Radical Anions in Solution

Thus, ketone enolates easily substitute chlorine in position 2 of the electrophilic nucleus of pyrazine (1,4-diazabenzene), and even in the dark, the reaction proceeds via the Sj l mechanism (Carver et al. 1981). It is expected that the introduction of the second chlorine in the ortho position to 4-nitrogen in the electrophilic nucleus of pyrazine promotes the ion-radical pathway even more effectively. However, 2,6-dichloropyrazine in the dark or subjected to light reacts with the same nucleophiles by Sr.,2 and not S nI mechanism (Carver et al. 1983). The authors are of the opinion that two halogens in the pyrazine cycle facilitate the formation of a-complex to the extent that deha-logenation of anion-radicals in solution and a subsequent nucleophilic attack of free pyrazine radical become virtually impossible. Thus, the reaction may either involve or exclude the intermediate a-complex, and only special identification experiments can tell which is the true one. 

TABLE 7.3. Relative Stabilities of Substituted Benzene Anion Radicals in Solution ... 

Figure 3 EPR spectra of radicals (A) benzoquinone anion radical in solution (B) the nitroxide TEMPOL in solution of photosynthetic reaction centres.

Another example is the m-dinitrobenzene anion radical in aqueous solution,11,12 where the effect is due to asymmetric solvation (one nitro group solvated, the other not), an effect very similar to that with dinitrodurene anions. In this case the mean lifetime of one solvation state was 0.8 ps at 291 K and 4.5 ps at 282 K. Still more examples are mentioned in the reviews by Atkins in the early 1970s.13... 

Rathore et al. (2006) studied the intramolecular single-electron transfer in anion-radicals formed from fluorenylidene derivatives. The derivatives used for the reduction were Me— Flu—CH2—Flu—CH2—Flu—CH2—Flu—Me and its deuterated analog, Me—Flu—CH2—(Flu-d8)— CH2—(Flu-dj)—CH2—Flu—Me. Each parent compound initially gave an anion-radical in which an unpaired electron was tunneled between the two internal Flu nuclei and then occured within the outer Flu nuclei. In the outer part, coordinative solvation of the anion-radical by HMPA proceeded much more effectively because of ready space accessibility. Such a solvation provides a driving force for electron tunneling. As the solution electron affinities of perdeuterated aromatic hydrocarbons are less than those of perprotiated hydrocarbons, the electron tunneling was found to be at least an order of magnitude faster only in the case of [Me—Flu—CH,—(Flu-do)—CH,— (Flu-d8)-CH2-Flu-Me]-. ... 

Fig. 3a,b First-derivative electron spin resonance benchmark spectra of a DNA anion radical in frozen 7M LiBr aqueous solution and of b one-electron reduced (MX ) in frozen 7M LiBr aqueous solution. The three markers are each separated by 13.09 G. The central marker is at g=2.0056 [7a]. Reprinted with permission from the J. Phys. Chem. Copyright (2000) American Chemical Society... 

Dissolved solids are those which are in solution and cannot be removed by filtration. The major dissolved materials in water are silica, iron, calcium, magnesium and sodium. Metallic constituents occur in various combinations with bicarbonate, carbonate, sulfate and chloride radicals, in solution these materials divide into their component parts called ions, which carry an electrical charge. The metal ions carry a positive charge and are referred to as cations. The bicarbonate, carbonate, sulfate and chloride ions are negatively charged and are referred to as anions. 

Chatgilialoglu C, Castelhano AL, Griller D (1985) Structures and optical absorption spectra of some sulfuranyl radicals in solution. J Org Chem 50 2516-2518 D Alessandro N, Bianchi G, Fang X, Jin F, Schuchmann H-P, von Sonntag C (2000) Reaction of superoxide with phenoxyl-type radicals. J Chem Soc Perkin Trans 2 1862-1867 Das S, von Sonntag C (1986) Oxidation of trimethylamine by OH radicals in aqueous solution, as studied by pulse radiolysis, ESR and product analysis. The reactions of the alkylamine radical cation, the aminoalkyl radical and the protonated aminoalkyl radical. Z Naturforsch 41b 505-513 Das S, Schuchmann MN, Schuchmann H-P, von Sonntag C (1987) The production of the superoxide radical anion by the OH radical-induced oxidation of trimethylamine in oxygenated aqueous solution. The kinetics of the hydrolysis of (hydroxymethyl)dimethylamine. Chem Ber 120 319-323... 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

