Friction dielectric

Aguilar, M., Bianco, R., Miertus, S., Persico, M. and Tomasi, J. Chemical reactions in solution modeling of the delay of solvent synchronism (dielectric friction) along the reaction path of an SN2 reaction, Chem. Phys., 174(1993), 397-407... 

G. van der Zwan and J. T. Hynes, Time-dependent fluorescence solvent shifts, dielectric friction and nonequilibrium solvation in polar solvents, J. Phys. Chem. 89, 418M188 (1985). 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

G. R. Fleming We have not yet looked at the issue raised by Prof. Rice. We understand much more about dielectric friction than about collisional friction at short times. The calculation you suggest would be very interesting. 

A number of analogous compounds to BA have been reported, including 5,5 -dibenzo-[a]-pyrenyl (BBPY) [116]. These compounds exhibit emission spectra similar to BA. It would be interesting to explore the ultrafast dynamics of BBPY in order to test the generality of the GLE model. It would also be interesting to study the femtosecond dynamics of BA as a function of applied pressure. Static experiments on the emission of BA, reported by Hara et al. [123], demonstrate that in low viscosity solvents an increase of pressure affects the emission similarly to an increase of solvent polarity. As the pressure is increased, however, the LE/CT interconversion is slowed down. It would be interesting to measure C(r) in these environments and compare the solvation dynamics with LE/CT dynamics, in order to test the generality of the GLE dielectric friction model. 

Another interesting class of molecules are stilbene derivatives with charge donating groups. These compounds offer the opportunity to explore the role of polar solvation dynamics (dielectric friction) in cis/trans isomerization. Interesting papers on this subject have been published by Waldeck et al. [145] and Rulliere et al. [146]. Other well-studied polar excited state isomerization examples include pinacyanol, l,l -diethyl-4,4 -cyanine, and crystal violet, which have been studied by Sundstrom, Gilbro and their coworkers [148] and Ben-Amotz and Harris [148] and others who are referenced in these papers [148,149],... 

M. Maroncelli, Continuum estimates of rotational dielectric friction and polar solvation, J. Chem. Phys., 106 (1997) 1545-55. 

IV. HOW DOES DIELECTRIC FRICTION EFFECT VIBRATIONAL ENERGY RELAXATION ... 

Fluorescence Solvent Shift, Dielectric Friction, and Nonequilibrium Solvation in Polar Solvents. 

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... 

Expecially when AGf is very small, such overdamped solvent relaxation due to dielectric friction may contribute significantly to v, so that in some cases... 

M. Aguilar, R. Bianco, S. Miertus, M. Persico, and J. Tomasi, Chem. Phys., 174, 397 (1993). Chemical Reactions in Solution Modeling of the Delay of Solvent Synchronicism (Dielectric Friction) Along the Reaction Path of an Sn.2 Reaction. 

Viscosity of aqueous cesium chloride (CsCl) solution was measured in the range of 0.1-5.0 mol kg-i and 0.1-375 MPa at 25 °C. The Jones-Dole B coefficient of CsCl was obtained from the concentration dependence of the viscosity. It is negative not only at atmospheric pressure but also at high pressure, having a maximum against pressure at about 160 MPa. Similar maximum of the B was observed for aqueous sodium chloride (NaCl) solution. The similarity is discussed in terms of the water structure and dielectric friction theory. 

The deviation of nitrobenzene from the solid line (the slope = 1) in Figure 2 is probably attributed to the frequency dependent dielectric friction for the reaction dynamics around the barrier top, i.e., the much slower dielectric fluctuation of nitrobenzene (tl - 6 ps at 298K) compared with the ET rate hardly works as friction for the barrier crossing. In such case, the friction is shows tl (a[Pg.400]

The 10 s order rate constants for the thermal-induced (ground state) intramolecular electron transfer rates of the mixed-valence biferrocene monocation were first elucidated in various solvents by the H-NMR relaxation measurements. The obtained solvent dependent frequency factors indicated significant contribution of the solvent dielectric friction on the barrier crossing. An existence of the faster processes compared with the ET rate such as the internal vibration as an escape route of the reaction dynamics along the solvent coordination was also proposed in some extent. 

