Proton decoupling, dephasing carbon

Carbon resonances arising from both nonprotonated and proto-nated aromatic carbons may appear at the same frequency under proton decoupling. Yet these two resonances could possess very different relaxation behavior and in a solid could evolve very differently due to local proton dipolar fields which attenuate with the carbon-proton distances as 1/rcH When the spin locking pulse for proton nuclei is turned off, carbons with directly bound protons such as methines and methylenes rapidly dephase in the local proton fields and their spectral response is rapidly diminished. The rapid internal motion of CH3 groups greatly decreases the effectiveness of methyl protons. Nonprotonated carbons are only dephased by remote and therefore... 

The solid state NMR measurements on the hard coke concentrates were carried out at 25 MHz on a Bruker DSX spectrometer with MAS at 4.5-5.0 kHz to give spectra in which the sideband intensities are only ca. 6-7% of the central aromatic bands. A contact time of 1 ms was used for the cross polarisation (CP) measurements and the H decoupling and spin-lock field was ca. 60 kHz. The FIDs were processed using a Lorentzian linebroadening factor of 50 Hz. To determine the fraction of protonated and non-protonated carbon, four delay periods between 1 and 100 is were employed in dipolar dephasing experiments. 

Fig. 16 CP/MAS NMR spectra of Cd(mtn)Ni(CN)4.1/2QHi2 as a function of temperature, showing fade-out of the cyclohexanol resonances when the molecule reorients at rates comparable to the strength of the decoupling field [53]. Normal spectra are shown on the left, and dipolar dephased spectra, where resonances of carbons strongly coupled to protons are much weaker, on the right.

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