Multielectron process

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Interpretation of pubhshed data is often comphcated by the fact that rather complex catalytic materials are utilized, namely, poly disperse nonuniform metal particles, highly porous supports, etc., where various secondary effects may influence or even submerge PSEs. These include mass transport and discrete particle distribution effects in porous layers, as confirmed by Gloaguen, Antoine, and co-workers [Gloaguen et al., 1994, 1998 Antoine et al., 1998], and diffusion-readsorption effects, as shown by Jusys and co-workers for the MOR and by Chen and Kucemak for the ORR [Jusys et al., 2003 Chen and Kucemak, 2004a, b]. Novel approaches to the design of ordered nanoparticle arrays where nanoparticle size and interparticle distances can be varied independently are expected to shed hght on PSEs in complex multistep multielectron processes such as the MOR and the ORR. 

Keywords. Metal-based dendrimers, electrochemistry, multielectron processes, luminescence, light harvesting. 

In principle, an organic molecule can accept as many electron pairs as it has low-lying vacant orbitals. In the same way, high-lying occupied orbitals can release not a single, but several electrons. Such multielectron processes can result in the formation of polyion-polyradicals. As will be seen from this section, the main topic of interest in poly(ion-radicals) consists of their spin multiplicity. 

In conclusion, the electrochemical data offer a fingerprint of the chemical and topological structure of the polynuclear compounds. Furthermore, made-to-OTder synthetic control of the number of electrons exchanged at a certain potential can be achieved. The presence of multielectron processes makes such polynuclear complexes very attractive in view of their possible application as multielectron-transfer catalysts. Examination over a more extended oxidation potential window (in a solvent like liquid SOj) should permit one to obtain an even larger variety of oxidation patterns. 

Synthetic polymers stabilize metal colloids as important catalysts for multi-electron reactions. Polynuclear metal complexes are also efficient catalysts for multielectron processes allowing water photolysis. 

As the energy of the excited states and the redox levels of each metal-polypyridine unit depend on metal and ligands in a predictable way, the simultaneous presence of different metals in a dendritic structures gives rise to intramolecular energy transfer processes as well to different redox patterns with multielectron processes. In particular, the tetranuclear [Os(2,3-dpp)3 (2,3-dpp)Ru(bpy)2 3]8+ (OsRu3) shown in Fig. 5.3 has been designed to achieve an efficient antenna effect. This species can also be considered a first-generation mixed-metal dendrimers.31... 

Multielectron storage devices can be used as (i) redox catalysts, also called electron mediators, for multielectron processes, (ii) electrochemical sensors with signal amplification, and (iii) molecular batteries that can be foreseen to power molecular machines in the future or that can be used to construct flexible rechargeable batteries.10... 

The general electrochemical behavior of surface-bound molecules is treated in Sect. 6.4. The response of a simple electron transfer reaction in Multipulse Chronoamperometry and Chronocoulometry, CSCV, CV, and Cyclic Staircase Voltcoulometry and Cyclic Voltcoulometry is also presented. Multielectronic processes and first- and second-order electrocatalytic reactions at modified electrodes are also discussed extensively. 

The multielectronic process presented in reaction scheme (6.1) is formally equivalent to that corresponding to the reduction of a molecule containing n electroactive redox centers ... 

The different assumptions needed to make a statement of this problem will be presented in the following section. Then the general solution corresponding to the application of a sequence of potential pulses to attached molecules giving rise to simple charge transfer processes and particular solution corresponding to Multipulse Chronoamperometry and Chronocoulometry and Staircase Voltammetry will be deduced. Cyclic Voltammetry has a special status and will be discussed separately. Finally, some effects that cause deviation from the ideal behavior and more complex reaction schemes like multielectronic processes and chemical reactions in the solution coupled to the surface redox conversion will be discussed. 

Kajzar, F. and Agranovich, M.V. (1999) Multiphoton and Light Driven Multielectron Processes in Organics ... 

This type of reaction sequence is common for most multielectron processes [e.g., Cu(II) - Cu(s) 02 - HOOH flCH(O) ACH2OH]. If both electron transfers are reversible and the chemical reaction is irreversible, the first peak should indicate an nrelectron process for small k values and an (n, + n2)-electron process for large k values. Therefore, an increase of the scan rate should decrease the apparent number of electrons involved in the overall process. Often the second reduction step occurs at a less negative potential than the first, which means that only a single irreversible peak is observed. Potential-scan reversal can provide anodic peaks and data for red and redx. 

Given that electrochemical rate constants are usually extremely sensitive to the electrode potential, there has been longstanding interest in examining the nature of the rate-potential dependence. Broadly speaking, these examinations are of two types. Firstly, for multistep (especially multielectron) processes, the slope of the log kob-E plots (so-called "Tafel slopes ) can yield information on the reaction mechanism. Such treatments, although beyond the scope of the present discussion, are detailed elsewhere [13, 72]. Secondly, for single-electron processes, the functional form of log k-E plots has come under detailed scrutiny in connection with the prediction of electron-transfer models that the activation free energy should depend non-linearly upon the overpotential (Sect. 3.2). 

We begin with a summary of the standard single-electron rigid-bridge model for electron transport [1,2], and then describe effects that arise from bridge dynamics. We next examine issues in multistep multi-center electron transfer. The closely related problem of two-electron transfer is then discussed. Multi-center and multielectron processes are of great relevance for ET in DNA, proteins, and catalytic reactions. 

Tripathy, S. K., Viswanathan, N., Balasubramanian, S., Bian, S., Li, L., and Kumar, J. Polarization dependent holographic write, read and erasure of surface relief gratings on azopolymer films, in Multiphoton and Light Driven Multielectron Processes in Organics New Phenomena, Materials and Applications, pp. 421-436, eds R Kajzar and M. V. Agranovich, 2000 Kluwer Academic Publishers, Netherlands. 

J. A. Bmce and M. S. Wrighton, Modified p-type Si photocathodes for photochemical hydrogen generation Surface texturing, molecular derivatizing reagents, and nobel metal catalysts for multielectron processes, hr. J. Chem. 22, 184, 1982. 

Because the latter is a multielectron process, n values for the oxidation of the monocation are > I in bulk electrolyses. 

Note that Eq. (n) applies only to one-electron reactions. Whereas the theoretical treatment can be extended formally to multielectron processes, such reactions commonly occur in microscopically separate, one-electron steps. If the first step is rate controlling, n can be set at unity in Eq. (b) in 12.3.7.1, regardless of the number of electrons transferred in the overall reaction. A general difficulty for such multistep processes is that AG, cannot be extracted from rate measurements unless the standard potential for the redox couple comprising the elementary reaction is known. For multielectron reactions, only the formal potential for the overall reaction normally can be obtained. Similar remarks apply to other multistep electrode reactions, such as those involving phase transfer (e.g., metal deposition or gas evolution). 

The first coupled-channel calculations for total and differential energy losses were performed for very simple systems such as H on H, He [11,12,61]. Later theses calculations have been extended to more complex systems such as the inner-shells of Al and Si [22,24]. Good agreement with experimental data has been found and the remaining discrepancies have been attributed to multielectron processes. 

At 30 keV/u we find the largest deviation between the measured stopping power and our calculated values of about 12%. This may be attributed to an overestimation of cross sections for multielectron processes because of the use of the independent particle model. We emphasize that the present calculation does not properly take into account events in which more than one electron is actively involved, e.g., double target ionization or excitation and simultaneous projectile and target ionization. 

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