Liquid-Phase NMR of Humic and Fulvic Acids

Present review focuses on the most recent advances in the NMR of the coal-derived humic and fulvic acids, covering exclusively the results of the liquid-phase NMR and leaving apart an overwhelming amount of publications dealing with the solid-state NMR investigations in this field (the latter are comprehensively reviewed elsewhere). Owing to the complexity of humic and fulvic acids together with other coal-derived products, their ¹H and ¹³C NMR spectra consist of a number of overlapping signals belonging to different hydrocarbon types. Comprehensive studies of humic and fulvic acids by means of NMR revealed characteristic functional groups of their composition together with spectral regions in which they resonate. Quantitative ¹H and ¹³C NMR spectra characterize aromatic and saturated carbons spread over many structural moieties, which provides a solid guideline into molecular structure of humic and fulvic acids together with parent coal-derived products. Nowadays, quantitative ¹³C NMR measurements yield information about a variety of structural parameters such as functional group distribution, aromaticity, degree of condensation of aromatic rings, and medium chain lengths together with many other more specific parameters. The structural NMR studies of the coal-derived products are developing on a background of a marked progress in experimental and computational NMR. Discussed in the present review are the most recent advances in the liquid-state NMR studies of the coal-derived humic and fulvic acids together with their processing products.