Electron spin resonance spectra radical-cation

The mass spectra of several unsaturated sulfur heterocycles show the presence of 1,2-dithiete cation radicals. 1,2-Dithiete cation radicals also are obtained by treatment of a-hydroxyketones or a-diketones with sodium sulfide, sodium thiosulfate or sodium dithionite, and sulfuric acid. ° Bis(trifluoromethyl)-l,2-dithiete yields a cation radical directly when dissolved in sulfuric acid. ° The electron-spin resonance spectrum of the benzo-1,2-dithiete cation radical (formed in chlorina-tions with sulfur-containing reagents) has been observed.Thermolysis of a 1,4-dithiin may go via a l,2-dithiete. ° ... 

Electron Spin Resonance.—Nitroxide radicals of varying structiu-e have been employed in studies of micelle structure and mobility. The basic spectrum is a triplet due to N-electron coupling, which may show hyperfine coupling to 3-C-H in appropriate cases. On micelle formation or incorporation of the probe the spectrum normally broadens because of reduction in the rotational correlation time and shows enhanced broadening and change in positions of the high-field line. Cationic micelles incorporate the probe (10) with an association constant of 3 x 10 and at low surfactant concentrations there is... 

Fig. 8. Electron spin resonance spectrum of the electrochemically formed nitropropane radical anion, (b) Calculated spectrum coupling constants On = 2.48 mX, flH = 0.998 mX. Most likely charge distribution in the radical cation. For further details, see [111].

Fig. 14. Tbp Electron spin resonance spectrum of the chemicaliy formed A ,Af-dimethylaniline radical cation in concentrated nitric acid 9.41 GHz, 79 mW, modulation 0.065 mT, sweep 2 min, T = 10° C. Bottom Calcu-iated spectrum coupling constants An = 0.775 mT, acH3 = 0.854 mT, Ah, ortho = 0.371 mT, AH, meta 0.095 mT, Ah, para = 0.69 mT. For further details, see [205].

Spectroscopic techniques such as electron spin resonance (ESR) offer the possibility to "probe" the chemical environment of the interlayer regions. With the ESR technique, an appropriate paramagnetic ion or molecule is allowed to penetrate the interlayer, and chemical information is deduced from the ESR spectrum. Transition metal ions, such as Cu2+, and nitroxide radical cations, such as TEMPAMINE (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) have been used as probes in this manner (6-14). Since ESR is a sensitive and non-destructive method, investigations of small quantities of cations on layer silicate clays at various stages... 

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

In the continuation of our work a study has also been made of the system benzene/silica gel. When irradiating this system at 77 °K. it was found that the silica gel could stabilize both monomeric and dimeric cation radicals of benzene (6). Furthermore, the high resolution of the electron spin resonance lines indicated a high degree of mobility for the benzene molecules in the adsorbed layer. No spectrum from trapped electrons could be observed although this could very well be hidden behind the strong cation absorption. However, ethylene, and isobutylene in the adsorbed state at low temperature gave spectra from shortlived species identified as trapped electrons (7). 

INDO and McLachlan-modified HMO calculations have been conducted to help interpret the electron spin resonance (ESR) spectrum of cation radical (120) <78JPC1181> which is generated by protonation of (98) in TFA followed by reduction with zinc of the resultant dication (119) (Scheme 1) <63TL95>. The calculated energy level of the HOMO for (120) is of lower energy than that of the isoelectronic fluorene anion radical in both the INDO and HMO approximations. Similar calculations have been conducted to explain the ENDOR spectrum of (120) <84CPL398>. 

One of the most studied examples of complete electron transfer in a complex where both donor and acceptor are organic species is the tetramethyl-p-phenylenediamine-chloranil case. In polar solvents (see also the section on solvent effects, p. 101) the absorption spectrum is a superposition of what one observes separately for the Wurster s blue cation radical (III) and chloranil radical anion (IV). A recent report contains a survey of the older literature concerning electron-spin resonance studies of this complex. Often the free-radical species initially produced in such reactions undergo further slow chemical changes, not infrequently to intractable products. 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

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