Chemical Reactions in Solution: The New Photochemistry

Abstract

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