Solvent dynamics and the delayed recognition of Kramers theory

6 Solvent dynamics and the delayed recognition of Kramer s theory 

With the advent of picosecond and subsequently femosecond laser techniques, it became possible to study increasingly fast chemical reactions, as well as related rapid solvent relaxation processes. In 1940, the famous Dutch physicist, Kramers [40], published an article on frictional effects on chemical reaction rates. Although the article was occasionally cited in chemical kinetic texts, it was largely ignored by chemists until about 1980. This neglect was perhaps due mostly to the absence or sparsity of experimental data to test the theory. Even computer simulation experiments for testing the theory were absent for most of the intervening period. 

The situation changed dramatically with the application of picosecond and, later, faster techniques. One stimulating study was that of Kosower and Huppert [41]. They found that the reaction time for a particular intramolecular charge transfer in a series of alcoholic solvents was equal to the respective slowest longitudinal dielectric relaxation time of the solvent. It was later pointed out that this equality of the reaction and dielectric relaxation times would apply for barrierless reactions (AG a 0) or, more precisely, for the reactions where the relevant solvent dielectric relaxation, or its fluctuation, are the slow step, i.e., slower than the reaction would be in the absence of any slow solvent relaxational process. 

Beginning in 1980 or earlier, numerous theoretical and experimental investigations were undertaken. Kramers theory was also extended in several ways, such as introducing a frequency-dependent friction to replace a constant one [42] and introducing multidimensional effects [43-46]. Both of these effects caused deviations from Kramers theory, but his work remained the seminal paper. 

A more direct way of exploring the dynamics of polar solvent in the presence of a solute is by the optical excitation of a solute to an intramolecular charge transfer state and then observing the time-dependent fluorescence, as shown in Fig. 1.7 [47, 52]. To follow the time-dependent fluorescence at short times, faster than the usual fluorescent lifetime of nanoseconds, lasers plus an up-conversion technique were used. A quantity frequently measured is the dynamic Stokes shift S(r), 

---