Aliphatic carbons dephasing

As noted in Section 2.4 there are a number of ways to improve resolution in the NMR spectra of fossil fuels and to obtain additional structural parameters. Of these, the technique of dipolar dephasing has received the most attention. Dipolar dephasing provides information about the amounts of protonated and nonprotonated aromatic and aliphatic carbons in the sample, from which other structural parameters can be derived. Dipolar dephasing measurements have been made on raw oil shales and kerogens ... 

Currently, there are no accurate methods available for quantifying the aliphatic bridges in the coal macromolecule. Quantitative nature of the application of infrared (IR) spectroscopy is limited to certain general types of functional groups or bond types. Nuclear magnetic resonance spectroscopy, despite the success of dipolar dephasing techniques to decipher the extent of substitution on carbon atoms, is still inadequate to distinguish distinct structural entities . 

The Effect of Dipolar Interaction on Cross Polarization and Carbon-Proton Dephasing, J. Am. Chem. Soc., in press. The dipolar dephasing data on an Illinois No. 6 coal has allowed the semi-quantitative separation of methyl, CH2 plus aliphatic methine, and protonated and nonprotonated aromatic carbons, L. B. Alemany, D. M. Grant, R. J. Pugmire, and L. M. Stock, Solid State Magnetic Resonance Spectra of Illinois No. 6 Coal and Some Reductive Alkylation Products, Fuel, (in press). 

The sidebands in C solid-state NMR spectra of 13 polycyclic aromatic hydrocarbon model compounds associated with kerogen structure were suppressed with C CP MAS TOSS NMR. The chemical shift values of these model compounds were obtained and were subsequently used to determine the chemical shifts of aliphatic and aromatic carbons in a series of kerogens via C CP MAS TOSS NMR. Dipolar dephasing (DD) was used to obtain the spectra of nonprotonated carbon present. ... 

High resolution NMR which utilizes both cross polarization with dipolar dephasing and magic angle spinning (MAS) is called C-CP/ MAS NMR. This technique directly measures the organic carbon distribution in terms of aliphatic and aromatic carbon structures in oil shales, coals, humic substances, melanoidins, and other polymeric substances. 

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