SPIN DYNAMICS IN CONTINUOUS-WAVE AND PULSED ENDOR SPECTROSCOPY OF COALS

ENDOR experiments on coals recorded using continuous wave (CW) and pul sed techniques appear to give qualitatively different spectra. A matri x proton signal dominates the ENDOR spectrum of coals recorded in the CW ENDOR experiment while both a matrix and local proton ENDOR signals with hyperfine couplings of up to 20 MHz are observed in spectra reco rded using pulsed excitation techniques. Analysis of these spectral le ad to different implications for the structure of the molecules that h ost the unpaired electron. Using a combination of pulsed EPR (Electron Spin Echo, FID detected hole burning) and pulsed Electron Nuclear Mul tiple Resonance (Sub-level relaxation, hyperfine selective ENDOR, EPR sub-spectra) experiments, we investigate the electron and nuclear spin dynamics in order to reconcile the different signal amplitudes observ ed in the CW and pulsed ENDOR spectra. In the CW ENDOR experiment, the results of the FID detected hole burning experiments prove that the l ow ENDOR signal intensity can not be attributed to spectral diffusion mechanisms competing with ENDOR mechanisms. Instead, we find that an u nfavorable ratio of the electron and nuclear spin relaxation rates res ults in small local ENDOR signals. The matrix line dominates the spect rum because of the large number of matrix protons. In the pulsed ENDOR experiment, the hyperfine contrast selectivity mechanism suppresses t he intensity of the matrix ENDOR signal and enhances the amplitudes of the local ENDOR signals. In addition, the ENDOR signal is not a funct ion of the ratio of the electron and nuclear relaxation rates.