C-H dipolar couplings

When r s, one has interconversion between operators Br and Bs, and Rrs is a cross-relaxation rate. Note that the cross-relaxation may or may not contain interference effects depending on the indices l and /, which keep track of interactions Cyj and C,. Cross-correlation rates and cross-relaxation rates have not been fully utilized in LC. However, there is a recent report41 on this subject using both the 13C chemical shielding anisotropy and C-H dipolar coupling relaxation mechanisms to study a nematic, and this may be a fruitful arena in gaining dynamic information for LC. We summarize below some well known (auto-)relaxation rates for various spin interactions commonly encountered in LC studies. 

Figure 4. Correlation of the chemical shielding and C—H dipolar coupling tensors of the formate group of 50% C-enriched methyl formate powder, a. Computer calculated 2D spectrum for a = 33° and p = 85°. b, Experimental spectrum recorded at 25-MHz C resonance and 90 K. The 2D spectrum permits the determination of the relative orientation of two molecular interaction tensors without the need of growing single crystals. (Reproduced, with permis n, from Ref.

Fig. 9.31 The measurement of the H —N-C -H torsional angle in N-labelled NAV The (a) f FID and (b) frequency-domain spinning-sideband pattern depend on the evolution under the N—and C —H dipolar coupling, and in particular the relative orientation of the two bonds. The (c) best-fit simulation corresponds to a torsional angle of —135°. (Reproduced by permission of the American Chemical Society from [149].)...

Cross-polarization in the solution state has been employed for the selective observation of sealar-eoupled quadrupolar nuclei in anisotropic phases. The method has been apphed to a mixture of beryllium fluoride complexes. Despite severe overlap in the Be speetrum, a quadrupolar splitting was observed for BeFs whereas no sueh sphtting was resolved for Bep4 . NMR spectra of C-iodomethane dissolved in thermotropic and lyotropic liquid crystalline solvents have been used to measure and C- H dipolar couplings. From the... 

Figure 3-7 shows a ID 13C chemical shift and a 2D PDLF spectrum of 5CB liquid crystalline compound in the nematic phase under static conditions [56]. Since the spectral lines are well resolved, C-H dipolar couplings can be directly measured from the 2D spectrum for each carbon site of the molecule. 

Figure 4-7. Experimental dipolar spectra of a methylene group with equal C-H dipolar couplings provided by the signal of the 3-carbon in static and oriented RufHSO sample in columnar phase at 85 °C. The smaller splittings in the SLF spectrum and the inner doublet in the PDLF spectrum are due to a weak coupling to the core proton H4. The frequency axes have been corrected for the scaUng factors...

To this end, Ilin et al. have developed the T-HMBC experiment,120 which is itself based on an experiment developed by Vincent and Zwahlen for measuring dipole-dipole cross-correlation in polysaccharides.121 These experiments allow determining the conformation around glycosidic bonds based on 3Jch couplings and C-H-dipolar cross-correlated relaxation. 

Dipolar couplings of side chains can provide valuable structural information. Dipolar couplings for methylene and methyl sites have been applied to protein structure refinement [49]. In a side chain with a C-CH3 moiety, rapid rotation of the methyl group results in an averaged C-H dipolar vector aligned in the direction of the C-C bond. This rapid rotation scales the real value of the C-H dipolar by a factor of —1/3. Therefore, when including methyl dipolar couplings in protein structure refinement, the orientation of the C-CH3 bond is the one that is restricted, rather than the C-H bond. 

Since the C- H dipolar interaction depends on an intemuclear distance rCH as l/r H, the coupling for proton bearing carbons (r 1.1 A) is much greater as... 

Zibn s solid-state powder NMR method, which measures the H H dipolar coupling, is less affected by motional problems and may give a better estimate of c hh Ligand deuteriation is, however, usually ueeded for optimum results. The kind of accuracy that can be achieved is illustrated by the for Kubas complex, 0.890 0.006 A. In the absence of neutron data, X-ray diffraction has been used but its reliability is questionable for H2 complexes. If J(H,D) is observable in the corresponding HD complex, Morris equation can be used. This reliably relates the J(H,D) value directly to ( hh (and to the bond order). 

Fig. 18. (a) Illustration of the different dynamic regimes considered in the reptation model, (b) Scaling behavior of the correlation function CR(f) available through H DQ MAS NMR in the different dynamic regimes shown in (a), (c) Experimental dipolar couplings for PB and polystyrene-PB diblock (PS-PB) and triblock (PS-PB-PS) copolymers. For details, see ref. 89. 

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