Electrochemical methods, study of reactive intermediates

Effective charge and transition-state structure in solution, 27, 1 Effective molarities of intramolecular reactions, 17,183 Electrical conduction in organic solids, 16,159 Electrochemical methods, study of reactive intermediates by, 19, 131 Electrochemical recognition of charged and neutral guest species by redox-active receptor molecules, 31, 1... 

The study of reactive intermediates by electrochemical means, as well as the electroanalytical methods, are broad topics which cannot exhaustively be covered in a single chapter. Here, only those electroanalytical techniques which have been reduced to practical application in this field will be considered. A great deal of effort has gone into the development of methods to describe electrode processes theoretically. Only a brief introduction to the theoretical methods for handling the diffusion-kinetic problems is included. The applications discussed cover both thermodynamic and kinetic aspects of reactive intermediate chemistry and are a sampling meant to give an indication of the current state of the field. 

A large number of electrochemical methods exist which are or have the potential to be useful in the study of reactive intermediates. The methods are conveniently categorized according to the quantity measured, usually the current, potential, or some optical property of the reactants or the intermediates. A further classification arises from the manner in which experiments are conducted, i.e. transient or steady state measurements. In this brief survey only those techniques which have been reduced to useful practice are discussed and even then the coverage is not exhaustive. More detailed discussion can be found in several excellent references sources (Bard, 1966-present MacDonald, 1977 Bard and Faulkner, 1980). 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

Reactions of hydrated electrons with organic compounds, 7, 115 Reactions in dimethyl sulphoxide, physical organic chemistry of, 14, 133 Reactive intermediates, study of, by electrochemical methods, 19, 131 Reactivity indices in conjugated molecules, 4, 73... 

The intensive electrochemical studies of polycyclic systems, especially cyclic volta-metry (CV) are now at a stage which justifies naming cyclic voltametry an electrochemical spectroscopy as was suggested by Heinze 65). Early electrochemical studies referred only to the thermodynamic parameters while CV studies provide direct insight into the kinetics of electrode reactions. These include both heterogeneous and homogeneous electron-transfer steps, as well as chemical reactions which are coupled with the electrochemical process. The kinetic analysis enables the determination of reactive intermediates in the same sense as spectroscopic methods do. As already mentioned, electron transfer processes occur in both the electrochemical and metal reduction reactions. 

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