Energy disposal in gas‐phase nucleophilic displacement reactions

The partitioning of reaction exothermicity into relative translational energy of the products of gas‐phase SN2 (F− + CH3Cl) and nucleophilic aromatic substitution (F− + C6H5Cl) reactions has been investigated using kinetic energy release Fourier transform ion cyclotron resonance spectroscopy. The chloride product ion is observed to be highly translationally excited for the SN2 reaction, indicating a cold internal energy distribution for the products. For the chlorobenzene reaction the products are not generated with large translational energies. The results are compared with a statistical model. Ion‐intensity profiles for the CH3Cl reaction deviate significantly from the statistical model whereas the chlorobenzene results are consistent with this model. The kinetic energy release for the CH3C1 reaction is compared with energy‐disposal results for the photodissociation and dissociative electron‐attachment processes of halomethanes. In all three cases a node in the molecular orbital between the carbon atom and the departing halogen results in a repulsive energy release. Ion‐retention curves for the nucleophilic aromatic substitution reaction are consistent with the existence of a long‐lived ion‐dipole complex on the exit channel for this reaction.