Hydrogen localized motion

Local motions and segmental orientation in supramolecular hydrogen bond assemblies... 

The characteristics of the dynamic mechanical spectrum of SMAA show drastic changes compared with those of the aPS homopolymer even at very low molar fractions of the added comonomer. All the changes observed reflect the ionic interactions. The a relaxation temperature increases with increasing methacrylic acid content as a consequence of a stable network of chemical crosslinks due to anhydride bridge formation. The y relaxation could be related to local motion of methacrylic acid due to the breakdown of the weakest hydrogen bonds. The 3 relaxation could be attributed to local motion of the backbone chain induced by the breakdown of stronger hydrogen bonds than those invoked for the y relaxation. 

High resolution solid-state NMR spectroscopy is also a very powerful method for characterizing the solid structure and the local motion of different solid polymers. We recently characterized the crystalline-noncrystalline structure for different crystalline and liquid crystalline polymers, such as polyolefins [7-12], polyesters [13-15], polyether [16], polyurethanes [17, 18] and polysaccharides, including cellulose [19-29], amylose [30, 31] and dextran [32]. On the basis of these analytical methods, we also investigated the intra- and intermolecular hydrogen bonds of PVA in both crystalline and noncrystalline regions as well as in the frozen solution state. In this chapter. 

The dynamical properties of hydrogen in Sc are also quite remarkable. NMR measurements of the proton spin-lattice relaxation rate in a-ScH [132] have revealed a localized H motion with the characteristic jump rate tf of about 10 s at 50 K. This localized motion is evident from an additional frequency-dependent peak at low temperatures (35-80 K). The structure of the sublattice of tetrahedral interstitial sites in a h.c.p. metal suggests that the localized H motion corresponds to jumps between two nearest-neighbor sites separated by about 1.0 A in the c direction. QENS measurements on a-ScH [133, 134] have revealed the existence of a still faster localized motion with the jump rate passing through a minimum of approximately 7x10 ° s i near 100 K and increasing to 10 s at 10 K. 

For example, 2-aminopurine (AP) has been used to probe the dynamics of mismatches in DNA [340]. AP can be excited at 320 nm, where the normal DNA bases do not absorb (much), and emits at 380 nm (Fig. 4.36). Time-resolved anisotropy decays of AP across from all four natural DNA bases were performed (Fig. 4.37). AP can hydrogen-bond to T nearly as well as the natural A. The data were fitted to sums of two exponential terms the long time corresponded to overall tumbling and the short time to local motions within the DNA base stack. It was found that the internal correlation time corresponding to local motions, at 4°C,... 

Local Motions and Segmental Orientation in Supramolecular Hydrogen Bond Assemblies, in the field of viscoelastic materials, the technique has been mainly applied to thermoplastic elastomers. One interesting elastomer belonging... 

Case (B) conesponds to the diffusion-controlled reactions of small molecules with functional groups on polymer side diains. Because the local mode of relaxation of a polymer main chain ( -dispersion) and the rotation of a polymer side diain or phenyl group in the main chain (y-dispersicm) occur below T, the reactions controlled by the side chain local motion begin at temperature correspondmg to the onset of rotational motion and show breals at and T. Quenching or hydrogen abstraction of the benzo d)enone triplet in pofy(methyl methacrylate) may be explained in terms of this idea. 

In any case, the observed quasi-elastic intensity shows that the spatial extension of this localized motion is rather small, on the average. We think that in view of the multiplicity of sites available for hydrogen in these amorphous alloys, it is difficult to put forward a quantitative model. We can say that this localized motion of the hydrogen occurs certainly inside cages defined by the surrounding metal atoms. 

Thus we have observed 1) a long range diffusive motion but only for about 50 7o of the hydrogen atoms in the lattice 2) a rapid localized motion with a small spatial amplitude. It is clear that the other hydrogen atoms should also diffuse but, at the measured temperature, this diffusion is probably too slow to be observed on the backscattering spectrometer. 

From these results, we have found that in these two hydrides, all the hydrogen atoms diffuse but the diffusion is much slower. By contrast, the rapid localized motion is qualitatively the same but slower in the LaNi Cu and LaNi Al hydrides than in LaNi Mn and LaNi 5... 

This arises because as the temperature in increased from ambient, the main initial effect is to loosen the hydrogen-bonded local stmcture that iitiribits reorientation. Flowever, at higher temperatures, the themial motion of the water molecules becomes so marked that cluster fomration becomes iitiiibited. 

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