Inversion-recovery quadrupole echo

Figure 8.2 Basic 1-D pulse sequences (a) the quadrupole echo (b) the spin-alignment echo (c) inversion-recovery quadrupole echo and (d) quadrupole echo with pre-saturation for amorphous/crystalline selection, t is the quadrupole echo pulse spacing in each sequence, DE is a delay for spin-lattice relaxation, of either (b) quadrupolar order or (c) and (d) Zeeman order. Flip angles are labelled above the pulses.

Figure 8.2(c) is an inversion-recovery quadrupole echo pulse sequence, which is used to measure the Zeeman spin-lattice relaxation time,, with quadrupole echo detection [8,9,115]. Pre-saturation (Figure 8.2(d)) or progressive saturation (variation of the delay between transients) are also used to measure T. Notably, pre-saturation with spectral subtraction can separate the spectra of domains with different and is used to obtain the individual spectra of the amorphous and crystalline regions of semicrystalline polymers [8]. Also, Void and co-workers have recently presented methods involving selective inversion for the measurement of slow molecular reorientation, which provide an alternative to spin alignment or multidimensional methods [116]. 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

Rg.6. Schematic representation of various pidse sequences, employed in dynamic NMR of I = I spin systems Quadrupole echo sequence (QE), inversion recovery sequence (// ), saturation recovoy sequence (SR) and Jeener-Broekaert sequence (JB)... 

This is illustrated in Fig. 9. The 2D spectra refer to quadrupole echo sequences and characterize two possible reorientation mechanisms of a methyl group (three-site jumps vs continuous diffusion). Drastic spectral differences are observed. Ajqjarratly, these 2D relaxation spectra sensitively indicate the type of motion. The same is true for the corresponding normalized contour dots (see Fig. 9). We note that similar 2D spectra can be obtained from inversion recovery or Jeener-Broekaert sequences (see Fig. 6) [68]. Thus, by applying this 2D technique to different pulse sequoic, the various motions can be differentiated over an extremely wide dynamic range, extending from the fast-rotational to the ultraslow motional re me. Sin<% the different motions (see Fig. 4) modulate different kinds of molecular order (see Fig 3) these orders can be differentiated, likewise. 

---