Echo pulse sequence, quadrupole

The former is a protein of 14.7 kDa involved in the multienzyme nucleotide excision repair (NER) pathway with a determined NMR solution structure . In this protein, the Zn + possesses rather a structural than a catalytic role. Zn NMR spectra were acquired using a rather sophisticated probe (for details, see Reference 87) and operating at temperatures 5-250 K. Data acquisition was performed with the application of spin-echo methods for enhanced sensitivity . Specifically, experiments were carried out at 25 K using a combination of CP (cross-polarization) and spikelet echo pulse sequences which provide a considerable increase in signal-to-noise ratio (of the order of 30) relative to a classical quadrupole echo pulse sequence. The proton field strength applied to the above measurements was 60 kHz with a matching field of 20 kHz for zinc and a contact time... 

The solid-state deuterium NMR experiments were performed on surface-adsorbed material from triethoxyaminopropylsilane-rf, (DAPES) and triethoxy-aminobutylsilane-dj (DABES) prepared as described previously [9]. The samples were heated to 90°C at 10 mmHg for 12 h after adsorption. These deuterated coupling agents have been characterized by IR and NMR spectroscopy. The solid-state deuterium NMR spectroscopy was done on a Varian VXR-200 at 30.7 MHz. A quadrupole echo pulse sequence was used with a 2 s interval, 2 /is 90° pulse, 30 ps echo time, 2 MHz sweep width, and, typically, overnight accumulation. 

H quadrupole powder patterns are generally fairly straightforward to record. Usually, a solid- or quadrupole-echo pulse sequence (90 v i 90, -t-FID) is used in order that dead time losses do not occur these would otherwise severely distort the lineshapes.2... 

Figure 3. The quadrupole echo pulse sequence. The time t] is usually set at 30 ns, and t2 is approximately 25 /ts.

Figure 4. Solid state deuterium NMR spectra of (a) poly (butylene terephthalate) (b) segmented co-polyester containing 0.96 mole fraction of poly (butylene terephthalate) hard segments and (c) segmented copolymer containing 0.87 mole fraction hard segments. (See text for specific deuterium labeling patterns.) All spectra were obtained with the quadrupole echo pulse sequence at 20 °C and 55.26 MHz, using 30 ts at the tj quadrupole echo delay time.

Figure 6. Experimental (a), calculated (b), and difference (c) solid state deuterium NMR spectra for the segmented copolymer with 0.87 mole fraction of hard segments. The spectrum in (a) was obtained at 55.26 MHz and 20 C, using the quadrupole echo pulse sequence. The dashed line in (b) represents the sum calculated for the broad and narrow components in a respective 10 1 ratio. (See text for details of the calculation.) The spectrum in (c) is the difference spectrum obtained by subtracting the spectrum of poly(butylene terephthalate) (Figure 4a) from the segmented copolymer spectrum (Figure 6a).

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... 

Fig. 8. Pulse sequences for nuclear quadrupole resonance (NQR) excitation (a) spin-lock-spin-echo (SLSE) (b) phase-alternated spin-lock-spin-echo (PASLSE) (c) spin-lock-inversion-midecho (SLIME) (d) non-phase-alternated pulse sequence (NPAPS) (e) phase-alternated pulse sequence (PAPS). Numbers above the pulses indicate the relative phase of the RF dashed lines represent expected NQR signals.

Fig. 13. 2H NMR spectra of DMS recorded with the QCMPG pulse sequence (see Fig. 12) at the temperatures shown, from reference 33. For each temperature, the normal quadrupole echo 2H spectrum is shown above the QCMPG spectrum for comparison. Also shown are simulated quadrupole echo and QCMPG spectra, also taken from reference 33.

A composite-pulse sequence to cancel the spurious signals has been described. " The main advantage of this sequence over the Hahn echo sequence has been shown to be in the simplicity of optimizing the line intensity the optimization of only one pulse duration for this sequence but of two pulse durations and the interpulse delay for the Hahn echo sequence. The effects of the first-order quadrupole interaction during the pulses have been considered (spin I = 3/2 nuclei). It has been shown that the size of the sample must be much smaller than that of the r.f. coil in order for the r.f. magnetic field to become homogeneous for the sample. 

Fig. 25. Random walk simulations for static 2H NMR powder lineshapes arising from a quadrupole echo 90°x-t-90°v-t-FID pulse sequence for the model of an isotropic 3° jump.36 (a) Jump correlation time, tj = 411 gs correlation time for the motion, xc = 100 ms, echo delays x as given in the figure. Dotted line is the spectrum for an isotropic random jump with xj = xc = 100 ms and an echo delay x — 200 gs. (b) Jump correlation times xj and motional correlation times xc as given in the figure, echo delay x = 100 gs.

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... 

---