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 reorienta-tional dynamics in ionic liquids. 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. 

The vibrational dynamics of water solnbilized in lecithin-reversed micelles appears to be practically indistingnishable from those in bulk water i.e., in the micellar core an extensive hydrogen bonded domain is realized, similar, at least from the vibrational point of view, to that occurring in pure water [58], On the other hand, the reorientational dynamics of the water domain are strongly affected, due to water nanoconfmement and interfacial effects [105,106],... 

NMR 13C spin-lattice relaxation times are sensitive to the reorientational dynamics of 13C-1H vectors. The motion of the attached proton(s) causes fluctuations in the magnetic field at the 13C nuclei, which results in decay of their magnetization. Although the time scale for the experimentally measured decay of the magnetization of a 13C nucleus in a polymer melt is typically on the order of seconds, the corresponding decay of the 13C-1H vector autocorrelation function is on the order of nanoseconds, and, hence, is amenable to simulation. 

G. B. Dutt and S. Doraiswamy, Picosecond reorientational dynamics of polar dye probes in binary aqueous mixtures, J. Chem. Phys. 96, 2475-2491 (1992). 

Observation of reorientational dynamics of dipolar groups surrounding the fluorophore in response to changes in the dipole moment of the fluorophore occurring upon electronic excitation. Such dynamics result in the appearance of spectral shifts with time,(1 ) in changes of fluorescence lifetime across the fluorescence spectrum,(7,32) and in a decrease in the observable effects of selective red-edge excitation.(1,24 33 34) The studies of these processes yield a very important parameter which characterizes dynamics in proteins— the reorientational dipolar relaxation time, xR. 

Rose and Benjamin studied the water dipole and the water H-H vector reorientation dynamics at the water/Pt( 100) interface and the results are reproduced in Fig. 4. As in the case of the translational diffusion, the effect of the surface is to significantly slow down the adsorbed water layer. We note that the effect is very short range, and that the rotational motion of water molecules in the second layer is already very close to the one in bulk water. 

Spohr found a significant reduction in the dipole reorientation time for a different model of water (but using the same water/Pt potential). In that paper, the reorientation dynamics are characterized by the spectral densities for rotation around the three principal axes of the water molecule. These calculations demonstrated the hindered rotation of water molecules in the plane parallel to the surface. In addition, a reduction in the frequency of rotation about the molecular dipole for water molecules in the adsorbed... 

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. 

Very recently, detailed further analyses of the translation and reorientation dynamics of SCW have been reported showing consistency with our results in the limit of very low-density supercritical states for water [47], The dynamical behavior of SCW in this study is also compared to that of supercritical benzene showing that the density dependence of the self-diffusion coefficient and rotational dynamics of SCW is smaller than that of supercritical benzene because SCW is capable of maintaining stronger degrees of structural correlations and orientational anisotropy than benzene, which tends to lose intermolecular correlations at a much faster rate upon decreasing density [47],... 

The population and reorientational dynamics are not indicated in the figure, but may be also derived from the pump-probe measurements. The population lifetime Ti in the first excited level can be inferred from the excited state absorption monitoring (lie v = I v = 2 transition. The molecular reorientation becomes experimentally accessible, introducing polarization resolution in the probing step. For known structural and population dynamics the temporal evolution of the width of the observed spectral hole also provides information on the dephasing time T2 of the vibrational transition (63). 

In this section we will discuss results of transient IR spectroscopy of different alcohols in solution in a wide concentration range from almost monomeric alcohol samples to strongly associated oligomers. In order to investigate the influence of hydrogen bonds on the dynamical properties of the molecules, we present first a discussion of the data on the vibrational and reorientational dynamics of the OH mode of isolated molecules in the solvent. 

Lastly, we consider the diffusive contribution to the signal. Since this portion of the signal arises from molecular reorientation, it should be completely depolarized unless these diffusive reorientational dynamics also have a significant DID component. The orientational decay will be made up of exponential components, the number of which depends on the molecular symmetry and the relationship between the principal axes of the diffusion and polarizability tensors of the molecules (8). If these tensors share no axes, the orientational decay will be composed of a sum of five exponentials. If the tensors share one axis, the decay will be composed of three exponentials. If the tensors share all three axes, the decay will be composed of two exponentials. If the molecule is further a symmetric top, then reorientation about the axis of symmetry cannot be observed, and the decay will be composed of a single exponential. In principle, considerably more information is available when the principal axes of the diffusion and polarizability tensors are not shared however, in practice it is virtually impossible to find a unique fit to the sum of five exponentials, some of which may have very small amplitudes. In the remainder of this chapter we will therefore concentrate on symmetric-top liquids. 

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

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

Since the late nineteenth century, dielectric spectroscopy has been used to monitor dynamical properties of solid and liquid materials. At that time, dielectric measurements were performed either at a single frequency or in a very limited frequency range now, however, measurement technique and instrumentation have developed to such an extent that dielectric spectroscopy is today a well-established method to probe molecular dynamics over a broad range in frequency or time (cf. reviews by Johari [1], Bottcher and Bordewijk [34], Williams [35,36], and Kremer and Schonhals [37]), even with commercially available equipment. Including the latest developments, one can even say that nowadays dielectric spectroscopy is the only method that is fully able to realize the idea of 0- to 1-THz spectroscopy. In data sets that cover the range of up to 10 6—1013 Hz—that is, from ultra-low frequencies up to the far infrared—the full range of reorientational dynamics in... 

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. 

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