Spin-echo technique

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

Speck O, Chang L, DeSilva NM, Ernst T. Perfusion MRI of the human brain with dynamic susceptibility contrast gradient-echo versus spin-echo techniques. J Magn Reson Imaging 2000 12 381-387. 

In many products, the spin-relaxation properties of the components can be different due to molecular sizes, local viscosity and interaction with other molecules. Macromolecules often exhibit rapid FID decay and short T2 relaxation time due to its large molecular weight and reduced rotational dynamics [18]. Mobile water protons, on the other hand, are often found to have long relaxation times due to their small molecular weight and rapid diffusion. As a result, relaxation properties, such as T2, have been used extensively to quantify water/moisture content, fat contents, etc. [20]. For example, oil content in seeds is determined via the spin-echo technique as described according to international standards [64]. 

This section presents results of the space-time analysis of the above-mentioned motional processes as obtained by the neutron spin echo technique. First, the entropically determined relaxation processes, as described by the Rouse model, will be discussed. We will then examine how topological restrictions are noticed if the chain length is increased. Subsequently, we address the dynamics of highly entangled systems and, finally, we consider the origin of the entanglements. 

We describe in some detail the techniques of nuclear magnetic resonance which are used for studying alumina-supported platinum catalysts. In particular, we describe the spin-echo technique from which the Pt lineshape can be obtained. We also discuss spin echo double resonance between surface Pt and chemisorbed molecules and show how the NMR resonance of the surface Pt can be separately studied. We present examples of experimental data and discuss their interpretation. 

Figure 4. Spin-echo technique for removing initial transients from signal. Reproduced with permission from Ref. 1. Copyright 1982, The American Physical Society.

The reduction of obtainable light-pulse durations down to subpicosecond pulses (halfwidth about 10 sec) allows fast transient phenomena which were not accessible before to be studied in the interaction of light with matter. One example is the extension of spin echoe-techniques, well known in nuclear-magnetic-resonance spectroscopy, to the photon echoes in the optical region. 

A NMR study of water adsorbed on silica gel has been made by Zimmerman el al. 18). Transverse (Ta) and longitudinal (Ti) relaxation times of various amounts of water adsorbed at 25° have been obtained with the use of the spin-echo technique and a two-phase behavior of both Ta and T relaxation times has been observed as illustrated in Figs. 10a and b. Generally only one T value is obtained, as for a single phase, except for x/m g HaO/g solid) values in the vicinity ol x/m = 0.126. Two values of Ta... 

The rehydrated samples were obtained by exposing dehydrated samples to water vapor at least three days over saturated NH4GI solution at room temperature. A duraction of 0.5 s between scans were allowed for nuclear spin to recover to their equilibrium magnetization. The one—dimensional Na NMR spectra were recorded by using the spin—echo technique. The strength of the radio-frequency field for the two dimensional nutation experiments was 80 kHz and 128 ti values were used (0 250 /is). Each two- dimensional experiment took about 12 hours of spectrometer time. 

In a recent work (207), in situ ll and Al MAS NMR spectroscopy was used to investigate the Bronsted acid sites of zeolite HZSM-5 during the conversion of methanol under CF conditions. As a consequence of the application of MAS instead of the spin-echo technique, only framework Al atoms involved in quadrupolar interactions according to Cqcc < 5 MHz could be observed. Upon injection of methanol into the MAS NMR rotor reactor filled with calcined zeolite HZSM-5, a single Al MAS NMR signal arose at 54 ppm at reaction temperatures up to... 

Fig. 16. Temperature dependence of T2 at 24.3 MHz for m-polybutadiene linear PB (A) and samples with the following average number of repeat units between crosslinks 140 (B), 44 (C) and 14 (D). Open symbols denote T2 measurements obtained by spin-echo technique (reprinted from Ref.731 with permission)...

In the neutron spin echo technique, the normalized intermediate dynamic structure factor I(q,t)/I(qfi) is measured directly. This method has been used to probe... 

An alternative NMR method has been to use pulsed spin-echo techniques with hydrocarbon adsorbates highly enriched in l3C. Those surface species that have l3C—l3C bonds give rise to modulated free-induction decays (FIDs), which depend on direct l3C/l3C dipolar coupling. The analysis of the modulation provides information about the C—C distances present. C, species or C2 species with l3C —l2C bonds give unmodulated FID components. Furthermore, double... 

C NMR has been used to investigate the phenyl ring motions occurring at room temperature in BPA-PC, by using the dipolar rotational spin echo technique [40], Indeed, the reduction in dipolar coupling between carbons and directly attached protons arising from molecular motion (of frequency... 

General overview of several studies of transport and intermolecular interactions in compressed supercritical fluids is presented. The unique aspects of the instrumentation used in these studies are emphasized. First, the results of NMR studies of self-diffusion in supercritical ethylene and toluene are discussed. These experiments used the fixed field gradient NMR spin-echo technique. Second, the novel NMR technique for the determination of solubility of solids in supercritical fluids is described. 

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