Electron-transfer reactions mechanistic studies

The binding of CO has in many studies been used as a model for the activation of dioxygen, since this molecule does not undergo any real activation in the systems studied it merely binds to the metal center. The absence of subsequent electron transfer reactions simplifies the kinetic analysis and reveals more mechanistic insight on the actual binding process. 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends... 

DR. WILLIAM WOODRUFF (University of Texas at Austin) I d like to make a brief comment in a similar manner which applies to both substitution and electron transfer reactions. By and large, I think mechanistic inorganic chemists lack the equipment to study reaction rates in the subnanosecond time range. Yet... 

Infrared spectroelectrochemical technique proved to be an excellent method to look at time and potential dependent changes of various types of chemical species. The employment of this technique will surely be significant on the mechanistic study of electron-transfer reactions of rhenium complexes. 

Bimolecular electron-transfer reactions in solutions frequently have rates limited by the diffusion of the donor and acceptor molecules, because one or both of the reactant species is usually at a low concentration relative to the solvent. To obtain a detailed mechanistic and kinetic understanding of electron-transfer reactions in solutions, chemists have devised ingenious schemes in which the two reactants, the donor and acceptor, are held in a fixed distance and orientation so that diffusion will not complicate the study of the intrinsic electron-transfer rates. Recent developments, however, have led to theoretical models in which the orientation and the distance are changeable (see Rubtsov et al. 1999). 

Electron transfer reactions are among the most widespread and significant in all of chemistry. Electron transfer (ET) within the double helical structure of DNA exhibits an extremely broad range of mechanistic behavior, and its exploration has become a focal point within the chemical community since the key studies of Barton and collaborators [1-3]. 

The second region is the mixed kinetic transport-controlled region, and the most negative part of it can also be used for kinetic and mechanistic studies of the electron-transfer reaction after the experimental currents have been compensated for transport limitations. Finally, a second wave is observed at potentials higher than 0.5 V vs. AglAgCl, which can be attributed to the oxidation of sulphite to sulphate. However, this wave is not further considered because the oxidation mechanism of sulphite showed poor reproducibility (see section 6.3), and sulphite detection in dyeing processes is not of great importance compared with dithionite detection. 

The use of ac electrolysis in all its variations is certainly an interesting and valuable technique for study of the mechanism of electron transfer reactions. The generation of a short-lived redox pair as chemical intermediates is an important feature of the ac electrolysis. In the future it may even be developed to synthetic applications irrespective of the mechanistic details. In some cases it could be a convenient alternative to photochemical reactions. In other cases it represents a new reaction type which has no precedent. 

Photoscience covers a broad spectrum of interdisciplinary and interrelated subjects and it may be subdivided into photomedicine, photobiology, photochemistry and photophysics (Fig. 3-1). Photochemistry, in general, studies the reactions that occur through electronically excited states of molecules. Specifically, photochemistry studies the change of substance quality and characteristics by the influence of UV/VIS radiation. The mechanistic interpretation of the formation of photoproducts and their characterization and identification are typical domains of photochemistry. This research concept is strictly based on photophysics, which investigates the primary event of photon absorption by a molecule, the properties of electronically excited states and their deactivation mechanisms, such as for example fluorescence, phosphorescence and energy or electron transfer reactions, and non-... 

One of the main factors influencing the activation barrier in fast electron-transfer reactions is the change in the polarization of the immediate space surrounding the activated complex in solution. The more-well-known salt effects as well as the relatively new field of micellar effects can be used as mechanistic probes in this context. Since micelles have a hydrophobic as well as a hydrophilic part, this creates two different kinds of interfaces where electron transfer can occur if one of either the oxidant or reductant is contained or associated with these molecular aggregates. A futuristic approach could be that studies of this kind may serve as models for enzymatic reactions with complex bioaggregates such as membranes. 

Electrochemical techniques are a convenient means of studying one-electron oxidations of amines. The reaction pattern of the anodic oxidation of amines depends greatly on the reaction conditions, including the nature of the electrode and the nucleophilicity of the solvent [1-3]. A major drawback of electrode oxidations is that unwanted secondary electron-transfer reactions can occur at the electrode surface. Also in electrochemical processes the effective reaction volume is limited at the electrode surface, thereby creating a high local concentration of reactive intermediates which can lead to dimerization and disproportionation reactions. These factors have to some extent, limited the synthetic utility of the anodic oxidation of amines. Because of this the anodic oxidation of amines has been intensively studied, although mainly from a mechanistic standpoint. 

One of the best known examples of electron transfer reactions involving hemi-carceplexes is the one investigated independently by Pina, Balzani and colleagues [99] and by Deshayes and coworkers [100], The latter study is focused mainly on triplet energy transfer, a process which can be mechanistically related to electron transfer [101],... 

Use of SAMs to study mechanistic details of electron-transfer reactions... 

While the practically complete decomposition of (49b) in aqueous solutions at ambient conditions takes hours (at pH 3-5) or minutes (at pH 1-3 or 5-10),202 this complex is stable for months in polar aprotic solvents (such as DMF or DMSO), or for years in the solid state.147 The dependence of the redox potential of the Crv IV couple on the nature of the solvent has been studied for (49b).147 A series of mechanistic studies of the electron-transfer reactions of (49b) with inorganic or organic reductants have been performed several reviews of these studies are available.13,155,203-205 Applications of (49b) for the selective oxidations of some alcohols or organic sulfides have been proposed.202,206,207... 

The [Cr(OH2)6]2+ ion has been among the most widely used reductants in kinetic and mechanistic studies of the electron-transfer reactions in metal complexes. Most of these studies were published before 1991, and have been covered by comprehensive annual reviews.7,1101 Mechanistic studies of the Cr aq. + 02 reactions have also been reviewed (see Scheme 14 in Section 4.6.5.4.2). Some... 

L. Eberson, Electron-transfer reactions in organic chemistry. 4. A mechanistic study of the oxidation of... 

This chapter illustrates the complementarity of photochemical and radiation chemical techniques to elucidate elementary pathways in mechanistically rich systems. Some of the mechanistic conclusions that have resulted from these studies in aqueous media are presented. Extreme (both high and low) oxidation states of transition-metal complexes are included. Reactivity with respect to electron transfer reactions and small-molecule activation are addressed. 

This chapter will describe methods used to generate cations for their photochemical studies, as well as the characterization of their singlet and triplet excited states. Mechanistic studies of excited state carbocations are described, including reactions with nucleophilic species, electron transfer reactions, oxygen quenching, and isomerization processes. 

More useful mechanistic information is obtained from intramolecular electron-transfer reactions if the kinetics for the electron-transfer step can be isolated from the effects of diffusion. The main stimulus for making such studies is the urge to design systems that mimic some of the essential features of the photosynthetic reaction centre complex and much attention has focussed on the study of porphyrin-based photoactive dyads. Thus, a series of N-alkylporphyrins linked to a quinolinium cation has been synthesized and found to display a rich variety of photoreactions. The singlet excited state of the quinolinium cation operates in both intramolecular energy- and electron-transfer reactions while the excited singlet state of the porphyrin transfers an electron to the appended quinolinium cation. Several new porphyrin-quinone dyads have been studied,including cyclophane-derived systems where the reactants are held in a face-to-face orienta-... 

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