Quadrupolar broadening

The isotope has a nuclear spin quantum number I and so is potentially useful in nmr experiments (receptivity to nmr detection 17 X 10 that of the proton). The resonance was first observed in 1951 but the low natural abundance i>i S(0.75%) and the quadrupolar broadening of many of the signals has so far restricted the amount of chemically significant work appearing on this rcsonance, However, more results are expected now that pulsed fourier-transform techniques have become generally available. 

Static 2H NMR spectra suffer from low sensitivity, as resonances are spread continuously over the 200 kHz range. In samples with multiple sites, they also lack resolution. However, both shortcomings can be remedied by using MAS. As was shown in Sect. 2.3.3, in a rigid sample MAS averages out the inhomogeneous first-order quadrupolar broadening in the same way as it eliminates the CSA in... 

It has recently been demonstrated that the analysis of MAS sidebands patterns can be used to study molecular dynamics in the solid state [85-88]. Indeed, the line narrowing effect of MAS can be partly offset, or completely eliminated, if the 2H quadrupole tensor is reoriented due to motion on a time scale comparable to (first-order quadrupolar broadening, such motion-induced effects should be less evident in the DQMAS spectrum, as has indeed been observed by Wimperis and colleagues in several deuterated solids [87, 88]. For example, the simulation of the SQ spectrum of tetrathionate dihydrate-cfi yielded the same reorientational rate constant as the previously described quadrupolar echo approach (Fig. 6). 

Since the second-order quadrupolar broadening is inversely proportional to the Larmor frequency [(35) and (36)], one obvious means for improving the resolution is to employ the highest available magnetic field strength [94], The use of several spectrometers operated at different magnetic field strengths can be helpful in... 

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. 

Since all transitions involved in MQMAS are symmetric (q = 0), the first-order quadrupolar broadening does not contribute to the dephasing (33). As in DAS, the refocusing condition at time t can be written as... 

In 2006 Wimperis et al. proposed a method called satellite transitions acquired in real time by MAS (STARTMAS) [142, 202], which allows for the real-time acquisition of high-resolution NMR spectra of spin-3/2 nuclei under MAS. This method combines a train of pulses, similar to CPMG [109, 110], with sample rotation at the magic angle to refocus the quadrupolar broadening in a series of echoes, while allowing the isotropic quadrupolar shift and chemical shift to evolve. 

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],... 

Nitrogen-14, with its natural abundance of 99.6%, is one of the most ubiquitous and, until recently, least studied NMR-active nuclei. Due to the integer spin number (/ = 1), its single-quantum transitions are affected by first-order quadrupolar broadening, which in most materials is on the order of a few megahertz. A new class of 2D HETCOR protocols has been recently developed, which makes it possible to indirectly observe well-resolved 14N sites via their spin-1/2 neighbors and obtain the related parameters of the quadrupolar tensors. 

When Be(OH)2 is dissolved in strong alkali (pH > 12) a narrow 9Be NMR signal is observed at ca. 2 ppm (91). This is probably due to the ion [Be(OH)4]2 which, with near tetrahedral symmetry, would be subject to little quadrupolar broadening. 

Substituent effects hae been observed in a series of substituted malonato complexes (167). The 9Be resonance frequencies move to higher field as the basicity of the ligand increases, as can be seen in Table XI. Quadrupolar broadening is considerable in complexes of the hydrolyzed trimer, so much so that unless a species of this sort is present in high concentration relative to the others, its signal may be buried in the baseline noise. The tetrahedron is significantly distorted from Td symmetry in these compounds. 

Mn quadrupolar broadening. On heating these compounds decompose via hetero-lytic cleavage of the p -H-Si bond, typical behavior for a cationic silane complex because the silicon center becomes electron deficient upon coordination of the H-Si bond to metal and thus activated toward nucleophilic attack. 

Paramagnetic broadening, due to unpaired electrons, is usually much greater than quadrupolar broadening, and the nuclear resonance... 

Carbon-13 NMR spectra of all nitro compounds are characterized by quadrupolar broadening of the a carbon signal. 

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