Cluster redox potential, determination kinetics methods

Figure 1. Principle of the determination of short-lived cluster redox potential by kinetics methods. The reference electron donor, S of a given potential and the metal atoms are generated by a single puke. During cluster coalescence, the redox potential of the couple E°(M -Mn) progressively increases, so that an effective transfer is observed after a critical time when the cluster potential becomes higher than that of the reference, constituting a threshold. Repeatedly, a new adsorption of excess cations, M, onto the reduced cluster, (n xkch (dlows another electron transfer from S with incrementation of nuclearity. The subcritical clusters Mn(n

Figure 5. Principle of the determination of short-lived cluster redox potential by kinetics method. The reference electron donor S of given potential and the metal atoms are generated by the same single pulse. Earing the cluster...

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

For clusters of higher nuclearity too, the kinetic method for determining the redox potential °(M]] /M ) is based on electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters M](. [31] Note that the redox potential differs from the microelectrode potential M /M ) by the... 

An obvious application of direct electrochemistry of proteins is the determination of redox potentials. In cases of thermodynamically inaccessible or kinetically reactive species for which traditional potentio-metric methods are inappropriate, direct electrochemistry offers an alternative approach. A good example is the 7Fe ferredoxin from Azo-tobacter chroococcum, which contains one [4Fe-4S] and one [3Fe-4S] cluster. The [4Fe-4S] + potential obtained (7i) by cyclic voltamme-... 

For clusters of higher nuclearity too, the kinetics method for determining the redox potential E°(M /M ) is based on the electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters M/. Note that the redox potential differs from the microelectrode potential E° (M. M /M ) by the adsorption energy of M onM (except for n = 1). The principle (Figure 5) is to observe at which step n of the cascade of coalescence reactions (14), a reaction of electron transfer, occurring between a donor S and the cluster M/ could compete with (14). Indeed n is known from the time elapsed from the end of the pulse and the start of coalescence. The donor S is produced by the same pulse as the atoms M", the radiolytic radicals being shared between M (reactions 1,7,8) and S (reactions 25, 26). 

The transient character of unstable species is intrinsically because of at least one fast reaction which they undergo as soon as they are formed (for example coalescence reaction in the case of atoms and clusters). This reaction therefore induces competition with any redox reaction which could be regarded as determining the redox potential of a transient entity. In particular, the competition does not enable the establishment of a reversible equilibrium of electron transfer with another suitable system. Thus, the redox potential of short-hved species must be evaluated from kinetic methods - the pulse technique enables us to observe whether or not electron transfer involving the transient species and a series of donor/acceptor couples, used as monitors, is elfective, and thus to establish by a bracketing method the value of the imknown redox potential. Only elementary monoelectronic transfers are considered. Thus, note that one of the forms of the reference couple, reduced or oxidized, can also be a transient radical. 

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