Nuclear Magnetic Resonance NMR Methods

NMR techniques provide a somewhat more convenient and widely used method for the measurement of self-diffusivities. The method is, however, restricted to species such as hydrocarbons which contain a sufficiently hi concentration of unpaired nuclear spins. In comparing the results of NMR and uptake rate measurements it follows from Eqs. (5.6) and (5.9) that one should compare the NMR self-diffusivity with the corrected diffusivity from the uptake rate measurements. Exact agreement can be expected only when the cross coefficient is zero, but this is normally a good approximation at low concentrations. 

In the earlier NMR studies the self-diffusivity was derived indirectly from measurements of the spin-lattice relaxation time over a range of temperatures. The correlation time which is roughly equivalent to the average time between successive molecular jumps, may be derived from such information and the self-diffusivity is then estimated from the Einstein relation using an assumed mean square jump length A. For an isotropic cubic lattice 

The obvious disadvantage of this approach is that the mean square jump distance is seldom known with any accuracy and cannot be easily estimated a priori. 

The pulsed field gradient method of self-diffusion measurement was originally developed by Stcjskal and Tanner for the measurement of diffusion in liquids. The self-diffusivity is measured directly and no prior estimate of jump distance is needed so the results are, in principle, more reliable. The development and application of this technique to the study diffusion in zeolites and zeolitic adsorbents has been achieved largely through the researches of Pfeifer, Karger, and their co-workers at the Karl Marx University in Leipzig, 

FIGURE 53. Schematic diagram showing the principal features of NMR pulse field gradient spectrometer. 

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Following hits, the lead compounds are purihed using chromatographic techniques and their chemical compositions are identihed via spectroscopic and chemical means. Structures may be elucidated using X-ray or nuclear magnetic resonance (NMR) methods. 

This is the first attempt to examine family-held ginseng landraces that have been in cultivation for several decades. Clearly, there is evidence of variation between these landraces. Although HPLC-DAD analysis clearly quantified ginsenoside levels and showed variability, it did not present unique characteristics that would rapidly identify each landrace. Currenfiy, Nuclear Magnetic Resonance (NMR) methods are... 

The cationic ring-opening polymerization of cyclic ethers has been the subject of many recent investigations (1.. Nuclear magnetic resonance (NMR) methods, particularly carbon-13 techniques, have been found most useful in studying the mechanism of these polymerizations ( ). In the present review we would like to report some of our recent work in this field. 

The nuclear magnetic resonance (NMR) method is based on the interaction between matter and electromagnetic forces, and can be observed by subjecting... 

Nuclear magnetic resonance (NMR) methods are routinely used for structural confirmation and a number of the papers referred to in this chapter contain NMR data in addition to C NMR data. 

The simulated distillation data (Table V and Figures 2, 4, 6) and the FIA analyses of the distillates (Table II) were obtained by standard ASTM methods D2887 and D1319, respectively. The mass spectrometric analyses (MS) of the saturates fractions (Table VI) were obtained by an in-house method similar to that of Hood and O Neal (44). The aromatic fractions were analyzed by the proton nuclear magnetic resonance (NMR) method of Clutter et al. (45), and the results are reported in Tables VII and VIII. 

A 400 MHz nuclear magnetic resonance (NMR) method for the identification of rocuronium bromide was reported by Fielding [10]. The aH spectra were referenced to an internal TMS standard, while the solvent signals at 77.0 ppm (CDCI3) or 39.6 ppm (DMSO-dg) were used as references for the 13C data. Based on the NMR spectra, it was concluded that... 

Certain spectroscopic techniques, such as nuclear magnetic resonance (NMR) methods, require that the membrane mimetic, i.e., the lipid aggregate is not too large, and that the lipids exhibit a high degree of motion. For such studies, the micellar membrane model is often preferred. Micelles are relatively small (Fig. 3, top), which means that they rotate rapidly, on the time-scale required for NMR. These micelles consist of detergent molecules that aggregate above the critical micelle concentration (CMC). The size of a micelle is defined by the... 

The measurement of complexation has been accomplished by a variety of techniques. For alkali metal cations with simple crowns, calorimetric and ion selective electrodes have been, by far, the most common. Nuclear magnetic resonance (NMR) methods have also been used commonly and increasingly as the receptors and their substrates become more complex. 

Remarkably, it took almost 25 years to unravel the structure of glycopeptide antibiotics. The reasons are based on the highly complex structure of glycopeptides and the limited power of analytical methods, especially of nuclear magnetic resonance (NMR) methods, by that time. With the progress in the development of NMR... 

Structural characterization by nuclear magnetic resonance (NMR) methods is routine and, in general, mention of the results has been deferred to the sections dealing with the synthesis of particular heterocycles. This section includes results where sufficient work has been performed to suggest patterns and where more unusual techniques have been discussed. 

A more serious complication for the betaine mechanism arose when it was found that oxaphosphetanes are more stable than betaines. The earliest evidence was encountered by Ramirez et al. (17), who found that certain phosphines react with two equivalents of hexafluoroacetone to give 1,3,2-dioxaphospholane derivatives 23 (Scheme 6). These compounds rearrange into unusually stable oxaphosphetanes 26 via fragmentation to 24, followed by a proton shift to generate an intermediate ylide 25 (17). Shortly thereafter, Vedejs and Snoble (18) used P nuclear magnetic resonance (NMR) methods to show that more typical Wittig reactions of nonstabilized ylides PhjP CHR also produce oxaphosphetanes and that these intermediates can be easily observed at temperatures below 0°. Since betaines did not accumulate in any of the experiments, their conversion to oxaphosphetanes could not be rate... 

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