Deuterium quadrupolar echo

It is well known that nmr is a powerful means for the study of the dynamics of polymer chains both in solution and in the solid state. The relaxation of 13C nuclei has been extensively employed for this purpose in this and other laboratories. I illustrate here a dilferent and particularly intriguing approach which as yet has seen only very limited application to synthetic polymers. This is deuterium quadrupolar echo spectroscopy, as employed in our laboratory by Dr. Lynn Jelinski and her collaborators (20). The presence of the nuclear electric quadrupole lifts the degeneracy of the two deuterium Zeeman transitions, and in the solid state produces a very broad (ca 200 kHz) powder pattern of transitions which can be interpreted to yield very specific motional information for those carbons labelled with deuterium. In Figure 7 are shown spectra of poly (butylene terephthalate) deuterated on the central carbons of the aliphatic chains ... 

Quadrupolar interactions can offer direct information on the dynamics of organics within zeolite crystals. Eckman and Vega (304) studied the 2H quadrupolar echo decay in perdeuterated p-xylene adsorbed on zeolite ZSM-5. The deuterium quadrupolar interaction usually dominates the spin Hamiltonian, so that the powder pattern can be used as a test for models of molecular motion. At -75°C and 25°C typical rigid-lattice spectra were obtained. At 100°C however, the resonance arising from the aromatic deuterons was motionally narrowed, while the methyl resonance was not, The authors conclude that p-xylene molecules reorient about an axis which passes through the C3 axes of the methyl groups. 

Inversion-recovery deuterium NMR spectra were obtained by performing a 180°-r-90° pulse sequence, followed by the quadrupolar echo sequence (12). Spin lattice relaxation times were estimated from the null points in the inversion-recovery spectra. 

It is noteworthy that the sharp component gives a signal with the quadrupolar echo pulse sequence. The observation of a quadrupolar echo can be interpreted to indicate that the constraints on the motion of the deuterons persist for a long time (16). Although the deuterium line for the T = 10 ms component is sharp, the observation of a quadrupolar echo proves that the line is not isotropically mobile on the deuterium NMR timescale. The sharp component also gives a spectrum with a... 

Fig. 5. Fourier transforms of deuterium NMR at 30 MHz for a quadrupolar echo sequence (9O-T-9O90 pulse sequence). The doublet has a splitting of 66 1 kHz, and the solid lines in the middle trace indicate the fit for the doublet portion only, assuming axial symmetry t] = 0). The narrow signals at the center of the traces correspond to 3 at. % D, whereas the doublet component corresponds to 21 at. % D. [From Leopold et al. (1982).]...

Fig, 15. for deuterium NMR in a-Si H. From the top down, the curves display T, values for the doublet component at 30 MHz as described in the test (filled triangles), the narrow NMR component at 30 MHz (filled circles) and 14.4 MHz (open circles), and calculations for deuterium in isolated paradeuterium molecules in a solid nonmagnetic host at 14.4 and 30 MHz (solid curves). The bottom portion of the figure gives spin-spin relaxation times 7] from the doublet quadrupolar echo envelopes at 14.4 MHz (filled triangles) and 30 MHz (open square) and from narrow component measurements which include 14.4. MHz FIDs (open circles) and 30 MHz FIDs (filled circles). [From Leopold et al. (1982).]... 

The solid state deuterium NMR spectra were recorded on a home-built spectrometer operating at 55.26 MHz for deuterium (360 MHz for protons). The spectrometer has been described previously (5). Routine spectra were obtained in quadrature using the quadrupolar echo pulse sequence (Figure 3)... 

The acquisition of such wide-line signals is more demanding than for high-resolution spectra because the receiver must be turned on immediately following the pulse and the data must be acquired very quickly. For this reason, deuterium spectra are typically acquired using the (90°-T -90°-T -acquire) quadrupolar echo-pulse sequence in which the data are acquired as an echo so the receiver is not turned on immediately after the pulse. 

Overall molecular reorientations and internal motions take place in the 10 10 s time window and information about them can be accessed using relaxation rate measurements. By far the best approach is to use NMR experiments where deuterium Zeeman and quadrupolar spin-lattice relaxation times are measured on selectively deuteriated mesogens or on deuteriated probes dissolved in the mesophase. Frequency, orientation and temperature dependence of the spectral densities obtained in these relaxation studies give indications about the various motional modes and are extremely useful for testing orientational diffusion and chain dynamics models. Different deuterium relaxation experiments can be employed to extend the observable dynamic range the quadrupolar echo sequence gives spin-spin relaxation times sensitive to motions in the range s, while extremely slow motions... 

For quadrupolar nuclei with integral spins STRAFI studies have been reported for H and N (both 7= 1). In this case there is no central transition and the full effects of quadrupolar broadening should be expected when solids are imaged. Deuterium has only a relatively low quadrupolar coupling constant (e.g. Cq < 200 kHz in heavy water ice) and there was little appreciable effect on the echo shapes produced by either the odd or even pulse sequences. Heavy ice was produced by freezing and maintaining heavy water samples ( H enriched to 99.8%) at 268 K, while deuteriated samples of copper sulphate and silica gel were obtained by the addition of heavy water to the anhydrous samples. The echo trains for the last two samples decayed relatively rapidly and only about 16 echoes could be obtained for each train. In contrast, very long echo trains (up to 9000 echoes) were obtained for both... 

Leopold et a/. (1982) observe two distinct deuterium lines, one of which is relatively narrow and one of which exhibits a resolved quadrupolar splitting. Figure 5 shows the Fourier transform of the echo observed after a 90° -t-90° pulse sequence in which the rf phase of the second pulse is 90° with respect to that of the first. The solid line in the 39 °K trace of Fig. 5 represents a calculated powder pattern of the broad (Pake doublet) component of the line. This component comprises 21 at. % of the sample. The... 

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