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Ion imaging experiments and direct chemical dynamics simulations were performed to study the atomic-level dynamics for the F-+ CH3I SN2 nucleophilic substitution reaction.[1,2] The simulations reproduce the product energy partitionings and the velocity scattering angle distribution measured in the experiments.The simulations reveal that the substitution reaction occurs by two direct atomic-level mechanisms,that is,rebound and stripping,and an indirect mechanism.Approximately 90%of the indirect events occur via a prereaction hydrogen-bonded complex.This mechanism may play an important role for other F-SN2 reactions due to the strong electronegativity of fluorine.The average product energy partitioning for the indirect mechanism agrees with the prediction of PST,even though a postreaction complex is not formed.