Structure-sensitive Diffusion Processes

Diffusion is defined as the random translational motion of molecules or ions that is driven by internal thermal energy - the so-called Brownian motion. The mean movement of a water molecule due to diffusion amounts to several tenth of micrometres during 100 ms. Magnetic resonance is capable of monitoring the diffusion processes of molecules and therefore reveals information about microscopic tissue compartments and structural anisotropy. Especially in stroke patients diffusion sensitive imaging has been reported to be a powerful tool for an improved characterization of ischemic tissue. 

We have presented two types of nonlinear IR spectroscopic techniques sensitive to the structure and dynamics of peptides and proteins. While the 2D-IR spectra described in this section have been interpreted in terms of the static structure of the peptide, the first approach (i.e., the stimulated photon echo experiments of test molecules bound to enzymes) is less direct in that it measures the influence of the fluctuating surroundings (i.e., the peptide) on the vibrational frequency of a test molecule, rather than the fluctuations of the peptide backbone itself. Ultimately, one would like to combine both concepts and measure spectral diffusion processes of the amide I band directly. Since it is the geometry of the peptide groups with respect to each other that is responsible for the formation of the amide I excitation band, its spectral diffusion is directly related to structural fluctuations of the peptide backbone itself. A first step to measuring the structural dynamics of the peptide backbone is to measure stimulated photon echoes experiments on the amide I band (51). 

Since translational diffusion process is sensitive to the microscopic structure in the solution, understanding the diffusion provides an important insight into the structure as well as the intermolecular interaction. Therefore, dynamics of molecules in solution have been one of the main topics in physical chemistry for a long time. 1 Recently we have studied the diffusion process of transient radicals in solution by the TG method aiming to understand the microscopic structure around the chemically active molecules. This kind of study will be also important in a view of chemical reaction because movement of radicals plays an essential role in the reactions. Here we present anomalous diffusion of the radicals created by the photoinduced hydrogen abstraction reaction. The origin of the anomality is discussed based on the measurments of the solvent, solute size, and temperature dependences. 

Those diffusion systems which have been marked with an asterisk in Table 67 represent metal pairs which form a continuous series of mixed crystals. As a rule in such systems the value of is small, an important exception being selfdiffusion processes (Pb in Pb iDq = 5-1 cm. sec." Au in Au Do = 1-26 X lO cm. sec." ). The normal range of values of Dq lies between 10 and 10 cm. sec.". There are exceptions, however for example, the diffusions of silicon and tin into copper give = 1-Ox 10 and 6-7 x 10 cm. sec. . Dq for the self-diffusion of bismuth, in a direction perpendicular to the c-axis, reaches a value of (1-33—16-3) x 10. It is difficult to assess the reproducibility of some of the data, since some of the diffusion processes listed are structure sensitive, but those for bismuth appear reasonably consistent (see Fig. 100). It is to be noted that the higher the... 

Although the foregoing discussion of structure evolution in polycrystalline films has focused on diffusive processes at the surface, it is generally recognized that the final structure is sensitive to the specific deposition method employed and to the various parameters that influence deposition conditions. As noted earlier, the diffusive processes, which include surface mobility, bulk diffusion and desorption of adatoms, scale with the ratio of... 

The transformation of the amorphous fields has to effect such important characteristics as permeability and diffusivity of water. The transport processes proceed exclusively in amorphous part of any blend matrix, and hence this process will be structure sensitive relative to change of structural organization in intercrystalline fields. 

Obviously, all these structure-sensitive factors influence the reaction kinetics in practical applications to such systems as refractories, ceramics, cement, glass, luminescent materials, semiconductors, catalysts, and pigments. The reactivity of the raw mixes of the constituents, the effects of calcination or burning conditions, and the various types of diffusing species in the course of solid reactions influence the quality of the final product. From an industrial viewpoint the reactions in ionic solid systems have a universal importance. This is so vast a field that for an illustration of this significance an arbitrary selection is necessary. It is hoped, however, that the examples discussed will convey the impact of the reactivity of ionic solids on modern industrial processes and products. 

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