Organic radical anions, reactions

With very few exceptions, organic compounds do not form stable products following single-electron transfer reactions thus, as a rule, the primary products are unstable organic radical anions or radicals. The final products of the e q reactions will therefore be the result of subsequent reactions of these primary products. 

Beside the electron transfer from the semiconductor to adsorbed molecular oxygen also the direct transfer to an organic molecule is possible. This type of photocatalytic reaction, yielding an organic radical anion, has been found to occur with 1,4-benzoquinone [18], tetrachloromethane [19], and several nitroaromatic compounds [20]. But electrons can also be transferred very efficiently to (adsorbed) metal cations [21]. 

Early studies of radiative charge recombination processes made use of chemical oxidation and reduction to create the reactive radical ion species. Despite the limitation of the use of chemically stable radical ions, a significant body of literature developed that discussed radiative charge recombination for cross-electron-transfer reactions. In particular, many studies involved reactions of organic radical anions with amine cation radicals [5, 15, 18, 19]. 

McClelland (1964) has reviewed the chemistry of the relatively stable organic radical-anions and has discussed the information obtained by e.8.r. about ion-pair formation, exchange reactions, and the effect of the cation on charge distribution. Carrington (1963) has discussed the properties of the singly occupied antibonding molecular orbital in aromatic radical-anions. This Section is concerned mainly with information derived from e.s.r. observations about the pathways involved in one-electron reductions. 

The question whether or not radical ions are formed upon irradiation of liquids and stabilized enough for detection or engagement in bimolecular chemical reactions has moved radiation chemists ever since the early days of this research field. This was particularly exciting with respect to low polarity solvents, but even for aqueous solutions conclusions had to rely mainly on indirect evidence. A real breakthrough came with the experimental discovery of the hydrated electron and other powerful one-electron reductants (e.g., a-hydroxyalkane radicals such as (CH3)2C OH). Applying these new tools a large number of organic radical anions were detected and characterized with respect to their optical and chemical properties, particularly by pulse radiolysis. 

In radiolysis reactions, excited molecules, cations, free electrons, anions, and radicals are the main intermediates. For the study of radical anions, cations and radicals formed in a solid matrix, e.g., in polymers, electron paramagnetic resonance (EPR) spectroscopy is used since the 1950s. Anion and cation species can also be studied by UV spectroscopy. Absorption spectra of many organic radical anions and cations were measured in tetrahydrofiiran or in halogenated hydrocarbon matrices (Shida 1988). 

Phys. 74 6746 (1981) b) G. L. Gloss, L. T. Calcaterra, N. J. Green, K. W. Penfield, and J. R. Miller, Distance, stereoelectronic effects, and the Marcus inverted region in intramolecular electron transfer in organic radical anions, J. Phys. Chem. 90 3673 (1986). a) S. Larsson, Electron transfer in chemical and biological systems. Orbital rules for nonadiabatic transfer, J. Am. Chem. Soc. 103 4034 (1981) b) S. Larsson, n Systems as bridges for electron transfer between transition metal ions, Chem. Phys. Lett. 90 136 (1982) c) S. Larsson, Electron transfer in proteins, J. Chem. Soc., Faraday Trans. 2 79 1375 (1983) d) S. Larsson, Electron-exchange reaction in aqueous solution, J. Phys. Chem. 88 1321 (1984) e) S. Larsson,... 

Figure 4, Correlations of anion, radical, and radical anion reactions with organic halides and the halide effect...

Figure C3.2.10.(a) Dependence of electron transfer rate upon reaction free energy for ET between biphenyl radical anions and various organic acceptors. Experiments were perfonned with the donors and acceptors frozen into...

When halide ions or anions such as thiocyanate or azide are present, these anions are incorporated into the organic radical generated by decomposition of the peroxide. This anion transfer presumably occurs in the same step as the redox interaction with Cu(II), and such reactions have been called ligand-transfer reactions. " ... 

Reactions involving a crossed anodic condensation are of practical interest when they combine organic radicals of different type. In solutions containing anions RCOO and R COO, condensation products of the type RR are formed together with the standard products RR and R R by the reactions described. 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

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