3 anion with (dienyl)Fe(CO)3+ cations [84, 179]. Thus, spontaneous electron transfer (A et) occurs upon mixing of ( / -cyclohexadienyl)Fe(CO)3 with CpMo(CO)3 in acetonitrile to afford the transient 19-electron iron radical and the 17-electron molybdenum radical which both rapidly dimerize (Eq. 51). 

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