Mechanism of successive electron transfers

The situation is even more complicated when a tris-electrophoric system is charged. The first question is which subunit will be charged initially. Then, if a dianion with the two electrons in separate electrophores has been formed, the charges can reside either on neighbouring electrophores or on those allowing the greatest possible distance between charges (see Fig. 1). 

What experimental evidence is available to clarify the resulting mode of spin-density and charge-density distribution If one characterizes the reduction of (i) a single annulene, (ii) a doubly layered and (iii) a triply layered analogue by cyclic voltammetry, one derives a first criterion (Alexander et al., 1989 Bohnen et al., 1992 Fry et a/., 1985). The potential 

While cyclic voltammetric experiments provide thermodynamic and kinetic information on the charging processes (Heinze, 1986), only indirect information on the structure of the redox products is available. Fortunately, independent evidence can be obtained from spectroscopic experiments. 

The detection of zero-field splitting for dianions of [18] or [21] is very important it reveals not only the existence of a triplet state, but it also provides information on the mode of spin density distribution. Even more 

A related finding appears for the dianions of the doubly layered system [14] their esr spectra taken in the glass provide evidence for a triplet structure, and from the D-value the inter-plane distance in the charged species can be readily determined (Irmen et al., 1984 Alexander et al, 1989). 

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As an analogous example, the behavior of sulfonium salts can be mentioned. At mercury electrodes, sulfonium salts bearing trialkyl (Colichman and Love 1953) or triaryl (Matsuo 1958) fragments can be reduced, with the formation of sulfur-centered radicals. These radicals are adsorbed on the mercury surface. After this, carboradicals are eliminated. The carboradicals capture one more electron and transform into carbanions. This is the final stage of reduction. The mercury surface cooperates with both the successive one-electron steps (Scheme 2.23 Luettringhaus and Machatzke 1964). This scheme is important for the problem of hidden adsorption, but it cannot be generalized in terms of stepwise versus concerted mechanism of dissociative electron transfer. As shown, the reduction of some sulfonium salts does follow the stepwise mechanism, but others are reduced according to the concerted mechanism (Andrieux et al. 1994). 

Syntheses of oligoporphyrins have targeted the construction of model systems to miinic the assembly of chlorophylls in photosynthesis of green plants and bacteria. In earlier times, a chlorophyll dimer was considered the key unit of antenna systems which gather sunlight for production of carbohydrate. Many model systems that elucidate the mechanism of photoinduced electron-transfer reaction of photosynthesis have been reported and well documented in many reviews. Syntheses of covalently linked electron donors and acceptors have been extended to model compounds appended to successive electron acceptors such as quinone derivatives. 

Along this line, the combination of electrochemistry and IR spectroscopy has been extremely successful in the identification and unraveling of molecular mechanisms of biological electron transfer and of catalysis coupled to redox reactions. 

The proposed mechanism for the conversion of the furanone 118 to the spiro-cyclic lactones 119 and 120 involves electron transfer to the a -unsaturated methyl ester electrophore to generate an anion radical 118 which cyclizes on the /3-carbon of the furanone. The resulting radical anion 121 acquires a proton, giving rise to the neutral radical 122, which undergoes successive electron transfer and protonation to afford the lactones 119 and 120 (Scheme 38) (91T383). 

Under the advisement of PhD mentor Professor Joseph T. Hupp, the PI successfully used spectroelectrochemical quartz crystal microgravimetry to elucidate the mechanism of charge transport... for both aqueous and nonaqueous sytems. This was the first demonstration of proton-coupled electron transfer at oxide semiconductor interfaces. These findings were then successfully applied to a new interpretation of photoinduced electron transfer at similar interfaces, which are of importance in the field of solar energy conversion. ... 

In this case, the formation of a surface oxide (Oads) occurs electrochemically with two successive electron transfers. Therefore, if step (7.28) is rate determining, the mechanism is EE with a predicted Tafel slope of 40 mV at low OHads coverage. 

The mechanism of polyamine hydrogenation (Fig. 6.8) is believed to involve successive electron transfer (from polyamine to fullerene) - proton transfer (from polyamine radical cation to fullerene radical anion) steps (Briggs et al. 2005 Kintigh et al. 2007). At or near room temperature, aliphatic amines and polyamines are known to hydroaminate [60]fullerene (Miller 2006), likely also involving preliminary electron transfer - proton transfer steps followed by free radical coupling of C and N based radicals (Fig. 6.8). At elevated temperatures in polyamine solution, however, this latter free radical coupling step becomes uncompetitive with... 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

Fig. 20 Pictorial illustration of the hypothetical mechanism of the action of oxalate on the reduction of [188Re04]. Oxalate ions react first with the teraoxo anion forming an intermediate Re(VII) complex and causing the concomitant expansion of the coordination sphere of the metal from tetrahedral to octahedral. Successively, electron transfer takes place from Sn2+ ions to the octahedral metal center...

The reason for the E selectivity lies in the mechanism of the elimination. The first step is believed to be two successive electron transfers from the reducing agent (sodium metal) to the sulfone. Firstly, a radical anion is formed, with one extra unpaired electron, and then a dianion, with two extra electrons and therefore a double negative charge. The dianion fragments to a transient carbanion that expels acetate or benzoate to give the double bond. 

Major advances have been made in recent years in the field of redox enzy-mology. In part this has been attributable to the wealth of structural information acquired for redox systems, principally by X-ray methods. The successful application of electron transfer theory to redox proteins has also put the study of biological electron transfer onto a sound theoretical platform. Coupled with the ability to interrogate mechanism by site-directed mutagenesis, spectroscopic and transient kinetic methods these developments have contributed to the major expansion seen in recent years of research activity in the field of biological electron and radical transfer. 

Various subclassifications exist according to the exact nature of the chemical step, which may eventually be a succession of elementary steps with formation of intermediate products. As explained earlier for the displacement of endergonic electron transfer steps, the C step occurs because it is continuously pulled to the right by the further chemical reaction of the Y species. Note that Chapter 28 is devoted to this class of mechanisms. 

Finally it should be pointed out that there is an alternate mechanism for the two successive electron transfer processes indicated by the sequence 14-> 15 -> 16- 13 a. It is quite possible that the ascorbate radical anion dissociates from the mixed ligand complex 15, prior to reoxidation of the Cu(I) ion, and recombines with another Cu(II) chelate prior to the final electron transfer step indicated by 16 17 +... 

Armstrong and Firman ° analyzed a mechanism that included two successive electron-transfer reactions. A general approach to multistep mechanisms involving soluble species in semi-infinite diffusion was presented recently hy Harrington. It allows determination of the number of breakpoint frequencies on the Bode magnitude plot for an arbitrary mechanism and, in consequence, for the determination of the reaction mechanism and kinetics. 

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