Dilute spins correlating

Further possibilities are offered by various two-dimensional NMR methods. For example, heteronuclear solid-state correlation spectroscopy (27) is capable of correlating the spectra of abundant and dilute spins in solids, simplifying spectral assignment and permitting determination of shielding tensors. Futhermore, spin diffusion among abundant spins can be directly observed by this method. 

The experiments based on proton detection of rare spin nuclei are usually the most sensitive methods of determining NMR parameters of magnetically diluted spin systems. Unfortunately, recording of 2D correlation maps is usually also a time consuming experiment, especially if wide spectral bands have to be covered in the indirectly detected dimension. The most frequently encountered situation is that only one or a few peaks are expected within a narrow spectral band. However, the position of this band is not known. Several attempts have been made to reduce the experimental time needed to perform such experiments. One approach would be to record a highly truncated data set and use the linear prediction [86,87] to reduce the effect of the data truncation on the appearance of the spectrum. This technique is now available with most... 

An interesting three-dimensional separated-local-field/dilute-spin-exchange solid state NMR experiment has been reported where homonuclear spin-exchange cross-peaks provide correlation in the un u>z plane. Resonances are resolved along the u> axis by their heteronuclear dipolar couplings. The experiment was demonstrated experimentally with the heteronuclear and N- N homonuclear dipole-dipole interactions in... 

The left-hand panel of fig. 116 shows in addition the relaxation data for CeRhSb doped with 10% La. They are quite in keeping with the results for CeNiSn doped with 15% La. There is no evidence for the formation of hole-hole correlations leading to an AFM state. The spin correlations of CeRhSb are merely weakened by the dilution of 4f electron... 

For dilute spins, such as and 31p, the dipolar broadening can be removed by sufficiently intense rf irradiation at the proton resonance frequency.2-4 Nevertheless, a substantial broadening due to the anisotropy of the chemical shift remains. In order to obtain "high resolution" NMR spectra, it has become customary to subject multllamellar dispersions to prolonged ultrasonic irradiation.5 This process, which results in particles of reduced size with reduced reorientational correlation times, does indeed improve the resolution of the NMR spectya however, its exact physical and chemical consequences are a subject of much debate.6 We describe below a method whereby high resolution spectra can be obtained without resorting to sonication. 

The ds correlation diagram (Fig. Il.37d) is particularly interesting. The ground state CA iP is the only state on the diagram with a multiplicity of 6. This means that for a d5 octahedral complex, all transitions are not only La pone forbidden but also spin forbidden. Absorptions associated with doubly forbidden transitions are extremely weak, with extinction coefficients several hundred times smaller than those for singly forbidden transitions. It is understandable, then, that dilute solutions of Mn(ll) are colorless and only with a substantial increase in concentration is the characteristic faint pink color of [Mn(H20)6]24 observable (Fig. 11.38b). 

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