CLASSICAL DESCRIPTION OF ACTIVATED CONFORMATIONAL PROCESSES IN MOLECULAR-SYSTEMS COUPLED TO SOLVENT DEGREES OF FREEDOM

Extended Fokker-Planck (FP) equations are generalized stochastic equat ions which describe the evolution of a set of coordinates, loosely ref erred to as solute, coupled with a relevant set of solvent variables. They are useful for the analysis of molecular dynamics in liquids, whe n a time-scale separation between probe motions and relaxation times o f interactions with surroundings particles cannot be performed, becaus e of persistence of slowly fluctuating components. In this article, we focus attention on a model system, made up of an angular coordinate a nd its conjugate momentum, submitted to a bistable potential, and coup led to a dissipative harmonic mode, to investigate the influence of po lar solvents on reactive dynamics. The results are appropriate to desc ribe dielectric effects, solvent-controlled conformational changes inv olving charge transfer which occur in photophysical processes, and the dynamic Stokes shifts observed in time-resolved fluorescence experime nts. (C) 1996 John Wiley & Sons, Inc.