  In addition to nucleophile and solvent, the leaving group has a significant
influence on nucleophilic substitution (SN2) reactions. Its role is frequently
discussed with respect to reactivity, but its influence on the reaction
dynamics remains obscured. Here, we uncover the influence of the leaving group
on the gas phase dynamics of SN2 reactions in a combined approach of
crossed-beam imaging and dynamics simulations. We have studied the reaction F-
+ CH3Cl and compared it to F- + CH3I. For the two leaving groups Cl and I we
find very similar structures and energetics, but the dynamics show
qualitatively different features. Simple scaling of the leaving group mass does
not explain these differences. Instead, the relevant impact parameters for the
reaction mechanisms are found to be crucial, which is attributed to the
relative orientation of the approaching reactants. This effect occurs on short
time scales and may also prevail under solution phase conditions.
