Quadrupolar echo

Solid-state NMR spectra of 2H, where the main concern is to refocus the reasonably small first-order broadening, are commonly obtained by measuring the quadrupolar echo on a static sample. In such a case, the o pq frequencies can be approximated... 

Fig. 5 The quadrupolar echo pulse sequence and coherence transfer pathways for I = 1. The pathway marked in red is eliminated by the EXORCYCLE but not by the conventional quadrupolar echo...

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). 

Fig. 6 (a) SQ and (b) DQ rotor-synchronized 2H MAS NMR spectra of sodium tetrathionate dihydrate-d4 (solid lines). The dashed line in (a) represents the exact numerical simulation of the SQ spectrum for random molecular motion with the rate constant k given in the figure, (c) The corresponding experimental and simulated static 2H quadrupolar-echo spectra, (d) Simulated SQ (solid line) and DQ (dashed line) linewidths as functions of k. (Reproduced with permission from [88])... 

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. 

The 14N spectra of single crystals of h-GaN (Fig. 3, bottom right) are doublets, corresponding to the two transitions — 1 <-> 0 and 0 +1, and also show the expected orientation dependence for a Cq of 24.14 kHz [49]. A 90 — t — 90v x quadrupolar-echo pulse sequence was used to eliminate probe ringdown. The spin-... 

The spin spin relaxation time T2a can be measured by a quadrupolar echo pulse train 90° — x — 90° — (2t — 90°) if the pulse spacing x < C 2/Av is satisfied (i.e., the deuteron spin relaxes as if the quadrupolar splitting Av is absent or if a non-selective excitation is achieved). Due to the large quadrupolar splitting often encountered in LC, the above condition for x cannot be fulfilled experimentally and the two lines relax independently (i.e., r> 21 Av or selective excitation of half of the doublet exists). The quadrupolar echo pulse train then measures50... 

The NMR experiments were performed using the quadrupolar echo pulse sequence 7i/2x—Ti—7i/2y—T2—acquisition with phase-cycling and quadrature detection. A Bruker MSL 400 spectrometer was used for the high pressure studies operating at a resonance frequency of 61.4 MHz. In the liquid-crystalline phase, perdeuterated lipids display NMR spectra, which are superpositions of axially symmetric quadrupolar powder patterns of all C-D bonds.From the sharp edges, the quadrupolar splittings... 

Gullion, T. (1995) Measurement of dipolar interacbons between spin-1 /2 and quadrupolar nuclei by rotabonal-echo, adiababc-passage, double-resonance NMR. Chem. Phys. Lett., 246, 325-330. 

Even if the optimisation of the use of DFS, RAPT or adiabatic inversion pulses is not straightforward for nuclei with low sensitivity, it is nonetheless worth applying one of these methods to improve sensitivity. As long as there is no influence on the CT resonance, these techniques are likely to produce an enhanced CT signal compared to standard spin-echo experiments. Therefore, for Mg (as well as for other insensitive half-integer spin quadrupolar nuclei such as S, K and Ca), it is always advisable to apply some population transfer technique before the excitation of the CT signal. 

The spectrum obtained by FT of the whole train of decaying echoes consisfs of a series of spikelets separated by the frequency vcpMG = l/ra. The envelope of these sidebands is defined by the second-order quadrupolar CT lineshape under MAS. The linewidth of each spikelet is determined by the true transverse relaxation time (T2) of the material, which is a measure of the decay time of the amplitude of the echoes in the... 

F. H. Larsen, H. J. Jakobsen, P. D. ElUs and N. C. Nielsen, Sensitivity-enhanced quad-rupolar-echo NMR of half-integer quadrupolar nuclei. Magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors. /. Phys. Chem. A, 1997, 101,8597-8606. 

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