Nuclear magnetic resonance diffusion experiments

From the above comparison it seems clear that there are considerable discrepancies in the degrees of slowing down between nuclear magnetic resonance (NMR) experiments and CMD. This is especially true for the translational diffusion constant. 

Carr H Y and Purcell E M 1954 Effects of diffusion on free precession in nuclear magnetic resonance experiments Rhys. Rev. 94 630-8... 

For a given sequence, Bloch equations give the relationship between the explanatory variables, x, and the true response, i]. The / -dimensional vector, 0, corresponds to the unknown parameters that have to be estimated x stands for the m-dimensional vector of experimental factors, i.e., the sequence parameters, that have an effect on the response. These factors may be scalar (m — 1), as previously described in the TVmapping protocol, or vector (m > 1) e.g., the direction of diffusion gradients in a diffusion tensor experiment.2 The model >](x 0) is generally non-linear and depends on the considered sequence. Non-linearity is due to the dependence of at least one first derivative 5 (x 0)/50, on the value of at least one parameter, 6t. The model integrates intrinsic parameters of the tissue (e.g., relaxation times, apparent diffusion coefficient), and also experimental nuclear magnetic resonance (NMR) factors which are not sufficiently controlled and so are unknown. 

Carr HY, Purcell EM. Effects of diffusion on free preces- 46. sion in nuclear magnetic resonance experiments. Phys. Rev. 1954 94 630. 

There are macroscopic (uptake measurements, liquid chromatography, isotopic-transient experiments, and frequency response techniques), and microscopic techniques (nuclear magnetic resonance, NMR and quasielastic neutron spectrometry, QENS) to measure the gas diffusivities through zeolites. The macroscopic methods are characterized by the fact that diffusion occurs as the result of an applied concentration gradient on the other hand, the microscopic methods render self-diffusion of gases in the absence of a concentration gradient [67]. 

Jost S, Bar NK, Fritzsche S, Haberlandt R, and Karger J. Diffusion of a mixture of methane and xenon in silicalite A molecular dynamics study and pulsed field gradient nuclear magnetic resonance experiments. J Phys Chem B 1998 102 6375-6381. 

Solid state nuclear magnetic resonance spectroscopy (NMR), e.g. [107-109]. This technique is sensitive to the local environment of certain nuclei, their mobility and orientation [108]. It provides information about the heterogeneity of polymer blends to c. 5 nm or less (spin diffusion experiments) or c. 0.3 nm in cross-polarization experiments, from which the direct (averaged) distance between two types of nuclei in a sample can be determined [107,108]. Motions of moleuclar groups in a polymer chain can be analyzed and correlations with dispersion areas in the mechanical spectra may be possible [109]. Solid state NMR is not a standard technique at the present time but it is becoming increasingly important. 

Several experiments have been carried out to confirm the physical properties of solitons in mns-polyacetylene [27]. Lately, this excitation has also been studied in another degenerate ground state conjugate polymer, poly(l,6-heptadiene) [28]. The onedimensional spin diffusion and associated spin dynamics are verified from electron magnetic resonance spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and electron nuclear double resonance (ENDOR) measurements [13]. The density of neutral solitons has been estimated by Motsovoy and co-workers [29]. For more details on the physical properties of solitons, the reader is referred to a review article by Heeger and co-workers [13]. However, more theoretical and experimental work is... 

Various experiments indicate that properties of the microemulsion phase change continuously with increasing salinity as inversion from a water-continuous to an oil-continuous microstrucmre occurs. For instance, electrical conductivity decreases continuously with increasing salinity (Bennett et al., 1982). In addition, the self-diffusion coefficient of oil as measured by nuclear magnetic resonance (NMR) techniques increases from small values at low salinities where oil is the dispersed phase to a value comparable to that of the bulk oil phase near and above the optimal sahnity. The self-diffusion coefficient of water, in contrast, decreases from a value comparable to that in pnre NaCl brine below and near the optimal sahnity to mnch smaller values at high salinities where water is the dispersed phase (Olsson et al., 1986). Thus the surfactant phase is bicontinuous near the optimal sahnity, as originahy proposed by Scriven (1976) and subsequently confirmed by electron microscopy (Jahn and Strey, 1988). 

In contrast with chemical diffusion, the self- (or intra-or tracer-) diffusion of ions simply is the manifestation of Brownian motion. It can be visualized by labeling a small amount of ions and observing their displacement in an environment of nonlabeled ions of the same type. Nuclear magnetic resonance (NMR) spin-echo experiments use spin labeling radioactive tracers are used in closed capillary methods. At infinite dilution the self-diffusion of an ion X, is linked to its limiting ion conductivity Eq. (43), and its generalized mobility a>,, Eq. (15), (F = CoNa, Earaday constant) ... 

Nuclear magnetic resonance (NMR) spectrometers offer spectral capabilities to elucidate polymeric structures. This approach can be used to perform experiments to determine comonomer sequence distributions of polymer products. Furthermore, the NMR can be equipped with pulsed-liied gradient technology (PFG-NMR), which not only allows one to determine self-diffusion coefficients of molecules to better understand complexation mechanisms between a chemical and certain polymers, but also can reduce experimental time for acquiring NMR data. Some NMR instruments can be equipped with a microprobe to be able to detect microgram quantities of samples for analysis. This probe has proven quite useful in GPC/NMR studies on polymers. Examples include both comonomer concentration and sequence distribution for copolymers across their respective molecular-weight distributions and chemical compositions. The GPC interface can also be used on an HPLC, permitting LC-NMR analysis to be performed too. Solid-state accessories also make it possible to study cross-linked polymers by NMR. 

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