Analytical Sciences

Dynamics of Cross Polarization in Solid State Nuclear Magnetic Resonance Experiments of Amorphous and Heterogeneous Natural Organic Substances

Pellegrino CONTE* and Anne E. BERNS**

*Dipartimento di Ingegneria e Tecnologie Agro-Forestali (DITAF), Università degli Studi di Palermo, ed. 4, v.le delle Scienze 13, 90128 Palermo, Italy
**Forschungszentrum Jülich GmbH, Institute of Chemistry and Dynamics of the Geosphere, Institute 4: Agrosphere, 52425 Jülich, Germany

Nuclear magnetic resonance (NMR) experiments on carbon-13 in the solid state were done with cross polarization (CP) and magic angle spinning (MAS) in order to overcome the low NMR sensitivity of ¹³C and the chemical shift anisotropy, respectively. In the present research, CPMAS ¹³C-NMR spectra were collected by modulating the contact time needed for cross polarization (variable contact times experiments, VCT) on two different humic acids (a soil-HA and a coal-HA). VCT data were fitted by a model containing either a monotonic or a non-monotonic cross polarization term. The non-monotonic model, which fitted the experimental results better than the monotonic one, provided two cross-polarization rates, thereby suggesting that two different mechanisms for the energy transfer from protons to carbons arise in amorphous and heterogeneous humic substances. The first mechanism was a fast proton-to-carbon energy transfer due to protons directly bound to carbons. The second mechanism was related to a slow transfer mediated by local segmental motions. Different domains in the humic acids were identified. Soil-HA was made of rigid domains, containing mainly aromatic and carboxylic moieties, and fast moving domains, containing alkyl, C-O and C-O groups. Coal-HA showed a rigid aromatic domain that was differentiated from a very mobile domain made of alkyl and COOH groups.

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