Pulsed NMR techniques

Pseudo-affine deformation scheme 96, 97 Pseudohexagonal (rotator) phase 67 Pseudorotator phase 67 Pulsed NMR techniques 30... 

The review aims to highlight some recent studies that involve liquid crystals and show the utility of newer pulse NMR techniques in LC. They may involve solutes dissolved in ordered phases and their applications, or may involve the molecular ordering, rotational and/or translational diffusion of solvent molecules. Deuterium NMR spectroscopy has demonstrated many advantages over other nuclei like H and 13C, but the need to specifically deuteriate mesogens is sometimes a major drawback. 13C NMR spectroscopy seems to be useful since non-enriched samples can often be used. However, the use of 13C NMR in semi-solids like LC often requires more sophisticated NMR techniques and instrumentation. There are indeed many uncharted... 

In the early stages of the development of NMR techniques (1950s-mid 1970s), the studies of polymers could be classified into two major domains broad line NMR of the solid state and the high resolution NMR of polymer liquids and solutions. In this period, crosslinked polymers were investigated by the broad line and pulsed NMR techniques, respectively. These studies in the solid state yield information primarily on macromolecular dynamics, and indirectly on the network structure. 

Pulse NMR techniques, both low-field and high-field, were applied to study the properties of water in food systems. All three possible nuclei, H, 2H and 170, were probed, and various models for data interpretation were developed. An extensive review of the subject may be found in Schmidt and Lai 0. 

A series of Al containing MCM-41 samples with Si/Al ratios in the range 62 to 2.5 were synthesized in our laboratory using sodium aluminate [132]. As show in Table 2, NMR data indicated that for both as-synthesized and for calcined samples at least 90-95% of all Al in all samples was located in tetrahedral positions. Table 2 also shows that the BET surface area decreases sharply as the Si/Al ratio drops below 10. This is consistent with TEM observation made by Kloetstra et al. [126] who found that in samples with low bulk Si/Al ratios, most of the iuminum was part of a separate dense phase displaying a tetrahedral environment. This indicates that the conventional one-pulse NMR technique does not discriminate between tetrahedral Al in the MCM-41 framework and tetrahedral Al in the so-called dense phase [126], and should be combined with adsorption and TEM measurements for proper characterization of the state of aluminum. 

We have studied a variety of transport properties of several series of 0/W microemulsions containing the nonionic surfactant Tween 60 (ATLAS tradename) and n-pentanol as cosurfactant. Measurements include dielectric relaxation (from 1 MHz to 15.4 GHz), electrical conductivity in the presence of added electrolyte, thermal conductivity, and water self-diffusion coefficient (using pulsed NMR techniques). In addition, similar transport measurements have been performed on concentrated aqueous solutions of poly(ethylene oxide)... 

The strength and sensitivity of the -H NMR signals of oil shales are sufficient such that pulsed NMR techniques afford a means of dynamically observing thermal transformation processes. Parameters can be extracted from the NMR data that relate to the hydrogen content, phase structure, molecular mobility and free radical content of the specimen. 

Pulse NMR techniques allow one to not only measure the NMR spectrum of the sample but also to evaluate the relaxation times of the nuclei present. The relaxation information is not readily obtainable from the continuous-wave experiment, and, therefore, in addition to being much faster, the pulse experiment also yields more information than the continuous-wave experiment. 

The earliest applications of NMR to analytical problems in the food industry employed CW techniques. Commercial pulsed NMR instruments were introduced at a later stage, but pulsed NMR is now the more widely used of the two methods. Practical aspects of low-resolution pulsed NMR techniques have been reviewed21,286 and the manufacturers (Bruker, Oxford) application notes also provide valuable information on specific procedures. 

The basic principle behind the multiple-pulse NMR techniques to achieve line narrowing (i.e., eliminate the H- H dipolar interaction) is to manipulate the H spin system with r.f. pulses rather than by motion of the whole system, as is done with MAS. This manipulation is performed by using a series of well-timed r.f. pulses such that the average Hamiltonian over the entire period of the pulse sequence does not include the homonuclear dipolar interaction, but still maintains a scaled-down chemical shift e ct. Because of the strict requirements on r.f. pulse widths, shapes, phasing and timing, the multiple-pulse techniques represent some of the most difficult solid-state NMR techniques to implement on a routine basis. The most popular multiple-pulse techniques are currently the eight-pulse MREV-8 and the 24-pulse BR-24 sequence. ... 

Ti and Tz are related to the width, shape and intensity of the NMR absorption line. These quantities can be derived from the measured absorption line or alternatively directly using pulsed NMR techniques (see e.gf. Ref 1). The latter can also be employed for measurements of Tip, which is also a spin-4attice relaxation time but measured at lower frequencies kHz as distinct from MHz for Tj. This offers the advantage of enabling relaxations to be detected with greater resolution. 

The remaricable efficacy of the dual-mode sorption and transport rturdel for description of pure component data has been illustrated by plots of the linearized forms of Eq. (20.4-16) for a wide number of polymer-pen nt systems. " Typical examples of such data are shown in Fig. 20.4-10 for various gases in polycarbonate. These linearized plots are stringent tests of the ability of the proposed functional forms to describe the phenomenological data. Assink also has investigated the dual-mo model using a pulsed NMR technique and concluded the following ... 

The polymer bound fraction, p, can be directly determined using spectroscopic methods such as NMR. The method depends on the reduction in the mobility of the segments that are in close contact with the surface. By using a pulsed NMR technique, one can estimatep. An indirect method for estimation of p is to use microcalorimetry. Basically one compares the enthalpy of adsorption per molecule with that per segment [9]. The latter may be obtained by using small molecules of similar structure to a polymer segment. 

S. Stapf, S. Kariyo and B. Bluemich, Correlating Molecular and Macroscopic Properties of Elastomers by NMR Relaxometry and Multi-Pulse NMR Techniques , in Modem Magnetic Resonance, Part 3, ed. G. A. Webb, Springer, Dordrecht, Netherlands, 2006, p. 1435. 

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