Multi-pulse sequences

Lesage et al. have shown that the resolution of the proton NMR spectroscopy of powdered solids can be improved significantly when multi-pulse sequences are employed [44a]. In the approach based on combined rotation and multipulse spectroscopy (CRAMPS) (Figure 7.9) the problem of dipolar line broadening is usually overcome. 

Multi-pulse sequences are designed in such a way, that the product of pulse operators of a cycle is the unit operator 1. Then, for sufficiently small pulse separations t, the evolution operator in (3.3.33) can be approximated by... 

A solid-state variant of the DANTE sequence (Fig. 5.3.15) is obtained by replacing the rf pulses and the free precession periods of the original sequence by line-narrowing multi-pulse sequences [Carl, Corl, Flepl, Hep2J. Such DANTE sequences can be used for selective excitation in solid-state spectroscopy (cf Fig. 7.2.8) and for slice selection in solid-state imaging (Fig. 5.3.16). 

In combination with multi-pulse excitation, rotating-frame imaging can also be used for imaging of solids [Choi]. A time-suspension multi-pulse sequence (cf. Table 3.1) is applied to the sample by use of an rf coil with a homogeneous B field so that both... 

In NMR, multi-quantum coherences can be excited by just two pulses [Eml, Muni] but for rigid samples multi-pulse sequences are more efficient (cf. Fig. 7.2.26) [Bau2, Muni]. Because the receiver coil in the NMR experiment corresponds to a magnetic dipolar detector, only dipolar single-quantum coherence can be detected directly and not multi-polar multi-quantum coherences. However, the latter can be detected indirectly by methods of 2D NMR spectroscopy [Eml]. 

Multi-pulse sequences based on magic echoes... 

Multi-pulse sequences for line narrowing based on magic echoes represent another possibility to realize time-suspension. Such sequences are less difficult to implement than pulse sequences based on solid echoes and consequently, appear to be more popular for use in imaging. Therefore, they are treated separately in Section 8.8. 

Chemical-shift selection The homonuclear dipole-dipole interaction among the abundant nuclei needs to be eliminated by use of a multi-pulse sequence like MREV8. Such... 

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