Determination of the apparent ratio of quaternary to tertiary aromatic carbon atoms in an anthracite coal by 13C- 1H dipolar dephasing n.m.r.

Abstract

13C- 1H heteronuclear dipolar dephasing n.m.r. techniques allow discrimination between different chemical species contributing to the 13C n.m.r. spectra of complex hydrocarbons. Model compound studies show significantly different effective transverse relaxation constants for carboxyl and quaternary carbon atoms (≈200 μs), secondary and tertiary (≈20 μs), and primary carbon atoms (≈80 μs). Use of these effective relaxation data, together with appropriately timed windows in the continuous wave decoupling applied in standard cross-polarization-magic-angle spinning experiments on anthracite coal allow discrimination between aromatic tertiary and aromatic quaternary ring carbon atoms in this coal. Within the accuracy of experimental error, and of the structural modelling experiments herein reported, the use of the dipolar dephasing technique together with results of X-ray diffraction on coals allows a reasonable estimate to be made of the average number of condensed polynuclear rings in an ‘average molecule’ in the anthracite studied. Based on a model of pericondensed aromatic rings, this number lies between 33 and 45.