Journal Article

PreJuSER-449

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Dynamics of Cross Polarization in Solid State Nuclear Magnetic Resonance Experiments of Amorphous and Heterogeneous Natural Organic Substances

Conte, P. ; Berns, A. E.FZJ*

2008
Japan Society for Analytical Chemistry Tokyo

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Please use a persistent id in citations: doi:10.2116/analsci.24.1183

Abstract: 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 C-13 and the chemical shift anisotropy, respectively. In the present research, CPMAS C-13-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-N groups. Coal-HA showed a rigid aromatic domain that was differentiated from a very mobile domain made of alkyl and COOH groups.

Keyword(s): J

Note: P.C. acknowledges Forschungszentrum Julich GmbH (Germany) for enabling him to perform experimental work as visiting scientist at the NMR center of the Institute of Chemistry and Dynamics of the Geosphere, Institute 4: Agrosphere. The authors are grateful to Dipl.-Ing. H. Philipp (FZJ) for his assistance in sample preparation. This paper was partially funded by University of Palermo within a project titled "Nuovi processi sostenibili per la bonifica di suoli contaminati".

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Research Program(s):

1. Terrestrische Umwelt (P24)

Appears in the scientific report 2008

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