Antiecho pathway

As stated before, the sign of the ratio R(Iyp) determines the sign of the triplequantum level necessary to refocus the second-order broadening and form an isotropic echo. For spin 1=312 nuclei the echo pathway corresponds to the correlation ofp=-3 andp=-l coherences (solid line in Fig. 4) whereas for spins 7=5/2,7/2,9/2 the echo pathway corresponds to the correlation ofp=3 andp=-l coherences (dashed line in Fig. 4). For the antiecho pathway, for which the anisotropic broadening is refocused at negative values of the acquisition time 2, the assignments are reversed. 

To overcome these difficulties, the z-filter experiment was adapted to MQMAS by Amoureux et al. [24]. In this three-pulse scheme the two hard pulses (excitation of the MQ coherences and conversion into OQ coherence) are followed by a short delay during which the magnetisation is stored along the z-axis as zero-quantum coherences and then transferred into observable IQ coherences using a selective n/2 pulse (Fig. 5a). The symmetrisation of the echo and antiecho pathways during the two hard pulses (p=0—> 3—>0) forces an equal intensity of the echo and antiecho signals, leading to amplitude-modulated FIDs and, thus, to pure absorption spectra. This is a robust method, easy to optimise. 

Fig. 5a-d Pulse sequences and coherence transfer pathways for 3QMAS NMR experiments a amplitude-modulated z-filter acquisition scheme b phase-modulated shifted-echo experiment for spin 1=3/2 E and AE represent the echo and antiecho pathways. Split-tii c z-filter d shifted-echo acquisition schemes for spin 7=3/2. ( ) represents the phase of pulse n... 

Since the experiment relies on a second echo formation, either the echo or the antiecho pathway can be chosen. However, choosing the antiecho pathway requires a much longer echo delay, which can lead to unwanted relaxation or exchange effects generating antisymmetric echoes. It is, therefore, preferable to choose the echo pathway, which is also the choice depicted in Fig, 5d and 5e. 

Fig. 10.4) it has been noticed that opposite phase shifts during phase cycling have to be implemented on Varian instruments when compared to Bruker, since phase shifts have been implemented differently by the spectrometer manufacturers). The remaining antiecho polarization transfer pathway, 5 - 13 connects a single transition of spin S with... 

If the phase cycling used selects only one coherence transfer pathway, a two-dimensional spectrum with phase-twist lineshapes is obtained. However, the experiment can be easily modified to ensure that pure absorption (pure phase) lineshapes are obtained. The most commonly used modified acquisition schemes that allow pure-phase MQMAS spectra to be obtained include amplitude-modulated experiments, with hyper-complex (States) acquisition, and phase-modulated experiments, with delayed acquisition such as shifted echo or antiecho and split-methods. 

An important shortcoming of the two-pulse methods is the difficulty of balancing the echo and antiecho amplitudes to obtain undistorted two-dimensional lineshapes, especially for nuclei with spin larger than 3/2. The two coherence-transfer pathways being different, it has been shown that their amplitude depends on the quadrupole couphng and the orientation of the crystallites. In a powder, it is nearly impossible to reach a perfect equalisation of these amplitudes. 

Fig. 16 a z-filter FAM-IIMQMAS pulse sequences, b Shifted-echo FAM-IMQMAS pulse sequences. For 1=5/2,7/2 and 9/2 nuclei the echo coherence transfer pathway is p=0 3 -l, while the antiecho coherence transfer pathway is p=0 -3 -l, where p is the coherence order. For 1=3/2 nuclei these assignments are reversed... 

Figure 6.9. Sequences for the gradient-selected HMQC experiment. Sequence (a) is suitable for the collection of absolute-value data. Sequence (b) provides phase-sensitive data via the echo-antiecho procedure. The N- and P-type pathways are selected with the last gradient whilst the first two gradients are placed within spin-echoes to refocus shift evolution.

Figure 6.10. A gradient-selected, phase-sensitive HSQC sequence using the echo-antiecho approach. The N- and P-type pathways are selected by the last gradient.

Fig. 11. Left. Experimental schemes for the acquisition of 3D DACSY spectra on quadrupolar spins. The pulse sequence ((a) top) is analogous to the one used in 2D DAS experiment it contains three 90° pulses with relative phase cycled so as to select antiecho coherence transfer pathways, and initial and final sample spinning angles chosen so as to average out second and fourth rank anisotropies, (b) Sequence of r-space cross-...

Figure 6.19 (a) Energy-level diagram for a spin / = 5/2 nucleus, illustrating the effects from the first- and second-order quadrupolar interactions on the singlequantum, triple-quantum and quintuple-quantum transitions, (b) rf pulse scheme for the basic wo-pulse MQ MAS experiment, selecting the coherence transfer pathv ay shown below for a triple-quantum MQ MAS experiment for an / = 5/2 nucleus, where p = 2m is the order of the coherence. The echo (solid lines) and antiecho (dashed lines) pathways are selected by phase cycling of the two pulses (P, and Pj) and of the receiver (Pr). The most simple phase cycle for the echo pathway is p, = 0°, 60°, 120°, 180° and 240° P2 = 0° and Pr = 180°, 0°, 180°, 0°, 180° and 0°. 

---