Redox potentials, calculations results

As a result, the electromotive force (EMF) of the cell is zero In the presence of fluoride ions, cerium(IV) forms a complex with fluoride ions that lowers the cerium(IV)-cerium(IIl) redox potential The inner half-cell is smaller, and so only 5 mL of cerium(IV)-cenum (III) solution is added To the external half-cell, 50 mL of the solution is added, but the EMF of the cell is still zero When 10 mL of the unknown fluonde solution is added to the inner half-cell, 100 mL of distilled water IS added to the external half-cell The solution in the external half-cell is mixed thoroughly by turning on the stirrer, and 0 5 M sodium fluonde solution is added from the microburet until the null point is reached The quantity of known fluonde m the titrant will be 10 times the quantity of the unknown fluoride sample, and so the microburet readings must be corrected prior to actual calculations... 

The Rieske protein II (SoxF) from Sulfolobus acidocaldarius, which is part, not of a bci or b f complex, but of the SoxM oxidase complex 18), could be expressed in E. coli, both in a full-length form containing the membrane anchor and in truncated water-soluble forms 111). In contrast to the results reported for the Rieske protein from Rhodobacter sphaeroides, the Rieske cluster was more efficiently inserted into the truncated soluble forms of the protein. Incorporation of the cluster was increased threefold when the E. coli cells were subject to a heat shock (42°C for 30 min) before induction of the expression of the Rieske protein, indicating that chaperonins facilitate the correct folding of the soluble form of SoxF. The iron content of the purified soluble SoxF variant was calculated as 1.5 mol Fe/mol protein the cluster showed g values very close to those observed in the SoxM complex and a redox potential of E° = +375 mV 111). 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

The program reports in its output the resulting redox potential for each redox couple, as calculated from the Nernst equation. The Nernst potentials, arranged in decreasing order, are... 

As discussed in detail in Section 7.4, the energy liberated by a redox reaction depends on the redox potential of the electron-donating half-cell reaction, relative to the electron-accepting reaction. In the calculation results, we can trace the redox... 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

To study the role of ion pairs, it is necessary to investigate electron-transfer reversibility in a solution and compare the results obtained with redox potentials of a donor and an acceptor. As a rule, ion-pairing phenomena define electrode processes too (Baizer and Lund 1983). However, the known equations for equilibrium calculations cannot take ion pairing into consideration because the equations do not contain ion-pair terms. One has to rely on experiments, which are able to take into account the equilibrium of electron transfer in solutions. 

The redox potential values of all metal atoms, except alkaline and earth-alkaline metals [60], are higher than that of °(H20/eaq) = —2.87 V he- However, some complexed ions are not reducible by alcohol radicals under basic conditions and thus ii°(M L/ M°L)< —2.1 Vnhe (Table 2). The results were confirmed by SCF calculations of Ag L and Ag L structures associated with the solvation effect given by the cavity model for L = CN [61] or NH3 [47], respectively. 

This is a striking result since it suggests that simple absorption band measurements can be used to calculate Ea for thermal electron transfer. Note that for unsymmetrical cases where AE 0, both Eop and AE must be known AE can sometimes be estimated from temperature dependent redox potential measurements. 

The studies discussed here, have been used to establish the scope and accuracy of the MM-AOM method. The results obtained indicate that combined MM-AOM calculations are potentially a powerful tool for the design of novel compounds with particular spectroscopic properties and for the determination of solution structures. The study involving EPR spectroscopy and MM-AOM calculations, discussed in the next section, was the first example of this approach. Others have been published since and confirm the general applicability of the method. Another, particularly instructive example, which also involves the computation of redox potentials is given in Chapter 11. 

Equilibrium constants have been reported for the association of NO with NO- and the association of the resulting N202 with H+ (281). In principle it would be possible to calculate redox potentials involving N202- and HN202, but because of the current uncertainty in the electronic state of NO in these reactions such a calculation is reserved for the future. 

Schellmann (unpublished results) calculated from the solubility products of manganese and iron hydroxides that interstitial water can be sufficiently supplied with manganese and iron even at higher redox potentials, because the time of dissolution is adequately long. 

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