Semiquinone formation constant

Table 1. Potentials Ej and EjCV) of i in acetonitrile vs. Ag/AgCl/CH N. = Semiquinone formation constant ( ) = irreversible...

Table 11. Comparison of semiquinone formation constants K m of iso-7r-electronic redox systems of the general types A, B and C in DMF versus Ag/AgQ in acetonitrile...

Semiquinone Formation Constant.—It was assumed in the foregoing treatment that the two stages of oxidation are fairly distinct, but when this is not the case the whole system behaves as a single two-electron process, as in Fig. 79. In view of the interest associated with the formation of semiquinone intermediates in oxidation-reduction reactions, methods have been developed for the study of systems in which the two stages may or may not overlap. The treatment is somewhat complicated, and so the outlines only will be given here. ... 

If actual oxidation-reduction measurements are made on a particular system during the course of a titration, it is possible, by utilizing the equation from which the data in Fig. 85 were calculated, to evaluate the semiquinone formation constant for that system. The standard potentials Eij Ei and E can also be obtained for the hydrogen ion concentration existing in the experimental solution. 

Dithioacetic acid derivatives add to 1,4-benzo- or 1,4-naphthoquinones to give, after oxidation of the adduct with silver oxide or chloranil, the quinones 217 and 218 (69LA103). Quinones 218 were prepared also from 2,3-dichloro-1,4-naphthoquinone and salts of dithiocarbamic acids (51JA3459) and those of type 219 by oxidation of the corresponding hydroquinones. From reduction potentials and the semiquinone formation constants, it was concluded that their anion radicals are thermodynamically stable (86CC1489). 

Further, the semiquinone formation constant , i.e. the propensity for mixtures of quinone and hydroquinone to form semiquinone radicals, equation (10), is related to the reduction potentials by equation (11) [11] ... 

O2 ( h202.2) to the positive side and of the first one-equivalent oxidation of electron donors to the negative side, as shown in Figure 1. The Em of these molecules in the bound state on the oxidase will be the same as in the free state provided the aflSnities of the oxidase for the reduced and oxidized forms are equal. The one-order-of-magnitude difference between these affinities will make 0.03 volt difference in the normal potentials (Em) between the free and bound state. It may be said that hydrogen donors and O2 are activated by the oxidase when it stabilizes the intermediates and increases the semiquinone formation constant, K. An increase in Ks results in a rise of Eo and a drop of Ei. 

All 3,7-diaminophenothiazines display two reversible oxidations with remarkable semiquinone-formation constants. The potentials can be fine-tuned by the electronic nature of the remote substituents on the / -substituted aniline moieties. All these compounds qualify as weakly luminescent and highly suitable hole conduction molecules for moleculcu- electronics applications or charge transport layers in organic light-emitting diode (OLED) and organic fidd-effect transistor (OFET) devices. 

Polarographic investigations showed that systems of type 112 display two-step polarograms from which the semiquinone formation constants k = 10 - can be derived. In the series of benzoxazoles and benzothiazoles,... 

Their semiquinone formation constants are derived from their two-step polarograms. Their preparation, chiefly by quaternization of 2,2 -bis-benzo-thiazole, was described in detail. ... 

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