New Structural Information on a Humic Acid from Two-Dimensional ¹H-¹³C Correlation Solid-State Nuclear Magnetic Resonance

Abstract

New information on the chemical structure of a peat humic acid has been obtained using a series of two-dimensional 1H-13C heteronuclear correlation solid-state NMR (HETCOR) experiments with different contact times and with spectral editing by dipolar dephasing and 13C transverse relaxation filtering. Carbon-bonded methyl groups (C-CH3) are found to be near both aliphatic and O-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 OCHn-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 1H 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 13C relaxation filtering under 1H 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.

Coupling NMR to NOM

“…Subsequently, the natural logarithm of each peak's intensity is plotted against the contact time to yield a cross polarization build-up curve for each resolvable carbon species ( [36,44,45,46,47]). T CH and T 1ρ I are obtained by mathematically fitting the cross polarization to Equation 1 (where T IS =T CH in this case) or to a more complex version of this equation.…”

“…The third method is known as phase adjustment of spinning sidebands (PASS) and involves separating sidebands according to their phase, simplifying the obtained spectrum [23]. The fourth, and by far the most utilized, is the CP-TOSS technique [22,23,24] and, as far as can be determined, it is the only one of the four techniques to be applied to NOM [42,44,45,46,47,51]. This cross polarization technique has been around for a long time and has been used extensively.…”

Medium- and Long-Distance 1H–13C Heteronuclear Correlation NMR in Solids

“…The correlation of 1 H and 13 C chemical shifts in solid-state NMR (1, 2) provides useful information on the structure of complex organic materials, including proteins (3), coals (4,5), soil organic matter (6), and multicomponent polymers (7,8). However, the information content of peaks correlating directly bonded protons and carbons is often trivial, given the limited 1 H chemical shift resolution achievable in solids.…”

Selective Dephasing of OH and NH Proton Magnetization Based on 1H Chemical-Shift Anisotropy Recoupling

“…1 H- 13 C heteronuclear (HETCOR) nuclear magnetic resonance (NMR) spectroscopy of solids (1-3) has proven useful for identifying structural units in complex organic materials such as coals (4) and humic substances (5). With magnetization filters such as the MELODI sequence, which dephases the often trivial signals of directly carbon-bonded protons (6 ), the HETCOR spectra can be simplified and their information content further enhanced.…”