2H powder lineshapes

The effect of chemical shift anisotropy on 2H powder lineshapes has also been discussed.35 Another study emphasizes the importance of recording 2H... 

Resolution of 2H powder pattern lineshapes can be a problem in uniformly or multiply labelled samples. In many cases, 2H labelling of specific sites is near impossible, especially in more complex molecules, or in naturally occurring samples. In other cases, it would be excessively time consuming and expensive to specifically label all sites of interest in a molecule, separately in successive... 

Figure 15.17 shows 2H spectra of powdered hexamethyl-benzene-D g. Since we are dealing with a powder, we expect powder patterns, and the back-to-back superimposition of a pair of asymmetric powder lineshapes is clearly seen. Quad-rupolar interactions can get quite large, and in most cases they will dominate the chemical shift spectrum. For deuterium, which has a rather tiny quadrupole moment, the quadrupolar interaction varies from about 25 kHz in hexamethylbenzene-D g to about 200 kHz in carboxylate deuterons. In the heavier nuclei, such as 127I, the quadrupolar interaction can reach 2000 MHz No one is ever likely to observe a spectrum that wide. 

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.

The different possible conformations of the furanose ring. Top C3 -endo conformation found in DNA-A bottom (V-endo conformation found in DNA-B. (c) Best-fit comparison of simulated 2H NMR powder lineshape to experiment for [2"-2H]-C3 from [d(CGCGAATTCGCG)]2.37... 

Selective inversion recovery experiments i.e. only select frequencies within the powder pattern are excited, have also been performed on 2H for the purposes of studying molecular motion. Initial experiments were performed on deuterated dimethylsulfone (DMS) to demonstrate the utility of the experiment.46 Selective inversion recovery curves were fitted to a suitable motional model, a two-site jump model in the case of DMS, to yield the motional rates as a function of temperature. A significant feature of this work is that the activation energy for the motion so obtained differs markedly from that obtained from earlier 13C chemical shift anisotropy lineshape studies. 

For these complexes, the isotropic and 15N chemical shifts and the 15N chemical shift tensor elements were measured as a function of the hydrogen bond geometry. Lineshape simulations of the static powder 15N NMR spectra revealed the dipolar 2H-15N couplings and hence the corresponding distances. The results revealed several correlations between hydrogen bond geometry and NMR parameters which were analysed in terms of the valence bond order model. It was shown that the isotropic 15N chemical shifts of collidine and other pyridines depend in a characteristic way on the N-H distance. A correlation of the and 15N... 

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... 

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