Reversible Two-Electron Transfer

In case (i), where the second electron transfer occurs at a more positive potential than the first = +0.25 V), only one voltammetric wave is observed. This 

It is interesting to note that in case (i) the reduction of A to C occurs at a potential greater than O.OV. This is due to the presence of 

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The Qf E curve for a reversible two-electron transfer taking place in a monolayer is independent of time (i.e., it has a stationary character) and, therefore, is independent of the potential-time waveform applied to the electrode, as in the case of a reversible one-electron transfer reaction. It is also important to highlight that the normalized charge, has a identical expression to that for the normalized transient current 7 v N obtained for solution soluble species when the NPV technique is applied to an electrode with any geometry (see curves in Fig. 3.16, and Eq. (3.141)), and also to the normalized stationary current obtained for solution soluble species when any potential-time waveform is applied for ultramicroelectrodes with any geometry. 

Miscellaneous Physical Chemistry. The electrochemical oxidation of /3-carotene by a single-step reversible two-electron transfer has been reported.106 The fluorescence lifetimes of a- and j8-carotene have been measured by the streak camera technique to be 5.5 10 ps (6 x 10 2 mol 1 1 in chloroform).107 Ellipsometric measurements of light-absorbing monomolecular layers of several compounds, including 8 -apo-/3-caroten-8 -oic acid (227) have been described.108 The kinetics of autoxidation of amorphous retinyl acetate have been determined.109... 

The oxidation of tetrakis-p-dimethylaminophenylethylene analogous to (27) in acetonitrile is a process which fits the theoretical criteria for a reversible two electron transfer and (28) is obeyed (Bard and Phelps, 1970). 

The reduction of 69 by iodide ion proceeds by an inner-sphere process involving prereductive association (173). The electrochemical reduction involves a single, chemically reversible two-electron transfer at 0.0 V (174). The large peak separations observed in the cyclic voltammograms of 69, at Hg, Pt, or Au electrodes, are due to slow charge-transfer kinetics for the reduction process the slowness of the electron transfer may arise from the accompanying structural rearrangement (174). 

Neither the neutral complex nor the dication react with the substrate on the time-scale of the electrochemical experiment under the experimental conditions used therefore, the species responsible for the catalytic process is an unstable radical cation intermediate in the single step, reversible two-electron transfer (Scheme 2). EHMO calculations suggest that the substratebinding site may result from the opening of one of the thiolate bridges at the radical cation stage. ... 

Complexes with pyridine-2,6-diimine ligands, five-coordinate [NiX2(174)] (X = C1, Br) or six-coordinate [Ni(174)2]X2, were usually assumed to have innocent neutral ligands. The X-ray structure and spectroscopic characteristics of [Ni(174)2](PF6) confirm that the complex contains the neutral ligand ([174] ) and a divalent nickel ion.579 The cyclic voltamogram of this complex in CH2C12 shows three reversible one-electron-transfer processes at = 1.19 V, —1.30 V, and — 1.82V (vs. Fc+/Fc), of which the first is a one-electron oxidation, while the other two correspond to two successive one-electron reductions. The spectroscopic data allow one to assign these processes as follows ([174]1 is a one-electron reduced radical form of [174] ) [Nini(174)°2]3+ [NiII(174)02]21 - " [NiI(174)°2]+ = " [NiI(174)°(174)1 ]°. 

Cyclic voltammograms of DTT-TTF, 86a and 86b, exhibited two reversible one-electron transfer processes corresponding to the successive formation for the stable cation radical and dication <2003JMC1324>. 

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... 

An irreversible chemical reaction interposed between two reversible one-electron transfers (case R-R). The so-called ErQEr process, if n — n2 = 1, can be written as ... 

Two reversible one-electron transfers. The simplest case to treat consists of two successive one-electron transfers (EE mechanism). 

Diagnostic criteria for two-electron transfers with different extents of reversibility. The voltammetric responses of EE processes can be qualitatively confused with processes complicated by coupled chemical reactions. The only distinctive criterium to define these processes is the constancy of the parameter ip/v112 relative to each step with the scan rate. 

Sulfoxide Reduction Sulfoxide reduction is a two-electron-transfer reversible reaction resulting in thioethers. Organic sulfoxides are used mainly as agrochemicals, and their reduction (abiotic and microbially mediated) has been found in anaerobic soils, sediments, and groundwater (Larson and Weber 1994). 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

CH3CN V = 0.2 V s ) indicated that the electrode process was not a Nernstian two-electron transfer but involved two successive one-electron steps, with the second thermodynamically more favorable than the first one [32]. Therefore, the reversible, overall two-electron process in Sch. 11 is better represented by two successive, reversible, one-electron steps involving a thermodynamically unstable and undetected cation intermediate (see Sch. 13 EE process, or ECE process, where the chemical step C is a fast, reversible deformation of the M2S2 core). In agreement with this, it should be noted that the oxidation of ds-[Mo2(cp )2(/x-SMe)2(CO)4] ds-13 also... 

The zinc electroreduction investigated [179] in urea-chlorides melts was found to be reversible with two-electron transfer. 

As with U and Np ions, flow coulometry experiments were conducted to further study the electrode reactions of Pu ions in acidic aqueous solutions [49]. The results from these studies confirm the reversibility of the one-electron couples Pu02 " /Pu02 and Pu" /Pu " " in nitric, perchloric, and sulfuric acid solutions. The further reduction of Pu02 resulted in an irreversible two-electron transfer yielding Pu +. The flow coulometry results in the mixed phosphoric-nitric acid solutions confirm the overall conclusions that have been reached from the stationary and rotated working electrode experiments described previously, in which PuO + is the primary Pu(IV) product from Pu02 reduction. 

A reversible one-electron transfer at + 1.34 V was observed, assigned to the metal-centered Ru(II)/Ru(III) oxidation. The complex cathodic pattern of this species, comprising 10 successive reversible one-electron transfer, is the sum of that of its two subunits, the C60 core and the Ru(II) tris-bipyridyl complex, thus excluding any... 

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