E.p.r. and spin echo study of bituminous coal radicals
1986; Elsevier BV; Volume: 65; Issue: 5 Linguagem: Inglês
10.1016/0016-2361(86)90365-0
ISSN1873-7153
AutoresDavid C. Doetschman, Devkumar Mustafi,
Tópico(s)Photochemistry and Electron Transfer Studies
ResumoElectron spin echo spectrometry was employed to isolate the vitrain maceral free radical spectrum in several bituminous coals by means of the relaxation time differences between the macerals suggested by the coal e.p.r. power and temperature dependence. E.p.r. power saturation corrections are necessary to obtain reliable numbers of spins and the results indicate that the density of spins in the fusain maceral may often be apprecialy less than in the vitrain maceral. The bituminous coal vitrain spin relaxation mechanism at 85–100 K occurs by means of electron-nuclear hyperfine interaction. The coal echo decays with a spin-spin relaxation time T2. Measured T2 times are 1.0–1.4 μsec in the temperature range 80–120 K. The echo decay is modulated at frequencies of 2.5 and 5 MHz. One type of radical, with whose CH fragments the observed modulation is consistent, is perinaphthyl. Calculations of hyperfine constants of meta and para protons based on solution spectra and the McConnell relation have been made. The observed echo modulations are predicted and the same model predicts most of the ENDOR lines found in an ENDOR study of a Pittsburgh bituminous coal. The experiments provide very strong evidence that the coal radical spin multiplicity is doublet, S = 12, with weak spin-spin couplings of less than a few hundreds of milliGauss. The optimum pulse duration observed for the coal radicals is 1.4 times longer than for a known triplet S = 1 molecule and relaxation to thermal equilibrium occurs with a single exponential dependence characteristic of a doublet manifold of spin states.
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