Peak oxidation potential

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

The peak potential is a function of scan rate, unlike the case for a reversible process when the peak potentials are independent of scan rate. As the scan rate increases, the voltammetric peak becomes wider. Thus, the peak oxidation potential shifts to more positive potentials as the scan rate increases. 

Peak oxidation potentials EJv vs. Ag/AgCl) in MeCN (4-XCgH4)2Se, MeCN ... 

As a second example of chemical properties we have considered peak oxidation potentials of 4,4 -disubstituted diphenyl selenides and diphenyl tellurides. We consider these quantities to be chemical properties because no covalent bonds have been formed or broken. The best regression equations obtained were for the selenides ... 

Both the C and N chemical shifts of a number of quinoxalines substituted at position 2 with the n-electron excess 2 -benzo[ ]furanyl substituent, which has at position 3 a hydroxy or amino group, could be satisfactorily calculated by the GIAO method on the basis of HF and DFT ab initio structures. Quantum mechanical calculations using the 3-21 G(d) basis-set were performed on some p-substituted diaryl tellurides and aryl Me tellurides, and the corresponding cationic radicals of these compounds. Calculated relative radical stabilization energies were shown to correlate with experimental data, and the peak oxidation potentials and Te chemical shifts were... 

Figure 8. Rate constants (log scale) for quenching (physical and chemical) of Ojf AJ as a function of quencher half-peak oxidation potentials for substituted N,N-dimethylanilines ( ), methoxybenzenes ( ), 2,6-ditertiarybutylphenols (O) and al-kylamines (+). From [101] with permission. Copyright 1978, Pergamon Press.

Electrochemical studies on pyridodithiapentalene (69) using cyclic voltammetry show scan rate-independent irreversible oxidation peaks. Oxidation potentials vary between -1-0.70 and -1-0.93 V depending on the substituents. However, no correlation is found between Hammet a constants and oxidation potentials, which can be rationalized in terms of the lack of Ti-orbital overlap between the substituents and the parent ring <92JOC3895>. 

Others have formed PANl-polystyrene core-shells and hollow spheres using chemical polymerization and decorated them with Au nanoparticles [84] (Figure 14.7). These composites were found to have improved conductivity over undecorated polymer nanoparticles. These were used to investigate dopamine oxidation using cyclic voltammetry. The peak oxidation potential was reduced due to the presence of the Au and the amperometric responses were increased significantly over Au nanoparticle or PANI-modified films alone, most likely due to the distribution of the Au nanoparticles and associated improvements in dopamine diffusion and access to the Au nanoparticle surfaces (Figure 14.8). 

The voltammogram obtained from the difference between that recorded before and during stimulation demonstrates that the detected substance Is dopamine. The current from the peak oxidation potential for dopamine recorded from sequential voltammograms demonstrates that the concentration of dopamine increases during stimulation, and then rapidly decreases after the stimulation is over. The disappearance of dopamine Is due to uptake back Into dopamine cells (21). The appearance of dopamine during the stimulation Is a combination of the synaptic release of this compound as well as the concurrent uptake. The combined effect of both of these factors, plus diffusion, can be modelled as Indicated by the solid line in Figure 8. 

Waltman et al. [325] found a linear correlation when the peak oxidation potentials, py, of 3-substituted thiophene monomers were plotted versus their Hammett substituent constants, o-" [324], as shown in Fig. 15. values, which are normally plotted versus cr", were not available in this case because these reactions were irreversible in nature. Regardless, the values should be similar to the values as determined for other families of polymers, such as substituted fluorenes [325]. Three parameters—polar, steric, and mesomeric effects—control the change in the Ep values [326]. The Hammett-Taft equation describes the behavior as... 

FIGURE 15 The peak oxidation potentials, p, of several 3-substituted thiophene monomers are plotted against their respective Hammet substituent constants, [Pg.798]

Figs. 4-9. 4-10 4-9 Peak oxidation potentials of poly(thiophene) analogues compared with thiophene monomers. Reproduced with permission from ref. [43]. 4-10 Comparative CVs of poly(aniline) and poly(OEt-aniline). Reproduced with permission from ref. [44]. 

However, no well defined oxidation peak for Rather a small gradual sloping oxidation wave rising towards 4.0 volts is observed. This is entirely consistent with the peak oxidation potential, E, of 0.96 volts vs. SSCE C 4.2 volts relative to lithium) observed fBr polythiophene in acetonitrile (12). This is anodic of peak oxidation potentials for AlCl oxidation to Cl and implies that AlCl oxidation would occur before the polymer becomes oxidized. 

The lower peak oxidation potential, E, in poly-3-methylthiophene can be explained if one considers the ele< ron as a nucleophile. When the chemical group attached to the thiophene is an electron donator either by induction or resonance, the oxidation potential should be lowered. Groups such as CH or CH donate electrons inductively while N donates electrons by resonance. The addition of an electron-donating methyl group to the thiophene allows the oxidation potential of the poly-3-methylthiophene to be lowered to below 4.0 volts. Although the peak oxidation potential at which polythiophene oxidizes is considerably above 4.0 volts in Li(S02) AlCl, it is predictably lowered and occurs at 3.9 volts in the poly-3-methylthiophene. Thus, poly-3-methylthiophene alone or electropolymerized on carbon black surfaces promises to be a suitable polymer for use in rechargeable L1/S02 cells. 

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