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@ARTICLE{Conte:449,
      author       = {Conte, P. and Berns, A. E.},
      title        = {{D}ynamics of {C}ross {P}olarization in {S}olid {S}tate
                      {N}uclear {M}agnetic {R}esonance {E}xperiments of
                      {A}morphous and {H}eterogeneous {N}atural {O}rganic
                      {S}ubstances},
      journal      = {Analytical sciences},
      volume       = {24},
      issn         = {0910-6340},
      address      = {Tokyo},
      publisher    = {Japan Society for Analytical Chemistry},
      reportid     = {PreJuSER-449},
      pages        = {1183 - 1188},
      year         = {2008},
      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".},
      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.},
      keywords     = {J (WoSType)},
      cin          = {ICG-4},
      ddc          = {540},
      cid          = {I:(DE-Juel1)VDB793},
      pnm          = {Terrestrische Umwelt},
      pid          = {G:(DE-Juel1)FUEK407},
      shelfmark    = {Chemistry, Analytical},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000259584800019},
      doi          = {10.2116/analsci.24.1183},
      url          = {https://juser.fz-juelich.de/record/449},
}