Quadrupole echo spectrum

Fig. 23.9. (A) quadrupole echo spectrum of (A) an oriented block and (B) unaligned silk fibroin fiber of [2,2- H2]Gly-labeled samples.

Figure 23.9(A) shows the quadrupole echo spectrum of an oriented block of [2,2- H2]Gly labeled silk fibroin fibers when the fiber axis was set parallel to the magnetic field direction. The quadrupole echo spectrum, Fig. 23.9(B), of unaligned [2,2- H2]Gly labeled silk fibroin fiber is also observed as a powder pattern. Both spectra are split into doublets, which give the value of the quadrupole splitting, Avq as 117.8 kHz for the [2,2- H2]Gly site the values are the same in each case. The full rigid-lattice width of about 126 kHz should be observed when the motion is frozen [72]. Thus, it is concluded that the motion of the methylene groups of the Gly residue is almost frozen at room temperature, which is in agreement with the prediction from the intermolecular hydrogen bonding network in the silk fibroin backbone chain with an antiparallel /3-sheet conformation.

A restricted (<, ifi) region for Gly in the Ramachandran map (-180° > 0°, 0° > i/r> 180°) was obtained as the overlap of each region obtained experimentally from the NH, NC, CN, and CO bond orientations as described above. The quadrupole echo spectrum of the oriented [2,2- HaJGly labeled silk fibroin fiber yields an angle, 0 = 90 2°, for the C H2 bond vector of Gly residue relative to the fiber axis. A further narrow... 

Deuteron NMR provides an excellent, albeit expensive, method for the determination of the glass transition, Tg. The quadrupole echo spectrum disappears (due to the echo distortion caused by isotropic motion) when the correlation time for chain motion approaches the quadrupole coupling frequency, typically about 30° above the calorimetric Tg. At higher temperatures, a narrow line appears due to fast isotropic motion. Early on, deuteron NMR and the spin alignment experiment (Figure 8.2(b)) were used to characterize slow motions of polystyrene [3]. More recently, deuteron NMR has been used to characterize... 

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.

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.

Fig. 23.20. A series of NMR speetra taken at 76.77 MHz of mouse KIF and mouse macrofibrils where, in both creases, the keratin chains have been labeled with l-[4,4,5,5- H4]lysine. Each spectrum was acquired using the quadrupole-echo sequence and is an average of 50000 acquisitions. The temperatures at which these equilibrium spectra were taken are (a,f) 25, (b,g) -10, (c,h) -20, (d,i) -35, (e,j) -45°C. Even at the lowest temperature, variations in the lineshapes indicate that the onset of the quenching of side chain motions is occurring more significantly for the lysines of the macrofibrils than for those of the KIF.

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).

The three-pulse spin-alignment echo [111,112] (90y-T-45 -DE-45-T-echo, Figure 8.2(b)) provides a second method with which to obtain a spectrum. For DE = 0, the experiment is similar to a quadrupole echo and the DE dependence is determined by slow reorientation (10" < < lO s). In the... 

Figure 8.9 (a) The pulse sequence for the 2-D deuteron exchange experiment where is the incremented period for the first-time domain, t2 is the aquisition period for the second time domain, t is the quadrupole echo spacing and is the mixing time period, (b) A 2-D exchange spectrum of crystalline i-PP. This confirms the 120° jump motion for the helix shown in the chain model. (Reprinted from D. Schaefer, H.W. Spiess, U.W. Sutter and W.W. Fleming, Macromolecules 23 (1992) 3431-3439 by permission of the publishers, American Chemical... 

It has recently been demonstrated that the analysis of MAS sidebands patterns can be used to study molecular dynamics in the solid state [85-88]. Indeed, the line narrowing effect of MAS can be partly offset, or completely eliminated, if the 2H quadrupole tensor is reoriented due to motion on a time scale comparable to (first-order quadrupolar broadening, such motion-induced effects should be less evident in the DQMAS spectrum, as has indeed been observed by Wimperis and colleagues in several deuterated solids [87, 88]. For example, the simulation of the SQ spectrum of tetrathionate dihydrate-cfi yielded the same reorientational rate constant as the previously described quadrupolar echo approach (Fig. 6). 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

Hg. 27 a. Electron spin echo envelope modulation of Co(acacen), temperature 4K. a) Nuclear modulation pattern of Co(acacen) diluted into a Ni(acacen) 1/2 H2O single crystal. Crystal setting rotation axis I, = 20" (From R. de Beer ). b) Fourier transform of the nuclear modulation pattern (From R. de Beer ) c) Stick spectrum ENDOR frequencies (Amu 1, 2) calculated from the hfs and quadrupole tensors in Ref. 59 dashed lines ms = -1/2, /iiH lines mj 1/2... 

Quadrupolar-echo experiments yield lineshapes related to those obtained from the CSA. For example, for an 7 = 1 nucleus such as H, the spectrum of a powder material with axially symmetric quadrupole tensors resembles overlapping mirror-image CSA lineshapes (Fig. 31). The... 

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