Semiquinone -charge

Most quinone reductions go through an intermediate radical or semiquinone stage, usually revealed by a one-electron step in the redox potential.100 The radical formed by the reduction of compound VI is especially stable, probably because of the additional involvement of the benzoyl group.101 The ordinary semiquinones are more stable in basic solution since some of the resonance structures of the neutral radical involve separation of charges. 

Radicals in which the odd electron is on a nitrogen next to an aromatic ring are stabilized by resonance analogous to that of tri-phenylmethyl. In the case of Wurster s salts, the nitrogen analogs of semiquinones, there are two equivalent resonance structures in the acid form, but in the less stable basic form one of the structures requires separation of charge. Evidence for the unpaired electron has been obtained by measurement of the paramagnetism.144... 

C-P-Q is converted to the charge separated diradical state C h-P -Q-. Then an electron is transferred from Q to the lipid-soluble 2,5-diphenylbenzoquinone Qs 133b yielding Qs. In the third step uncharged semiquinone QsH is formed when the latter accepts a proton from the external aqueous solution. The basic function of a proton shuttle is carried out when the semiquinone radical diffuses through the membrane in the fourth step. It is then oxidized to Qs + by the carotenoid radical cation... 

Stade observed an interesting oxidation with tetrachloro-p-benzoquinone. In methylene chloride an intense red coloration appears, but no signal in the ESR spectrum. Apparently only a charge-transfer complex 61 is formed, without electron transfer. A similar observation has been made in the reaction of N, N, N, N -tetramethyl-p-phenylenediamine with tetrachloro-p-benzoquinone in non-polar solvents Here, as in our case, electron transfer does not take place until a polar solvent such as acetonitrile is added. The ESR spectrum initially shows the doublet of 55 (23,2 Gauss) overlapping with the sharp sin et of tetrachloro-semi-quinone 62 (which has a somewhat smaller g factor). The semiquinone signal slowly disappears until finally only the doublet of 58 remains. The following scheme summarizes the reaction course ... 

FIGURE 2. Zinc(II) porphyrin-o-quinone dyads and two typical reactions (a) photoinduced charge separation, (b) chelation of in situ formed semiquinone anions. PQ = porphyrin quinine M = metal... 

These violet substances, like semiquinones in general, are deeply colored. The color is correlated with resonance involving the transfer of electric charge from one end to another of a large molecule, as in the triphenylmeth ne dyes and other deeply colored substances.62... 

It has been noted (79JCS(Pl)2563) that chromenes form adducts with DDQ and that these may be produced during attempts to dehydrogenate chromans. It is proposed that the charge transfer complex between the chromene and DDQ breaks down by a one-electron process to give the semiquinone radical (129) and the phenoxyl radical (130). The p-quinoneallide (131) which results rearranges to the 3-substituted chromene (Scheme 14). 

The reduction of a quinone requires two electrons, and it is possible that these electrons could be transferred either together or one at a time. The product of a single-electron transfer leads to what appropriately is called a semiquinone, 1, with both a negative charge and an odd electron (a radical anion) ... 

In some early applications [96] to ETs involving compounds with quin-one-like compounds Q, QA Q -A Q=, it was necessary to examine some data on the formation constants of the semiquinone QH, Q + QH2 — 2QH (The H is typically attached to an O or an N.) My impression, after looking at available data, was that the equilibrium constant was approximately the same, provided all three species had the same charge. Looking at the structures, one could see that every atom in a molecule on the left in this reaction corresponded to one on the right that had the same nearest and next nearest neighbor. I then looked at many examples of other pairs of compounds, which I termed conformal pairs and found that the total of heat of combustion of a pair was approximately the same as its conformal pair [97]. 

Although charge transfer in macromolecules is of fundamental interest and has been studied piecemeal, often with model systems, this field is in its infancy. DNA and RNA complexes have been discussed in Section III. D. The oxidized and reduced forms of flavin mononucleotide produce charge-transfer complexes as precursors of the semiquinone free radical... 

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