Dipolar broadening carbon-proton

In an organic solid representative broadenings are 150 ppm for aromatic carbon chemical shift anisotropy and 25 kHz (full width at half-height) for a rather strong carbon-proton dipolar interaction. At a carbon Larmor frequency of 15 MHz, the shift anisotropy corresponds to 2.25 kHz. In high magnetic fields the forms of the respective Hamiltonians are... 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

Heteronuclear dipolar coupling can be a major broadening mechanism, especially as in a number of materials protons are present which often have dipolar couplings to other nuclei in excess of 50 kHz. This means that MAS is usually not sufficient to remove the effect of the dipolar coupling in the spectra. C NMR spectra are, for instance, often obscured by dipolar coupling to the protons. In order to remove the carbon-proton coupling the protons are irradiated with an rf field while, at the same time, the carbon spectrum is measured. Consider the CH system where the heteronuclear dipolar... 

These techniques depend on the availability of uniformly highly enriched 13C and 15N proteins, which can usually be prepared by recombinant DNA methods. In the following discussion we assume that the protein is enriched to 95% or more in both 13C and 15N. In addition, for proteins larger than about 200 residues, it becomes important for some experiments to reduce line broadening from H-H and 13C—H dipolar interactions by replacing most or all carbon-bonded protons in the protein by deuterium (also by rDNA methods). This substitution provides substantial line narrowing, because 1/T2 varies as y2, as we saw in Chapter 8. For other experiments, such as measurement of H—H NOEs, the presence of protons is, of course, essential. 

On the other hand, LD/MAS spectra of the nylon 4 single crystal sample shown in Fig. 12.2(c), show an obvious change from 20 to 100°C. In the spectrum at 20°C, no peak is observed owing to the broadening caused by dipolar interactions between carbons and protons. Such broadening at 20°C cannot be overcome by the low power decoupling for protons used in... 

A common situation is exemplified by the usual organic solids in which there are protons in profusion and carbon-13 only in natural abundance of about 1%. For the NMR spectra of 13C in such a sample, most of the broadening comes from the heteronuclear dipolar interaction between the 13C and the protons and very little from the homonuclear dipolar interaction between the carbons. Thus, if it were possible to eliminate the heteronuclear dipolar interaction, you could have a high resolution 13C spectrum even though its S/N may be quite poor because of the small number of 13C. 

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