Benzene anion radical spectrum

Fig. 8.13 The first-derivative ESR spectrum of the benzene anion radical C6H. The relative intensities are 1 6 15 20 15 6 1.

Proton hyperfine splitting pattern in the ESR spectrum of the benzene anion radical. 

The anion-radicals from aromatic nitro compounds preserve the second-order axis of symmetry. The analysis of superfine structure of the ESR spectrum of the nitrobenzene anion-radical reveals equivalency of the ortho and meta protons (Ludwig et al. 1964, Levy and Myers 1965). With the anion-radical of nitrosobenzene, the situation is quite different. This was evidenced from the ESR data (Levy and Myers 1965, Geels et al. 1965). Following electron transfer, the bent nitroso group fixes in the plane of the benzene ring to a certain extent. This produces five different types of protons, since both meta and ortho protons become nonequivalent. The nonequivalence of the ortho and meta protons has also been established for the anion-radicals of acetophenone (Dehl and Fraenkel 1963) and 5-methylthiobenzoate (Debacher et al. 1982 Scheme 6.17). 

Biczok, L, Linschitz, H., and Walter, RL, Extinction coefficients of fullerene triplet and anion radical and one-electron reduction of the triplet by aromatic donors, Chem. Phys. Lett., 195,339,1992. Dimitrijevic, N.M. and Kamat, P.V., Triplet excited state behavior of fullerenes pulse radiol) is and laser flash photolysis of fullerenes (C60 and C70) in benzene, /. Phys. Chem., 96,4811, 1992. Bensasson, R.V., Hill, T., Lambert, C., Land, E.J., Leach, S., and Truscott, T.G., Pulse radiol) is study of buckminsterfuUerene in benzene solution. Assignment of the 60 triplet-triplet absorption spectrum, Chem. Phys. Lett., 201, 326, 1993. 

Fig. 12.2. EPR spectra of small organic free radicals, (a) Spectrum of the benzene radical anion. [From J. R. Bolton, Mol. Phys. 6 219 (1963). Reproduced by permission of Taylor and Francis, Ltd.] (b) Spectrum of the ethyl radical. [From R. W. Fessenden and R. H. Schuler, J. Chem. Phys. 33 935 (1960) J. Chem. Phys. 39 2147 (1963). Reproduced by permission of the American Institute of Physics.]...

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

In order to characterize electron acceptor (basic type) properties of the samples, tetracyano ethylene compound, known to be easily ionizable in TCNE radical anion, was introduced at room temperature in the samples outgassed at different temperatures up to 800°C. No ESR signal was observed. As steric hindrance could preclude the experiment, smaller molecules as SO and p-dinitro benzene were also introduced. Then too, no ESR spectrum could be detected although the ESR technique is extraordinarly sensitive. It may thus be concluded that the ZSM-5 and ZSM-11 materials did not exhibit electron donor (basic) properties as detectable by ESR. 

EPR spectrum of electrochemically generated benzene radical anion, C6H6-. The hyperfine interaction between the free spin and the six H1 nuclei generates a seven-line spectrum of nominal relative intensities 1 6 15 20 15 6 1. The hyperfine splitting constant is 0.375 mT. 

The radical anion of molecular oxygen (O ) has been prepared and trapped in a range of alcohols, water and benzene but not in aliphatic hydrocarbons (Bennett et al., 1968a). In contrast to COg the e.s.r. spectrum shows that 0 interacts strongly with its immediate environment. This interaction which alters the separation of the upper molecular orbitals of the anion is strongly dependent on the nature of the matrix. Previously, the Oj" radical ion has been stabilized only in ionic materials such as the alkali halides thus it is of particular interest to find that this anion can be trapped successfully in a non-polar matrix (benzene). There is some evidence (Evans, 1961), from optical spectroscopic studies that molecular oxygen can form a weak charge transfer complex with the 77-electron system in benzene and it seems probable that O2 is stabilized in benzene by the formation of a similar complex. 

For the benzene radical anion, one thus expects a seven-line pattern with intensity ratios of 1 6 15 20 15 6 1, in good agreement with the ESR spectrum shown in Fig. 2. 

For the molecular case, the essential conclusion is that the orbital must have some s (or cr) character for the impaired electron to interact with a magnetic nucleus. Consider however the case of the benzene radical anion, in which the electron is usually described as being in a tt orbital with a node in the molecular plane. As a consequence no coupling with the proton nuclei is expected, a prediction clearly in conflict with the hyperfme splitting of 3.75 gauss seen in the ESR spectrum of this species as shown in Fig. 2. Flow, then, does the unpaired tt electron density appear at the Ft nucleus ... 

Like the other aromatic substrates with electron-withdrawing groups, the radical anions derived from 1,3-benzenedicarbonitrile dimerize reversibly as indicated by the persistence of a—low-intensity—ESR spectrum of the radical anion [262 and refs, therein]. Results obtained by CV put a value of 10 M on K i, and also in this case kdim increases upon addition of water [262]. The reversible dimerization of the radical anions of dialkyl benzene-1,3-dicarboxylate (92a), dialkyl pyridine-1,3- dicarboxylate (92b), and their sulfur analogues (Y = S) [Eq. (58)] has been characterized by CV and ESR spectroscopy [263], and values of 10 —lO" M s and of kpreparative scale, a competing slow, but irreversible, first-order reaction gave the monocarboxylate anions as the sole product [263]. 

Bowers and Greene (1963) reported the e.s.r. spectrum of the radical-anion of cyclopropane and Bowers ealkali-metal reduction of the parent compound. However, Gerson et al. (1966) have found that none of these compoimds is reduced under these conditions (i.e. the e.s.r. signal due to the solvated electron is not quenched) and Jones (1966) has foimd that the signal from the supposed adamantane radical-anion is that of the benzene radical-anion. 

Certain aromatic nitro compounds which contain electronegative substituents undergo elimination when reduced. Fujinaga et al. (1964) found that the electrochemical reduction of several halogenonitro-benzenes gives initially the corresponding radical-anion whose e.s.r. spectrum is then replaced by that of the nitrobenzene radical-anion, e.g. 

Q The ESR spectrum of the benzene radical anion, [C6H(,], in I which the unpaired electron is in a molecular orbital delocalized round the benzene ring, shows a septet, with a coupling constant of 0.375 mT. Comment on the spectrum, in relation to the quartet I shown by the methyl radical, with n(C-H) = 2.30 mT. 

Unpaired electrons can be present in ions as well as in the neutral systems that have been considered up to this point. There are many such radical cations and radical anions, and we consider some representative examples in this section. Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals. Benzene and naphthalene are examples. The ESR spectrum of the benzene radical anion was shown earlier in Figure 11.2a. These reductions must be carried out in aprotic solvents, and ethers are usually used for that purpose. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representative compounds are given in... 

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