New structural information on a humic acid from two-dimensional 1H-13C correlation solid-state nuclear magnetic resonance

New information on the chemical structure of a peat humic acid has been obtained using a series of two-dimensional H-1-C-13 heteronuclear correlation solid-state NMR (HETCOR) experiments with different contact times a nd with spectral editing by dipolar dephasing and C-13 transverse relaxation filtering. Carbon-bonded methyl groups (C-CH3) are found to be near both aliphatic and 0-alkyl but not aromatic groups. The spectra prove that most OCH3 groups are connected directly with the aromatic rings, as is typical in lignin. As a result, about one-third of the aromatic C-O groups is not phenolic C-OH but C-OCH3. Both protonated and unprotonated anomeric O-C-O carbons are identified in the one- and two-dimensional spectra. COO groups are found predominantly in OCH, COO environments, but some are also bonded to aromatic rings and aliphatic groups. All models of humic acids in the literature lack at least some of the features observed here. Compositional heterogeneity was studied by introducing H-1 spin diffusion into the HETCOR experiment. Comparison with data for a synthetic polymer, polycarbonate, indicates that the separation between O-alkyl and aromatic groups in the humic acid is less than 1.5 nm. However, transverse C-13 relaxation filtering under H-1 decoupling reveals heterogeneity on a nanometer scale, with the slow-relaxing component being rich in lignin-like aromatic-C-O-CH3 moieties and poor in COO groups.