Scheme of squares

Of course, proton transfer can also occur between two reactants in the solution. As such, it is not an electrochemical reaction, unless it is combined with an electron exchange with the electrode. Such a combined electron-proton transfer can be represented by the scheme of squares shown in Fig. 2.8. Both electron and proton transfer... 

Initially, it was thought that all two-electron reaction pathways were of this general sort, and a so-called scheme of squares was elaborated, which, for the system shown, would take the form ... 

Studies in the area of electrochemical molecular recognition deal with bifunctional receptor molecules that contain not only binding sites but also one or more redox-active centres whose electron transfer reaction is coupled to the receptor s complexation. Such systems can be described by the scheme of squares as shown in Scheme 1. 

Scheme 1 Scheme of squares for cis-trans redox isomerism. 

Convenient analysis of these complex electrode processes with separation of electron and proton charge transfer steps can be achieved by the use of a scheme of squares [13]. 

Also, in complex electrode reactions involving multistep proton and electron transfer steps, the electrochemical reaction order with respect to the H+ or HO may also vary with pH, indicating a change of mechanism with pH. In this respect, the use of schemes of squares outlined in Sect. 2.2 is very useful in the analysis of these complex kinetics [13]. 

Given that there are a limited number of possible intermediates, we can draw up a scheme of squares of the form shown above [3], where the x symbolises a Pt—C bond, and which summarises the following reactions ... 

Figure 7. a) The tl-orbital scheme of square-pyramidal bispidine-copper(II)-coligand complexes... 

We have applied this technique to the study of the proton flux that takes place when a modified electrode, the thionine-coated electrode, is either oxidised or reduced. We were particularly interested in the question as to whether the proton and electron fluxes were in time with one another or not. Typical results for proton and electron fluxes for reduction and oxidation at a number of different values of pH are displayed in Fig. 7. At first sight, we were bewildered by the variety of behaviour. However, we can explain the different transients as follows. In Table 2, we set out the scheme of squares [18, 19] for the thionine/leucothionine system with a number of vital pKk values. Starting at pH 4 in the oxidation direction (LH + - Th+ + 2e + 3H+), we see that the proton flux is indeed larger than the electron flux and that both fluxes are in time with each other. In the opposite reduction direction, the electron flux is similar but the proton flux is smaller and delayed. The reason for this is that, to start with, protons are used up and the pH crosses the pKa at 5.5 (Th+ + 3H+ + 2e - LH +). However, for pH > 5.5, the reaction can utilise the H+ stored in the coat (Th+ + 2 LH2+ + 2 e - 3 LH2+). This means that bulk H+ is not consumed, leading to a smaller H+ transient. When the electron flux dies away, the pH drifts back to the equilibrium value of 4. As it does so, there is an H+ flux from the relaxation LH2+ + H+ - LH +. The explanation of the transients at pH 5 is similar. In the reduction direction, the H+ flux has almost completely collapsed. In this case, the pH crosses the pKa boundaries at 8.5 where there will be no H+ flux (Th+ + 2e -> L ). The relaxation flux after the electron flux has died away will also be small since the bulk concentration of H+ (pH = 5) is so small. At pH 6, the reduction transients are similar to those at pH 5. In the oxidation direction, the pH rapidly crosses the pKa = 5.5 boundary. Now the coat mops up the H+, releasing no H+ to the solution (3LH2+ - ... 

Scheme 1 Scheme-of-squares representation of the redox-switching mechanism of a polythionine film exposed to acetic acid solution. Th represents a monomeric thionine unit, A represents acetate, and X can be either a water or acetic acid molecule. (Reproduced from Ref [124] with permission from the American Chemical Society.)... 

SCHEME 18.3 Scheme-of-squares for polyhexylthio-phene, exhibiting two redox states, each of which existing in one solvation state. (Reprinted from Brown, M.J., A.R. Hillman, S.J. Martin, R.W. Cernosek, and H.L. Bandey. /. Mater. Chem., 10, 115-126, 2000. With permission.)... 

Neurotransmitters, such as dopamine (DA) and epinephrine (EP), are catecholamines that undergo complex multistep oxidation processes in aqueous solution via coupled ET, proton transfer (2e , 2H at physiological pH), but with the complication of side reactions to form melanin-like compounds that can block electrode surfaces [193]. Such processes are expected to follow a classical scheme of squares [194], and are of considerable interest for the practical detection of neurotransmitters, as carbon electrodes have become the electroanalytical platform of choice [5, 60, 195]. This is due to a desirable range of properties including biocompatibihty, chemical inertness, and low background current that are responsible for lower detection hmits, wide potential windows, and low... 

This is a scheme of squares where the subscripts 1 and 2 denote adjacent sites in a redox polymer film. Electron hopping occurs vertically, and electroinactive counterion displacement occurs horizontally. Note that the species A ", BC, BCJ" and AJ are produced by strongly uphill... 

FIGURE 1.16. Scheme of squares outlining ion association processes in electroactive polymer films. 

Scheme of squares showing the possible intermediates involved in the oxidation of methanol. 

The scheme of squares is used to describe mechanistic pathways involving electron and proton transfers, and was first proposed by J. Jacq [/. Electroanal. Chem. 29 (1971) 149]. The scheme is based upon the assumption that the reactions occur in a stepwise manner. Figure 4.7 depicts a simple one-proton one-electron scheme. In the following, assume E = 0.0 V, E2 = -1-0.473 V, pKai = 3 and pKa2 = 11. 

Fig. 4.8 The variation of the midpoint potential for the scheme of squares, as outlined in Fig. 4.7.

This cube was based ou a previous scheme of squares visual approach. Here, the axes x, y and z, respectively, represent coupled electron/proton transfer, solvent transfer and acetic acid coordination. Four equilibrium constants describe the coordination reactions for the four pairs of species on the left and right faces of the cube. The authors interpreted their data on partial redox switching of poly(vinylferrocene) films imder permselective conditions in aqueous perchlorate bathing electrolytes which produce films that reach... 

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