Solvent dynamical effects on relaxation and reaction process were considered in Chapters 13 and 14. These effects are usually associated with small amplitude solvent motions that do not appreciably change its configuration. However, the most important solvent effect is often equilibrium in nature—modifying the free energies of the reactants, products, and transition states, thereby affecting the free energy of activation and sometime even the course of the chemical process. Solvation energies relevant to these modifications can be studied experimentally by calorimetric and spectroscopic methods, and theoretically by methods of equilibrium statistical mechanics. With advances of experimental techniques that made it possible to observe timescales down to the femtosecond regime, the dynamics of solvation itself became accessible and therefore an interesting subject of study. Moreover, we are now able to probe molecular processes that occur on the same timescale as solvation, making it necessary to address solvation as dynamic in addition to energetic phenomenon. This chapter focuses on the important and most studied subclass of these phenomena—solvation dynamics involving charged and polar solutes in dielectric environments. In addition to their intrinsic importance, these phenomena play a central role in all processes involving molecular charge rearrangement, most profoundly in electron transfer processes that are discussed in the next chapter. Consider, as a particular example, a neutral (q = 0) atomic solute embedded in a dielectric solvent, that undergoes a sudden change of its charge to q = e, where e is the magnitude of the electron charge. This can be achieved, for example, by photoionization. The dipolar solvent molecules respond to this change in the local charge distribution by rotating in order to make their negative end point, on the average, to the just formed positive ion. Thus, the solvent configuration changes in response to the sudden change in a local charge distribution. The driving force for this change is the lowering of overall free energy that accompanies the buildup of solvent polarization.
Solvation Dynamics Studied by Picosecond Fluorescence:
Microscopic Reorientation and Longitudinal Relaxation of the Solvent