Spectator modes in reaction dynamics revisited: Reaction cross sections and rate constant for Cl− + CH3Br → ClCH3 + Br− from quantum scattering

Initial-state selected total cross sections for the reaction Cl− + CH3Br → ClCH3 + Br− have been calculated by means of time-independent quantum scattering theory. The totally symmetric vibrations of the methyl group and the two carbon-halogen stretching modes are explicitly included. The initial states with a single quantum in either of the three totally symmetric modes yield, as a function of translational energy, almost the same cross section summed over all product states. This strongly questions the generality of the concept of ‘spectator’ modes in reaction dynamics which applies only for thermal rate constants where the influence of these modes is small due to their low population.