Proton dynamic nuclear polarization in coal

Spin dynamics in the carbon-13 nuclear magnetic resonance spectrometric analysis of coal by cross polarization and magic-angle spinning

“…5 is 2.00287 (39). In addition, Wind et al have indicated from their dynamic-polarization/ cross-polarization experiments that a greater portion of the free radicals in coal are located in the aromatic region (22), in agreement with suggestions from ESR work (40,41).…”

“…Of course, the possibility exists that with both techniques some fraction of the carbons are missed, e.g., by large paramagnetic shifts or broadening. This possibility is being addressed by electron-nuclear polarization experiments (22). In any case, the excessive time requirements of the Bloch-decay technique dictated by 13C T1 values of a few seconds preclude its consideration as a serious competitor for CP/MAS in coal analysis.…”

“…If a significant fraction of the 13C resonances are sufficiently broadened and/or shifted by paramagnetic centers to render them outside the limits of detection of a typical CP/MAS spectrometer, then major distortions of integrated intensities could result. These points are being explored here and elsewhere by electron-nuclear dynamic polarization approaches (22), which have the promise of elucidating the nature of the paramagnetic centers in coal and the effects they have on 13C CP/MAS spectra.…”

Optimal sample formulations for DNP SENS: The importance of radical-surface interactions

“…In spite of the early success of 1 H and 13 C DNP using intrinsic radicals for the characterization of coal samples, [48][49][50][51][52] attempts to apply DNP SENS methodology using radical dopants to carbonaceous materials have been met with great difficulty. For example, very recently, Leskes and co-workers studied the solid electrolyte interface on reduced graphene oxide materials and were unable to record a DNP enhancement for the reduced graphene oxide layer.…”

Determination of the fraction of organic carbon observable in coals and coal derivatives measured by high-resolution solid-state carbon-13 nuclear magnetic resonance spectrometry

“…The presence of unpaired electrons and attendant electron-nuclear dipolar interactions can render a fraction of the carbon resonance signal invisible, broadened, and/or paramagnetically shifted (20). The consequences of electron-nuclear interactions in coals with respect to CP/MAS-13C-NMR measurements are being examined by dynamic nuclear polarization experiments (21)(22)(23).…”