Solute-solvent interactions, mode coupling

Since the goal is to study the friction on a solute which is different in size from the solvent, in the mode coupling expressions of friction the terms representing the coupling between the solute and the solvent are calculated using the solute-solvent interaction potential. Thus the binary terms cu q) and y dn are all calculated from V12 (r), and all the other solvent static and dynamical quantities are calculated from v(r). 

While the model employed in the present work provides a reasonable picture of a unimolecular reaction involving a large molecule in solution, other ingredients not considered here may play a role in some systems. The possible role played by intramolecular friction (nonlinear coupling between the reaction coordinate and other nonreactive modes near the barrier) has been discussed in Section IV. Also, the dependence of the molecular potential surface, in particular the activation barrier on the molecule-solvent interaction, may dominate in some cases the observed solvent effect on the rate. Such may be the case (see Section VIII) in a polar solvent when the reaction involves a change in the molecular dipole moment (such as a charge transfer reaction). 

Schweizer and collaborators have elaborated an extensive mode-coupling model of polymer dynamics [52-54]. The model does not make obvious assumptions about the nature of polymer motion or the presence or absence of particular long-lived dynamic structures, e.g., tubes it yields a set of generalized Langevin equations and associated memory functions. Somewhat realistic assumptions are made for the equilibrium structure of the solutions. Extensive calculations were made of the molecular weight dependences for probe diffusion in melts, often leading by calculation rather than assumption to power-law behaviors for various transport coefficients. However, as presented in the papers noted here, the model is applicable to melts rather than solutions Momentum variables have been completely suppressed, so there are no hydrodynamic interactions. Readers should recall that hydrodynamic interactions usually refer to interactions that are solvent-mediated. 

This idea of predominantly displacing translational solvent modes upon excitation leads to an appealing microscopic picture for the coupling of the mechanical and dielectric solvent response. Upon photoexcitation, the free energy will be lowered by both solvent molecule translations, accommodating the new solute size and shape, and solvent molecule rotations, creating favorable electrostatic interactions with the new... 

The other enhancement method is to use molecular electronic resonances [19, 22, 23]. A most interesting topic in this enhancement method is so-called molecular near-held effect reported by Kano et al. [34]. This effect, explained by intermolecular vibronic coupling, makes HRS spectroscopy highly sensitive to intermolecular interactions one can observe HRS modes of solvent molecules adjacent to the solute dye molecules via the intensity borrowing from the dye molecules. That is, solvent-solute interactions are selectively observed without impairment from signals of the bulk solvent. It is almost impossible to observe a similar effect in resonance RS spectroscopy because of the unavoidable contribution from Franck-Condon type resonance. 

In reality there are several molecular vibrations that can couple to an electronic transition although the basic phenomenology is retained, namely the resonant character of the S St) 0-0 transition and the mirror symmetry of the vibronic satellites. In case of a jr—Mt transition the dominant vibrational modes are those of the polymer backbone, notably of the phenyl ring. An example is the absorption and fluorescence of it-conjugated molecules, such as tetracene, in the gas phase [22], In fluid solution there is interaction between the transient dipole of the molecule with the permanent and induced dipoles of the solvent. It gives rise to (i) a bathochromic shift of the spectra, (ii) a Stokes shift between the... 

One application of PI-QTST to PT has been to study a model A-H-A PT solute in a polar solvent [77]. This computational study provided a detailed examination of the specific features of PT, including the competition between proton tunneling and solvent activation, the influence from intramolecular vibrational modulation of the PT barrier, and the role of electronic polarizability of both the solute and the solvent. Changes in the total quantum activation free energy, and hence the reaction probability, due to these different effects were calculated (cf. Fig. 18). By virtue of these studies, it was found that to fully understand the rate of a given PT reaction, one must deal with a number of complex, nonlinear interactions. Examples of such interactions include the nonlinear dependence of the solute dipole on the position of the proton, the coupling of the solute dipole to both the proton coordinate and to other vibrational modes, and the intrinsically nonlinear interactions arising from both solute and solvent polarizability effects. Perhaps the most important conclusion... 

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