Reduction potentials measurement

The reduction potentials, measured by scanning voltammetry, are substitution dependent.282 As shown in Table 12 for a series of triaryl tetrazoliums (169), the reduction becomes easier when electron-withdrawing substituents are present. This is in agreement with the polarographic data on a series of benzothiazolyl tetrazolium salts (170).283 With bis tetrazolium... 

Although the same limitations apply to the use of F as those described above for the anodic counterpart, the global trend in Fig. 7 shows that gas-phase electron affinities also generally reflect the trend in the reduction potentials measured in solution for the large variety of (uncharged) acceptor structures included in Table 4.71... 

A different view of the OMT process is that the molecule, M, is fully reduced, M , or oxidized, M+, during the tunneling process [25, 26, 92-95]. In this picture a fully relaxed ion is formed in the junction. The absorption of a phonon (the creation of a vibrational excitation) then induces the ion to decay back to the neutral molecule with emission (or absorption) of an electron - which then completes tunneling through the barrier. For simplicity, the reduction case will be discussed in detail however, the oxidation arguments are similar. A transition of the type M + e —> M is conventionally described as formation of an electron affinity level. The most commonly used measure of condensed-phase electron affinity is the halfwave reduction potential measured in non-aqueous solvents, Ey2. Often these values are tabulated relative to the saturated calomel electrode (SCE). In order to correlate OMTS data with electrochemical potentials, we need them referenced to an electron in the vacuum state. That is, we need the potential for the half reaction ... 

Electrochemical data for the solid state are rare, but one example comprises reduction potential measurements of oligo(p-phenylene-vinylenes) with a convergence limit of —2.18 V vs. Fc/Fc+ for long chains. In contrast to the solution measurements, all solid-state voltammograms exhibit a strong hysteresis [240]. 

Reduction Potentials Measured for Myoglobin and the Four Nitrophorins, and the Derived Ligand Dissociation Constants of the Nitrophorins (49, 50, 55)... 

Reduction of alkyl and benzyl halides proceeds in two one-electron addition steps. The first detectable product is the alkyl or benzyl radical and this is reduced further to the carbanion. Some alkyl iodides show two polarographic waves corresponding to the two steps. Alkyl bromides show only one two-electron wave and alkyl chlorides are not reducible in the available potential window. Benzyl halides also show only one wave and benzyl chlorides are reducible in the available potential range. Reduction potentials measured in dimethylformamide are collected in... 

A comprehensive series of oxidation-reduction potential measurements have shown the FAD moiety to have the following one-electron couples PFl/PFIH = = —290 mV and PFIH 7PFIH2 = —365 mV while the FMN moiety exhibits the following PFl/PFl- = -110 mV and PFIH /PFIH = -270 mV. The FMN and FAD smiquinones were found to both be the neutral form as judged from absorption and ESR spectral data. The overlap of oxidized/semiquinone potential of the FAD moiety withkhe semiquinone/hydroquinone couple of the FMN moiety demonstrates the thermodynamic facilitation of flavin-flavin electron transfer via a one-electron mechanism. Stopped-flow kinetic data are also consistent with this view in... 

Calculated e v values likewise show a reasonably linear relation with polarographic reduction potentials measured on the dyes in solution. The values likewise correlate well with experimental oxidation potentials. 

Makino R, Chiang R, Hager LP (1976) Oxidation-reduction potential measurements on chloroperoxidase and its complexes. Biochemistry 15 4748—4754... 

Conroy CW, Tyma P, Daum PH et al (1978) Oxidation-reduction potential measurements of cytochrome c peroxidase and pH dependent spectral transitions in the ferrous enzyme. Biochim Biophys Acta 537 62-69... 

Water quality and oxidation-reduction potential measurements. 

The electron affinity of 2- and 5-nitroimidazoles calculated by a semiempirical quantum-mechanical method satisfactorily correlates with experimental values of one-electron reduction potentials measured by pulse radiolysis [942],... 

Example 32 A solution contains 0-05m FeCl2 and 01 5m FeCI3. What is the oxidation-reduction potential measurable in the solution ... 

Weller correlation Empirical correlation for the energy of full charge-transfer exci-plexes relative to the ground-state in n-hexane as a function of the electrochemical one electron oxidation and reduction potentials measured in a polar solvent for the donor (D) and the acceptor (A) involved (see e.g. A. Weller, in The Exciplex , Gordon and Ware eds., Academic Press Inc., N.Y., 1975) ... 

The electron affinities (EA) of the nucleobases have not been determined experimentally. Calculated values for the vertical and adiabatic EA obtained by scaling experimental and calculated values for other aromatic molecules are summarized in Table 1 [33a]. The vertical values follow the order U>T>C>A>G, with U having the largest (most positive) EA. The calculated adiabatic EA for C is less positive than the values for T or U. Chen and Chen [36] have asserted that the electron affinities of the purines are larger than those of the pyrimidines. However, this claim appears to be based upon questionable reduction potential measurements (see p. 114). The nucleobase anion radicals are estimated to be stabilized by c. 3 eV in aqueous solution. 

Oxidation-reduction potential measurements were made with a platinum-calomel electrode system connected to a Beckman Model H-2 pH meter. The buffered bacterial suspensions were similar to those in the dose-contact time experiments. To these suspensions, successive equal aliquots of an ozone solution were added at 1°, and the oxidation-reduction potential was measured in millivolts. 

Under what conditions are standard reduction potentials measured (21.1)... 

It was possible to obtain better resolution for these compounds because there are three independent experimental methods for estimating the electron affinities that could be iterated to consistency. These were the ECD measurements, half-wave reduction potential measurements, and electronegativity values. In addition, these electron affinities had been calculated with the MINDO/3 procedure. We also calculated the values using the CURES-EC procedure and obtained estimates of the charge densities. These procedures were extended to a larger set of 80 compounds, some without gas phase Ea that will be discussed in Chapter 10. 

The Ea from half-wave reduction potential measurements and energies of charge transfer complex absorption in solution support the gas phase measurements from all the cited techniques. The major difference between the gas phase measurements and the solution or solid phase measurements is the interaction between the solvents or solid phase and the anions. These measurements provide a transition between the low values of valence-state electron affinities measured in the gas phase and the negative valence-state electron affinities. Thus, the valence electron affinities for naphthalene and pyridine are 0.17 eV and 0.0 0.2 eV by solution phase techniques [39, 77]. 

Polarographic half-wave reduction potential measured in aprotic solvents. 

Figure 12-8 summarizes the information available as far as the HOMO/LUMO positions of the compounds is concerned. Being inferred from oxidation/reduction potentials measured by cyclic voltammetry in polar solution and from HOMO/ LUMO gaps, respectively, absolute values should be viewed with some caution. 

Oxidation-reduction potential measurements in natural waters and wastewaters are difficult to interpret. The only potentials that will register in the ORP cell are those from species that can react at the indicator electrode surface—we call these electroactive species. In natural waters only a few reactions proceed at the electrode surface, for example,... 

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