Reaction mechanism multistep electron transfers

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

V,/V-dimethylaniline, especially when those strong donors are paired with the relatively electron-poor MES derivative of the bis(arene)iron(ll) acceptor. As such, the dark reactions arise via essentially the same multistep mechanism as that for charge-transfer de-ligation, the difference arising from an adiabatic electron transfer (10) as the initial step that is thermally allowed when the driving force -AGET is sufficient to surmount... 

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]. 

Multistep electrode reactions can also be complicated by homogeneous chemical reactions. The most studied case is that the product of a first electron transfer undergoes a homogeneous chemical transformation with an electro-inactive species present in a large excess under these conditions, the reaction scheme is that corresponding to a pseudo-first-order ECE mechanism given by... 

Importantly, all photoinduced processes share some common features. A photochemical reaction starts with the ground state structure, proceeds to an excited state structure and ends in the ground state structure. Thus, photochemical mechanisms are inherently multistep and involve intermediates between reactants and products. In the course of a photoinduced charge transfer reaction the molecule passes through several energy states with different activation barriers. This renders the electron transfer pathway quite complex. 

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). 

In the context of the present discussion, it is worth noting that virtually all the experimental systems that exhibit such "anomalous temperature-dependent transfer coefficients are multistep inner-sphere processes, such as proton and oxygen reduction in aqueous media [84]. It is therefore extremely difficult to extract the theoretically relevant "true transfer coefficient for the electron-transfer step, ocet [eqn. (6)], from the observed value [eqn. (2)] besides a knowledge of the reaction mechanism, this requires information on the potential-dependent work terms for the precursor and successor state [eqn. (7b)]. Therefore the observed behavior may be accountable partly in terms of work terms that have large potential-dependent entropic components. Examinations of temperature-dependent transfer coefficients for one-electron outer-sphere reactions are unfortunately quite limited. However, most systems examined (transition-metal redox couples [2c], some post-transition metal reductions [85], and nitrobenzene reduction in non-aqueous media [86]) yield essentially temperature-independent transfer coefficients, and hence potential-independent AS orr values, within the uncertainty of the double-layer corrections. 

Electron-transfer reactions between a donor D and an acceptor A have been widely studied in liquid solutions. Marcus first proposed the following multistep mechanism to account for these reactions [2, 3] ... 

The mechanisms of the electron-transfer event in such systems, involving solvational reorganization of the reactant, have been treated in much detail in the literature of complex-ion chemistry in inorganic chemistry (25) and by Marcus (26), Hush (27), and Weaver (28) for corresponding redox processes conducted at electrodes. The details of these works are outside the scope of this article, but reviews (29,30) will be useful to the interested reader. Chemisorbed intermediates, produced in two- or multistep redox reactions, are not involved except with some organic redox systems such as quinones or nitroso compounds. 

Armstrong and Firman ° analyzed a mechanism that included two successive electron-transfer reactions. A general approach to multistep mechanisms involving soluble species in semi-infinite diffusion was presented recently hy Harrington. It allows determination of the number of breakpoint frequencies on the Bode magnitude plot for an arbitrary mechanism and, in consequence, for the determination of the reaction mechanism and kinetics. 

The rate expression for the multistep consecutive electron-transfer reaction of Scheme 1 [i.e., Eq. (31)] is able to relate complex consecutive electron-transfer reaction mechanisms to experimental potential vs. logarithmic current-density relations. When p is assumed to be 1/2, the Tafel slopes (1/a/) predicted by this relation can only have values less than or equal to 118 mV dec i (at 25 °C) for electron-transfer limited reactions, since electrons transferred in non-rds steps will add integers (to P) in the expected a values and therefore decrease the Tafel slope below 118 mV dec 1. For instance, the usual cathodic Tafel slope of 118 mV dec-i for a one- electron transfer over a synunetric harrier is decreased to 39 mV dec for one preceding quasi-equilibrium electron transfer and to 24 mV dec for two, etc., and the anodic Tafel slopes are similarly decreased for one and two following (where the reaction steps are still written as reductions, as in Scheme 1) electron transfers, respectively. It should be noted that the Tafel slopes that are determined hy a values involving y-i- P differ substantially and discontinuously from the value for a = P = 1/2, and therefore should be easily distinguishable. 

