Chemical Reactions in Solution: The New Photochemistry

Understanding the dynamics of chemical reactions in the condensed phase reaches anew plateau with each technological advance in time-resolved spectroscopy. Submicrosecondstudies of the past revealed the role of long range molecular diffusion incondensed-phase chemistry and photochemistry. The picosecond (10 −12 –10 −9 s) timescale, combined with the use of a high concentration of reactants, can provide newinformation about the “microdynamics” in the local region of the reaction itself. Therole of solvent is particularly important: how it attaches to an activated reactantmolecule, how it is displaced by the other reactant molecule preparatory to reaction,and how the solvent behavior affects the dynamics of single- and multi-channelprocesses, thus the relative yields of products in competing reactions. The theorypresented here divides itself into two types: one that depends on a diffusion equationthat also contains terms describing a distance-dependent reaction sink function and areaction barrier; and a second type that deals phenomenologically with rate equations,including the rate of reactant/solvent interchange. Experiments subdivide naturallyinto steady state and transient measurements, the former dealing with quantum yieldsand steady state spectroscopic studies, the latter with picosecond transient spectroscopy.The two theoretical approaches can be interrelated in certain useful limits. The twotypes of experimental data, in combination with the theory, supply fundamentalinformation about solvent participation in the local reaction region.