Radicals anion

Radical Anions.—Mention has already been made of the reverse cycloaddition which occurs on reduction of (66) to (67) and which might be a concerted allowed process in the radical anion. Another example is reported. The radical anion (426) is made by reduction of the hydrocarbon with Na-K alloy at —110 °C. On warming to — 20°C it rearranges to the radical anion (427) of pleiadene—perhaps a [ 2 -I- 2J cyclo-reversion allowed because of the extra electron.  

A large number of radical ions have been studied by ESR since its applications to free radical chemistry started in the middle of the 1950s. They were generally prepared in the liquid phase by redox (reduction-oxidation) reactions, electrolysis or photolysis, and rather stable radical ions possessing conjugation system such as aromatics have been studied. The results have been summarized, for example, in a book Radical Ions by Kaiser and Kevan [30]. Since the middle of the 1960s a 

We do not intend to show full coverage of the ESR studies of radical anions reported so far, but to show some typical examples and to demonstrate general trends. Thus, here we present our recent ESR studies combined with quantum chemical computations on electron delocalization in perfluoroalkane radical anions [3, 31, 32] and on structure distortion in perfluoroalkene radical anions [33] and in the acetylene radical anion [34]. 

Radical anions of c-CnF2n (n = 3-5) were generated and stabilized in y-irradiated (plastically crystalline) tetramethylsilane (TMS) and (rigid) 2-methyltetrahydrofuran (2-MTHF) matrices. The identification of radical anions was confirmed by generating the identical ESR spectra in photoionization 

Higher-order hyperfine splitting ESR transitions with isotropic hyperfine (hf) and g values can be expressed as follows [36b]  

Radical anion Matrix T/K g value (State) = References 

In addition to formation of radical anions via process (2), they can sometimes also be generated by the dissociative resonance capture process 

Many other H27 abstractions by O from a single carbon atom can be explained in a similar way. For example, 0T abstracts a proton from the methine position of cyanocyclopropane to give the (M — H)- ion, but abstracts H2 from one of the corners of the ring to generate the (M — H2)t ion as shown by deuterium labelling (Dawson and Nibbering, 1980). Abstraction of H2 from different carbon atoms would lead in this case either to the radical anion of 1-cyanocyclopropene or to that of 3-cyanocyclopropene. By comparison with the radical anion of acrylonitrile, 

The fact that interesting radical anions can be generated with Ov is further demonstrated by the recently published observation of the H2Ot ion (de 

Finally, it must be noted that in a few cases radical anions have also been observed to be generated from even electron anions One example concerns the CID loss of a methyl radical from the (M — H) ion of methoxyaceto-nitrile (Dawson and Nibbering, 1980) as shown in (80). The capto-dative character (Viehe et al., 1979 Crans et al., 1980) of the generated radical 

1 Structures. - A very comprehensive review of the literature on atomic and molecular electron affinities has been published142. The possibilities of per-fluoroaroinatic molecules as electron acceptors has been examined using a DFT 

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M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

Zhou F and Bard A J 1994 Detection of the electrohydrodimerization intermediate acrylonitrile radical-anion by scanning electrochemical microscopy J. Am. Chem. See. 116 393... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

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...

For a limited range of substances, negative radical anions (M ) can be formed rather than positive ions (Equation 3.3). Negative radical anions can be produced in abundance by methods other than electron ionization. However, since most El mass spectrometry is concerned with positive ions, only they are discussed here. 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

Gain of an electron by a molecule gives a (negative) radical anion (M ). 

The kind of reaction which produces a dead polymer from a growing chain depends on the nature of the reactive intermediate. These intermediates may be free radicals, anions, or cations. We shall devote most of this chapter to a discussion of the free-radical mechanism, since it readily lends itself to a very general treatment. The discussion of ionic intermediates is not as easily generalized. 

These green radical ions react with styrene to produce the red styrem radical anions ... 

On the basis of these observations, criticize or defend the following proposition Regardless of the monomer used, zero-order Markov (Bernoulli) statistics apply to all free radical, anionic, and cationic polymerizations, but not to Ziegler-Natta catalyzed systems. 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Several alternative methods followed this early work. In one, aromati2ation is effected by treating the ketal of androstadienedione with the radical anion obtained from lithium and diphenyl in refluxing tetrahydrofuran. Diphenylmethane is added to quench the methyllithium produced from the... 

For example, naphthalene radical anion L -I with counterion (Li, ). 

Aromatic Radical Anions. Many aromatic hydrocarbons react with alkaU metals in polar aprotic solvents to form stable solutions of the corresponding radical anions as shown in equation 8 (3,20). These solutions can be analyzed by uv-visible spectroscopy and stored for further use. The unpaired electron is added to the lowest unoccupied molecular orbital of the aromatic hydrocarbon and a... 

Sodium naphthalene [25398-08-7J and other aromatic radical anions react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions. Although the equihbtium (eq. 10)... 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

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). 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

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