Isomerization dynamics in solution

Recent advances in the dynamics of photochemical isomerization reactions in solution are reviewed. The isomerization reactions in solution show diverse behaviour depending on the nature of the solvent, the viscosity of the solution and the sharpness of the activation barrier. The results of recent time-resolved spectroscopic measurements on several isomerizing molecules in different solvents are discussed. The rate constants of these isomerization reactions lie in the picosecond regime. We briefly discuss the stochastic, Markovian theories of chemical kinetics in solution, including the well known Kramers' theory. Contrary to the traditional belief, recent experiments reveal an apparent breakdown of Kramers' theory at large viscosities. This breakdown is most dramatic for isomerizing molecules with sharp barriers. The failure of Kramers' theory for these cases can be traced to the assumption that the solvent forces on reactive motion are delta-correlated in time. We discuss the recent generalization of Kramers' theory by Grote and Hynes who removed the white-noise assumption of Kramers' theory by including the frequency dependence of friction. This generalized theory can qualitatively explain the new experimental data. We also review both experimental and theoretical studies on isomerization reactions in the absence of an activation barrier. We discuss why the earlier theory of Forster and Hoffmann is inconsistent, and review the recent theory of Bagchi, Fleming and Oxtoby on isomerization in solution in the absence of a barrier. We also discuss the importance of non-equilibrium solvation dynamics on fast isomerization reactions in solution. Recent computer simulation studies are reviewed. Lastly, we discuss the outstanding problems of this field for future work