Quinoid radicals

The computational characterization of the structures and spectroscopic properties of the quinones and quinoidal radicals generated during the course of photosynthetic charge separation form the subject of this review. The thermodynamics of quinone reduction has been reviewed elsewhere(i) and reviews discussing the strengths and limitations of different computational methods and basis sets are also available. (4, 5)... 

Although quinone structures and short, non-covalent contacts between quinones and proteins are available from X-ray diffraction structures, analogous information for the quinoidal radicals usually must be inferred indirectly from spectroscopic data. The primary spectroscopic methods used to infer the structures, side-chain conformations, and intermolecular contacts of quinoidal... 

TESTS OF COMPUTATIONAL METHODS FOR CALCULATING PROPERTIES OF QUINOIDAL RADICALS... 

CALCULATED PROPERTIES OF QUINOIDAL RADICALS IMPORTANT IN PHOTOSYNTHESIS. 

Wheeler, R. A. "Quinones and quinoidal radicals in photosyntfaesis,"7%eoret. Comput. Chem. 2001,9,655-690. 

The orientation of the second phenolic unit in the usnic acids resembles that of all the known dibenzofuran derivates in that the position involved in the carbon-carbon coupling is meta to a carbonyl substituent and ortho and para to 0-substituted positions. It differs from the dibenzofurans in having a second ortho-hydroxyl substitoent instead of an alkyl group. Dibenzofuran derivates could form from the methylphloroacetophenone precursor of usnic acid by a change in the orientation of the C ring with respect to the ether linkage, and such an orientation would still allow a para-quinoid radical. 

Occasionally, equilibria between a quinoid and a diradicaloid form of tetraazafulvaleiies of type 77 have been discussed (66AG303 72NKK100 79JOC1241). Based on ESR measurements, only traces of radicals (0.1% at 200°C) could be observed and therefore 77 (Ar = Ph) exists at room temperature predominately in the quinoid structure. Other authors stated that the thermochromism of 77 mainly results from a change in intermolecular interaction, not from biradical formation (84MI1030). 

One of the most important properties of quinoid compounds is the two step redox reaction. Quinoid compounds undergo one electron reduction to so-called semiquinone anion radicals, and further one electron reduction of semiquinone anion radicals gives dianions (Scheme 14). 

Perhaps due to oxidizing quinoid type electronic structure of benzotriazol-2-yl derivatives, some of their properties are completely different from those of isomeric benzotriazol-l-yl derivatives. Thus, anions derived from 2-alkylben-zotriazoles 388 are rapidly converted to appropriate radicals that undergo coupling to form dimers as mixtures of racemic 289 and meso 390 forms <1996LA745>. When the reaction mixture is kept for an extended period of time at —78 °C, (Z)- 391 and (E)- 392 alkenes are formed. When benzophenone is added to the reaction mixture, alcohols 387 are obtained in good yields however, benzaldehyde does not react under these conditions (Scheme 63). 

To understand the overall biological activity of quinoid compounds, it is necessary to study the chemical properties of these substances. Inasmuch as the cellular damage that is induced by quinones resembles that seen after radiolysis, the most prominent reactions involving quinones are probably DNA damage and generation of oxygen free radicals. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

Acceptor substituents in nitrophenyl anion-radical frequently form quinoid structures. Donor substituents cannot participate in the formation of such structures. As to electron spectroscopy, it is very sensitive to changes in the electron structure of a chromophore system. The influence of acceptor groups is, therefore, stronger than that of donor groups. If changes in chromophore systems are absent, the method of spectrophotometry remains relatively less informative. 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

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