Multiple quantum MAS

Figure Bl.12.12. Pulse sequences used in multiple quantum MAS experiments and their coherence pathways for (a) two-pulse, (b) z-filter, (c) split-t with z-filter and (d) RIACT (II).

We note that optimal control is a universal tool for experiment design and has also, in solid-state NMR spectroscopy, found additional applications in the design of homonu-clear dipolar recoupling [41], broadband rf pulses and quantum gates [71], building blocks of symmetry-based recoupling experiments [129], quadrupolar multiple-quantum MAS experiments [165], and improved pulses for quadrupolar nuclei [166]. Numerous references to further applications with regard to liquid-state NMR can be found in [72]. 

S. P. Brown and S. Wimperis, Two-dimensional multiple-quantum MAS NMR of quadrupolar nuclei a comparison of methods. /. Magn. Reson., 1997,128,42-61. 

Multinuclear (isocyanide)gold complexes, reactivity, 2, 287 Multi-phase organometallic catalysis, in ionic liquids, 1, 856 Multiple-quantum MAS, half-integer spin quadrupolar nuclei central transition NMR studies, 1, 466 Multistate magnetization transfer, in dynamic NMR magnetization, 1, 410 Multistep catalytic cycles... 

In 1995 Frydman et al. realized that the line narrowing of the central transition can be achieved by changing the coherence state of the spins instead of spatial reorientation of the spinning axis [57,58], and proposed a third method of avoiding second-order quadrupolar broadening, known as Multiple-Quantum MAS (MQ-MAS). This two-dimensional method uses fast MAS and cor-... 

T. Vosegaard, F. H. Larsen, H. J. Jakobsen, P. D. Ellis and N. C. Nielsen, Sensitivity-enhanced multiple-quantum MAS NMR of half-integer quadrupolar nuclei. J. Am. Chem. Soc., 1997, 119, 9055-9056. 

Most recently, the multiple quantum MAS NMR technique was applied to obtain a high resolution solid-state NMR spectrum [16-17], but it seems to need much more time before this technique becomes useful for solid-state NMR. 

Another illustration of how powerful structural NMR tools can be is found in the multiple quantum MAS (MQMAS) NMR study of a G-quadruplex channel. A guanine nucleoside derivative forms tetramers, G-quar-tets. which, in the presence of monovalent cations, assemble to result in a G-quadruplex channel consisting of the stacking of four 6-quartets, coordinating in the channel four cations in this case, three Na" and one While the ID Na-MAS NMR spectrum exhibits only a broad, umesolved signal the 2D MQMAS spectrum gives a clear and neat separation for the three crystallographically distinct Na sites (Fig. 2). 

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. 

J.P. Amoureux, C. Fernandez, Triple, quintuple and higher order multiple quantum MAS NMR of quadrupolar nuclei. Solid State Nucl. Magn. Reson. 10 (1998) 211-223. 

S.E. Ashbrook, S. Wimperis, Novel two-dimensional NMR methods that combine single-quantum cross-polarization and multiple-quantum Mas of quadrupolar nuclei,... 

Two Dimensional Multiple Quantum MAS. For non-integer spin quadrupolar nuclei a two dimensional (2D) Multiple Quantum Magic Angle Spiiming (2D MQMAS) technique emerged in 1995 that has had a profound effect on their study, and has meant that much higher resolution spectra can be produced from these nuclei in sol-gel materials. This experiment correlates the (m, -m) multiple quantum transition to the (1/2, -1/2) transition (Fig. 3-2(c)). The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. At specific times, the anisotropic parts of the quadrupole interaction are refocused and an echo forms. The amplitude of the echo is determined onlyhy isotropic interactions, so that the experiment is repeated at different echo times which forms the 2D data set where the echo amplitude is modulated only by isotropic effects. FT of the echo amplitude then produces an isotropic spectrum that... 

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