Radicals, reduction singlet

The practical significance of the reaction depicted in Scheme 7.43 consists of the development of novel antioxidants carrying chalcogen atoms. Divalent organochalcogen compounds react readily with many types of oxidants (peroxide, peroxyl radicals, peroxynitrite, singlet oxygen, and ozone). The tetravalent organylchalcogenides formed are reduced by many mild reductants. Therefore, compounds of this sort have the potential to act as catalytic antioxidants. 

Cr(VI) is reduced intracellularly by a wide variety of enzyme systems microsomal (Alexander et al. 1986), mitochondrial (Rossi and Wetterhahn 1989), and cytosolic (Banks and Cooke 1986). It also is reduced nonenzy-matically. Reactive moieties generated in these reduction processes include Cr(V) (Rossi and Wetterhahn 1989), hydroxyl radicals, and singlet oxygen (Kawanishi et al. 1986). 

The latter system employs electrons from NADPH for the univalent reduction of oxygen to superoxide radical (SOR), and is under the influence of the enzyme NADPH oxidase. Subsequent reactions lead to the production of hydrogen peroxide, hydroxyl radicals, and singlet oxygen, all of which possess antimicrobial activity... 

Burke M, Edge R, Land EJ, McGarvey DJ, and Truscott TG. 2001. One-electron reduction potentials of dietary carotenoid radical cations in aqueous micellar environments. FEBS Letters 500(3) 132-136. Bystritskaya EV and Karpukhin ON. 1975. Effect of the aggregate state of a medium on the quenching of singlet oxygen. Doklady Akademii Nauk SSSR 221 1100-1103. 

According to Hercules 5> a measure of the relationship between direct excitation of the first excited singlet state by radical-ion recombination and triplet-triplet annihilation is the entropy factor FAS, estimated to be on average 0.2 eV. The enthalpy of the radical cation-radical anion recombination can be measured as the difference between the redox potentials 1/2 Ar—Ar (oxidation) and 1/2 Ar—Ar<7> (reduction). This difference has to be corrected by the entropy term. If this corrected radical-ion recombination enthalpy is equal to or larger... 

The ground state (0 kJ/mol) for the CL molecule is represented by the term symbol 3v . The first excited state (92 kJ/mol above the ground state) is a 1 singlet (electrons spin paired with both electrons in either the n x or the n y level). The 1 v state with paired spin electrons, one each in the 7i v and n y levels, is the next excited level 155 kJ/mol above the ground state. Reduction of 02 by one electron yields the superoxide ion (02), a radical anion. Reduction by two electrons yields the peroxide ion, (02 ). Bond lengths and bond orders for these are given in Table 4.2. As noted in equation 4.2, the reduction potential for 02 in the presence of protons is thermodynamically favorable. Therefore, reversible binding of O2 to a metal can only be achieved if competition with protons and further reduction to superoxide and peroxide are both controlled.8... 

The first of these radical ions was the stannylene radical anion, [(Mc3Si)2CH]2Sn, which was obtained by the reduction of the stannylene with sodium in THF. In the ESR spectrum, it showed a broad singlet withg= 2.0177 and... 

Figure 5.2. Pathways of oxygen reduction. The sequential reduction of molecular oxygen, ultimately to water, is shown. Abbreviations 02 % superoxide free radical H202, hydrogen peroxide OH, hydroxide ion -OH, hydroxyl free radical e, electron H+, hydrogen ion. Singlet states of oxygen are also shown as i +02and Ag02.

The photochemical reduction of 1-methylquinolinium ions by (TMS)3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water-acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation-quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 

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