The Role of Noncovalent Bonding in Coal

Publisher Summary The primary structure of coal is principally aromatic in nature, with about 55–90% aromatic carbon. The ring positions are mostly occupied by carbon atoms, although there are many heterocyclic rings that include sulfur, nitrogen, or oxygen atoms. Coal and coal liquids contain significant amounts of oxygen and nitrogen compounds and a high aromatic content, which suggests that hydrogen bonding is an important factor in determining the physical properties of coal. The hydrogen bonds can be either intramolecular or intermolecular. This chapter describes the measurement of hydrogen bonding in infrared (IR) spectroscopy, which is done by observing changes in the frequency, intensity, and shape of the absorption bands. Intramolecular hydrogen bonding is virtually unaffected on dilution. Thus, dilution studies offer a means of differentiating between intermolecular and intramolecular types of hydrogen bonds. Similar to IR spectroscopy, nuclear magnetic resonance spectroscopy can also be used as a diagnostic tool for hydrogen bonding. In proton magnetic resonance spectra run in a nonpolar solvent at infinite dilution, the proton resonance of the hydroxyl group in phenol is known to absorb in the range of 4–5 ppm; at higher concentrations, this signal is shifted to 6–11 ppm.