Solid-state 13C-NMR dipolar dephasing experiments for quantifying protonated and non-protonated carbon in soil organic matter and model systems

Dipolar dephasing was investigated as a means for quantitatively differentiating protonated and non-protonated and molecularly mobile and rigid components in soil organic matter. We experimented on cellulose, hemicelluloses, lignin, a protein, chitin, charcoal, palmitic acid and two waxes which served as model systems for components of soil organic matter. Dipolar dephasing decay curves showed significant deviation from the expected exponential decay (for non-protonated and methyl carbons) and Gaussian decay (for non-methyl protonated carbons), partly due to rotational and dipolar modulation. No signal is observed for most protonated carbon (excluding methyl carbon) for a dipolar dephasing delay of 45 mus, and so we generated three subspectra, representing non-protonated, non-methyl protonated and methyl carbon classes, requiring the acquisition of just the 0- and 45-mus dipolar dephased spectra. This methodology was applied to eight samples of soil organic matter, allowing the determination of relative contributions of overlapping resonances such as C-substituted and H-substituted aromatics (110-145 p.p.m.), carbohydrate anomeric and tannin (90-110 p.p.m.), and amino acid and methoxyl (45-65 p.p.m.). The waxes behaved aberrantly, probably because some of their components are highly mobile. We determined accurately the exponential dipolar dephasing decay rates of non-protonated carbon resonances, free from the interference of rotational and dipolar modulations.