Solvent reorientation

The solvents themselves are adsorbed on the electrode surface, as is shown by the capacitance-potential graphs illustrated in Fig. 9 (Payne, 1967, 1970) potassium hexafluorophosphate, the electrolyte in each of the solvents, is thought to be adsorbed only very weakly. The solvents show somewhat differing curves and the peaks have been interpreted both in terms of competition between the solvent and anions for sites at the surface and also in terms of solvent reorientation. Ethers are adsorbed from the amide solvents most strongly at the potentials around the peaks and this has been postulated to be due to an increase in freedom for the solvent to rotate at these potentials (Dutkiewicz and Parsons, 1966). 

Statement number 6 has to do with carbon acids and is supported by reference (7). There are, in fact, other references that suggest solvent plays a much more direct role in the kinetics of protonating carbanions than statement number 6 would imply. For example, there is evidence that nuclear reorganization and rehybridization of the carbon atom are too rapid to have much kinetic importance when compared with solvent reorientation. The strong dependence of carbanion protonation rates on the solvent supports this view. These rates are typically much faster in organic solvents, such as DMSO, than in water. A particular reaction that was studied in different solvents (17) is... 

Here, dx is the change in the surface potential of the metal brought into contact with the solvent and represents a modification of the electronic distribution in the metal and<5 ic is the change of the surface potential due to solvent reorientation after contact with the metal and gf(ion) is the contribution of ions to the potential drop in the presence of free charges on the metal. At the potential of zero charge, (ion) = 0, and... 

In Strickler and Berg theory, it is assiuned that fluorescence occurs from the original excited state without taking into account possible solvent reorientation and subsequent formation of a new lower energy excited state. Hence, the relationships between absorption and emission spectra may be more complicated than simply following Strickler and Berg theory, and ti may differ from These results indicate the importance of considering the effect of medium on fluorescence properties for these compounds. 

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],... 

This equilibrium hypothesis is, however, not necessarily valid for rapid chemical reactions. This brings us to the second way in which solvents can influence reaction rates, namely through dynamic or frictional effects. For broad-barrier reactions in strongly dipolar, slowly relaxing solvents, non-equilibrium solvation of the activated complex can occur and the solvent reorientation may also influence the reaction rate. In the case of slow solvent relaxation, significant dynamic contributions to the experimentally determined activation parameters, which are completely absent in conventional transition-state theory, can exist. In the extreme case, solvent reorientation becomes rate-limiting and the transition-state theory breaks down. In this situation, rate con-... 

Fig. 6-7. Effect of solvent reorientation in the excited state on the fluorescence band of a dipolar molecule with dipole flip on excitation. S[ and Sq are the Franck-Condon excited and ground states, respectively Si and Sq are the corresponding equilibrium states tr < Te.

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. 

An ultrafast intermolecular electron transfer (ET) from electron donating solvent to an excited dye molecule was found. A temperature-dependent non-exponential time dependence was observed in aniline, and a temperature-independent single exponential process for Nile blue (160 fs) and oxazine 1 (260 fs) was observed in N,N-Smethylaniline. The solvation times of solvent anilines were obtained by dynamic Stokes shift measurements. The rate of ET in some systems was observed to be much greater than the solvation time of anilines. The dynamic behavior was simulated by the 2-dimen ional potential energy surface for reaction, taking into account of the effects of both solvent reorientation and nuclear motion of reactants. 

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 still faster ( nsec) dynamics of local motions of polymer molecules in dilute solutions have been investigated by Ediger and coworkers (Zhu and Ediger 1995, 1997). They find that the rates of these local motions of a few bonds are not proportional to the solvent viscosity, unless the solvent reorientation rate is fast compared to the polymer local motion. Thus, for local motions (such as bond reorientations) of polymer molecules, Stokes law of drag does not always hold. 

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

On the other hand is comparable to solvent structural relaxation times and is shorter than solvent reorientation and conformational relaxation times. In this case the effective friction for motion in the vicinity of Rj will be less than the zero frequency friction which describes diffusion for R > R. ... 

The adsorption system is in many respects similar to biological systems. Especially the smallness of the solvent reorientation energy, with Ag < and -AGg = A, of a donor-acceptor system with ion-paired product state in a diffusionless, semi-rigid environment, opens many possibilities to study certain aspects of electron transfer which are of importance to biological systems. 

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