C Dipolar Couplings

TABLE 12. Li- C dipolar couplings and calculated Li—C distances between the... 

The anisotropic heteronuclear dipole-dipole interaction is a function of not only the distance between two nuclei but also the orientation of the internuclear vector with respect to Bq. Therefore, the dipolar coupling values directly provide geometrical parameters that can be translated into molecular conformations. For example, H- N, C- N, and H- C dipolar couplings are commonly used to determine the relative orientations of the chemical bonds, which are converted into the conformation and/or the topology of molecules using NMR methods.5 5" ... 

Fig. 11. The LG-CP experiment of Fig. 9(a) applied to - C dipolar couplings in polycarbonate.15 (a) The full two-dimensional spectrum, (b) The LG-CP dipolar lineshape for the quaternary 13C (carbon 4), which as expected shows only very small - C dipolar couplings (as evidenced by the lack of observable splitting of the lineshape). (c) The LG-CP dipolar lineshape for carbon 3. (d) The best-fit simulation to the lineshape in (c). The motion used in this simulation (fast limit) assumes that the phenyl ring containing the carbon reorients about the local twofold axis of the ring through an angle of 120°, with the reorientation axis wobbling with an amplitude of 20°.

Since all possible C—bond orientations with respect to magnetic field are present for powder samples, the peaks for — C dipolar coupling... 

Fig. 7.3, peak positions are given by the dashed lines for C— C dipolar coupling based on usually accepted structural parameters (the bond length between carbons is 0.153 nm and C—C—C bond angle 112°). As seen from the figure, these peak positions agree with those predicted from X-ray data. 

Figure 19 Demonstration of the rotational resonance effect for the framework-bound ethoxy species obtained on CsX from ethyl iodide-1,2- C. The rotational resonance condition was achieved when the spinning speed was set to match the frequency difference (4646 Hz) between the signals at 68 and 17 ppm. Rotational resonance reintroduced the C- C dipolar coupling for the rigid ethoxy species, resulting in a splitting pattern that reflects the short intemuclear distance.

Spinning at 54.7° (the magic angle) from static field axis to achieve averaging of the chemical shift anisotropy to zero. This sharpens the signals of soMd state spectra to solution-state-Mke widths. Suppression of C dipolar couplings is also achieved. Also used in gel-phase NMR (e.g. beads) and in NMR of tissue samples. 

It has been shown by Griesinger and co-workers that it is possible to measure long-range H-C dipolar couplings for the water insoluble compounds in orienting media. The measurements have been performed for menthol dissolved in solution of poly-y-benzyl-glutamate in chloroform. The results have been used to check the usefulness of the method for elucidation of the stereochemistry of the studied compound. 

The surface dynamics of bacteriorhodopsin has been studied by Barre et al. by solid-state NMR. They have measured site-specific H-C dipolar couplings in [3- C] Ala-labelled bacteriorhodopsin, which allowed an insight into internal fluctuation motions on the millisecond or microsecond timescale relevant to a variety of biological functions. 

Shimizu, R. N., Asakura, N., Ando, I., Abe, A., and Furuya, H., Conformational analysis of the alkyl spacer of a dimer liquid crystal in the solid and liquid crystalline states by C- C dipolar coupling, Afagn. Reson. Chem., 36, S195-S199. (1998). 

This delay allows the carbon magnetization to decay because of the H— C dipolar coupling, resulting in spectra where CH and CH2 are substantially removed. This experiment allows identifying quaternary carbon signals and methyl groups. Separate CH and CH2 spectra can also be obtained by a procedure known as CP—polarization inversion. In this pulse sequence, a 40 is CP step is followed by another step (typically 35 ps) in which the phase of the radio-frequency pulse on proton channel is... 

There are additional 2D methods that make use of information associated with nuclear spin interactions other than the H- C dipolar coupling [10]. As an example, Hu et al. [72] described a 2D experiment involving the correlation between C isotropic chemical shift and CSA in a subbituminous coal. The results, combined with spectral editing data, allowed further separation of different chanical groups due to the occurrence of distinct C CSA tensors for protonated, bridgehead, and alkyl-substituted aromatic carbons. 

Figure 3-7. (a) Chemical structure of aliquid crystalline compound, 4-n-pentyl-4 -cyanobiphenyl (5CB). (b) Carbon-13 chemical shift spectrum of 5CB in the nematic phase obtained via cross-polarization under static condition. Thirteen carbon sites are well resolved, (c) A 2D proton-encoded local field spectrum of 5CB in the nematic phase under static conditions. H-C dipolar coupling spectra for each carbon site are shown. Reprinted from reference 47, with permission from the Journal of Physical Chemistry... 

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