Reaction cross sections and thermal rate constant for Cl− + CH3Br → ClCH3 + Br− from J-dependent quantum scattering calculations

Abstract

Employing dimensionality-reduced time-independent quantum scattering theory and summation over all possible total angular momentum states, initial-state selected reaction cross sections for the exothermic gas-phase bimolecular nucleophilic substitution (S_N2) reaction Cl⁻ + CH₃Br → ClCH₃ + Br⁻ have been calculated. The carbon–halogen bonds and the rotation of the methyl halides are taken into account. In agreement with previous calculations for J = 0, initial rotational motion of CH₃Br decreases the reaction probability and consequently the cross sections. The experimentally obtained thermal rate constant for 300 K is reproduced within the experimental error. For lower temperatures, it is calculated to be below the experimental values but shows the same strong increase for T → 0.