Thus, if a charged particle moves fast enough in the medium, it will experience a retarding force (friction) due to the fact that, during its trajectory, the orientation and the position of the solvent molecules are not in equilibrium with respect to their actual position this effect, which is expressed by -Psiow> is called dielectric friction. In addition to this, we may invoke another naive picture. During its motion a particle (which, for simplicity s sake, may be assumed as uncharged, but with anon-zero collisional diameter) collides with solvent molecules, and thus experiences a different retarding force, i.e. the mechanical friction. 

The other model for the ionic friction concerns the dielectric response of solvent to the solute perturbation. When an ion is fixed in polar solvent, the solvent is polarized according to the electrostatic field from the ion. If the ion is displaced, the solvent polarization is not in equilibrium with a new position of the ion, and the relaxation of the polarization should take place in the solvent. The energy dissipation associated with this relaxation process may be identified as an extra friction. The extra friction, called the dielectric friction, decreases with increasing ionic radius, thereby, with decreasing electrostatic field from the ion. The dielectric friction model developed by Born [66], Fuoss [67], Boyd [68] and Zwanzig [69, 70] has taken a complete theoretical form due to the work by Hubbard and Onsager [71, 72] who proposed a set of continuum electrohydrodynamic equations in which the electrostatic as well as hydrodynamic strains are incorporated. 

As far as qualitative aspects of the ion-size dependence of the friction coefficients are concerned, which has a minimum with increasing ionic radius, both models (the solventberg and dielectric friction models) explain the experimental observations to a certain extent. Then one might ask which model of the two is more faithful to the real physics of the ionic friction and/or how do the two effects interplay if both are coexisting For instance, use of the same parameter for the effective ionic radius will not be justified since increase in the effective radius should give rise to increase in the Stokes friction but decrease in the dielectric friction. The question should be answered by microscopic theory, not by treatment based on the continuum model. 

Here we address the problem from a different point of view, namely, in terms of a response of collective excitations in solvent to the ionic field. In Sec. 5.3 we have succeeded in abstracting the collective excitations in a model diatomic liquid which can be identified as acoustic and optical modes. The two modes arise essentially from the translational and rotational motions of solvent molecules. Since the Stokes and dielectric frictions originate basically from the energy dissipation due to the translational and rotational motions of solvent molecules, respectively, it is reasonable to ask how the ionic field couples with the collective excitations and/or how the two drag forces are related to the two col-... 

Equation (5.161) is very similar to that used by Bagchi et al. in their recent study [76]. However, since their expression is written in terms of the longitudinal ion-dipole direct correlation function and the orientational intermediate scattering function of the solvent in place of Cux k) and F fi k,t) in our formula, its application is limited to the calculation of the dielectric friction. As we have clarified in Sec. 5.3, both the translational and rotational motions of solvent molecules manifest themselves in Fxfi k, t), and Eq. (5.161) can be applied to the calculation of the friction coefficient which comprises the hydrodynamic as well as dielectric contributions. Thus Eq. (5.161) can be regarded as a more general microscopic expression for the friction coefficient. 

Cnn and (zz as the hydrodynamic (or Stokes) and dielectric frictions, respectively, and (nz as their cross term. 

The solute-size dependence of Czz (the dash-dotted line in Fig. 5.11) is in accord with that of the dielectric friction picture, which also justifies our convention to call Czz the dielectric friction. On the other hand, the solute-size dependence of Cnn (the dashed line in Fig. 5.11) shows a peculiar behavior. One might expect, if Cnn were indeed the hydro-dynamic friction, that Cnn would obey a linear solute-size dependence... 

Summarizing the results in this section, we have found that both the solventberg and dielectric friction mechanisms are responsible for the large C, of small ions, and they are reflected in nn (the Stokes or hy-... 

Figure 5.20. Decomposition of C based on Eq. (5.204) for cations and anions in water. Circles, the total friction C triangles, the Stokes part Cnn squares, the dielectric friction part Czz diamonds, the cross term Cnz multiplied by a factor of 2, see Eq. (5.204). The solid and dashed lines are to guide the eyes.

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