Radical 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. 

ENDOR spectroscopy has proven to be a valuable technique to provide information on both free and protein bound flavin radicals. Since flavin radical ESR spectra can be partially saturated at moderate microwave power, ENDOR spectra may be observed as nuclear spin transitions by detection of changes in the partially saturated ESR signal as a function of nuclear radio frequency. The resonance condition for nuclei (when I = Vz) is described by the following equation ... 

The ESR method provides information only on particles that exist for more than 1 X 10 3 sec. In order to investigate short-lived radicals, ESR spectra are recorded in a steady flow. The steady-flow methods allow one to study reactions with conversion times below 10 4 sec. 

Since identical ESR spectra were obtained on U.V. irradiation at liquid nitrogen temperature of PVC, copolymer of vinyl chloride with vinyl bromide and 3-chloropentane it is concluded that this radical is the "Radical I." This radical may play an important role in PVC degradation. To further firmly establish the identity of this radical, theoretical analysis of "Radical I" and computer simulations of ESR spectrum were performed. The polymer radical ESR spectra are very sensitive to the conformation of the chain. The characteristic chain conformation of vinyl syndiotactic sequences in solution has been shown (23) to consist chiefly of trans-trans groups, separated by gauche units . .. (TT)X (GG)i (TT)y (GG)y (TT)Z. .. 

Berdnikov, B. M., Doctorov, A.B., and Makarshin, L.L. (1980) Dipole-dipole broadening of free radicals ESR spectra in the presence of paramagnetic ions, Theoretical and Experimental Chemistry (Kiev) 16, 765-771. 

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). 

The methyl radical has attracted much attention since it is the most simple and fundamentally important alkyl radical. ESR spectra have been observed for methyl radicals generated in liquids [36c], isolated in inert gas matrices [46, 75], adsorbed on... 

In order to understand the penultimate unit effect in dimeric model radicals, ESR spectra of monomeric model radicals were studied for comparison. The ESR spectra... 

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