Pulsed field gradients observation

Fig. 21.1 Pulse sequence to selectively observe solvent-exposed amide protons with TROSY (SEA-TROSY). Narrow and thin bars represent 90 and 180° rf pulses, respectively. Unless specified otherwise, pulse phases are along the x axis. The pulsed field gradients are of 500 ms duration with strengths of gi =20 G cm-1, g2 = 30 G errf1, g3 = 40 G errf1, g"4 = 15 G cm-1, g5 = 55 G errf1. The bipolar gradient gj is 0.5 G errf1 and is used to avoid...

Water self-diffusion coefficients (Dh o) have been determined by pulse field gradient NMR for Nation (Nil/) , BPSH, and SPEEKK and have been shown to increase with increasing acid and water content. At high water contents, the values for PEMs approach the corresponding value for pure water. This is due to the increase in volume fraction of free water. At low water content, however. Nation exhibits greater Dnp values than either BPSH or SPEEKK. In corranon with the observations for EOD, this has been attributed to the smaller channels in aromatic-based polymers, leading to a considerably lower dielectric constant for the water in the channels. 

Pulsed field gradient (PFG)-NMR experiments have been employed in the groups of Zawodzinski and Kreuer to measure the self-diffusivity of water in the membrane as a function of the water content. From QENS, the typical time and length scales of the molecular motions can be evaluated. It was observed that water mobility increases with water content up to almost bulk-like values above T 10, where the water content A = nn o/ nsojH is defined as the ratio of the number of moles of water molecules per moles of acid head groups (-SO3H). In Perrin et al., QENS data for hydrated Nation were analyzed with a Gaussian model for localized translational diffusion. Typical sizes of confining domains and diffusion coefficients, as well as characteristic times for the elementary jump processes, were obtained as functions of A the results were discussed with respect to membrane structure and sorption characteristics. ... 

A(g) is the signal intensity observed with an applied gradient g, A(0) is the intensity in the absence of an applied gradient, y is the nuclear gyromagnetic ratio, and (5 and A are time intervals of the pulsed field gradient spin—echo sequence. 

The D-HMBC pulse sequence can also be used in combination with the pulse field gradient (PFG) technique [12]. Figure 5(c) shows the successful observation of cross peaks between the methyl group at C-5 of an oxazole unit and adjacent carbons in promothiocin. These cross peaks are hidden by the strong t noise of the solvent peak in the HMBC and D-HMBC spectra. The above results clearly indicate that D-HMBC is a quite useful technique for structural studies of complicated natural products. 

Selective probe heads are used for optimal sensitivity for a particular nucleus. Sensitivity of a selective H probe head is normally greater than that of a switchable probe head with indirect observation. With a selective X-nucleus (a nucleus other than proton) probe head, decoupling of protons is normally possible. Because of their limited usefulness, selective probe heads are rare in NMR laboratories. Other probe heads are also available, for example, those for triple resonance experiments and experiments utilizing pulsed-field gradients. In addition to their suitability for 2-D experiments, the gradients are particularly suitable for solvent suppression (20). 

A simple way to view a pulsed field gradient experiment is to add up the twist acquired by the sample magnetization in each gradient pulse and make sure they add up to zero for the desired pathway. If the twist is not zero at the beginning of acquisition of the FID, there will be no observable signal. For example, in the INEPT experiment (Fig. 8.26)... 

For comparison, we have also reported in Fig. 3 the data obtained by Appel and Fleischer [51] with PDMS at 60°C, using the pulsed field gradient NMR technique to measure the self-diffusion coefficient. Except for the slightly higher values due to the increased temperature, it is remarkable that exactly the same trends are observed for the two sets of data corresponding to N = P. 

The obtained parameter values are summarized in Table 3. The self-diffusion coefficient of the proton decreases with lowering temperature, suggesting that the translational motion of water molecules is restricted at the low temperatures. This is probably because of the water-water interaction will be enhanced with decreasing temperature. The self-diffusion coefficients obtained from QENS are less than those determined by the H-NMR pulsed-field gradient method. A probable reason for the difference in Z h is ascribed to different time scales observed by both methods ( 200 peV for LAM40). 

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