Abstract
The origin of spin paramagnetism of coals is still under question as well as its correlation with chemical and technological properties of different coals. Recent applications of high-frequency EPR (150 GHz) together with electron spin echo at 10 GHz revealed new details about the EPR signals of conjugated aromatic hydrocarbon radicals in coal. The shape of the high-frequency EPR spectra clearly demonstrates both g anisotropy and ‘strain broadening’ caused by different local g shifts for non-equivalent radical centres. T1 and T2 values, as measured by two- and three-pulse spin echo, depend on the degree of excitation of inhomogeneous spectra.The model of strong spin exchange between centres with different conjugation areas is suggested in order to explain observed effects.
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