Shifted-echo pulse sequences

Fig. 15. The FAM-RIACT-FAM split-Z] shifted-echo pulse sequence for sensitivity enhancement in the 3QMAS experiments of spin- systems. Also shown are the coherence transfer pathways.

The aluminium species in activated zeolites have been studied using NMR spectroscopy. Aluminium and iron in the framework of zeolites have been analysed quantitatively by Al NMR spectroscopy. The templating of high-silica zeolites by THF has been studied by C NMR spectroscopy. O NMR spectroscopy has been used to study the atomic multipoles in Fl-form aluminosilicate frameworks. A comparison of fast amplitude modulation shifted-echo pulse sequences for O 3Q- and 5Q-MAS NMR experiments has been carried out for oxygen sites in zeolites. By using a combination of the Monte-Carlo technique with Si NMR spectroscopy, cation sites in zeolites have been located. " The growth of zeolites on kaolin microspheres has been investigated by NMR spectroscopy. F1 MAS spin echo NMR spectroscopy,... 

A few relatively recent published examples of the use of NMR spectroscopy for studying polymer degradation/oxidation processes will now be discussed briefly. At the early stages of degradation, the technique can be used to provide chemical identification and quantification of oxidised species for polyolefins, oxidation sites can be identified by the chemical shifts of -CH2- groups a and ji to carbons bonded to oxygen [85]. Spin-spin relaxation times may be determined by a pulse sequence known as the Hahn spin-echo pulse sequence. 

Fig. 14. (a) The two-pulse MQMAS sequence. The refocusing echo occurs at t2 = kt (solid line) and the anti-echo at t2 = —kt (dashed line), (b) The shifted-echo MQMAS sequence. An intermediate storage of the p-multiquantum coherence in the (+1) quantum coherence leads to a shift of the echo to t2 = kt + mtr. Fourier transformation of the whole echo gives a pure-absorption multiquantum spectrum, (c) z-Filter MQMAS sequence. Symmetrical pathways make the anti-echo and echo intensities equal. It allows a pure p-multiquantum spectrum. (Adapted from Charpentier el alP with permission.)... 

In Figures 2 and 3 are shown the GAMMA C++ code for a simulated spin-echo pulse sequence using ideal RF pulses and a simulated PRESS pulse sequence that accounts for a non-ideal RF pulse envelope, crusher gradients and spatially varying RF refocusing due to resonance group chemical shift offset and... 

FIGURE 9.1 Use of a spin echo pulse sequence to refocus two magnetizations precessing at different frequencies because of difference in chemical shift. 

The spin-echo pulse sequence can also be applied to produce homonuclear (e.g., H) J-resolved spectra. For such a system it is not possible to apply broadband decoupling, so the F2 dimension might be expected to display the ordinary coupled H spectrum. However, because the coupling information is independently available, it is not difficult to process the data in such a way that only the chemical shifts are displayed in the F2 dimension, as illustrated in Fig. 10.7. 

For the morphological discussions, NMR measurements must be carried out in the solid state. The cross-polarization and magic angle sample spinning (CPMAS) for and the solid echo pulse sequence for are practical methods. From CPMAS measurements, the spectrum similar to the solution NMR spectrum is observed. NMR chemical shifts are easily obtained from the CPMAS spectrum. Furthermore, the application of the sequence developed by Torchia supplies Tj in the solid state. However, C NMR measurements are time consuming because of the low natural abundance of C nucleus. Tj of H can be observed easily by the solid echo... 

Figure B2.1.10 Stimulated photon-echo peak-shift (3PEPS) signals. Top pulse sequence and iuterpulse delays t and T. Bottom echo signals scaimed as a fiinction of delay t at tluee different population periods T, obtained with samples of a tetrapyrrole-containing light-harvesting protein subunit, the a subunit of C-phycocyanin.

Spin-echo The refocusing of vectors in the xy-plane caused by a (t-180°-t) pulse sequence produces a spin-echo signal. It is used to remove field inhomogeneity effects or chemical shift precession effects. 

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