Molecular Reorientation Dynamics

The highly detailed results obtained for the neat ionic liquid [BMIM][PFg] clearly demonstrate the potential of this method for determination of molecular reorientational dynamics in ionic Hquids. Further studies should combine the results for the reorientational dynamics with viscosity data in order to compare experimental correlation times with correlation times calculated from hydrodynamic models (cf [14]). It should thus be possible to draw conclusions about the intermolecular structure and interactions in ionic liquids and about the molecular basis of specific properties of ionic liquids. 

Antony, J. H., Mertens, D., Dolle, A., Wasserscheid, R, and Carper, W. R., Molecular reorientational dynamics of the neat ionic liquid l-butyl-3-methyl-imidazolium hexafluorophosphate by measurement of C nuclear magnetic relaxation data., Chem. Phys. Chem., 4, 588-594, 2003. 

II. Selected Methods to Study Molecular Reorientation Dynamics... 

II. SELECTED METHODS TO STUDY MOLECULAR REORIENTATION DYNAMICS... 

The dynamic window of a given NMR technique is in many cases rather narrow, but combining several techniques allows one to almost completely cover the glass transition time scale. Figure 6 shows time windows of the major NMR techniques, as applied to the study of molecular reorientation dynamics, in the most often utilized case of the 2H nucleus. Two important reference frequencies exist The Larmor frequency determines the sensitivity of spin-lattice relaxation experiments, while the coupling constant 8q determines the time window of line-shape experiments. 2H NMR, as well as 31P and 13C NMR, in most cases determines single-particle reorientational dynamics. This is an important difference from DS and LS, which access collective molecular properties. 

The NMR frequencies at two times separated by the time tm, and thus the corresponding orientations [cf. Eq. (15)] are correlated via cosine functions. The correlation function CSin(fm tp), where the cosine functions are replaced by sine functions, may also be accessible modifying pulse lengths and pulse phases in an appropriate way. This is possible for both CSA and Q interactions. Rotational jumps of the molecules during the mixing time tm lead to 0)>(0) / tm), and hence to a decay of CCOSjSin(fm tp). Therefore, these correlation functions provide access to the details of the molecular reorientation dynamics. 

The (3-process provides a further puzzle There are almost no reports of its detection by depolarized light scattering, even though the latter is believed to probe the same molecular reorientation dynamics as DS. In part, this must be related to the fact that the relevant kilohertz-to-megahertz range is not easily accessible to LS. However, from the rare reports where LS and DS results of 13-processes are compared, it appears that (3-processes only weakly show up in LS, if at all [65,160,161]. As an example, Fig. 36 shows normalized correlation... 

The very detailed results obtained for the neat ionic liquid [BMIM][PFs] demonstrate clearly the potential of the method to determine the molecular reorientational dynamics in ionic liquids. An even more detailed study is given elsewhere [12aj. 

R 226 A. Doelle and W.R. Carper, Molecular Reorientational Dynamics [in Ionic Liquids] , p. 168... 

Dutt GB, Doraiswamy S, Periasamy N. Molecular reorientation dynamics of polar dye probes in tertiary-butyl alcohol—water mixtures. J Chem Phys. 1991 94(8) 5360. http // dx.doi.org/10.1063/1.460521. 

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