Polar solvents reorientation

Even if we consider a single solvent, e g., water, at a single temperature, say 298K, depends on the solute and in fact on the coordinate of the solute which is under consideration, and we cannot take xF as a constant. Nevertheless, in the absence of a molecular dynamics simulation for the solute motion of interest, XF for polar solvents like water is often approximated by the Debye model. In this model, the dielectric polarization of the solvent relaxes as a single exponential with a relaxation time equal to the rotational (i.e., reorientational) relaxation time of a single molecule, which is called Tp) or the Debye time [32, 347], The Debye time may be associated with the relaxation of the transverse component of the polarization field. However the solvent fluctuations and frictional relaxation occur on a faster scale given by [348,349]... 

So the temperature decrease results in an increase in the dielectric constant of the liquid polar solvent. However, freezing a solvent has the opposite effect. On freezing with glass formation, the effective solvent dielectric constant decreases drastically, because the solvent dipoles cannot reorient. The frozen glass, therefore, cannot stabilize the newly formed ions (Liddell et al. 1997). 

A surprising aspect of SD is how rapidly C i) in highly polar solvents decays relative to other relaxation processes such as reorientation of solvent dipoles. This very rapid time scale cannot be ascribed to dynamical solvent-solvent correlations, which, as illustrated in Fig. 6, are modest even for the longest ranged A . Thus the key to imderstanding the reasons for the rapid decay of C i) is in examining how solvent-solvent correlations contribute to it and to what extent their contributions can be accounted for in terms of static correlations measured by ((5A ) ), Eq. (32). The initial cmvature of C(t), which characterizes its short-time Gaussian-like behavior is often characterized in terms of the solvation frequency co o/v... 

This form of the equation is valid for linear molecules and symmetric rotors (for which I corresponds to reorientation of the symmetry axis) and can be used for nondipolar solvents. A somewhat more complicated expression would hold in the absence of axial symmetry. Maroncelli et al. estimated the value tti for AP corresponding to a charge shift of a spherical ion in a continuum model of a polar solvent of dielectric constant s and showed that it increases with increasing solvent polarity and works well when tti is significantly larger than one. 

The situation is very different for polar solvents, i.e., solvents that have a relevant permanent dipole moment. In such solvents the greatest part of the dielectric response originates from the slight reorientation of the applied external field, and only a small part from electronic polarization. For water, with s = 78.4 (at 25 °C), the electronic polarizability contribution is only... 

Much of the research on solvation dynamics has been devoted to polar solute-solvent systems. In these media, it has been found that the response to a change in solute dipole is due primarily to collective solvent reorientation and that it can be predicted reasonably well using information on pure solvent dipolar reorientation, for example, from dielectric permittivity measurements, as input [1,6,7,9],... 

It has be emphasised that no account has taken of any possible interaction between permanent dipoles, so that the equation will be valid only for polar gases and for dilute solutions of polar molecules in non-polar solvents. Moreover, the equation is not valid at high frequencies, because in that case the dipoles are not able to reorient sufficiently quickly in the oscillatory electric field, so that at optical frequencies a = oca-... 

Dielectric friction is the measure of the dynamic interaction of a charged or dipolar solute molecule with the surrounding polar solvent molecules. This concept has been applied, by Hynes et al. [339] and others [486], to solvent- and time-dependent fluorescence shifts resulting from the electronic absorption by a solute in polar solvents. If the solvent molecules are strongly coupled to the charge distribution in ground- and excited-state molecules, the relatively slow solvent reorientation can lead to an observable time evolution of the fluorescence spectrum in the nano- to picosecond range. This time-dependent fluorescence (TDF) has been theoretically analysed in terms of dynamic... 

Electron transfer reactions are categorized as outer or inner sphere. In outer-sphere processes, structural changes during ET largely involve solvent reorientation (polarization). Inner-sphere reactions involve changes in the bonding to the redox unit and are more difficult to describe using the ET theory as it is explained here. 

The nature of this dual fluorescence phenomenon has been the subject of intense interest. The currently most widely accepted explanation assumes that dielectric polarization of the solvent permits excited-state rotational isomerization, leading to a highly polar fluorescent TICT state with a conformation of the D+ and A moieties close to perpendicular [12, 13, 15-24]. This model predicts that the 7r-electronic decoupling of the D+ and A subunits leads to full charge separation and, consequently, to a large dipole moment and a considerable solvent reorientational en-... 

The strong field of an ionic solute restricts the reorientational motion of the a polar solvent, which directly effects the EFG-TCF. This fact has been used to determine the structure in the solvation shell around the quadrupolar nucleus by comparing the performance of different theoretical models for the quadrupolar relaxation. In the models, the solvent electrical dipoles have been assumed either to be radially oriented to the solute or randomly oriented, which gives different expressions for the EFG-TCF [60,61]. The assumptions in these models has been examined in MD simulations [52,62], and different ways to describe the EFG-TCF in terms of different reorientational TCF was suggested. 

Another important source of information is dielectric spectroscopy, because dipolar ion pairs contribute to the static dielectric constant of the solution [42, 43], In polar solvents the dielectric spectra reflect two modes caused by the reorientation of solvent and of ion-pairs. In non-polar solvents one solely observes ion pair reorientation. For Bu4N-iodide (BU4NI) in dichloromethane (CH2CI2) an increase of the total dielectric constant e with the concentration of the salt is found as result of the ion pair formation. A decrease in the particle density of the solvent causes a minute decrease of the solvent contribution. The dielectric constant does, however, not increase linearly with the salt concentration. A decreasing slope at high salt concentrations may result from the redissociation of the ion pairs but at a quantitative level, redissociation alone is... 

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