Comproportionation equilibrium

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

Complexes, see also specific type in solution, structures, see X-ray diffraction n-Complexes, 4 178-184 Complex formation constant, outersphere, 43 46, 55 electrovalent interaction in, 3 269-270 Compressibility coefficient of activation, 42 9 Comproportionation constants, class II mixed-valence complexes, 41 290-292 Comproportionation equilibrium, 41 280-281 Compton effect, 3 172 Conantokins, calcium binding, 46 470-471 Concanavalin A, 36 61, 46 308 Concensus motif, 47 451 Concentration-proportional titrations of poly-metalates, 19 250, 251, 254 Condensation... 

Returning now to the comproportionation equilibrium, it is interesting to notice that for large systems (2 mm co) the comproportionation constant tends towards a statistical limit of 4 (see below). Then a single electrochemical wave is observed, a fact which has frequently dissuaded researchers from further studies on such binu-clear complexes. However, even in this apparently unfavorable case, the proportion of mixed-valence species at half-oxidation (reduction) is, according to Eq. (3), a comfortable 50 %, nevertheless allowing a meaningful correction. 

Male and Allendoerfer draw the conclusion28 from their electrochemical measurements that Kcom, the comproportionation equilibrium constant, must be less than 1. In a more extensive study29 the value Kcom = 0.027 was determined in DMSO and found to be independent of the activity of the protons in solution (the concentration of H2SO4 was varied in the range between 0.1 and 0.5 M). It was proved, moreover, that the decay of radical cation 13 could be described by a kinetic equation based on a second-order dimerization process and a pre-equilibrium for the radical formation by comproportionation. The corresponding second-order rate constant was found to be k = 34 M-1 s-1 at room temperature29. 

Cyclic voltammetry of 5 and 6, in a 0.1 M tetrabutylammonium hexa-fluorophosphate solution in methylene chloride V5. the ferrocene/ferrocenium reference, reveals two two-electron oxidations ( 1/2 = 200 mV, 1000 mV) and two one-electron reductions ( 1/2 = —H60mV, —ISOOmV). The splitting in the reduction waves, A , is 340 mV, and corresponds to a comproportionation equilibrium constant of 5.6 x 10. The total electrochemical splitting reflects both the electronic interactions typical of a strongly electronically coupled... 

IV)0, RudlDOH, and RudDOHa, which all agree well with the known spectra (120). The only calculated spectrum that is not known explicitly from independent experiments is that of I, but this spectrum strongly resembles the calculated spectra of complexes of Ru(III) with bound oxidized substrates, especially bound epoxides (130, 190). The rate constants for the comproportionation equilibrium [Eq. (10)] are known (127). The determination of using this model and time window is well defined Experiments conducted over the concentration ranges of 0.03 to 0.23 mM Ru(tpy)(bpy)0 and 1 to 33 mM GMP give = 9 2M-is ... 

Whenever an E Er reaction takes place, one must consider the possibility of a dispro-portionation-comproportionation equilibrium,... 

The stability of a mixed-valence complex [III,II] relative to its oxidized and reduced forms can be quantified by the comproportionation equilibrium. 

In a recent study, Evans et al. showed that it was possible to factor out AGr from the comproportionation data of three families of similar mixed-valence complexes. The value of AGs may be calculated by recognizing that it stems directly from the statistical distribution of the comproportionation equilibrium, which implies Kc = 4 for a symmetrical system, and which... 

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