Spectroelectrochemical experiments

FIGURE 2-11 Spectra for a series of applied potentials (mV vs. Ag/AgCl) during thin-layer spectroelectrochemical experiment on 1.04 x 10 3 M [Tc(III)(dmpe)2Br2]+. Medium is dimethylformamide containing 0.5 M TEAP = tetraethylammonium perchlorate. (Reproduced with permission from reference 27.)... 

Spectroelectrochemical experiments can be used to probe various adsorp-tion/desorption processes. In particular, changes in the absorbance accruing from such processes can be probed utilizing the large ratio of surface area to solution volume of OTEs with long optical path length (29). Additional information on such processes can be obtained from the Raman spectroelectrochemical experiments described later. 

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21... 

The electrogeneration of [(TPP)Co] from (TPP)Co, and the reaction of this species with CHjI can be followed by cyclic voltammetry as shown in Figures lc and Id. In the absence of any added reagent, there are two reversible reduction waves which occur at Ei/2 = 0.85 jind -1.86 V (see Figure lc). These are due to the formation of [(TPP)CoJ and [(TPP)Co]2-, where the second reduction has occurred at the porphyrin ir ring system. The first reduction of (TPP)Co is not reversible in the presence of CH3I, and occurs at Ep = -0.86 V (see Figure Id). A new reversible reduction also appears at Ej/2 = -1.39 V. This process is due to (TPP)Co(CHj) which is formed as shown by Equation 8. The formation of (TPP)Co(CHj) as the final product of the electrosynthesis was confirmed by spectroelectrochemical experiments which were carried out under the same experimental conditions(26). 

Three types of spectroelectrochemical experiments are useful for mechanistic studies ... 

The classical Hoijtink mechanism and the dianion mechanism have been observed at electrodes with a high hydrogen overvoltage, such as mercury. If mercury is replaced by platinum with its low hydrogen overvoltage, a radical pathway seems to be favored [199], which is closely related to catalytic hydrogenations of hydrocarbons. Spectroelectrochemical experiments provided evidence for an additional hydride mechanism (Eqs. 25-27) [200]. ... 

Fig. 3 Spectroelectrochemical experiment of a Sg -I- DMF solution (2.5 mM, 293 K 0.005 V s 200 pm working electrode Pt-electrode pseudoreference electrode Pt-wire). O.d. optical density.

The spectroelectrochemical experiments in a wide range of temperature and scan rate have been a powerful tool for understanding the complex redox properties of sulfur and polysulfides in DMF and numerical simulations have been essential to validate the proposed electrochemical mechanisms. 

Normal reflection optics have been used to advantage with bulk electrode materials. Examples of this type of spectroelectrochemical cell are shown in Figure 9.11 [67]. Simple bifurcated fiber-optic waveguides are used to direct source light onto reflective bulk electrode surfaces and to collect the reflected light for transmission to a detector. This is a simple means for performing spectroelectrochemical experiments at bulk metal electrodes that cannot be as... 

Spectroelectrochemical experiments have been extended to the infrared spectral region using simple optical and electrochemical cell arrangements. Figure 9.12 shows a spectroelectrochemical cell that permits acquisition Fou-... 

A spectroelectrochemical experiment can be conducted at an optically transparent electrode (OTE) to determine the diffusion coefficient of a species. Single potential step chromoabsorptometry experiments are conducted so that the diffusion... 

Thin-layer cell — An electrochemical cell with the reactant solution confined to a thin layer. -> mass transport can be neglected as long as the layer thickness / is smaller than the diffusion layer thickness / (2 Dtf 2 for a given experimental time t. Thin-layer cells are frequently employed in spectroelectrochemical experiments and in - cyclic voltammetry. 

In a spectroelectrochemical experiment at modified gold for substrate-bound P450cam an of — 373 mV was found. The surface interaction and direct electrochemical transformation does not affect the enzyme structure as was confirmed spectroscopically. Both, upon direct electrochemical reduction and upon ligand binding the spectral changes clearly indicated the native state of P450cam during reversible reduction and oxidation [181]. 

Absorbance of intermediates formed during the anodic oxidation of 68e in 0.5 M H2SO4 in spectroelectrochemical experiments on ITO glass electrodes was observed in the range 490-520 nm and it was found46 that the ensuing reactions are slower than those for unsubstituted anilines, but the nature of intermediates was not resolved. However,... 

Spectroelectrochemical experiments in which the potential sequence is applied in both directions in a repetitive fashion allow an assessment of the stability of the electrogenerated species, which was indeed confirmed for both the Fe(III)TMPyP and Co(III)TMPyP systems. Yet another virtue of this methodology is the expedient preparation and spectroscopic characterization of reduced and oxidized forms of materials without the need for synthesizing often highly reactive and thus difficult to handle species. 

A general idea of the anticipated redox range of the system under investigation is usually adequate to determine the ideal optically transparent electrode material however, the success of the spectroelectrochemical experiment may be determined by the pristine condition of the electrode. The working electrode. 

Spectroscopic data obtained from spectroelectrochemical experiments require careful and case-specific analysis. The Fe /Fe redox couple has a unique role in diflferent iron-containing proteins. It is hypothesised that the mammalian iron-transport protein transferrin uses the Fe /Fe redox couple as a switch that controls the time and site-specific release of iron, while other iron-containing proteins, such as myoglobin, are able to hold on to iron in both oxidation states. Therefore, it is very important to evaluate the protein and its interaction with both the oxidised and reduced states of iron and accordingly develop a data-analysis model. The spectroelectrochemical response of an iron binding protein can be ideal Nernstian, non-Nernstian resulting from coupled... 

Flowever, the ideality of the Nernstian plot (Figure 2.5) indicates that there is little or no Fe " dissociation occurring in the electrochemical cell during the spectroelectrochemical experiment. The occurrence of significant dissociation would result in a Nernstian plot whose slope (n /i rmV ) would not yield a... 

For a deeper understanding of the interplay of the porphyrin ligands, the metals and (if present) the axial coligands in these various oxidation states, thorough electrochemical and spectroscopic investigations, and of course spectroelectrochemical experiments are sought. This chapter will report on how... 

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