The relative kinetic energy distributions for the Y-+ CH3X nucleophilic substitution products from dissociation of metastable X-(CH3Y) (X = C1, Br; Y = Br, I) have been analyzed by means of ion kinetic energy spectroscopy, and the results modeled using statistical phase space theory. Comparison of the experimental distributions with the theoretical distributions predicted for statistical partitioning of the available 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 partition to relative translation. This unavailable energy must correspond to internal energy, most likely vibrational excitation, in the CH3X products. These results are consistent with recent theoretical predictions of non-RRKM dynamics in gas-phase s N 2 reactions.

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