MQMAS spinning

In the STMAS experiment,like in MQMAS, spin coherences are manipulated rather than the spinning angle. However, instead of creating symmetric MQCs, the singlequantum satellite transition coherences. In — 1/2 >< n -f l/2l, are now excited. After an evolution period tj, these are then refocused with the symmetric CT SQCs evolving during t2- The relevant accumulated phase thus becomes... 

In the vast majority of cases where NMR is used to localise cations in zeolites, one or both of the following techniques is used Magic Angle Spinning (MAS) or Multiple Quantum Magic Angle Spinning (MQMAS). 

In the following section, we explain the basic protocols used for removing the second-order quadrupolar broadening based on the refocusing of the second-order quadrupolar interaction. These protocols rely on mechanical reorientation of the rotor axis (DAS) or use a combination of sample spinning and rf manipulation of the spins evolution (MQMAS and STMAS). Experimental aspects of these methods, as well as methods for data processing and analysis, are described in Sects. 5.3 and 5.4. 

The quadrupolar effects of order higher than two (7) are usually assumed to be negligible, especially at high magnetic fields. However, once the first- and second-order effects are removed, the measurement of third-order contributions becomes realistic. It can be easily shown that, similar to the first-order case, the CT and all symmetric MQ transitions (q = 0) are free of the third-order contribution, which thus can be safely ignored in DAS, DOR, and MQMAS experiments [161,162]. This is not the case for transitions between non-symmetric spin states, such as the STs. Indeed, numerical simulations of the third-order effect have explained the spectral features that have been observed in 27A1 STMAS spectra of andalusite mineral [161]. 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

The CP can also be used for polarization transfer to MQ coherences of half-integer quadrupolar nuclei [222, 223, 228-231]. This type of transfer is mainly used in the context of MQMAS [228,229,231], although the spin-locking of MQ coherences is also featured in experiments involving homonuclear dipolar recoupling experiments [232]. 

Several basic 2D schemes have been applied in these experiments, based on (1) the use of the SQ evolutions during t and f2, in analogy to the NOESY experiment, (2) the MQ-SQ experiment, in which the evolutions of MQ coherences during t are correlated with the CTs of recoupled spins to enhance the resolution in analogy to MQMAS, and (3) the DQCT-SQ protocol similar to that used in DQMAS NMR of spin-1/2 nuclei. 

MQMAS NMR multiple quantum magic-angle spinning NMR... 

A. P. M. Kentgens and R. Verhagen, Advantages of double frequency sweeps in static, MAS and MQMAS NMR of spin I = 3/2 nuclei. Chem. Phys. Lett., 1999,300, 435-443. 

An extensive analysis of the MQMAS sideband pattern (for both spin- and half-integer quadrupolar systems) was given by Griedrich et a/.141... 

Fig. 15. Comparison between the static, MAS and MQMAS 23Na NMR spectra of multisite sodium salts. Contour levels in the 2D plots were taken at 70%, 35%, 16%, 8%, and 4% of the maximum spectral intensities asterisks indicate the spinning sidebands. (Reproduced, with permission, from Medek et a/.209)...

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