Mobility, translational

Polymerizations Above Tg. Let the polymerization begin in pure monomer. As the concentration of polymer chains increases initially one observes a relatively small increase in the termination rate constant. This is related to the effect of polymer concentration on coil size. A reduction in coil size increases the probability of finding a chain end near the surface and hence causes an increase in k-. Soon thereafter at conversions 15-20 polymer chains begin to entangle causing a dramatic reduction in radical chain translational mobility giving a rapid drop in k-j. ... 

Since the components of a compoimd are linked to each other in such a way that they have lost same of their free translational mobility, the volume of hydrated molecule is always smaller than the sum of the volumes of its components, the hydration is accompanied by a decrease of the total volume. 

PGSE-NMR provides direct information on the translational mobility of a liquid medium capable of swelling a given CFP. The self-diffusion coefficient of the swelling agent is found to be related to the nanoporosity of the matrix as determined from ISEC and to the rotational correlation time of a suitable paramagnetic probe (ESR) [22]. 

Viscosity is a useful quantity, in that both rotational and translation mobility of molecules in solution are viscosity dependent and can be related to viscosity through the Stokes-Einstein equation ... 

To summarize, there is a sizable and self-consistent body of data indicating that rotational and translational mobility of molecules inside swollen gel-type CFPs are interrelated and controlled mainly by viscosity. Accordingly, T, self-diffusion and diffusion coefficients bear the same information (at least for comparative purposes) concerning diffusion rates within swollen gel phases. However, the measurement of r is by far the most simple (it requires only the collection of a single spectrum). For this reason, only r values have been used so far in the interpretation of diffusion phenomena in swollen heterogeneous metal catalysts supported on CFPs [81,82]. 

By using NFS, information on both rotational and translational dynamics can be extracted. In many cases, it would be favorable to obtain separate information about either rotational or translational mobility of the sensor molecule. In this respect, two other nuclear scattering techniques using synchrotron radiation are of advantage. Synchrotron radiation-based perturbed angular correlations (SRPAC) yields direct and quantitative evidence for rotational dynamics (see Sect. 9.8). NIS monitors the relative influence of intra- and inter-molecular forces via the vibrational density of states (DOS) which can be influenced by the onset of molecular rotation (see Sect. 9.9.5). 

S. Stapf, R. Kimmich 1997, (Translational mobility in surface induced liquid layers investigated by NMR diffusome-try), Chem. Phys. Lett. 275, 261. 

Figure 7. The original behaviour of the neo-pentane translational mobility (Dt is constant in the loading range 20 % - 65 %) suggests that the neopentane confined phase behaves as a lattice fluid phase.

The continued use of aw in foods does not preclude the use of other concepts or measurement methods, such as the food polymer science approach proposed by Slade and Levine (1991) or rotational and translation mobility as measured by NMR. Rather, it may be most useful to combine these various approaches, recognizing the strengths, perspective (i.e., distance and time scales), and limitations of each. Then, each approach can be utilized where it is most applicable so as to build a multilevel understanding of the workings of specific food systems. 

One final but very important point needs to be made regarding the glass transition. As highlighted by Ludescher et al. (2001), it must be kept in mind that the glass transition is a macroscopic manifestation of cooperative changes in molecular mobility (specifically, translational mobility) of individual molecules in a continuous amorphous phase—where the change in... 

Several material properties exhibit a distinct change over the range of Tg. These properties can be classified into three major categories—thermodynamic quantities (i.e., enthalpy, heat capacity, volume, and thermal expansion coefficient), molecular dynamics quantities (i.e., rotational and translational mobility), and physicochemical properties (i.e., viscosity, viscoelastic proprieties, dielectric constant). Figure 34 schematically illustrates changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. A number of analytical methods can be used to monitor these and other property changes and... 

Method involves measuring the change in molecular mobility (rotational and translational mobility) experienced by nuclei associated with solid components (e.g., 111 and 13C). The temperature associated with an increase in solid component mobility is assigned as Tg... 

An important feature of liquids at surfaces or interfaces is that generally the molecules at the surface are translationally mobile with... 

Comparison with the experimental value indicates that the conditions that best describe the experiments are those in line 4 in solution the HS04 translates in a restricted way and upon adsorption the ion loses its translational mobility and retains only a restricted rotation around its axis perpendicular to the surface. The degrees of freedom (d.f.) allowed for die movement of each species as well as the area covered in their movement (one cell, communal) are indicated in each case. 

