Electron spin resonance studies of coals and coal-derived asphaltenes. [Book chapter]

ESR spectra were obtained for vitrains and fusains from a large number of coals. The objectives of the investigation were: (1) to better characterize the immediate chemical environment of the unpaired electrons; (2) to deduce information about the metamorphic changes that occur during vitrinization and fusinization; and (3) to provide needed background information for future studies of the role of free radicals in coal liquefaction. To further pursue the latter objective, ESR spectra of asphaltenes were also obtained. The more significant correlations of the data with coal composition or coal rank are shown in plots. The concentrations of unpaired electrons in the vitrains, as estimated by comparing the ESR intensity of each sample with that of a standard sample of diphenylpicrylhydrazyl, are shown as a function of the carbon contents of the samples. These data show that, in general, the concentrations of unpaired electrons increase with increasing coal rank up to a carbon content of approximately 94% after which the spin concentrations decrease rapidly. The free radical electrons are thought to be delocalized over aromatic rings and thus stabilized by resonance. Resonance stabilization of the radicals is greater for the vitrains from higher rank coals since these presumably contain themore » larger polynuclear condensed aromatic ring systems. The precipitous decrease in spin concentration above 94% C results from the increased conductivity of the samples. The spin concentrations of the fusains exhibit no readily discernable dependence on carbon content. The changes that occur in ESR linewidths and g values during vitrinization and fusinization are discussed. (LTN)« less