Reaction pathways redox couples

Analysis of the above data led to the conclusion that all of the redox reactions proceed with electron transfer through the [CuL2]2+/+ redox couple, and that the change in number of ligands occurs in the Cu(I) oxidation state. This interpretation is given as pathway I in Fig. 5. 

Upon addition of Ba2+ cations, the 2+/l+ bipyridinium redox couple is shifted anodically by 45 mV and the l+/0 couple is shifted cathodically by 10 mV. K+ and NH4 produce similar effects (Table 15). However, addition of Na+ cations causes a small cathodic shift to the 2+/l+ couple and an anodic perturbation to the l+/0 couple. This is in agreement with the proposed conformational change pathway for coupling the complexation and redox reactions. 

There are multiple possible current pathways through a DSSC, as shown in Fig. 1, because the nanoporous cell consists of two interpenetrating, bicontinuous chemical phases. The relative conductivity of these two phases and of the connection between them, Rct, depends on the illumination intensity, applied potential, kinetics of the redox couple, and so forth. Therefore, the distribution of current pathways depends also on these variables. In the DSSC, the dark current will take the distributed path of least overall resistance (Sections III. A-III.C), meaning it will flow primarily through solution [50] under the expected conditions of Rct < / 2- The dark current is thus mainly a measure of reaction (5) in this potential range, even though reaction (4) is expected to be the dominant recombination... 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

The formal potential of the NAD+/NADH redox couple is -0.56 V vs. SCE at pH 7 [15, 17]. However, at platinum and glassy carbon electrodes NADH, oxidation occurs at 0.7 V and 0.6 V vs. SCE, respectively [18]. From these oxidation potentials, it is clear that the direct electrochemical oxidation of NADH requires a substantial overpotential. In nature, NADH oxidation is thought to occur by a one-step hydride transfer. However, on bare electrodes the reaction has been shown to occur via a different and higher energy pathway which produces NAD radicals as intermediates. 

Fig. 2. Schematic plots outlining outer-shell free energy-reaction coordinate profiles for the redox couple O + e R on the basis of the hypothetical two-step charging process (Sect. 3.2) [40b]. The y axis is (a) the ionic free energy and (b) the electrochemical free energy (i.e. including free energy of reacting electron), such that the electrochemical driving force, AG° = F(E - E°), equals zero. The arrowed pathways OT S and OTS represent hypothetical charging processes by which the transition state, T, is formed from the reactant.

A major application of eqn. (47) is to diagnose the presence of catalytic, presumably inner-sphere, electrochemical pathways. This utilizes the availability of a number of homogeneous redox couples, such as Ru(NH3)e+/2+ and Cr(bipyridine) +,2+ that must react via inner-sphere pathways since they lack the ability to coordinate to other species [5]. Provided that at least one of the electrochemical reactions also occurs via a well-defined outer-sphere pathway, the observation of markedly larger electrochemical rate constants for a reaction other than that expected from eqn. (47) indicates that the latter utilizes a more expeditious pathway. This procedure can be used not only to diagnose the presence of inner-sphere pathways, but also to evaluate the extent of inner-sphere electrocatalysis (Sect. 4.6) it enables reliable estimates to be made of the corresponding outer-sphere rate parameters [12a, 116, 120c]. 

Few standard rate constants are available for simple one-electron redox reactions known to follow inner-sphere pathways. One reason is that reactions are required that are free from coupled chemical steps (i.e., are chemically reversible) so that values of E, and hence k can be obtained. Such redox couples [e.g., Ru(III)/(II)] exhibit rapid electrode kinetics even for outer-sphere pathways, so that the inner-sphere rate constants commonly are immeasurably large. 

A reaction pathway similar to that in Scheme 9 may be drawn for the BQMI 57, R =H. 0-Alkylate 91, alkylated aminophenol 92 and an analogous dialkylate were isolated [4]. BQMI 57 reacts with R directly. The presence of a redox couple with 4-hydroxy-DPA 83 is not necessary. When using the couple, the reactivity increases and is higher than that of the sole BQMI. A reactivity series of various quinones with R" may be drawn [4] (Scheme 10). 

In the case of the selective catalytic reduction using ammonia as reductant but in excess 02, Ce-exchanged sodium-type mordenite (CeNa-MOR) has been reported as an active catalyst in the 250-560°C temperature range with respect to non-redox La,Na-and H-mordenite catalysts (Ito et al. 1994). In this case, the reduction of nitric oxide is thought to proceed with crucial involvement of a Ce3+/Ce4+ redox couple, although the intermediate reaction pathway depends on the reaction temperature. 

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