Quadrupole echo pulse sequence, solid

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 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).

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... 

In another work the MAS NMR technique is compared to the static powder quadrupole echo (QE) and Jeener-Brockaert (JB) pulse sequences for a quantitative investigation of molecular dynamics in solids. The line width of individual spinning sidebands of the ID MAS spectra were found to be characteristic of the correlation time from 10 to s so that the dynamic range is increased by approximately three orders of magnitude when compared to the QE experiment. As a consequence, MAS NMR is found to be more sensitive to the presence of an inhomogeneous distribution of correlation times than the QE and JB experiments which rely upon line shape distortions due to anisotropic T2 and Tiq relaxation, respectively. All these results have been demonstrated experimentally and numerically using the two-site flip motion of dimethyl sulfone and of the nitrobenzene guest in the a-p-tert-butylcalix[4]arene-nitrobenzene inclusion compound. 

Figure 12 Offset dependence of the intensity of the CT resonance in MAS NMR spectra of Na2MgEDTA-4H20, recorded with a single HS pulse placed before the spin-echo sequence. The experimental data (square points) were recorded using a 7mm probe (with 7kHz MAS rate) the solid red line is just a guide for the eyes. Also shown (blue line) is the simulated first-order quadrupole static powder pattern for the single Mg site in this compound.

---