Quadrupolar echo sequence

Fig. 16. (a) QCPMG pulse sequence consisting of an initial quadrupolar echo sequence (part A)... 

Inversion-recovery deuterium NMR spectra were obtained by performing a 180°-r-90° pulse sequence, followed by the quadrupolar echo sequence (12). Spin lattice relaxation times were estimated from the null points in the inversion-recovery spectra. 

Fig. 5. Fourier transforms of deuterium NMR at 30 MHz for a quadrupolar echo sequence (9O-T-9O90 pulse sequence). The doublet has a splitting of 66 1 kHz, and the solid lines in the middle trace indicate the fit for the doublet portion only, assuming axial symmetry t] = 0). The narrow signals at the center of the traces correspond to 3 at. % D, whereas the doublet component corresponds to 21 at. % D. [From Leopold et al. (1982).]...

Overall molecular reorientations and internal motions take place in the 10 10 s time window and information about them can be accessed using relaxation rate measurements. By far the best approach is to use NMR experiments where deuterium Zeeman and quadrupolar spin-lattice relaxation times are measured on selectively deuteriated mesogens or on deuteriated probes dissolved in the mesophase. Frequency, orientation and temperature dependence of the spectral densities obtained in these relaxation studies give indications about the various motional modes and are extremely useful for testing orientational diffusion and chain dynamics models. Different deuterium relaxation experiments can be employed to extend the observable dynamic range the quadrupolar echo sequence gives spin-spin relaxation times sensitive to motions in the range s, while extremely slow motions... 

When the spectral width of the CT is too large to apply spinning-based techniques, the Hahn-echo or quadrupolar-echo sequence can be used to acquire the static spectrum to minimize the distortion of the line-shape owing to the probe ringing. The stationary spectra also contain information about the CSA. If the quadrupolar and the CS interactions both contribute to the line-shape, both the EFG and the CS tensors may be determined. 

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...

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 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... 

The faithful representation of the shape of lines broadened greatly by dipolar and, especially, quadrupolar interactions often requires special experimental techniques. Because the FID lasts for only a very short time, a significant portion may be distorted as the spectrometer recovers from the short, powerful rf pulse. We saw in Section 2.9 that in liquids a 90°, t, 180° pulse sequence essentially recreates the FID in a spin echo, which is removed by 2r from the pulse. As we saw, such a pulse sequence refocuses the dephasing that results from magnetic field inhomogeneity but it does not refocus dephasing from natural relaxation processes such as dipolar interactions. However, a somewhat different pulse sequence can be used to create an echo in a solid—a dipolar echo or a quadrupolar echo—and this method is widely employed in obtaining solid state line shapes (for example, that in Fig. 7.10).The formation of these echoes cannot readily be explained in terms of the vector picture, but we use the formation of a dipolar echo as an example of the use of the product operator formalism in Section 11.6. 

The H-NMR experiments were performed using the quadrupolar echo pulse sequence ti/2x-T,-7i/2y-T2-acquisition with phase-cycling and quadrature detection. A Bruker MSL 400... 

It is noteworthy that the sharp component gives a signal with the quadrupolar echo pulse sequence. The observation of a quadrupolar echo can be interpreted to indicate that the constraints on the motion of the deuterons persist for a long time (16). Although the deuterium line for the T = 10 ms component is sharp, the observation of a quadrupolar echo proves that the line is not isotropically mobile on the deuterium NMR timescale. The sharp component also gives a spectrum with a... 

The fact that the echo decays very nq>idly in the time domain leads to a problem common in solid-state NMR work the appearance of a pulse feedthrong . This artifact results from the recovery of the receiver circuits and a ringdown of the transmitter and probe circuits after the high-power excitation pulses. To reduce this problem a pulse sequence used to eliminate ringdown in quadrupolar echo NMR was employed [10]. In this sequence, the phase of the tc/2 and the n pulses of the Hahn-echo sequence are varied in such a way that the arti cts add destructively and are essentially eliminated. 

Typical pulse sequences, commonly employed in relaxation studies of 1 = 1 spin systems are shown in Fig. 6. The quadrupole echo (QE) sequence (top) [51] provides the spin-spin relaxation time Tjg [52]. Since Tjg is most sensitive to motions with correlation times equal to the inverse quadrupolar coupling constant, quadrupole echo sequences of deuterons ( H) offer a means to study molecular dynamics in the... 

The quadrupolar-echo experiment represents the most widely used experiment for the observation of quadrupolar nuclei.For half-integer nuelei, it may be tuned to observe only the central transition (1/2-1/2), which is perturbed by the quadrupolar interaction only to second order, thus allowing the observation of less dominant anisotropic interactions. A significant improvement in sensitivity can be obtained by ncorporating a spin-echo method such as the Carr-Purcell-Meiboom-Gill sequence into the detection period. " The powder line-shape splits into a manifold of sidebands, from which information on the homogeneous and inhomogeneous interactions can be extracted from the line-shape of the sidebands and their envelope, respectively. 

Figure 14 Pulse sequence and detected signal for the quadrupolar-echo experiment used to record solid-state spectra.

MHz on a Bruker CXP200 spectrometer using a quadrupolar spin echo sequence with phase cycling. 

The solid state deuterium NMR spectra were recorded on a home-built spectrometer operating at 55.26 MHz for deuterium (360 MHz for protons). The spectrometer has been described previously (5). Routine spectra were obtained in quadrature using the quadrupolar echo pulse sequence (Figure 3)... 

The acquisition of such wide-line signals is more demanding than for high-resolution spectra because the receiver must be turned on immediately following the pulse and the data must be acquired very quickly. For this reason, deuterium spectra are typically acquired using the (90°-T -90°-T -acquire) quadrupolar echo-pulse sequence in which the data are acquired as an echo so the receiver is not turned on immediately after the pulse. 

CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence to odd-half-integer quadrupolar spin systems. It was found that QCPMG (controlled CPMG) can be one order of magnitude more sensitive than quadrupolar echo MAS experiments. Assisted by numerical simulation, the QCPMG MAS experiments can be used to determine the quadrupolar as well as chemical shift tensors. ... 

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