C-13 NMR SPIN-LATTICE RELAXATION PROPERTIES AND QUANTITATIVE ANALYTICAL METHODOLOGY OF C-13 NMR-SPECTROSCOPY FOR COATS

The quantitative reliability of C-13 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. C-13 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 C-13 spin-lattice relaxation times that are significantly larger than those previously reported in the literature. The relaxation time characteristic of the most slowly relaxing carbons, T-1s, was determined for the aromatic carbons and for the aliphatic carbons of each coal. Then, if one uses repetition delays that are >5T(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 C-13 spins detected. The role of unpaired electron spin dynamics on the effective electron magnetic moment is considered, and the corresponding influence on C-13 NMR detection is discussed.