MULTIFREQUENCY EPR STUDIES OF ARGONNE AND ILLINOIS SAMPLE BANK COALS

Eight coals from the Argonne Premium Coal Sample Program (APCSP), and an Illinois No. 6 coal from the Illinois Coal Basin Sample Program (ICBSP) have been investigated, employing a novel very high frequency (VHF) e.p.r. spectrometer operating at a microwave frequency of about 95 GHz and a magnetic field of 3.4 Tesla. Macerals separated from the ICBSP sample by density gradient centrifugation were also investigated. The high fields used in these experiments provide a 10-fold improvement in the dispersion of electronic Zeeman terms over that provided by conventional X-band instruments (9.5 GHz; 0.34 T), enabling us to resolve spectral features not seen at lower frequencies. Model compounds (perylene(+) and dibenzothiophene(+) radicals) were also studied for comparison purposes. It is proposed that low-field peaks in some of the coal and maceral VHP e.p.r. spectra are the result of spin-orbit coupling of unpaired electrons to heteroatoms, primarily sulphur, and this effect is modelled theoretically employing a simple, two-species model. Preliminary pulsed e.p.r. data on a separated fusinite taken at S-band (3 GHz) are also presented to support the electron exchange model for the lineshape of this maceral. © 1990.