Translational reorientation

Figure 1 (b), (c) The schemes of the dimer performing (b) harmonic translations/reorientations... 

Lindman and co-workers [21-23] demonstrated that the organisation and structure of microemulsions can be elucidated from self-diffusion measurements of all the components (using pulse-gradient or spin-echo NMR techniques). Within a micelle, the molecular motion of the hydrocarbon tails (translational, reorientation and chain flexibility) is almost as rapid as in a liquid hydrocarbon. In a reverse micelle, water molecules and counter ions are also highly mobile. 

The probability of a molecular arrangement is given by the Boltzmann distribution law when the temperature and volume of the system as well as the number of molecules therein are constant. Three types of trial are used during the simulations attempts to translate/reorient the molecule, attempts to delete particles, and attempts to create a molecule in the simulation cell. The decision regarding the acceptance of each trial or its rejection to return to the old configuration is based on the probabihty ... 

Models for description of liquids should provide us with an understanding of the dynamic behavior of the molecules, and thus of the routes of chemical reactions in the liquids. While it is often relatively easy to describe the molecular structure and dynamics of the gaseous or the solid state, this is not true for the liquid state. Molecules in liquids can perform vibrations, rotations, and translations. A successful model often used for the description of molecular rotational processes in liquids is the rotational diffusion model, in which it is assumed that the molecules rotate by small angular steps about the molecular rotation axes. One quantity to describe the rotational speed of molecules is the reorientational correlation time T, which is a measure for the average time elapsed when a molecule has rotated through an angle of the order of 1 radian, or approximately 60°. It is indirectly proportional to the velocity of rotational motion. 

In addition to the described above methods, there are computational QM-MM (quantum mechanics-classic mechanics) methods in progress of development. They allow prediction and understanding of solvatochromism and fluorescence characteristics of dyes that are situated in various molecular structures changing electrical properties on nanoscale. Their electronic transitions and according microscopic structures are calculated using QM coupled to the point charges with Coulombic potentials. It is very important that in typical QM-MM simulations, no dielectric constant is involved Orientational dielectric effects come naturally from reorientation and translation of the elements of the system on the pathway of attaining the equilibrium. Dynamics of such complex systems as proteins embedded in natural environment may be revealed with femtosecond time resolution. In more detail, this topic is analyzed in this volume [76]. 

In typical QM-MM simulations, no dielectric constant is included. Orientational dielectric effects come naturally from reorienting and translation of the elements of the system, providing the system comes to equilibrium. What is left out of the model is electronic polarization of molecules, which makes a minor contribution. 

Translation and reorientation of CD4 molecules in zeolites as studied by deuteron NMR relaxation... 

Nuclear magnetic resonance provides means to study molecular dynamics in every state of matter. When going from solid state over liquids to gases, besides mole- cular reorientations, translational diffusion occurs as well. CD4 molecule inserted into a zeolite supercage provides a new specific model system for studies of rotational and translational dynamics by deuteron NMR. 

The non-collective motions include the rotational and translational self-diffusion of molecules as in normal liquids. Molecular reorientations under the influence of a potential of mean torque set up by the neighbours have been described by the small step rotational diffusion model.118 124 The roto-translational diffusion of molecules in uniaxial smectic phases has also been theoretically treated.125,126 This theory has only been tested by a spin relaxation study of a solute in a smectic phase.127 Translational self-diffusion (TD)29 is an intermolecular relaxation mechanism, and is important when proton is used to probe spin relaxation in LC. TD also enters indirectly in the treatment of spin relaxation by DF. Theories for TD in isotropic liquids and cubic solids128 130 have been extended to LC in the nematic (N),131 smectic A (SmA),132 and smectic B (SmB)133 phases. In addition to the overall motion of the molecule, internal bond rotations within the flexible chain(s) of a meso-genic molecule can also cause spin relaxation. The conformational transitions in the side chain are usually much faster than the rotational diffusive motion of the molecular core. 

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. 

For reorientational motions the hole in the embedding medium does not change and Eq. 4.29 is valid for arbitrary reorientations. In the case of translational... 

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

Fig. 3. Schematic representation of the topological space of hydration water in silica fine-particle cluster (45). The processes responsible for the water spin-lattice relaxation behavior are restricted rotational diffusion about an axis normal to the local surface (y process), reorientations mediated by translational displacements on the length scale of a monomer (P process), reorientations mediated by translational displacements in the length scale of the clusters (a process), and exchange with free water as a cutoff limit.

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