Radical anion dimerization

Hydroxyl radicals were generated radiolytically in NaO-saturated aqueous solutions of thiourea and tetramethylthiourea. Conductometric detection showed that HO and a dimeric radical cation were produced. The dimeric radical cation is formed by addition of a primary radical to a molecule of thiourea. In basic solution, the dimeric radical cation decays rapidly to a dimeric radical anion, which is formed via neutralization of the cation and subsequent deprotonation of the neutral dimeric radical (Scheme 16). This was not observed in tetramethylurea. These dimeric radical cations of thiourea and tetramethylurea are strong oxidants and readily oxidize the superoxide radical, phenolate ion, and azide ion. 

Figure 4 (a) Singly occupied molecular orbital (SOMO) of dimer radical anion of acetonitrile... 

While it was initially suggested that anion-1 and anion-2 are the monomer and the dimer radical anions of acetonitrile [53], respectively, more recent work suggests that anion-1 cannot be a monomer anion (which in any case has a dilferent absorption... 

Figure 5 (a) Typical end-of-pulse absorption spectra obtained in pulse radiolysis of room temperature liquid acetonitrile (7-nsec fwhm pulse of 20 MeV electrons). The 500-nm peak is from anion-2 (dimer radical anion) the 1450-nm peak is from anion-1 (cavity electron), (b) Energy diagram and sketches of anion-1 and anion-2 (see the text). 

While anion-2 is clearly the dimer radical anion of acetonitrile, identification of anion-1 as a cavity electron requires caution. First, we stress that anion-1 cannot be the monomer anion of acetonitrile. The monomer anion does not absorb in the NIR [30,46,52]. For the monomer anion to occur at all, the neighboring acetonitrile molecules should all be oriented in the same direction, as in p-acetonitrile otherwise, coupling to a neighboring (antiparallel) molecule reduces the overall energy and causes instant dimer formation. It is difficult to see how such a fortuitous orientation could persist for 0.3-3 nsec in a room temperature liquid. Also, it is not clear why a monomer anion would... 

At room temperature, the cyclic voltammogram features an irreversible two-electron reduction (peak a) with oxidation of the monomeric anion evident on the return sweep (peak b). As the temperature is lowered, a new peak (peak c) grows in on the positive-going scan. This peak, due to oxidation of the dimeric radical anion to starting material, grows at the expense of peak b until the latter is entirely absent at -76°C. These results are qualitatively in accord with... 

Formation of Inorganic Radicals and Their Dimeric Radical Anions 89... 

Table 5.1. Compilation of equilibrium constants of some dimeric radical anions...

Dimeric radical anion Equilibrium constant/dm3 mol-1 Reference... 

A number of studies are concerned with the free-radical reactions of typical nucleobase lesions. For example, the cyclobutane-type Thy dimer can be split by one-electron reduction [Heelis et al. 1992 reactions (307) and (308)], a process that is relevant to the repair of this typical UV-damage by the photoreactivating enzyme (photolyase, for a review see Carrell et al. 2001, for the energetics of the complex reaction sequence, see Popovic et al. 2002). At 77 K, the dimer radical anion is sufficiently long-lived to be detectable by EPR (Pezeshk et al. 1996). 

An analogous stereoelectronic influence on the rate of the reductive repair process was also observed by Carell et al. [28]. The ring opening of the dimer radical anion also proceeds stepwise, but with the C(5)-C(5 ) bond being broken first. The C(5) and C(5 )-methyl groups of thymine-derived dimers, which were found to be repaired more slowly than the uracil-derived dimers, lead to distortion of the geometry, which results in deceased overlap of the n C(4)-0(4) orbital with the a C(5)-C(5 ) orbital. 

The spin density in the dimer radical anion is concentrated mainly at one azolyl cycle because of uncoplanarity of the cycles [851],... 

The halogen and pseudohalogen dimer radical anions, X2 (X = Cl, Br, I, SCN), react efficiently with phenols and phenolate ions to give the corresponding phenoxyl radicals (equation 6). 

Ph.CH CH2 Ph.CH.CH2.CH2.CH.Ph or react with monomer, yielding then dimeric radical-anions. 

Most likely is smaller than the propagation constant, i,e, k < 250 M sec l, However, since the concentration of the monomer is at least 10 times larger than that of the monomeric radical-anions, the addition competes efficiently with the dimerization. Nevertheless, the resulting dimeric radical-anions, M.M , play no role in the pol3nnerization because their diffusion controlled disproportionation (rate constant 10 M lsec" ) destroys them as soon as formed. Hence, radical propagation is imperceptible in such systems. 

Based on the known chemistry of flavin photolysis reactions, it appears unlikely that thymine dimer cleavage occurs via a direct energy transfer mechanism (160). One proposal suggests that in the model reaction with 1-deazariboflavin, the thymine dimer radical anion is formed via electron donation from the excited sensitizer (164). Alternatively, electron abstraction by the excited flavin could occur, resulting in the thymine dimer radical cation (159, 160), although it is unlikely that reduced flavin would act as an electron acceptor. A schematic for this mechanism is illustrated in Scheme 33, where the initial formation of a sensitizer-dimer complex is consistent with the observed saturation kinetics. The complex is activated by excitation of the ionized sensitizer (pH > 7), and electron donation to the dimer forms the dimer radical anion and the zwitterionic, neutral 1-deazariboflavin radical (162). Thymine dimer radical would spontane-... 

An alternate mechanism involving reduced FAD as sensitizer [pathway B] indicates that an electron is directly donated to the pyrimidine dimer, forming FADH and dimer radical anion. Pyrimidine dimer radicals are unstable (157) and will spontaneously decompose to the monomer and monomer radical anion. The latter species then regenerates reduced FAD in a manner similar to that described for pathway (A). 

Kinetics of the forward and backward reactions reveal that the decomposition of the dimeric radical-anion, viz. 

With the model systems used in these investigations, dimer splitting can be induced both by reduction and by oxidation, depending on the sensitizer. While it was inferred from the existence of dimer radical anions D that splitting via the former route occurs in two steps, it has been debated for some time whether this also holds for the latter route, that is, whether dimer oxidation and cleavage are concerted or successive. CIDNP spectroscopy is particularly well suited to answer such questions because the intermediates leave their EPR spectrum (their polarization pattern) in the products. Thus, not only can the intermediates be identified by this signature, even if they are rather short lived, but the occurrence of their polarizations in a product... 

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