In-situ electrochemical ESR

In situ electrochemical ESR studies, employing an ESR spectrometer to which an electrochemical cell is attached, have also proved very useful in elucidating the redox mechanisms of CEPs and their precursors.179180... 

As has already been described in the Introduction, the first electrochemical ESR experiment was performed by Ingram and co-workers [2] who demonstrated the formation of aromatic radical ions in electroreductions. However, the real potential of the joint electrochemical ESR technique was demonstrated first by the work of Maki et al. [3, 4,12-14], They obtained the first solution spectra of electrochemically generated radicals and by doing so performed the first in-situ electrochemical ESR experiment. This was performed in a two-electrode electrolysis cell, the anode being a platinum wire within a 3 mm o.d. capillary tube, the lip of which was positioned centrally, along the axis of a cylindrical cavity. The cathode was separated from the anode by sinters and was outside the ESR cavity. The system... 

The in-situ techniques of Cauquis, Kastening, and Dohrmann described above, although showing advantages over ex-situ methods, did not contribute significant advances towards a kinetic analysis of electrode mechanisms. Improvements upon this are shown by cells developed by several groups of workers for in-situ electrochemical ESR and are described in the following section. 

This section deals with contemporary in-situ electrochemical ESR methodology and concentrates on the use of such techniques for the investigation of kinetics and their decay mechanism. 

The work of Bard and Goldberg clearly showed how an in-situ electrochemical ESR cell with stagnant electrolyte can be used in the determination of reaction mechanisms and rate constants. 

This section describes typical applications of the in-situ electrochemical ESR methodology. Specific examples of radical identification, determination of radical decay mechanisms, polymer-coated electrodes, and spin trapping will be included. 

The oxidation of TPA is a good example of the use of in-situ electrochemical ESR to provide details of electrode mechanisms but, at the same time, the above reveals how the results obtained and their subsequent interpretation are easily affected by poor cell design, as is observed from the work of Kondrikov. 

That the fate of the Ph3CO radical was further oxidation and not dimerization was confirmed by in-situ electrochemical ESR experiments. Day performed oxidising-reducing potential sequences as carried out with TPA. When the potential was held at + 2.20 V before stepping to - 1.80 V, the resultant ESR spectrum was identical to that shown in Fig. 29, i.e. a mixture of benzophenone and benzoquinone radical anions produced by the mechanism as detailed earlier for the fate of Ph3CO+ species from TPA oxidation. 

The three examples described above show the use of in-situ electrochemical ESR in the investigation of the mechanism of electrode reactions. The work of Compton et al. [67] upon the reduction of fluorescein in aqueous media illustrates how in-situ techniques can provide information of both electrode mechanism and kinetics. In buffered solutions in the pH range 9-10 Compton et al. observed an apparent two-electron reduction of fluorescein (F) to leuco-fluorescein (L). In-situ electrochemical ESR experiments using the channel electrode revealed an ESR spectra as shown in Fig. 32 which was attributed to semi-fluorescein (S) where... 

Recently, in-situ electrochemical ESR has been applied to the field of polymer-coated electrodes and conducting polymers. The sensitivity of such cells is sufficient to detect radicals in thin polymer films on electrode surfaces. Various examples are given below. 

The main constraint in in-situ electrochemical ESR experiments is the ESR cavity. In-situ electrochemical experiments offer many opportunities for developing a proper understanding of methods of synthesis and the magnetic properties obtained. 

In this way, it was shown that the anion radical of 2-nitropropane formed during the reduction of the parent compound in aqueous solution (pH 10.2) at a mercury-plated foil electrode undergoes a first-order decay with a rate constant of 0.36 s A combination of the in situ electrochemical ESR data and voltammetric measurements made simultaneously at the channel electrode produced the spectrum shown in Figure 7. They, additionally, permitted the deduction that the reduction of 2-nitropropane under the specified conditions proceeded via an ECE mechanism in which the chemical step was ratedetermining proton uptake by the radical anion. In the buffered media employed, this was seen as a pseudo-first-order step. The electrochemical reaction sequence can therefore be written ... 

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