Relaxation of 13C and 15N nuclei by solvent-refined coal☆

Cross polarization and magic angle sample spinning NMR spectra of model organic compounds. 3. Effect of the carbon-13-proton dipolar interaction on cross polarization and carbon-proton dephasing

“…As reported previously, CH and CH2 carbon atoms are strongly coupled to protons and therefore dephase more rapidly in the absence of a proton locking field.1'8'9•11-14'17 Destructive interference arising from the dipolar interactions causes the disappearance of most CH2 carbon signals after 35 gs, aromatic CH carbon signals after 40 gs, and the aliphatic CH carbon signal after 55 gs. Opella and Frey8 indicate that other aliphatic CH carbon signals disappear within [40][41][42][43][44][45] gs. Indeed, as one report13 noted, the dipolar dephasing experiment is "extremely discriminating when a delay time of ca.…”

Carbon-13 solid-state NMR of Argonne-premium coals

“…It should be noted that when C is less than 14 carbons (i.e., benzene through either phenanthrene or anthracene, Xbr governs the relationship for Xb. but when the number of carbons exceeds 24 (coronene), one may expect Xb" to approximate best the dependence of Xb °n C. The hyperbolic tangent function may be used to transfer the dependence between the two limiting functions Xb' and Xb". and the following empirical function relates Xb to C: (18) Xb" = 1 -Ve/Vc (17) A plot of Xb" vs C *s shown as the upper dashed curve in Figure 6.…”

“…12 The aromaticity, /a, has been reported for whole coals, macerals, soil, and other fossil fuel related materials.12-17 Other researchers have also used dipolar-dephasing (DD) tech-niques18-22 along with the normal CP/MAS integrations over selected chemical shift ranges to subdivide /a values into the amount of protonated and nonprotonated carbons. [23][24][25][26][27] The quantitative accuracy of these aromaticity values has been open to question for two reasons. First, competing effects in the spin dynamics of cross-polarization do not polarize all spins at the same rate and differences in the spin lattice relaxation in the rotating frame may alter the signals such that they do not achieve full polarization at the same contact time.…”

An Essay on the Organic Geochemistry of Coal

“…Thus the rate of relaxation of excited nuclei depends on the proximity of protons and other factors, and by using a time-resolved measurement system (dipolar dephasing), one can extract more information and can classify the carbon atoms into as many as six different molecular environments Gerstein et al, 1982;Wilson et al, 1982Wilson et al, , 1983aPugmire et al, 1983a,b;Havens et al, 1983). Thus the rate of relaxation of excited nuclei depends on the proximity of protons and other factors, and by using a time-resolved measurement system (dipolar dephasing), one can extract more information and can classify the carbon atoms into as many as six different molecular environments Gerstein et al, 1982;Wilson et al, 1982Wilson et al, , 1983aPugmire et al, 1983a,b;Havens et al, 1983).…”