Dimeric cyclic voltammetry

The redox characteristics, using linear sweep and cyclic voltammetry, of a series of (Z)-6-arylidene-2-phenyl-2,3-dihydrothiazolo[2,3-r][l,2,4]triazol-5(6//)-ones 155 (Figure 24) have been investigated in different dry solvents (acetonitrile, 1,2-dichloroethane, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO)) at platinum and gold electrodes. It was concluded that these compounds lose one electron forming the radical cation, which loses a proton to form the radical. The radical dimerizes to yield the bis-compound which is still electroactive and undergoes further oxidation in one irreversible two-electron process to form the diradical dication on the newly formed C-C bond <2001MI3>. 

As noted in Section 2.2.5, the effect of dimerization may also be seen on the second wave, the wave that corresponds to the reduction of the radicals formed at the first wave. The example presented in Figure 2.35 shows the cyclic voltammetry of benzaldehyde in basic ethanol.26 The second wave represents the reduction of the benzaldehyde anion radicals formed at the first wave that have escaped dimerization. In other words, Scheme 2.29 should be completed by Scheme 2.30. 

Although separate determination of the kinetic and thermodynamic parameters of electron transfer to transient radicals is certainly important from a fundamental point of view, the cyclic voltammetric determination of the reduction potentials and dimerization parameters may be useful to devise preparative-scale strategies. In preparative-scale electrolysis (Section 2.3) these parameters are the same as in cyclic voltammetry after replacement in equations (2.39) and (2.40) of Fv/IZT by D/52. For example, a diffusion layer thickness S = 5 x 10-2 cm is equivalent to v = 0.01 V/s. The parameters thus adapted, with no necessity of separating the kinetic and thermodynamic parameters of electron transfer, may thus be used to defined optimized preparative-scale strategies according to the principles defined and illustrated in Section 2.4. 

The governing dimensionless partial derivative equations are similar to those derived for cyclic voltammetry in Section 6.2.2 for the various dimerization mechanisms and in Section 6.2.1 for the EC mechanism. They are summarized in Table 6.6. The definition of the dimensionless variables is different, however, the normalizing time now being the time tR at which the potential is reversed. Definitions of the new time and space variables and of the kinetic parameter are thus changed (see Table 6.6). The equation systems are then solved numerically according to a finite difference method after discretization of the time and space variables (see Section 2.2.8). Computation of the... 

In a reaction scheme where dimerization of an intermediate and its reduction compete as in Scheme 6.1 (taking reductions as an example), the location and characteristics of the second wave in cyclic voltammetry at which the intermediate B is reduced are governed by the outcome of this competition. 

It is worthwhile emphasizing that there are important differences between the 2 + /3 + oxidation of the [Mn(terpy)2]2+ complex, which is reversible on the time scale of cyclic voltammetry, and that of the [Mn(phen)j]2+ and [Mn(bipy)3]2+ complexes, which is complicated by following chemical reactions. A study of these chemical complications has shown that oxidation of the [Mn(phen)3]2 + and [Mn(bipy)3]2+ complexes leads to the formation of the oxo-dimers [Mn202(phen)4]" + and [Mn202(bipy)4]"+ (n = 3, 4), respectively. Figure 24 shows the molecular structures of the dimers with n = 3.40,41... 

The oxidation step of thioureas can be reversed, as was shown by cyclic voltammetry to regenerate thiourea from anodically produced dimeric salt (in AN/BU4NCIO4, potentials vs Ag/Ag+) (Scheme 1). 

Chiral 2-imidazoline dianions undergo one-electron oxidation in the presence of TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) to form a radical anion that is either trapped stereoselectively by TEMPO or undergoes dimerization. Oxidation of bis-diazene oxides leads to novel (9-stabilized 4N/3e radical cations and 4N/2e dications. These were detected by ESR spectroscopy and cyclic voltammetry. B3LYR/6-31G calculations confirmed the nature of the 4N/3e and 4N/2e systems. ... 

An enantiopure dimer 149 with a l,l -binaphthyl-bridge was prepared via the bis-tosylhydrazone (see Table 4.4, page 130/131) [122], The electronic properties of these dimers, such as the electronic absorphon spectra and cyclic voltammetry, are indistinguishable from those of other methano-bridged fullerenes. CV-data show clearly that the two CgQ-imits of the binaphthyl-dimer are reduced independently [122],... 

As demonstrated by various techniques, including cyclic voltammetry, ESR and NMR spectroscopy, instead of 1, which is an open shell system, the diamagnetic dimer 2 was isolated. The suggested mechanism for the formation of 2 involves, as first step, the acid-catalyzed cleavage of the MEM group followed by an intramolecular ring formation to an 1,3-oxazetidinium system. This intermediate then loses formaldehyde and CO to form the azafulleronium ion which is... 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

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