VIBRATIONAL-EXCITATION IN PRODUCTS OF NUCLEOPHILIC-SUBSTITUTION - THEDISSOCIATION OF METASTABLE X(-)(CH3Y) IN THE GAS-PHASE

The relative kinetic energy distributions for the Y-+CH(3)X nucleophil ic substitution products from dissociation of metastable X(-)(CH3Y) (X = Cl, Br; Y = Br, I) have been analyzed by means of ion kinetic energ y spectroscopy, and the results modeled using statistical phase space theory. Comparison of the experimental distributions with the theoreti cal distributions predicted for statistical partitioning of the availa ble energy reveals that the substitution products are translationally cold. The theoretically calculated distributions can be made to agree with experiment if most of the energy released in the dissociation is assumed to be unavailable for randomization, such that it cannot parti tion to relative translation. This unavailable energy must correspond to internal energy, most likely vibrational excitation, in the CH(3)X products. These results are consistent with recent theoretical predict ions of non-RRKM dynamics in gas-phase S(N)2 reactions.