Electron transfer reactions, energetics

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

A first type of reaction that may affect the first electron transfer intermediate is its reduction (or oxidation) at the electrode. In most cases, the second electron transfer is energetically more costly than the first (for a discussion of exceptions to this rule, see Section 1.5). The two processes thus occur at successive values of the electrode potential. There is therefore no difficulty in preventing the occurrence of the second reaction by an appropriate adjustment of the electrode potential. 

Electron affinities for 35 substituted nitrobenzenes have been reported and provided a comprehensive data set for the examination of substituent effects38. The data were used to derive Taft gas-phase substituent parameters and discussed qualitatively based on frontier orbital molecular theory38. The rate constants for the exo-energetic electron-transfer reactions were found to be close to those predicted by the ADO (average dipole orientation) theory38. 

Some Comparisons Between the Energetics of Electrochemical and Homogeneous Electron-Transfer Reactions... 

Designing a conjugated polymer sensor based on FQ, however, is not only a matter of making a fluorescent polymer for which the photoinduced electron transfer reaction is energetically favorable. There are other important factors that must be considered and requirements that must be met to rehably detect any analyte of interest, including TNT, from the vapor phase. In the broadest sense, these considerations distill to the two primary considerations for any sensing system, sensitivity and selectivity. 

Table II presents the vadues of v, the rate constant for the electron transfer reaction with the donor and acceptor in contact, calculated by deconvolution of the fluorescence decay curves for a number of excited porphyrin-cOkyl halide systems. It appears that the rate parauneter depends strongly on the calculated exothermicity for these reactions. Parauneter i/ contadns information about the Framck-Condon factor of the electron-tramsfer reaction, which is in itself dependent on the reaction exothermicity and reorgauiization energy (22.23). Whether the rate constauit for the electron-transfer reactions depends on the exothermicity in the manner predicted by theory, that is with a simple Gaussian dependence (22), cannot be ainswered at present because of the uncertainties in the energetics of the particular reactions studied here.

The reactions of dioxygen have been amply documented The reduction can occur by a one-, two- or four-electron transfer reaction, the first of which is energetically unfavorable. The one-electron reduction of hydrogen peroxide yields the extremely reactive hydroxyl radical (Scheme 1). 

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