QENS experiments

Figure 8 shows a comparison of experimental and calculated energy spectra in the QENS experiments for ethane in NaX at different Q values. Table III contains the molecular mobility data determined by QENS. In Fig. 9 the self-diffusion coefficients obtained by QENS (cf. Table III) are com-... 

An ND study of Ag" shows a coordination (Fig. 5) intermediate between that of Li+ and with a value of Ago 4 (Table II). Based on the assumption that the coordination of monovalent ions scales with hare-ion size, a recent XD experiment has been used to demonstrate that Ag (aq) and Na+(aq) are the same to a first approximation (71, 73). Consequently, difference methods have confirmed the trend that Li+ > Na+ > K in terms of the stability of their hydration shells. The lability of the aquaions in the alkali series is further confirmed in QENS experiments (31b), in which results show that the translational dynamics of the protons of the water molecules are not appreciably perturbed for alkali metal cations other than Li+. Results of computer simulation studies of models in which alkali ion-water potential is based on ab initio calculations give good overall agreement with the... 

We also observe a slowing down of the relaxation time T2 related to internal motions. The relaxation time T2 increases from 2.3 ps at ambient pressure to 3.6 ps at 6000 bar. This increase of T2 is accompanied by a slight decrease of the proportion of mobile protons (from 43% at ambient pressure to 39% at 6000 bar) and of the radius of the sphere in which motions occurs (from 3. 6 A at ambient pressure to 3.1 A at 6000 bar). At atmospheric pressure, QENS experiments on other proteins have shown that the motions observed, at similar wave vector and energy transfer ranges, concern protons belonging to the side chains at the surface of the protein . Thus the observed modifications of internal dynamics can only concern lateral chains. Moreover, molecular dynamics simulations have shown that backbone of BPTI is not affected by pressures up to 5000 bar ". A possible explanation about the pressure effects on internal protein motions is that the effect of pressure is to increase the density of first hydration layer at the protein surface. This water density increase induces... 

For cubic Laves-phase compounds with RpJR > 1-35, the Z2/rj ratio becomes less than 1. In this case, each g site has only one nearest neighbor lying at the adjacent hexagon. Such a transformation of the g-site sublattice may lead to a qualitative change in the microscopic picture of H jump motion the faster jump process is expected to be transformed into the back-and-forth jumps within pairs of g sites belonging to adjacent hexagons. The results of recent QENS experiments... 

Figure 26.6 The hydrogen jump rates t," and Tj" in Cl 4-type HfCrjHj, as functions ofthe inverse temperature. The jump rates are obtained from QENS experiments [84]. The solid lines are the Arrhenius fits to the data.

For data fitting, it is easier to work with the scattering function than with the double-differential cross-section since there are no experimental parameters. During a normal QENS experiment, one measures in Q-o) space motions, which occur in real space and in time. The scattering fimctions are the four-dimensional Fourier transforms of the van Hove correlation function G(r,t) [10]. 

The dynamics of hydration water was investigated by quasi-elastic neutronscattering (QENS) experiments in a completely deuterated penta-alanine peptide at different levels of hydration (7, 30, 50, and 90%) and of dried powder. The last... 

For a detailed comparison between QENS experiments and molecular simulations (Jobic and Theodorou 2006) and for an excellent review of molecular simulations, the reader is requested to refer to articles by Smit et al. (2000) and Smit and Krishna (2001,2003). 

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