ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
For the Cl−+CH3Cl SN2 nucleophilic substitution reaction transition-state theory predicts that crossing the central barrier region of the potential-energy surface is the rate-controlling step. In this work classical trajectories are initialized at the central barrier. Four different models are considered for the potential-energy surface. A significant amount of central barrier recrossing is observed in the trajectories, which suggests that transition-state theory is an incomplete model for calculating the Cl−+CH3Cl SN2 rate constant. Two types of recrossings are observed in the trajectories: intermediate recrossings in which trajectories linger near the central barrier and complex recrossings in which trajectories trapped in the Cl−⋅⋅⋅CH3Cl complex return to the central barrier region. Intermediate recrossings are important if, in the trajectory initial conditions, zero-point energy is added to the vibrational modes orthogonal to the reaction coordinate. Rice–Ramsperger–Kassel–Marcus (RRKM) theory predicts extensive dissociation of the Cl−⋅⋅⋅CH3Cl complex to Cl−+CH3Cl and negligible complex recrossings in the trajectory calculations. In contrast to this prediction, negligible Cl−+CH3Cl formation is observed and continual complex recrossings occur, on a time scale longer than the complex's RRKM lifetime. These results indicate the ergodic assumption is invalid for the Cl−⋅⋅⋅CH3Cl complex. Phase-space bottlenecks which give rise to the intermediate and complex recrossings are considered.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.462331
Permalink