Molecular dynamics simulation study on the isomerization reaction in a solvent with slow structural relaxation

Abstract

• Isomerization reaction in viscous solvent was studied by MD simulation. • Slow structural relaxation of solvent is coupled to reaction dynamics. • Retardation of reaction in viscous solvent was explained by two-step model. The trans–gauche isomerization reaction of model 1,2-dichloroethane in water and ethylene glycol (EG) was studied by molecular dynamics (MD) simulation. With low barrier height, the reaction in EG was slower than that in water, and their difference decreased with increasing barrier height. Compared with the time-dependent diffusion model, in water, the effective diffusion coefficient was almost independent of time, whereas it decreased with time in EG. The trends were reproduced by Langevin dynamics simulation with a time-dependent friction coefficient from MD simulation. The effective diffusion coefficient in water agreed well with the prediction of the Grote–Hynes (GH) theory, whereas for EG, the GH theory overestimated the effective diffusion coefficient. It was suggested that the coupling with the slow structural relaxation of EG slows down the dynamics far from the transition state, which may slow down the overall reaction dynamics when the activation barrier is not high.