13C NMR Spin-Lattice Relaxation Properties and Quantitative Analytical Methodology of 13C NMR Spectroscopy for Coals

Abstract

The quantitative reliability of {sup 13}C NMR spectroscopy is examined and discussed. Studies were performed on eight Argonne Premium coals, using the nonoverlapping carbonyl peak of 4-(N-methylpyrrolidino)bicyclo[3.2.1]-octan-8-one triflate as an intensity reference. {sup 13}C spin-lattice relaxation times measured by cross-polarization (CP) detection depend on the CP contact time. A series of experiments with different CP contact times yielded observed {sup 13}C spin-lattice relaxation times that are significantly larger than those previously reported in the literature. The relaxation time characteristic of the most slowly relaxaing carbons, T{sub 1S}, was determined for the aromatic carbons and for the aliphatic carbons of each coal. Then, if one uses repetition delays that are >5T{sub 1S}, quantitative results can be expected for the NMR-observable carbons in a direct polarization (DP) experiment. Carbon spin counting experiments performed by using DP instead of CP yielded numbers for the fraction of carbon spins that are 81-97% of those present in the sample. A relationship was observed between the number of unpaired electron spins and the fraction of {sup 13}C spins detected. The role of unpaired electron spin dynamics on the effective electron magnetic moment is considered, and the corresponding influence on {sup 13}C NMR detection is discussed. 42more » refs., 9 figs., 2 tabs.« less