Radical linewidths

Electron spin resonance determinations of g-values, linewidths, radical densities and saturation properties have been performed on carbon radicals in samples of coal macerals isolated by density gradient centrifugation techniques. These data are compared with elemental analyses and density measurements. Each maceral type exhibits a different ESR signature" which can be understood in terms of the nature of the starting organic and the extent of coalification. 

The reagent alkali metal/naphthalene in tetrahydrofuran reacts with graphite, polynuclear aromatics, and various coals to form chemically reduced products. In the present paper, we emphasize the use of electron paramagnetic resonance data, in the form of g values, linewidths, radical densities, and saturation characteristics, to analyze the reduced coal products and to infer certain differences between the reduced coals and the anions of graphite and simple aromatic hydrocarbons. Additionally, because the interaction of coals with alkali metal/naph-thalene requires much time for completion, we have investigated internal decomposition pathways for the... 

The oxidation (doping) process in the case of iodine has been monitored by electron spin resonance (esr) and optical spectroscopies. Esr reveals a narrowing of the initially broad signal at g 2.0 (AH 20G vs. 9G for cis- and 0.8G for trans-polyacetylene) Po lOG upon iodine vapor treatment. The number of spins increases with first order kinetics. In contrast, with polyacetylene there is no change in linewidth radical population decreases with first order kinetics. Optically, oxidation witl iodine causes the peak at 2eV (cf. 1.9eV for polyacetylene) to completely disappear and a new geak to appear at 0.9eV (Figure 3) (cf. 0.8eV for polyacetylene). 

Even for a single radical tire spectral resolution can be enlianced for disordered solid samples if the inliomogeneous linewidth is dominated by iimesolved hyperfme interactions. Whereas the hyperfme line broadening is not field dependent, tire anisotropic g-matrix contribution scales linearly with the external field. Thus, if the magnetic field is large enough, i.e. when the condition... 

Table B2.5.5. The photochemical decomposition of methyl radicals (UV excitation at 216 nm). ris tire wavenumber linewidth of the methyl radical absorption and /ris the effective first-order decay constant [54].

FIGURE 9.5 CW ENDOR spectrum of 1-carotene radicals, (a) Experimental spectrum of Figure 9.4. (Reported in Wu, Y. et al., Chem. Phys. Lett., 180, 573, 1991.) (b) Simulated ENDOR powder pattern (using linewidth of 0.6MHz) for the sum of radical cation and neutral radicals in 5 3 1 1 ratio. (Reported in Gao, Y. et al., J. Phys. Chem. B, 110, 24750, 2006. With permission.)... 

FIGURE 9.8 HF-EPR spectra of canthaxanthin radical cation adsorbed on silica-alumina (solid line)—experimental spectrarecorded at5 K (dotted line)—simu lated spectra using g-tensor values g = 2.0032 and g =gyy = 2.0023 and linewidth of 13.6G. (From Konovalova, T.A., J. Phys. Chem. B, 103, 5782, 1999. With permission.)... 

Methyl radicals formed on a silica gel surface are apparently less mobile and less stable than on porous glass (56, 57). The spectral intensity is noticeably reduced if the samples are heated to —130° for 5 min. The line shape is not symmetric, and the linewidth is a function of the nuclear spin quantum number. Hence, the amplitude of the derivative spectrum does not follow the binomial distribution 1 3 3 1 which would be expected for a rapidly tumbling molecule. A quantitative comparison of the spectrum with that predicted by relaxation theory has indicated a tumbling frequency of 2 X 107 and 1.3 X 107 sec-1 for CHr and CD3-, respectively (57). 

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

It is also possible to form radical cations and radical anions on the same alumina or silica-alumina surface (88). One of the more interesting observations was that a marked enhancement of the radical anion spectrum for trinitrobenzene results when perylene is adsorbed on an alumina surface, and similarly the radical cation signal is reenforced by adsorption of trinitrobenzene. The linewidths of the spectra confirm that the radical ions are separated by a distance greater than 10 A. This means that the electron must be transfered through the lattice or that the ions separate after the transfer step, which seems unlikely. Oxygen was still required for the formation of the radical cation. 

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

Impurity-related defects (S03 and Cff3) Irradiation of solid C02 doped with water vapour, gaseous S02 and CH4 produces signals of impurity-related defects.124 Contamination of water gave OH radicals with similar g factors but different linewidths. Methyl radicals (CH3 ) and S02 were observed in addition to C02 and C03. H02 radicals generated by electric discharges in C02 atmosphere with moisture were observed in the dry ice frost condensed on the cold tip from the C02 atmosphere. The lifetimes of defects at ambient temperatures of outer planet were obtained by extrapolating the decay time as a function of the absolute reciprocal temperature, 1 /T.m... 

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