SEDOR experiment

Detailed analyses of a combination of n3Cd and 31P MAS-NMR and spin-echo measurements and 113Cd-31P SEDOR experiments led to the conclusions that,... 

By referring to their position with respect to Cs+ in a chemical reference (CsCl), i.e., their shift, we will call these lines NS (not shifted), S (shifted), and 2S (twice shifted), respectively. Finding three Cs lines contradicts the expectations of structural studies, for which there is only one site for Cs in this phase, namely the octahedral site [23]. We confirmed this by investigating the same sample batch as for the NMR and, in very similar quenching conditions, through x-ray studies at Brookhaven with RW. Stephens and G.M. Bendele. The particular phase diagram of CsC60 allows one to be convinced rather directly that all these lines are intrinsic they all disappear irreversibly at the transition to the dimerized structure. In addition, SEDOR experiments [21] demonstrate that the different Cs sites are mixed on the microscopic scale. 

Each of these experiments has pros and cons. The SEDOR experiment is easiest to set up but lacks the resolution since it is made on a static sample, and is often limited in the range of accessible dipolar couplings by T2. The REDOR experiment is more elaborate to set up and requires stable spinning, but its increased resolution enables multisite systems to be explored, while the longer T2 enables longer distances to be determined. TEDOR is less sensitive than REDOR as it involves a transfer step with a theoretical maximum efficiency of 50 percent. However, it is not a difference experiment, and is therefore less prone to experimental errors. An important application of TEDOR is its use in 2D-correlation spectroscopy. 

The location of the lead cations in a series of lead-containing A, X and Y-zeolites and their mobility on exposure to water have been studied by a combination of ° Pb NMR and Al- ° Pb double resonance (SEDOR) experiments (Niessen et al. 2001). Systematic changes in the ° Pb relaxation time and the average chemical shift with water content suggest that the chemical environment of the lead cations is strongly affected by the sorption of water and supports a 2-site exchange model for the formation of Pb OH entities (Niessen et al. 2001). 

Fig. 5 Naj Li (left side) and Naj Li (right side) SEDOR results on the model compound LiNaS04. Solid curves show fits to the equation shown, where the inversion factor f takes into account the incomplete spin population inversion caused by strong quadrupolar splittings. The dotted curve shown on left is the theoretical prediction for f=1.00 in the Naj Li SEDOR experiment. Reproduced from [56]...

To probe the potential influence of cation size differences on the distribution, Na[ Cs SEDOR experiments have also been conducted on mixed sodium cesium borate glasses containing 30 mol% alkali [57]. Being a nucleus (1=7/2) with a moderately small nuclear electric quadrupolar moment, the Cs nucleus features similar spectroscopic characteristics as Li. Usually sizeable first-order quadrupolar splittings reduce the inversion efficiency of the n-... 

Slichter s spin echo approach is a variation on the spin echo double resonance (SEDOR) experiment originally proposed by Hahn I03. To see how this experiment works, consider the spin echo sequences shown in Fig 53. The first case, Fig 53a, describes Hahn s original echo experiment 103. In a hetero-nuclear case (always observing the rare spin) any heteronuclear dipolar interactions (along with the chemical shift and inhomogeneous line broadening processes) will be refocused as a result of this sequence. The second example. Fig 53b, is Hahn s SEDOR experiment (74). Here, the heteronuclear dipole-dipole interaction is not refocused because of the application of the second tt pulse to... 

Figure 53 (a) Hahn s original spin echo experiment. The pulse spacing is t. (b) The SEDOR experiment as proposed by Kaplin and Hahn. Again the pulse spacing in t. (c) The SEDOR experiment as proposed by Slichter, et al. [74]. The abundant spin it pul.se here is at All times begin at the end of the first pulse. 

A useful feature of the SEDOR experiment is that by measuring the dipolar coupling between selected nuclei, it probes only their immediate vicinity. This localization is achieved because the dipolar interaction is inversely proportional... 

Fig. 5. Schematic representation of a SEDOR experiment. The pulse sequence applied to the resonating S-spins corresponds to the weU-known Hahn s echo experiment...

The main problem is that the SEDOR experiment is carried out imder static conditions, i.e., without magic angle spinning. Therefore, SEDOR measurements only yield average values for the internuclear distance if a sample contains two or more different types of 7-S-spin pairs with different internuclear distances, r/j. Typical double resonance experiments for the measurement of internuclear distances under magic angle spinning conditions are the REDOR (rotational-echo double resonance [50]) and the TEDOR (transferred-echo double resonance [51]) techniques. 

Ethylidyne (8) has been recognised on Pt/SiOa at 300 K using the SEDOR NMR technique applied to heavily C-labelled ethene the C—C bond length was 149 pm. This seemed to occur on large platinum particles, where areas of (111) face are most likely it was also seen by SIMS on platinum black, but on small particles vinylidene (17) predominated. Similar SEDOR experiments with ethyne showed 75% vinylidene and 25% ethyne as 12A or 15. " Adsorbed benzene was shown to rotate freely at 300 K, and cyclopropane was adsorbed, but not strongly, i.e. without loss of hydrogen. ... 

It is established therefore that imaging of by direct methods is feasible and can be improved by N H experiments even when the resonance is broadened by rapid exchange of attached protons. But if the compound of interest has resolved interactions, then indirect detection by a SEDOR experiment for liquids offers... 

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