Intramolecular electron exchange

Upon further contact with a redox reagent or at higher redox potentials, additional electrons can be transferred. After a two-electron transfer, each redox unit can accept one charge with formation of a singlet or triplet dianion, or (less favourably from an electrostatic point of view) both charges can enter one redox unit. Here again, an intramolecular electron-exchange process is possible. 

As an extension of the intermolecular self-exchange described above, the solvent-induced intramolecular electron exchange kinetics in radical anions of 1,3-dinitrobenzene [47] and benzene 1,3-dicarbaldehyde [48] have been studied by several authors (Freed and Fraenkel, 1964 Grampp et al., 1989, 1990b Shohoji et al, 1987). The advantage of [47] and [48] is their structural simplicity and their high stability, which allows measurements even in protic... 

The intramolecular electron exchanges among the different metal sites of polynuclear /3-diketonate-metal complexes are a matter of intrinsic interest, especially as far as the stabilization of mixed-valent oxidation states is concerned " . Obviously, the pertinent electrochemical pattern, while becoming richer and richer, also tends to become more... 

The hyperfine structure may also indicate, in a rather elementary way, the symmetry and thus intramolecular electron exchange situation of radicals. For instance. 

The interesting feature about these dioxetanes is that they possess easily oxidized groups, for example, 3-indolyl, / -dimethylanilinyl, and acridinyl " groups. Consequently, it has been proposed that these types of dioxetanes chemienergize 7T-rr singlet excited states by the intramolecular electron exchange mechanism. This topic will be discussed in detail in Section V.2. 

Rate constants and EM s for intramolecular electron exchange in NfCHjl N in HMPT at ao e... 

When interfacial electron exchange rate(s) are sufficiently high and the response is free from mass transport hmitations, the catalytic current will be determined by the inherent activity of the enzyme. Variation of current (activity) with potential can be explained by an extension of the Michaelis-Menten description of enzyme kinetics that relates activity to oxidation state through incorporation of the Nemst equation." " The resulting expressions describe the catalytic cycle, and include rates of intramolecular electron exchange, chemical events, substrate binding and product release, together with the reduction potentials of centres in the enzyme, and the influence of... 

Figure 4-5. Points of electrochemical control in enzyme voltammetry. The voltammetry reports on the electrochemical control centre , the redox site (solid) up to which electron exchange with the electrode is fast. For single-centre redox enzymes (A), the electrochemical control centre is the active site. For multi-centered redox enzymes the electrochemical control centre may be a relay centre (B), or the active site redox group if catalysis is not impeded by the rate of intramolecular electron exchange (C). Further scenarios are also possible, as described in the text.

The number of protons involved in the redox-Bohr cooperativity was titrated, showing that this effect involves two protons [60]. Furthermore, kinetic NMR studies have shown that the intramolecular electron exchange (heme to heme) is extremely fast [61], that these two redox-Bohr protons have diffusion controlled exchange rates and that they titrate with the same pKa [54]. 

A good example to illustrate these features is the intramolecular electron exchange in 1,3-dinitrobenzene (3-DNB) radical anion, which has rates of 1.2-2.2 x 10 s in alcohols and in aprotic solvents 2.8-4.2 x 10 s H64]. In polar solvents a structure such as... 

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