Slow reorientation

In steady state spectroscopy under the condition of slow reorientation of molecules in solution (xr>x), the following peculiarities take place ... 

The importance of the magnetic coupling is easily seen in Fig. 17 which shows two water proton MRD profiles for serum albumin solutions at the same composition (89). The approximately Lorentzian dispersion is obtained for the solution, and reports the effective rotational correlation time for the protein. The magnetic coupling between the protein and the water protons carries the information on the slow reorientation of the protein to the water spins by chemical exchange of the water molecules and protons between the protein and the bulk solution. When the protein is cross-linked with itself at the same total concentration of protein, the rotational motion of the protein... 

The molecular motions underlying the dynamic mechanical and dielectric f3 transition in PMMA have been studied in detail [77] by using the 2D exchange NMR experiment. This detects slow reorientations that occur during a mixing time, fm, by measuring the angular-dependent NMR frequencies (expressed in ppm) before and after tm. The 2D frequency spectrum S( >i,... 

When a fluorophore is encapsulated in heterogeneous media or immobilized on a surface, single exponential emission decays are rarely observed. Multi-exponential kinetics are attributed to the slow reorientation of the molecular environment after photoexcitation, and the heterogeneity of the microenvironment. Different species in the excited ensemble are oriented differently or exist in different microenvironments on the timescale of the emission which influences the excited-state lifetimes of the immobilized species. Studying the number and distribution of decays can provide information on the microenvironment of the immobilized fluorophore. When combined with fluorescence depolarization studies, detailed information on the motion of these species and their interaction with their environment can be obtained. 

As a simple example a three-level system with slow reorientation is considered. States (0) and (1) are directly coupled to the excitation pulse, while the intermediate level (2) is populated in the relaxation path of the excess population Ni of the upper level on its way back to the ground state. The temporal evolution of the population numbers and the pump intensity Ipu is given by ... 

At room temperature, these molecules occupy well-defined locations in their respective crystal lattices. However, they tumble freely and isotropically (equally in all directions) in place at their lattice positions. As a result, their solid phase NMR spectra show features highly reminiscent of liquids. We will see an illustration of this point shortly. Other molecules may reorient anisotropically (as in solid benzene). Polymer segmental motions in the melt may cause rapid reorientation about the chain axis but only relatively slow reorientation of the chain axes themselves. Large molecular aggregates in solution (such as surfactant micelles or protein complexes or nucleic acids) may appear to have solidlike spectra if their tumbling rates are sufficiently slow. There are numerous other instances in which our macroscopic motions of solid and liquid may be at odds with the molecular dynamics. Nuclear magnetic resonance is one of the foremost ways of investigating these situations. 

It may be appropriate to discuss the NMR findings on the /1-process in the context of results from other experimental methods. Unlike 2H NMR, the vast majority of experimental techniques are not capable of resolving slow reorientation about very small angles. In particular, several studies on the /1-process of molecular glasses may have overlooked the small-angle contribution of the majority of molecules and concluded that a small fraction of molecules is involved in the secondary relaxation process. In contrast, straightforward analysis of 2H NMR solid echo (cf. Section 3.2.1) and spin-lattice relaxation data (cf. Section 3.2.4 and in particular Ref. 115) clearly shows that essentially all molecules participate in the /1-process. However, the amplitude of the reorientation differs among the molecules. The mean... 

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... 

Extremely Slow Reorientation Dynamics of Moiecular Tracers in Glassy Polymers... 

Anew experimental method based on the polarization-selective photochromic reactions is proposed to monitor extremely slow reorientation dynamics of molecular tracers in glassy polymer matrix. The correlations between the local relaxation processes of polymers and the reorientation dynamics of the tracers with different sizes are found from the experimental results obtained by this method. 

Dielectric relaxation data for a 0.08 M Mg2S04 solution are shown in fig. 4.11. On the basis of an analysis of these data by Barthel and coworkers [29, 32], three relaxation processes may be discerned. The first one, involving the ion pair, occurs between permittivity values of 82.9 and 75.2 and involves a relaxation time of 181 ps. The second process, which is attributed to the slow reorientation of water clusters, takes place between the permittivity values of 75.2 and 8.4 with a relaxation time of 8.4 ps. Finally, the high-frequency process, which occurs between 8.4... 

Naphthalene, in contrast to benzene, did not show any NMR-spectra line-width narrowing up to its melting temperature of 353 K. The mean experimental second moment was 9.1 compared to 10.1 G, estimated for the rigid crystal. Measurement of spin-lattice relaxation times indicated, however, also a slow reorientational jump motion about an axis normal to the molecular axis An activation energy of 105 kJ/mol was derived. Molecular dynamics simulations suggest that this reorientation about the axis of greatest inertia occurs with a frequency of 100 MHz within 20 K of fusion (353.6 K) Still, no plastic crystal behavior as found in cyclohexane and related compounds (see Sect. 3.1.1) is indicated for benzene or naphthalane, even close to the melting temperature. 

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. 

The three-pulse spin-alignment echo [111,112] (90y-T-45 -DE-45-T-echo, Figure 8.2(b)) provides a second method with which to obtain a spectrum. For DE = 0, the experiment is similar to a quadrupole echo and the DE dependence is determined by slow reorientation (10" < < lO s). In the... 

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