Magic-angle spinning MAS

In 1959 Andrew et al. [37] and Lowe [38] proposed, independently, this method to suppress the magnetic dipolar interaction in solids. In 1962, Andrew and Fades [39] showed that MAS could also be applied to eliminate other anisotropic interactions (to first order). To introduce this method, let us take as example again the magnetic dipolar interaction between the nuclei and H. The z-component of the dipolar magnetic field produced by the nucleus on the site is given by  

9 = 9m= 54.74°, where 9 is called the magic-angle. Of course, it is impossible to put all the spin pairs aligned along this specific orientation, except in very special cases of oriented monocrystalline samples. 

As already discussed, it is experimentally difficult to obtain spectra from solid samples for rare nuclei (low natural abundance) or with small gyromagnetic ratios, such as C. The low sensitivity is associated with two problems small NMR signal and long spin-lattice relaxation times [29]. In 1973, Pines at al. proposed the method called Cross-Polarization in order to circumvent these problems [41]. This method involved once more a doubleresonance experiment, based on the transference of polarization from the abundant H spins, with short spin-lattice relaxation times Ti, to the rare nuclei. After this polarization transfer, the rare nucleus signal intensity is increased by a factor equal to the ratio 

For many sequences it is often better to have a uniform rf field over the sample. This can be achieved by decreasing the volume occupied by the sample within the rotor, and spacers can be used. In CRAMPS experiments spacers with spherical cavities often give 

A number of factors result in residual broadening of MAS NMR spectra (Andrew 1972)  

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Solid-state Alnmr spectroscopy has been much used in recent years to study the composition and structure of aluminisilcates (pp. 351 -9) and other crystalline or amorphous Al compounds. The technique of magic angle spinning (MAS) must be used in such cases. ... 

A. R. Tate, P. J. D. Foxall, E. Holmes, D. Molca, M. Spraul, J. K. Nicholson, J. C. Lindon 2000, (Distinction between normal and renal cell carcinoma kidney cortical biopsy samples using pattern recognition of H-l magic angle spinning (MAS) NMR spectra), NMR Biomed. 13, 64-71. 

Although limited by sensitivity, chemical reaction monitoring via less sensitive nuclei (such as 13C) has also been reported. In 1987 Albert et al. monitored the electrochemical reaction of 2,4,6-tri-t-butylphenol by continuous flow 13C NMR [4]. More recently, Hunger and Horvath studied the conversion of vapor propan-2-ol (13C labeled) on zeolites using 1H and 13C in situ magic angle spinning (MAS) NMR spectroscopy under continuous-flow conditions [15]. 

These two techniques are called cross-polarization (CP) and magic angle spinning (MAS) in combination, these are thus called CP-MAS. 

Synthesis of compounds on solid-phase supports became quite popular in the late 1990s and, though interest might have waned a little more recently, there may still be a demand for it in some establishments. If monitoring reactions carried out on resins is what is required, then a magic angle spinning (MAS) probe is the only way to go. 

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). 

Nuclear magnetic resonance (NMR) spectroscopy is most frequently used to analyze liquid samples, but in the magic angle spinning (MAS) mode, this spectroscopy can also be employed to characterize solid catalysts, zeolites in particular [116-120], For example, the 29Si NMR signal can... 

There are also reports of interesting / coupling effects in some magic angle spinning (MAS) spectra as well as new experiments for purposes such as spectral editing. 

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