Fast reorientation

Such a value appears to be intermediate between the two types of motion mentioned above. The explanation is that the above values apply to fully dispersed systems. We are monitoring here the "fast reorientation of a local director for clay platelets that have stacked-up into tactoids and that therefore are much slower in reorienting. 

The relaxation time for this new dynamic equilibrium varies from femtoseconds to picoseconds. The fast reorientation of solvent molecules causes a fast solva-tochromic shift in the fluorescence band of the organic chromophores. Solvation dynamics is measured in terms of (8v (0) 8v (/)), where the fluctuating frequency v(t) is the difference in solvation energies between the two electronic states involved, i.e., v(t)= sE(t)/h [110]. In time-resolved emission spectroscopy the time dependence of the excited-state distribution is monitored via the frequency shift of the emission... 

For rotational diffusion of the protein molecules two extreme situations can be described analytically in the case of slow reorientation of the EFG, the amplitude of the perturbation function is damped exponentially, but oscillations are stiU present, and in the other extreme of fast reorientation of the EFG, the oscillations in the perturbation... 

Diffusional motion. Many rotational and translational diffusion processes for hydrocarbons within zeolites fall within the time scale that is measurable by quasielastic neutron scattering (QENS). Measurements of methane in zeolite 5A (24) yielded a diffusion coefficient, D= 6 x lO" cm at 300K, in agreement with measurements by pulsed-field gradient nmr. Measurements of the EISF are reported to be consistent with fast reorientations about the unique axis for benzene in ZSM-5 (54) and mordenite (26). and with 180 rotations of ethylene about the normal to the molecular plane in sodium zeolite X (55). Similar measurements on methanol in ZSM-5 were interpreted as consistent with two types of methanol species (56). 

The sorbed benzene molecules perform fast reorientations about their C-6 axes. Even at 125 K, the correlation time of this motion is much shorter than 1 ills. 

It appears (see Refs. 13 and 14) that during cooperative fast reorientations of hydrogen atoms, occurring inside a rather stable tetrahedral-like set of oxygen atoms, some favorable configuration may arise, in which a proton attached to a water molecule jumps to form another H-bond. From this viewpoint the lifetime... 

In water the lifetime Tor of the LIB state is commensurable with a part of a picosecond and is shorter in ice. An important feature of this state is a fast reorientation (sometimes termed disorder) of hydrogen atoms inside a rather stable tetrahedral-like network of oxygen atoms. As noted above and in the Appendix I, this reorientation is cooperative, since a rotating proton, attached to a water molecule, jumps in a certain favorable configuration to form another H-bond. 

For more rapid motions, however, characteristic deviations occur since, depending on the particular type of motion, at least a certain part of the quadrupole interaction is averaged to zero. Some theoretical spectra for perdeuterated benzene molecules undergoing different types of fast (t. o) ) motion are collected in Table 1 [66]. The transition of the spectra from the rigid case to the limiting case of fast reorientation appears at values of t,. for which... 

Structure 3 shows large Jhd of 29.5-32.8 Hz, giving equal to 0.87-0.94 A from Eqs. (5.3) and (5.4), which is consistent only with the dim calculated from the fast-rotation formula (0.86-0.90 A versus 1.09-1.15 A for slow rotation). The uncorrected neutron is 0.82(3) A for [RuH(H2)(dppe)2]" (d,m corrected for H2 rotation is 0.94 A). For many complexes, dim lies between dgn (slow) and dim (fast). W(CO)j(P Pr3)2(H2) clearly has a rapidly reorientating Hj with rotational barrier near 2 kcal/mol in the solid state and dim of 0.89 A. However as seen in Table 5.1, the fast reorientation correction gives a much too low d of 0.76 A, whereas no correction gives a value of 0.96 A (the potential disparity between solution and solid state values must also be considered). To rationalize this as well as why some Hj complexes require a correction and some do not, the character of H2 reorientation must be examined, such as the angle of torsional libration, Inelastic... 

Solvent molecules of water and acetone trapped in calix[4]hydroquinone supramolecular nanotubes have been studied by solid state and fast MAS NMR and DFT calculations. It has been shown that both water and acetone molecules occupy well-defined average positions, but undergo fast reorientation motions during which their protons interchange their positions. 

To describe the relaxation phenomenon theoretically, Kowalewski and his coworkers have developed a formalism based on the Stochastic Liouville Equation (SLE). In recent work, Kowalewski et aP have compared theoretical simulations of Ri field dispersion profiles computed using the SLE formalism with the results of more restricted theory developed in Florence that neglects reorientational motion of the zfs tensor. Kruk and KowalewskP describe a theoretical model that incorporates fast reorientational motion of the zfs tensor. These workers also present theoretical calculations describing the situation where collisional distortions of the zfs tensor are much larger than the time-averaged zfs tensor. 

A polymer composite of PVK/TNF, doped with DMNPAA can still be improved by modifying the structure of DMNPAA. DMNPAAs modified with certain alkyl substituents have fast orientational response to an external electric field and keep large anisotropy in polarizability. 4-But-oxy-3-propyl-l-(4 -nitrophenylazo)benzene has the shortest reorientation time constant of 19 ms and photorefractive time, which are 2,300 times and 63 times faster than those of a simple DMNPAA composite. The fast reorientational response results from the improvement of the dispersivity in the polymer composites and the decrease of the glass transition temperature. 

Figure 8.7 Order parameter concept, (a) Molecular order (b) intramolecular (bonds) order (c) correlation between order and membrane thickness, dt, left, ordered membrane with little bond or molecule fluctuations (large dt, right, less ordered membrane with bond and molecule fast reorientations within the membrane (small db)...

Figure 14.3). The rotational diffusion is fast with a low activation barrier, as demonstrated experimentally [58, 59, 60] and theoretically [53, 54, 55, 56). Spectroscopic data (IR spectra [61] and neutron diffraction experiments [62]) indicate strong hydrogen bonds, giving favor to a fast proton transfer rather than a fast reorientation step, which needs breaking of the hydrogen bond.

DOP also shows fast reorientation (much faster than that of polymer component). The presence of polymer affects these motions only at lower temperatures... 

Concentrated polymer systems containing plasticizer show the existence of slow and fast relaxations which are temperatnre dependent Slow motions are characteristic of a polymer. Fast reorientations are characteristic of relatively small molecnles of a plasticizer. The dynamics of molecules and their rates of motion can be estimated based on... 

The relaxation time can be deduced from the Debye-like drcular arc. A plot of T values versus the inverse of temperature (10 /r) (Fig. 25.4) allows a measure of activation energy (Fig. 25.5). The separation of domains already discussed above is clearly visible, from fast reorientational motions of dipolar polyatomic ions such as HX04 and HjO" to slow reorientation of NH4 ions. Motions of water molecules cover a broad region they are slow in gel, medium in superionic materials (e.g. HUP) and fast in liquid water. 

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