Magic spinning angle

Solution NMR spectra are routinely sharpened by spinning the NMR tube. The spectra of solids can likewise be sharpened, but the orientation of the spinning axis is as important as fast spinning. 

Such performance was beyond the capabilities of the NMR spectrometers used in the first few years of CP/MAS NMR studies of lignin. The problem of SSB strength was at first avoided by using low-field spectrometers, but this was unsatisfactory because of the poor sensitivity associated with low fields. The problem was later partly overcome by SSB-suppression pulse sequences (Dixon et al. 1981, Barron et al. 1985), or by applying correction factors to the centerband strength (Hemmingson and Newman 1985). 

Special probes have been developed for solid-state NMR that automatically position the sample at the magic angle. Modern instruments with MAS make the analysis of solid samples by NMR a routine analytical procedure. MAS, combined with two RF pulse techniques called cross-polarization and dipolar decoupling (discussed in Section 3.6.4), permits the use of the low-abundance nuclei C and Si to analyze insoluble materials by NMR, including highly cross-linked polymers, glasses, ceramics, and minerals. 

FIGURE 7.8 Schematic representation of a powder pattern for anisotropic chemical shielding, (a) Typical pattern for x22 033. (b) Typical pattern for axial symmetry where Tn = ct12 = crx and 33 = o-.  

This expression shows that the chemical shielding anisotropy (CSA) leads to a variation of the resonance frequency with orientation according to the familiar (3 cos2 0—1) dependence and thus produces typical powder patterns, as illustrated in Fig. 7.8 for axial and nonaxial symmetries. 

In 1958, long before dipolar decoupling and multiple pulse methods were developed, a very different approach was introduced in an attempt to narrow magnetic dipolar broadened NMR lines in solids. This method focuses on the geometric part of the Hamiltonians (Eqs. 7.6,7.8, and 7.17), namely the term (3 cos2 0—1). 

As we have seen, this term vanishes if 0 is averaged over all space, but clearly it also vanishes if cos2 0 = /3, which implies that 6 = 57.4°. By transforming from molecular coordinates to those that describe the orientation of a macroscopic cylindrical sample, one can show that rapid rotation of the macroscopic sample at an angle of 54.7° to B0 eliminates the 0 dependence, hence all dipolar interactions and CSA effects. 

MAS is normally applied concurrently with the dipole line-narrowing methods in order to eliminate the effects of CSA and provide true high resolution NMR spectra in the solid phase. For heteronuclear systems the combined method is usually referred to by the initials CP-MAS, and for homonuclear systems the acronym CRAMPS (combined rotation and multiple pulse spectroscopy) has been coined. 

MAS is the most common technique employed in solid-state NMR to remove line broadening. It involves fast mechanical sample rotation about an axis inclined at 54°44 (the value where 3 cos G -1 = 0, Equation 5.4) to the direchon of the external magnehc field. It can remove line broadening from dipolar interactions (averaged 

MAS NMR mimics the rapid time-averaged tumbling molecular motion that occurs in Hquid samples. The overall result is that the signal becomes much narrower and the isotropic peaks can be individually observed. In practice, MAS is achieved by loading the sample into a rotor that is axially symmetric about its axis of rotation. The sample is then spun, via an air turbine mechanism, at the magic angle to the applied magnehc field. MAS NMR is discussed in detail in a number of references, for example [17, 18]. 

A typical example of such an approach is investigation of nanoparticle surfaces where the formation and dynamics of lithium species is studied. The resulting high resolution spectra measured at different temperatures show a multitude of environments for the measured species, corresponding to different complexes, in some cases between which ionic exchange can 

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Chemical shift (isotropic component) 5. ISO Magic-angle spinning Chemical bonding coordination number... 

Figure 6 Solid state static and magic-angle spinning NMR spectra of a-Mg2V2(>7.

Nuclear Magnetic Resonance Magic-Angle Spinning... 

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. ... 

J. w. Geus, The interlayer collapse during dehydration of synthetic Na -beidellite a %a solid -state magic-angle spinning NMR study. Clays Clay Minerals. 25 457 (1992). 

Lindberg, J. J. and Hortling, B. Cross Polarization — Magic Angle Spinning NMR Studies of Carbohydrates and Aromatic Polymers. Vol. 66, pp. 1—22. 

As a consequence of restricted internal mobility in molecules in the crystalline state, nuclei in different conformation environments, but identical in other respects, can produce different signals in 13C cross polarization, magic angle spinning (CPMAS) solid-state NMR. This analysis is not necessarily limited to crystalline regions, since signals of different conformations are resolved if the exchange is slow with respect to the time scale of the NMR experiment. 

Sometimes decomposition reactions can be avoided by carrying out diazotizations in concentrated sulfuric acid. By this method Law et al. (1991) obtained the 1,5-bisdiazonium salt (incorrectly called tetrazonium salt) of l,5-diamino-4,8-dihy-droxy-anthraquinone, which is deprotonated to 2.28. The structure was verified by cross-polarization magic angle spinning (CPMAS) 13C NMR spectroscopy. 

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