%0 Journal Article
%A Conte, P.
%A Berns, A. E.
%T Dynamics of Cross Polarization in Solid State Nuclear Magnetic Resonance Experiments of Amorphous and Heterogeneous Natural Organic Substances
%J Analytical sciences
%V 24
%@ 0910-6340
%C Tokyo
%I Japan Society for Analytical Chemistry
%M PreJuSER-449
%P 1183 - 1188
%D 2008
%Z 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".
%X 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.
%K J (WoSType)
%F PUB:(DE-HGF)16
%9 Journal Article
%U <Go to ISI:>//WOS:000259584800019
%R 10.2116/analsci.24.1183
%U https://juser.fz-juelich.de/record/449