Internal Excitation in the Products of Nucleophilic Substitution from the Dissociation of Metastable Ion Complexes

The relative kinetic energy distributions for the products of the dissociation of four metastable gas-phase ion clusters have been analyzed by means of ion kinetic energy spectroscopy, and the results modeled using statistical phase space theory. The systems studied represent reaction intermediates in bimolecular nucleophilic substitutions (SN2). These studies build on previous investigations that demonstrated vibrational excitation in the products of the substitution reactions of halide ions with methyl halides. The present studies explore the effects of molecular structure, reaction exothermicity, and nucleophile and leaving group variation. The experimental kinetic energy distributions are compared with theoretical distributions calculated for statistical partitioning of energy among internal modes and relative kinetic energy of the products. In each reaction, the calculated distributions agree with the experimental distributions only if a significant fraction of the energy released in the exothermic reactions is assumed to be unavailable for randomization in the dissociation. The results suggest that the products of these SN2 reactions are internally excited.