Pulse sequences MQMAS

MQMAS is a very robust technique widely used at present. However, in spite of considerable efforts in optimising the pulse sequence, MQMAS remains non-quantitative and suffers from low sensitivity, particularly when large quadrupole coupling constants are present. This is because coherences must be transferred between the observable and unobservable parts of the density matrix. Recently, Gan proposed an ingenious new technique, known as satellite transition (ST) MAS NMR, which affords nearly quantitative spectra (at least for a... 

Fig. 19 Pulse sequences and coherence transfer pathways used in MQMAS- >-HETCOR experiments. In MQMAS-/-HETCOR, CP is replaced by /-based transfer...

Figure 43 (A) 3D 31P 27Al MQMAS pulse sequence and (B) spectrum providing isotropic/anisotropic and bonding characterisation of the Al-O-P...

The MQMAS has now become the most widespread method for the acquisition of high-resolution solid-state NMR spectra of quadmpolar nuclei. The MQMAS step has been incorporated into the other pulse sequences focusing on achieving the correlation between the quadmpolar and spin-1/2 nuclei (i.e., HETCOR-MQMAS). ... 

Figure 5.23 AI MQMAS spectrum of 8wt% vanadium VO /AbOs- In contrast with the Al MAS spectrum (Figure 5.22), different Al coordination environments are more readily distinguishable. Spinning rate = lOkHz. 60 = 9.40T. Shifted-echo spectra were acquired using the pulse sequence of Brown and Wimperis [163].

Recently, Asbrook and Wimperis[89] proposed a novel approach for combining SQ cross-polarization and MQMAS the SQ coherences created in the CP step are transferred directly to 3Q coherences, rather than via a population state. This results in a pulse sequence with a minimum number of coherence-transfer steps. Two types of experiments have been proposed, using either a z-filter or a reversed split-ti, where pure-phase hneshapes are recorded via acquisition of the whole-echo. In addition, the authors showed that the sensitivity of both experiments is considerably improved by incorporating FAM pulses in the SQ to MQ excitation step. 

Fig. 5. Schematic of the various MQMAS pulse schemes along with their coherence transfer pathways, (a) The basic two-pulse MQMAS scheme (denoted as Ph h), (b) z-filter, (c) z-filter with split-ri where the dotted pathway indicates the required coherence transfer for experiments on a spin I correlating pQC with SQC with I/ I <2/ and the solid pathway corresponds to the case Ip I = 21, (d) shifted-echo split-ri pulse sequence for the case Ip I = 21, and (c) shifted-echo split-ri pulse sequence for the case p < 21. The value of k is positive here.

The pulse sequences for the amplitude-modulated z-filter MQMAS experiment are shown in Fig. 5, with Fig. 5b showing the regular three pulse sequence and Fig. 5c... 

In general, the best MQMAS sequence for the highest resolution in any quadrupolar spin system is obtained with the use of an MQNQ experiment." Here, for instance, in a spin- system, instead of correlating either the 5Q or 3Q with SQC, the second-order quadrupolar frequencies resulting from a 5QC evolution period are refocused with 3QC evolution and only then transferred and detected via SQC. The pulse sequence for this is shown in Fig. 8a where both SQC and 3QC are subjected to a fj evolution (split-fj) to refocus second-order quadrupolar broadening. This, though yielding a better resolution, is prone to sensitivity problems more than other MQMAS schemes. 

We now outline pulse sequences that have been found to be the optimal ones for MQMAS spectra of spins-1, and j using FAM for signal sensitivity enhancement. Here, we discuss some specihc cases, showing in addition the experimental results. All the pulse sequences are based on the shifted-echo split-f j scheme. 

After a 2D Fourier transformation the resonances will show up as ridges lying along the quadrupole anisotropy (QA) axis. The isotropic spectrum can be obtained by projection of the entire 2D spectrum on a line through the origin (v j = V2 = 0) perpendicular to the QA axis, figure B1.12.12 shows some of the many different pulse sequences and their coherence pathways that can be used for the 2D MQMAS experiment. 

Fig. 10. Coherence pathways of the echo signals associated with two commonly employed MQMAS pulse sequences. The case of spin 3/2 was taken as an illustration.

MQMAS (Multiple Quantum MAS) is used to resolve resonances of noninteger quadruplar nuclei complete narrowing of the second-order quadrapolar line shape caimot be achieved by MAS alone, but this is feasible when MAS is combined with multiple quantum pulse sequences.This was used extensively in studies of zeolites. 

Fig. 14. (a) The two-pulse MQMAS sequence. The refocusing echo occurs at t2 = kt (solid line) and the anti-echo at t2 = —kt (dashed line), (b) The shifted-echo MQMAS sequence. An intermediate storage of the p-multiquantum coherence in the (+1) quantum coherence leads to a shift of the echo to t2 = kt + mtr. Fourier transformation of the whole echo gives a pure-absorption multiquantum spectrum, (c) z-Filter MQMAS sequence. Symmetrical pathways make the anti-echo and echo intensities equal. It allows a pure p-multiquantum spectrum. (Adapted from Charpentier el alP with permission.)... 

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