Nonequilibrium Solvation Dynamics Research Articles

Isomerization dynamics in solution

Abstract 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.

A simple dipole isomerization model for non-equilibrium solvation dynamics in reactions in polar solvents

Abstract A simple model for dipole isomerization reactions in polar solvents is studied. A full dynamical treatment at the generalized Langevin equation level reveals several non-equilibrium solvation regimes in which the rate constant differs considerably from the standard equilibrium solvation, transition state theory predictions. In addition, the solvent is found to be heavily involved in the reaction coordinate, in contrast to the standard equilibrium solvation view. The marked differences from a Kramers Langevin equation level description are also described. In each regime, the rate constant and reaction coordinate are found analytically, and typical reaction system-solvent trajectories are displayed. A related normal-mode description convenient for strong reaction system-nearest-neighbor solvent dipole interactions is constructed. In this perspective, non-equilibrium solvation effects on rates and reaction coordinates are interpreted in terms of a bias of the reactive motion towards the direction of least friction.

Nonequilibrium solvation dynamics in solution reactions

We construct a theoretical framework for the description of nonequilibrium solvation and solvent participation in the reaction coordinate for solution reactions. The framework is illustrated by a model of reactive dipole isomerization. We show that a multidimensional reaction coordinate picture is equivalent to a one dimensional description in which a generalized friction characterizes and quantifies nonequilibrium solvation effects on the reaction rate. The adiabatic regime where equilibrium solvation and mean potential ideas are correct is identified. Several distinct regimes of nonequilibrium solvation are identified and described in molecular terms. In the effective mass regime, equilibrium solvation ideas give the reaction barrier curvature correctly, but solvent inertia modifies the barrier passage rate. In the nonadiabatic regime, the solvent is ‘‘frozen’’ during the barrier passage and cannot provide equilibrium solvation. In the polarization caging regime, the reacting species adjust to the moving solvent, rather than vice versa, and the solvent is heavily involved in the reaction coordinate. The rate constant in each of these regimes is related to reactive and solvent dynamics.