Radical cations, ESR spectra

The redox properties of cyclic polysilanes are interesting because they resemble those of aromatic hydrocarbons. For instance, cyclic polysilanes can be reduced to anion radicals or oxidized to cation radicals. ESR spectra for both the cation and anion radicals indicate that the unpaired electron is fully delocalized over the ring [17,19,20]. The aromatic properties of the cyclic polysilanes are ascribed to a high energy delocalized HOMO and a relatively low energy LUMO. Because the HOMO and LUMO levels lie at similar level to those of benzene, cyclic polysilanes can serve either as electron donors or electron acceptors. 

The pairing of anion radicals with their counter cations is a wide-spread and now well documented phenomenon (Szwarc, 1972). In contrast, ion-pair phenomena in cation radical systems are not common, but see p. 222. Shifts in -values caused by halide-ion interaction with the tetramethoxybenzene cation radical have been reported (Sullivan, 1973), and halide-ion splittings of metalloporphyrin cation radical esr spectra have been demonstrated (Fajer et al., 1973). 

Recently the anion and cation radicals of heptafulvalene (XXIII) have been prepared and their ESR spectra have been investigated by... 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

ESR spectra of the radical anions of 3-phenoxy-, 3,4-diphenoxy-, and 3,4-dichloro-l,2,5-thiadiazole and of the radical cations of various 3-aryloxy-4-morpholino-l,2,5-thiadiazoles have been obtained by the electrochemical generation of the ions in 3 x 10-3M solutions of the thiadiazoles in the system MeCN/Et4NC104 (ca. 0.1 M) on a platinum helix electrode directly in an ESR resonator at first wave potentials <2003RJC806>. 

Prasad and coworkers60 studied the ESR spectra of cation radicals of dienes, trienes, tetraenes and pentaenes formed in CFCI3 matrix by X-ray irradiation. The structures of the resulting cation radicals were deduced by comparing the experimental coupling constant to those derived from INDO calculation. The unpaired spin density decreases with increasing chain length. 

Oxidative Fluorination of Nitrones to a-Fluorosubstituted Nitroxyl Radicals Formation of nitroxyl radicals by the radical cation route was observed in reactions of various nitrones with xenon difluoride in dry methylene chloride (520, 523). In this reaction, more than 40 nitrones, including 4H -imidazole N,N -dioxides (219), 4H -imidazole TV-oxides (223) and (224), 2H -imidazole N -oxides (225), 2H -imidazole TV,TV-dioxides (226), 3,3,5,5-tetramethylpyrroline N -oxide (TMPO), derivatives of 3-imidazoline-3-oxides (231) and (232), have been examined. ESR spectra of nitroxyl radicals containing one or two fluorine atoms at a-C have been registered (Scheme 2.108) (523). In the case of... 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

The copolymerization of styrene and maleic anhydride was studied by the spintrapping technique using 2-methyl-2-nitrosopropane as a spin trap. Four types of ESR spectra were obtained, of which three corresponded to trapping of the growing polymer chain at a centre originating from the styrene part or from two centres originating from the maieic anhydride part. The fourth EPR spectrum may be due to a cyclic flve-membered aminoxyl or a six-membered 1,2-oxazine radical cation. ... 

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

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