This description is elaborated below with an idealized model shown in Figure 17. Imagine a molecule tightly enclosed within a cube (model 10). Under such conditions, its translational mobility is restricted in all three dimensions. The extent of restrictions experienced by the molecule will decrease as the walls of the enclosure are removed one at a time, eventually reaching a situation where there is no restriction to motion in any direction (i.e., the gas phase model 1). However, other cases can be conceived for a reaction cavity which do not enforce spatial restrictions upon the shape changes suffered by a guest molecule as it proceeds to products. These correspond to various situations in isotropic solutions with low viscosities. We term all models in Figure 17 except the first as reaction cavities even... 

Fluorescein-labeled proteins are also used to measure the translational mobility of proteins and lipids by the Fluorescence Recovery After Photo-bleaching technique [54-59]. The uniformly labeled fluorescent sample is flashed with an intense light source to bleach a spot, thus producing a concentration gradient. The rate of recovery of fluorescence in that bleached area is measured and used to calculate the diffusion coefficient of the probe dye into the bleached zone. Such diffusion coefficient measurements have been used to determine the association constants of proteins in cells [60], to measure the exchange of tubulin between the cytoplasm and the microtubules [61,62], to study the polymerization-depolymerization process of actin [63-65] and to monitor the changes that occur upon cell maturation [66,67]. 

Fluorescence microphotolysis, or photobleaching, has been widely used to study translational mobility of lipids and proteins in membranes. An attenuated laser beam may be focused down to the diameter of a cell or less. Then the intensity can be suddenly increased by several orders of magnitude, bleaching any fluorescent material present. The return of fluorescent material by free diffusion from a neighboring region (fluorescence recovery after photobleaching) or by diffusion through a membrane into a cell can then be... 

For an outer-sphere reaction, given the translation mobility of the reactants, electron transfer may occur over a range of distances. The problem can be treated in a general way since from statistical mechanics the equilibrium distribution of intemuclear separations can be calculated based on pairwise distribution functions. Integration of the product of the distribution function and ket(r) over all space gives the total rate constant et-32b 48... 

Let us re-examine the notion of a point defect in this context. If a molecular subgroup of a molecule is imperfect, this damaged molecule constitutes a point defect in the crystal, although the defect has no immediate influence on the molecule s translational mobility. Point defects that induce (translational) motion are vacancies or interstitials. We can infer from the form of the Lenard-Jones potential that vacan-... 

The question about the difference between the macroscopic and microscopic values of the quantities characterizing the translational mobility (viscosity tj, diffusion coefficient D, etc.) has often been discussed in the literature. Numerous data on the kinetics of spin exchange testify to the fact that, with the comparable sizes of various molecules of which the liquid is composed, the microscopic translational mobility of these molecules is satisfactorily described by the simple Einstein-Stokes diffusion model with the diffusion coefficient determined by the formula... 

On the basis of these considerations it can be expected that the Einstein Stokes model will be applicable for vitrified solutions consisting of the particles with comparable sizes. In other words, for the order of magnitude estimations of the microscopic translational mobility of molecules in such solutions, the value of the diffusion coefficient, which is obtained by using the macroscopic viscosity t] in eqn. (1), can be used. 

The given considerations of the character of the translational mobility of molecules in vitreous matrices, of course, do not claim to be theoretically strict. Nevertheless, they are in qualitative agreement with the existing ideas about the structure of vitreous matrices and, apparently, can be useful for rough preliminary order of magnitude estimates of the mobility. 

Due to the extremely low translational mobility of the molecules in vitreous matrices, the kinetics of the chemical reactions in these matrices depends substantially on the form of the initial spatial distribution of the reagents. The study of the kinetics of electron tunneling reactions in vitreous matrices is often conducted in such a manner that one of the reagents is generated after vitrification of the solution by means of y- or / -radiolysis or photolysis, and the other is either generated in the similar manner or is introduced into the solution prior to freezing. In this connection, let us dwell upon the spatial distribution of both these types of reagent in vitreous matrices. 

The effect of unfreezing the translational mobility of reagents on the kinetics of electron tunneling reactions will be discussed in greater detail in Sect. 4. 

Tunneling reactions of e near the temperature of matrix devitrification. Effect of translational mobility of reagents on the kinetics... 

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