1H n.m.r. Zeeman relaxation in premium coals and other coals of various rank

Solid-state nuclear magnetic resonance (NMR) spectroscopic analysis were carried out on recent and archaeological wood. Cross-polarization-magic-angle spinning ~3C NMR spectra obtained from samples of poplar (Populus sp.), oak (Quercus sp.), and silver fir (Abies alba) were examined in this study. The most relevan t peaks were assigned according to the extensive literature in the area, and the differences observed discussed in terms of lignin and cellulose composition: by fixing a lignin reference signal intensity, the eellulose and hemicellulose moieties showed a strong depletion compared to the lignin signals in archaeological wood.

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“…Therefore, only a qualitative analysis was performed on the basis of the extensive literature in this field [15][16][17][18]. Fig.…”

Section: Resultsmentioning

confidence: 99%

13C CPMAS NMR spectroscopic analysis applied to wood characterization

et al. 2005

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“…in the frequency range [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] GHz. DNP Spectrometer.…”

Section: Introductionmentioning

confidence: 99%

“…Wind et al 38 have provided a detailed analysis of this model. A useful model for describing this behavior consists of a diffusion limit defined by the diffusion barrier, b, the scattering radius, , and a radius, R, that may be related to the average distance between electrons when the relationship (4/3)πR 3 N e ) 1 holds.…”

Section: Introductionmentioning

confidence: 99%

“…It is reasonable to assume the same b value in the case of carbazole doped with BDPA. Given the Larmor frequency (60 MHz) for protons, is found to be 7.33 Å by following the calculation given by Wind et al 38 Using the experimentally determined radical concentration and assuming a homogeneous distribution of radicals, the R-values for the four doped samples are easily calculated and the results are given in Table 2. The average volume for one proton is thus 22.9 Å 3 .…”

Section: Introductionmentioning

confidence: 99%

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Enantioselective Synthesis of Six-Membered Palladacycles Having Metal-Bound Stereogenic Carbons: Isolation and Reactivity of Palladacycles Containing Readily Accessible β-Hydrogens

Burke

Overman

2004

J. Am. Chem. Soc.

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Five enantiopure palladacycles containing palladium bonded to a stereogenic carbon and an N-coordinated oxindole were synthesized by the reaction of alkenyl aryl triflates 2 and 9 with Pd(0) bisphosphine complexes. Two palladacyclic complexes, 3beta and 10alpha, were characterized by single-crystal X-ray crystallography. The reactivity of neutral palladacycles 3beta and 10beta was studied in detail. These unusual palladium alkyls, which have three accessible beta-hydrogens, are thermally stable at temperatures as high as 120 degrees C. At higher temperature, or at room temperature in the presence of weak acids, these complexes epimerize at the stereogenic carbon bonded to palladium. The mechanism of the acid-promoted epimerization was studied in detail. During this epimerization, cationic palladium alkyls 13/14 and 33 and cationic palladium hydride alkene complexes 31 and 32 are in rapid equilibrium.

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“…Laboratory-Frame 1 H Spin − Lattice Relaxation in Coal. Laboratory-frame proton spin−lattice relaxation ( T 1 ) in coal has been investigated quite extensively via wide-line 1 H NMR techniques. − A wide variety of relaxation behaviors have been found for various coals. Both exponential and nonexponential relaxation behaviors have been observed.…”

Section: Introductionmentioning

confidence: 99%

Variable-Temperature High-Resolution Proton NMR Study of Laboratory-Frame and Rotating-Frame Spin−Lattice Relaxation in Coals

Xiong

Maciel

1997

Energy Fuels

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We have carried out the first systematic in situ variable-temperature (25−180 °C) high-resolution proton NMR study of laboratory-frame and rotating-frame proton spin−lattice relaxation of coal samples, based on the CRAMPS technique. For coal samples that have been exposed to air, we confirmed the fact that paramagnetic oxygen is the main source of laboratory-frame proton spin−lattice relaxation (T 1). We demonstrate that paramagnetic oxygen trapped in coal can be used as a sensitive probe for monitoring structural and dynamical changes in coal as the temperature is varied. High-temperature spin−lattice relaxation experiments help to reveal the structural heterogeneity of coal because of reduced proton and electron spin-diffusion rates at high temperature. Large domains, on the order of 200−800 Å, with distinctively different paramagnetic oxygen concentrations, were found in all three coal samples studied, consisting of one low-volatile and two high-volatile bituminous coals from the Argonne Premium Coal bank. In particular, we found that aliphatic-rich domains with a length-scale larger than 500 Å exist in Premium Coal 601. The observed dependences of the rotating-frame 1H spin−lattice relaxation time T 1 ρ on the strength of the spin-lock field and temperature support the view that the main relaxation mechanism is time-dependent 1H−1H dipolar interactions in coals. From these dependences, we estimate that the correlation time of molecular motion responsible for rotating-frame proton spin−lattice relaxation in coals is on the order of 5 μs, which is in agreement with conclusions drawn from previous proton dipolar-dephasing studies. Two T 1 ρ values were identified for each of the three coal samples studied, indicating the existence of structural heterogeneity in coal on a spatial scale of at least 50 Å. The sizes of heterogeneous domains in coal are estimated on the basis of measured spin−lattice relaxation times and the analysis of proton spin-diffusion processes.

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1H n.m.r. Zeeman relaxation in premium coals and other coals of various rank

Cited by 20 publications

References 27 publications

13C CPMAS NMR spectroscopic analysis applied to wood characterization

13C CPMAS NMR spectroscopic analysis applied to wood characterization

Enantioselective Synthesis of Six-Membered Palladacycles Having Metal-Bound Stereogenic Carbons: Isolation and Reactivity of Palladacycles Containing Readily Accessible β-Hydrogens

Variable-Temperature High-Resolution Proton NMR Study of Laboratory-Frame and Rotating-Frame Spin−Lattice Relaxation in Coals

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