Homonuclear linewidth

Among several indicators of motional state of organic solids H linewidths has been used from very beginning of solid-state NMR. However, inherently low resolution of H MAS NMR spectra caused by the very strong homonuclear interactions and relatively low MAS speed lead to a very limited application of H MAS NMR for studies of mobility.1 A 2D wideline separation (WISE) experiment correlates carbon chemical shifts recorded under MAS with broad H lines.1,24 As a result a broad ll lines are separated in the components corresponding to the 13C sites in their close proximity. For mobile domains in a sample, the H - ll coupling are relatively weak which leads to relatively narrow 11 static lines. The opposite is observed for the rigid domains. 

Inadvertent homonuclear Hartmann-Hahn transfer during the application of heteronuclear decoupling sequences in a detection period can give rise to undesirable linewidth anomalies (Barker et al., 1985 Shaka and Keeler, 1986). However, no application of Hartmann-Hahn transfer during the detection period of an NMR experiment is known to the authors from the literature. Potential applications include the direct (single shot) acquisition of Hartmann-Hahn coherence-transfer functions in the detection period rather than in an evolution period (Luy et al., 1996). 

Ruthenium is an element with two NMR active isotopes Ru and Ru, but only the former is of practical use since it has the lowest quadrupole moment. Ruthenium-101 has a better natural abimdance but the linewidths are about 33 times larger than with Ru. The observability of Ru is three times lower than that of Mo. Only two homonuclear clusters have been reported Ru3(CO)j2 in C6D6 5( Ru)=-1208 with Avi/2 = 80Hz (reference K4[Ru(CN)6] [42] and [HRu3(CO)ii] N(PPh3)2 with two resonances, one at 5( Ru)= —633 and the second at 6( Ru)= —1202 in intensity ratio 2/1 [43]. 

Figure 9.1. Influence of the MAS speed on the H NMR linewidth from OH groups and molecular water in hydroxyapatite. Note the marked effect on the OH protons reflecting the homonuclear dipolar contribution. After Yesinowski and Eckert (1987), by permission of the American...

In an abundant h spin system, H- h homonuclear dipolar coupling in solids is very large and the H signal is broadened to about 50 kHz. Since this large ll linewidth is sufficiently larger than the typical MAS rate of several kHz, we cannot observe the H high resolution NMR spectrum in solids by only using the MAS method. Under these situations, it is very useful to use multiple... 

The magic-echo frequency-encoding method of Fig. 8.8.1 has been demonstrated on a phantom of adamantane and hexamethylbenzene [Mat 1 ]. The spatial resolution achieved was better than (100 p,m) . For both materials the H linewidth at half-height is about 15 kHz without homonuclear decoupling. 

Highly efficient spin diffusion can be achieved if the chemical-shift difference is scaled below the linewidth due to the homonuclear dipolar couplings of the carbons alone. For uniformly C-labeled samples, this is a few kilohertz and for natural-abundance samples typically in the order of 15 Hz. Assuming a chemical-shift difference of 10 kHz (100 ppm at 10 Tesla), this would (in natural abundance) lead to the requirement of a spin-lock field with ft>rf/(27r) = 3.3 MHz to achieve the necessary scaling (see Equation... 

For such an experiment to work, we have to be able to distinguish the different domains during the evolution and the detection period of the two-dimensional experiment. Since proton spectral resolution in typical solids is very poor, we have to use homonuclear dipolar-decoupling methods to narrow the lines sufficiently to obtain spectral resolution. The 2D spin-diffusion CRAMPS spectrum was first recorded by Caravatti et al. [68] for blends of polystyrene (PS) and polyvinyl methyl-ether (PVME). There are other methods to generate an initial nonequilibrium polarization based on differences in linewidth or relaxation times. The reader is referred to the excellent book of Schmidt-Rohr and Spiess [67] for an overview. 

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