Chemical Shift Filter

If the functions of the second and the third pulses are interchanged (Fig. 7.2.6(b)), then the third pulse can be repeated for different frequencies to read out stimulated echoes from different slices. The CHESS multi-slice method is comparatively insensitive to spatial variations in flip angles from B inhomogeneities and makes efficient use of the available rf power. 

In solid-state imaging, frequency-selective pulses are difficult to apply because of the rapidly decaying FID. Nevertheless, for rare spins, selective excitation may be possible [Davl], but also saturation sequences which suppress longitudinal magnetization at selected frequencies for durations of the order of the spin-lattice relaxation time Ti can be applied (cf. Section 5.3.2) [Dodl, Nill]. For abundant spins, demanding line-narrowing sequences composed of hard pulses must be employed. 

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An alternative to using selective pulses in selective ID TOCSY has been proposed [52]. The frequency selection is instead accomplished by using a homonuclear ( H) chemical shift selective filter (CSSF) [53, 54]. The chemical shift filter for frequency selection consists of a non-selective 90° pulse which is set at the frequency of the selected signal, and a systematic increment of the chemical shift evolution between this pulse and the... 

Fig. 20. Experimental setup for applications of the SPECIFIC CP experiment in the context of triple-resonance solid-state NMR experiments. After an initial broadband adiabatic CP step from protons to the I nuclei, SPECIFIC transfer to the observed 5 nucleus occurs during the mixing time tm. The resulting signal represents a dipolar and chemical shift-filtered spectrum and can be controlled by variation of the carrier frequencies and the radiofrequency during the SPECIFIC transfer. A conventional HETCOR experiment is obtained by the introduction of an evolution time t. (Adapted from Baldus et al.215 with permission.)...

More sophisticated experiments may involve binomial coefficients [69,73] for the number of t experiments and application of selective pulses [74,75] in order to minimize signal overlap. The advantage of chemical shift filters over corresponding 2D experiments is that the number of t experiments can be significantly reduced and considerably higher digital resolution can be obtained. 

Figure 10 The chemical shift filtered INEPT experiment. The delay is equal to ti + 2A2 (see Figure 9) and is incremented as in a 2D spectrum. Dec. = decoupling...

There are many situations in organometallic NMR when the magnitude of the spin-spin coupling between rare spins is known and only the relative sign of the coupling has to be determined. In such cases the 2D correlation experiment can be reduced to a chemical shift filtered ID experiment. For example, a chemical shift filtered INEPT pulse sequence [71] can be used for this purpose [70,72] (see Figure 10). Alternatively, selective excitation can be used [70], or both the... 

Alternatively, ID analogues of 2D HSQC, HSQC-TOCSY, and HETLOC experiments and a selective version of/ -resolved spectroscopy using selective excitation and/or chemical shift filtering of proton or carbon resonances may be used for the measurement of coupling constants 49,51-59 Band-selective decoupling of some of the protons during acquisition leads to reduced multiplicity, and so facilitates the multiplet analysis.56... 

Fig. 6.1.10. Comparison of H CRAMPS and C high-resolution spectra of a blend of PEI (U) and PEEK (FI), under cross-polarization from H to " C, before and after, a chemical shift filter (see text) selectively destroys the magnetization of protons in the aromatic region between 5 and 10 ppm [51].

Fig. 10.12. H CRAMPS and CP/MAS spectra of PEI/PAEK (a) spectra without any selection (b) spectra after selection of the methyl proton magnetization of PEI (depicted by U) by chemical shift filter and (c) spectra taken after redistribution of H magnetization from spectrum (b) by spin diffusion. (Reprinted with permission from Ref. [108], 1990 John Wiley, New York.)...

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