Carbon relaxation

A. N. Garroway I would still hold out the hope for using these carbon relaxation rates to interpret mechanical properties, but the onus is on the experimenter to show he is actually measuring the effects of motion. Once that is done 1 think the idea will compete as freely as any other scientific concept. 

COMPARISONS OF INTERPRETATIONS OF METTHINE CARBON RELAXATION IN DISSOLVED POLYSTYRENE... 

Numbers are methyl carbon relaxation times in seconds. 

FIG. 2. Carbon relaxation times (s) for 1 m cholesteryl chloride in carbon tetrachloride. (31)... 

When a molecule is rigid and rotates equally well in any direction (isotropically), all the carbon relaxation times (after correction for the number of attached protons) should be nearly the same. The nonspherical shape of a molecule, however, frequently leads to preferential rotation in solution around one or more axes (anisotropic rotation). For example, toluene prefers to rotate around the long axis that includes the methyl, ipso, and para carbons, so that less mass is in motion. As a result, on average, these carbons (and their attached protons) move less in solution than do the ortho and meta carbons, because atoms on the axis of rotation remain stationary during rotation. The more rapidly moving ortho and meta carbons... 

When molecules are not rigid, the more rapidly moving pieces relax more slowly because their is shorter. Thus, in decane (5-3) the methyl carbon relaxes most slowly,... 

Solid state NMR was used to recognize guest-free TNP matrix and ICs with guests, e.g. benzene, tetrahydrofuran and p-xylene. Carbon relaxation times were used to study the motion mechanism of the guests. P CP/MAS spectra were particularly informative about the symmetry of the phosphazene ring. Figure 20 presents C MAS spectra of TNP ICs with benzene and THF guests in the matrix. 

If the total experimental time is not critical, direct observation of carbon signal without the CP step can be used. Although the direct polarization carbon signal is fully quantitative, its signal-to-noise ratio will be inferior to that for CP-MAS because of the reasons discussed above. Not only is the signal per scan weaker, but also the relaxation delay must be set much longer. Alternatively, when the carbon relaxation properties of both polymorphs are known, the intensities can be corrected for relaxation without waiting for full relaxation between successive scans. 

One of the principal advantages of CPMAS experiments is that resolution in the solid state allows individual-carbon relaxation experiments to be performed. If a sufficient number of unique resonances exist, the results can be interpreted in terms of rigid-body and local motions (e.g., methyl rotation, segmental modes in polymers, etc.) (1,2). This presents a distinct advantage over the more common proton relaxation measurements, in which efficient spin diffusion usually results in averaging of relaxation behavior over the ensemble of protons to yield a single relaxation time for all protons. This makes interpretation of the data in terms of unique motions difficult. 

Those familiar with the routine acquisition of 13c NMR spectra are aware of the consequences of the nuclear Overhauser effect (NOE). Saturation of protons has the effect of increasing the net 13c magnetization of those carbons relaxed by the protons of up to a factor of three times the equilibrium magnetization. Most analytical or survey 13c spectra are obtained with continuous broadband proton decoupling and any resultant NOE. Characteristics of this mode of operation are, (1) the possibility of variable NOE, (2) repetition rate governed by 13c T and (3) both protonated and non-protonated carbons are detected. The first aspect makes quantitation difficult. The second affects net sensitivity, and the third has the prospect of having undesirable signals in certain situations. 

It follows from Table 7 that the carbon relaxed chemical potentials correctly predict the meta-directing influence of typical electron withdrawing substituents, X = CN, NO2, COOH, CF3, although the differences in xc values are very small indeed. Again, a more emphasized meta-directing preference can be seen in the list of FF data (Table 8). 

Carbon relaxation with proton spin-spin coupling was investigated for several n-alkanes, e.g. in selectively deuteriated nonane C4D9CH2C4D9 with one isolated CH2 group. For this isolated AX2 spin system, dipolar relaxation was analysed in terms of spectral densities Jch hch> chh hh- The results indicate that motion at each segment of the chain has local prolate symmetric-top character, and correlation times point to greater motional anisotropy at the middle of the chain rather than at the ends. ... 

---