Quantum dynamical study of the Cl-+CH3Br SN2 reaction

Abstract

Reduced dimensional time-dependent and time-independent quantum dynamics calculations are performed for the Cl ^- +CH ₃ Br S _N 2 reaction. In the time-dependent study, a wavepacket is initiated at the Cl ^- ···CH ₃ Br pre-reaction complex to study its unimolecular dynamics. A large portion of the wavepacket is trapped in the pre- and post-reaction complexes for a time substantially longer than that predicted by RRKM theory. Many resonance states are seen in the power spectrum of the wavepacket's time autocorrelation function. In the time-independent study, the cumulative reaction probability for Cl ^- ···CH ₃ Br→ClCH ₃ ···Br ^- isomerization is obtained. It is found to be much smaller than the statistical number of states/flux at the central barrier, suggesting that dynamics effects are important. A reduced dimensionality approach, appropriate for a complex reaction, is used to extract the full dimensional Cl ^- +CH ₃ Br→ClCH ₃ +Br ^- bimolecular substitution rate constant from the reduced dimensional quantum dynamics calculation. For the model Hamiltonian used here, this quantum dynamics rate constant is in much better agreement with experiment than that of statistical theory.