Quadrupolar echo pulse sequence

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 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 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. 

The excitation schemes, or pulse sequences, that optimize the SNR depend on the relaxation times and frequencies inherent to the quadrupolar nuclei under observation and on the characteristics of the excitation/detection hardware. Focusing first on relaxation times, we define two broad categories (i) 7 , 1 and (ii) 7 2 Tl. Most materials fall in the former category, and the pulse sequences discussed in Sector 4.1.1. all involve the creation and detection of multiple spin echoes, which generally requires that r2e be at least several tens of milliseconds. The most notable exception is RDX, which is in the second category under ambient conditions. For materials with very short 7 ,s such as RDX at elevated temperatures, a single echo pulse sequence, or even a single-pulse FID acquisition, may be required. 

The lanthanum environment in LaAlOs is a symmetric LaOi2 unit, giving rise to a static La NMR resonance sufficiently narrow to produce a recognisable quadrupolar lineshape without the use of an echo pulse sequence which is narrowed further by MAS (Dupree et al. 1989). The highly symmetric La site in LaBg has also allowed the La NMR spectrum of this compound to be obtained readily (Lutz and Oehler 1980). 

The quadrupolar (solid) echo pulse sequence (90° -r - 90°) allows measurement of the spin-spin relaxation time Tir The double-quantum spin-spin relaxation rate T can be determined using a double quantum spin-echo pulse sequence 90° - r - 90° - t l 2 - 180° - - 90°. The first two 90° pulses create... 

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. 

In multidimensional NMR studies of organic compounds, 2H, 13C and 31P are suitable probe nuclei.3,4,6 For these nuclei, the time evolution of the spin system is simple due to 7 1 and the strengths of the quadrupolar or chemical shift interactions exceed the dipole-dipole couplings so that single-particle correlation functions can be measured. On the other hand, the situation is less favorable for applications on solid-ion conductors. Here, the nuclei associated with the mobile ions often exhibit I> 1 and, hence, a complicated evolution of the spin system requires elaborate pulse sequences.197 199 Further, strong dipolar interactions often hamper straightforward analysis of the data. Nevertheless, it was shown that 6Li, 7Li and 9Be are useful to characterize ion dynamics in crystalline ion conductors by means of 2D NMR in frequency and time domain.200 204 For example, small translational diffusion coefficients D 1 O-20 m2/s became accessible in 7Li NMR stimulated-echo studies.201... 

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