Construction of a model structure for upper freeport coal using 13C NMR chemical shift calculations

Upper Freeport coal was extracted in the high yield at room temperature with a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent. A solid-state C-13 NMR spectrum was measured for the mixed-solvent insoluble residue. On the basis of the spectrum, a model structure for the residue was estimated using chemical shift calculations and structural data, including the ultimate analysis, and was then iterated until spectra calculated for the model matched the experimentally obtained spectrum in the excellent way. In earlier work, the original extract had been further fractionated with acetone and pyridine, and H-1 NMR spectra and the elemental composition had been obtained on the resulting subfractions. These data were used to construct model structures for each fraction. With these structures from the earlier work and the residue's structure based on the C-13 NMR spectrum, a seven-molecule associated model structure for Upper Freeport raw coal was constructed; the structure had a continuous molecular weight distribution from the lightest to the heaviest extract fraction, allowing us to explain the high extraction yields (60-85 wt % (daf)) for the coal even at room temperature. The lowest energy conformation of the model structure was determined using a molecular mechanics-molecular dynamics method under periodic boundary condition. A gross anisotropic structure was produced. The physical density estimated from the energy-minimized conformation of this structure was 1.28 g/cm(3), in good agreement with the experimentally determined value of 1.30 g/cm(3). The average distance between the aromatic planes was calculated 4.1 Angstrom, a little higher than the reported value of 3.6 Angstrom.