Radical anion Subject

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

Dopamine (10) has also been the subject of some study. Maity and coworkers17 have studied the pulse radiolysis or /-irradiation induced reduction of the protonated form. In this instance the addition of an electron affords the radical anion 11 with a bimolecular rate constant for the reaction of 2.5 x 108 M-1 s-1. 

It is obvious that the isobutene unit provides a good starting point for the generation of trimethylenemethane radical anions. However, even isobutane units can be used to produce these more highly unsaturated species. In a preliminary work aimed at two-and three-fold deprotonation processes in solution, Kuck, de Meijere and coworkers165 have subjected triquinacene (41) and the tribenzotriquinacenes 44 to NCI conditions with... 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

The diffuse reflectance spectra reported in Table I show that it is possible to assign a CN stretching frequency to both neutral and radical-anion TCNQ in crystalline samples of metal-TCNQ complexes because the reflectance peak for neutral TCNQ is shifted 20 cm higher in frequency than for radical-anion TCNQ". Specifically, the reflectance data for Cu-TCNQ when compared to other metal-TCNQ salts of known composition strongly suggests that neutral TCNQ is not present in the unswitched Cu-TCNQ films. On the other hand, the additional peak that appears in the spectra of Cu-TCNQ subjected to an applied field shows a peak superimpos-able with the peak recorded for neutral TCNQ in Cs2(TCNQ" )3. 

The carbon-poor sulfur-nitrogen bicyclic [l,2,5]thiadiazolo[3,4-f][l,2,5]thiadiazole 6, its radical anion 7, and its dianion 8 have been the subject of a number of theoretical studies <1997JP033, 2004JA11202, 2005IC7194>. 

The presence of radical anions as intermediates has been clearly established in many organic reactions and has been the subject of theoretical studies due to their importance in different types of organic reactions including DNA dam-agei8t,i82 jn these intermediates, the it-acceptor can be linked to the C-X bond... 

Although ASCF methods are more likely to be successful, it is critical that diffuse functions be included in the basis set so that the description of the radical anion is adequate with respect to the loosely held extra electron. In general, correlated methods are to be preferred, and DFT represents a reasonably efficient choice that seems to be robust so long as the radical anion is not subject to overdelocalization problems. Semiempirical methods do rather badly for EAs, at least in part because of their use of minimal basis sets. 

Electrochemical and Electrocatalytic Reduction. The one-electron reduction of C02 yields the radical anion C02-, which reacts with an H source to give the formate ion. The reaction, however, is not selective because various other reactions may take place. An alternative and more promising approach is the two-electron reduction of C02 in the presence of a proton source to afford formic acid. The latter process requires a considerably lower potential (—0.61 V) than does the one-electron reduction (—1.9 V) consequently, the electrolysis in the presence of catalysts may be performed at lower voltages. The control of selectivity, however, is still a problem, since other two-electron reductions, most importantly reduction to form CO and H2, may also occur.101127 The reduction of C02 to CO, in fact, is the subject of numerous studies. Electrochemical and electrocatalytic reductions of C02 in aqueous solutions have been studied and reviewed.11,128-130... 

Before getting into the subject, classifying in accordance with their process and mechanism the electrolytic initiation reactions which have appeared in the literature will afford the reader a better understanding. The reactions are classified firstly into two types cathodic and anodic. For the cathodic reaction, generation of free-radical and radical-anion, and formation of unstable monomer and active catalyst are visualized from the corresponding references. 

Esr spectra are subject to exchange effects in the same way as nmr spectra. A specific example is provided by electron exchange between sodium naphthalenide and naphthalene. Naphthalene has a set of ten 77-molecular orbitals, similar to the six 7r-molecular orbitals of benzene (Figure 21-5). The ten naphthalene it electrons fill the lower five of these orbitals. In a solvent such as 1,2-dimethoxyethane, which solvates small metal ions well, naphthalene accepts an electron from a sodium atom and forms sodium naphthalenide, a radical anion ... 

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