One of the most important and impressive applications of transient RR and TR studies deals with investigation of the strutural dynamics and mechanism of action of the terminal oxidase in aerobic respiration namely, cytochrome-c oxidase (CcO). In contrast to the previous section which dealt with the dynamics involved in protein structural rearrangements, the following TR studies to be summarized for CcO deal with the detection and characterization of the complex, multistep proton and electron transfer reactions which follow O2 binding by CcO. Whereas in the case of Hb structural dynamics the essential data consist of frequency shifts of key modes, the TR spectra acquired during the multistep redox reaction involved in CcO function reveal several distinct sets of transient spectral features which are characteristic of specific intermediates. The... 

The product of the reduction of [Co(bpy)3] by Cr, upon aerial oxidation, is a red dimeric species, postulated to have the structure [(H20)4Cr(/x-OH)2Cr(OH2)2]. This product and the stoichiometry of the reaction suggests a two-electron process, with the bpy ligand serving as a temporary bridging radical. An investigation of the Cr(II) reduction of [Co(pd)3] (pd = pentane-2,4-dione) in water/acetone mixtures reveals outer-sphere, and mono- and di-bridged ([H ] dependent) pathways.The effect of the cosolvent on the activation parameters is observed at an acetone mole fraction of 0.06, at which point its solvation of the activated complex becomes important. The reduction of [Co(en)2(dppd)] (dppd = l,3-diphenylpropane-l,3-dione) by Cr occurs by a multistep mechanism in which the first step is the formation of the [Co(en)2(dppd )] radical, which catalyzes the inner-sphere Co(III)/Cr(II) electron transfer process. " A molecular orbital study indicates that the [Co(en)2(dppd)] reduction likely involves attack of Cr " at the methine carbon of dppd, in contrast to the attack on an oxygen in the [Co(en)(pd)2] reduction. 

As will be discussed later, two or more mechanisms have been proposed for the "net hydride transfer reactions in mimetic systems. One is, of course, the one-step "hydride transfer mechanism and the other is a multistep mechanism involving the initial "electron" transfer process. The latter mechanism is further subdivided into two categories the two-step electron-hydrogen atom transfer mechanism and the three-step electron-proton-electron transfer mechanism as shown in Scheme 8. 

Roberts et al. (1982) concluded that the multistep mechanism involving an electron transfer process can be excluded, considering of the a-value of a Br nsted plot (a=0.5) observed over a wide range of cationic substrates, which agrees with the Marcus theory for atom transfer. The Br nsted a for an atom or group transfer depends on the position of a substituent and the tightness of the transition state (t) as well as on the resemblence of the transition state to reactants and/or products. The Marcus theory predicts that t can be related to the rates of symmetrical reactions. Rates and equilibrium constants were measured for the reactions of 10-methylacridane with a series of 1-benzyl-3-cyanopyridinium ions substituted in the... 

Not only the nitroaromatic species, such as IV, but also some simpler compounds, which used to be considered as typical substrates of the 8 2 reactions, can be involved in multistep radical-forming nucleophilic substitutions. Evidence has been accumulating over the last years that the nucleophilic substitution with alkyl halides occurs, at least in some instances, by the single-electron transfer mechanism. It has been suggested [29,30] that the SET and Sn2 mechanisms represent the extremes of a wide spectrum of mechanistic possibilities for substitution reactions. It has been deduced on qualitative theoretical grounds that the propensity of alkyl halide R—X to react with nucleophiles via an electron-transfer step depends crucially on the stability of the three-electron bond R—X in the initially formed radical-anion species. A more electronegative R will stabilize this bond and bring about a shift in the mechanism from the Sn2 to the SET type, which has then experimentally been shown to be a correct conclusion, see Ref. [30]. 

Here, M represents the electronically conducting electrode material (e.g.. Ft) that is not involved in the overall reaction and plays the role of an electrocatalyst for the reaction. The last intermediate step occurs in two identical consecutive steps since electron transfer occurs by quantum mechanical tunneling, which involves only one electron transfer at a time. When multistep reactions take place, there is the possibility of parallel-intermediate steps. The parallel-step reactions could lead to the same final product or to different products. Direct electro-oxidation of organic fuels, such as hydrocarbons or alcohols, in a fuel cell exhibits this behavior. For instance, in the case of methanol, a six-electron transfer, complete oxidation to carbon dioxide can occur consecutively in six or more consecutive steps. In addition, partially oxidized reaction products could arise, producing formaldehyde and formic acid in parallel reactions. These, in turn, could then be oxidized to methanol. 

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