The first two processes are common reactions for complexes in their intraligand singlet or triplet (it, 7t ) excited states. Since excited molecules are both better reductants and better oxidants than those in the ground state [1], the same complex can be oxidized to its ic-cation radical (when quinones, nitroaromatics, halocarbons, viologenes or other oxidizing agents are present) or reduced to its n-anion radical (in the presence of a reductant, e.g. NAHD or l-ascorbic acid). The radicals formed are usually unstable species the anion radical [SbO(UroP )]6- formed at a photolysis of SbO(UroP)5- in alkaline solutions is one of the rare exceptions [216]. 

The synthesis of metal graphite compounds is carried out by the reduction of layer graphite compounds and metal chlorides by using different reducing agents such as, for instance, solutions of aromatic anion radicals in THF, metallic sodium in liquid ammonia, complex boron and aluminum hydrides, and vapor or liquid potassium. As a result, the complex compounds containing one (Sn, Co, Mn, Cr, Mo, W, Pd) and two (Fe-Mo, Fe-W, or Mo W) metals were synthesized. 

The outline of the remainder of this contribution is as follows. In Section 3.7.2, we discuss radical anion dissociation in solution, in which a conical intersection has an important impact on the ground state reaction barrier, rate constant and reaction path, all of which are also influenced by nonequilibrium solvation. The excited electronic state conical intersection problem for the cis-trans isomerization of a model protonated Schiff base in solution is discussed in Section 3.7.3, focusing on the approach to, and passage through, the conical intersection, and the influence of nonequilibrium solvation thereupon. Some concluding remarks are offered in Section 3.7.4. We make no attempt to give a complete discussion for these topics, but rather focus solely on several highlights. Similarly, the references herein are certainly incomplete. We refer the interested reader to refs [1-9] for much more extensive discussions and references. 

Electronic absorption spectra have been reported for a variety of diazine anion-radicals in both liquid and solid solution.99 163,166 Polarographic reduction potentials have been measured for a wide range of azines and related to Hiickel LUMO energies.167... 

The first experimental hint for a finite lifetime of an alkyl halide anion radical in homogeneous etheral solution was provided by Garst and coworkers in 1977. From the reduction of 5-hexenyl chloride, bromide and iodide with disodium tetraphenylethylene in 2-methyltetrahydrofuran (MTHF) at 20°C they concluded that radical anions R-X Na were involved and that the order of their stability was R-I Na" >R-Br Na > R-CrNa. ... 

Free radicals are well known to add on activated olefins [241], Thus, in electrochemistry, the indirect formation of a free radical in solution may happen concomitantly by reaction on the mediator inside a solvent cage. Two examples follow. First is the indirect reduction of RX types compounds by the radical anion of a spin marker [242] such as a nitroso derivative. In this case the reaction is monoelectronic, since the nitroxyde radical is totally electroinactive (note that a nucleophilic action of the radical anion cannot be totally excluded (Scheme 46). 

Short-lived free radicals derived from aromatic amines can be generated and detected by the ETSF method. Long-lived free radicals in solution, such as tris(4-bromophenyl)amine radical cation (129), react with aromatic amines to yield short-lived radical anions (130a-d), as shown in equation 20. Solutions of 129 are generated electrolytically and... 

MV+ or DQ+ radicals incorporated in the cavities of zeolites-Y exhibit EPR spectra analogous to those of the radicals in solution, consisting of a broad single line (A//pp j 17 G) with g-factor values of 2.0030 (MV+ ) and 2.0029 (DQ+ ).24 Although incorporated in the zeolite cavity, the radicals react with dioxygen at -78 °C to form the superoxide radical anion through an electron-transfer reaction, and the spectra are replaced by the anisotropic spectra of the 02 radical anion interacting with either MV2+ (g/ = 2.084 and g2,3 = 2.005)-or DQ2+... 

---