Trajectory Studies of SN2 Nucleophilic Substitution. 4. Intramolecular and Unimolecular Dynamics of the Cl----CH3Br and ClCH3---Br- Complexes

Haobin Wang, Gilles H. Peslherbe, William L. Hase

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Classical trajectory calculations, performed on an analytic potential energy function derived from ab initio calculations, are used to study the intramolecular and unimolecular dynamics of the Cl----CH3Br complex with initial mode specific excitation. Two distinct patterns are observed in the dynamics of this complex. When the low-frequency modes are excited, the complex preferentially dissociates to Cl- + CH3Br. However, when the high-frequency CH3Br intramolecular modes are excited, the above is a negligible reaction path and, instead, Cl----CH3Br → ClCH3---Br- becomes important. Contrary to RRKM theory, the ClCH3---Br- complexes formed by this isomerization do not immediately dissociate to C1CH3 + Br but remain trapped in the central barrier region of the potential energy surface, with extensive barrier recrossings. The intramolecular dynamics of Cl----CH3Br and ClCH3---Br- are interpreted in terms of intermolecular and intramolecular complexes, with the former accessing the dissociation products and the latter the central barrier region. There is a dynamical bottleneck for transitions between these two complexes. The ClCH3 + Br- product energies, for ClCH3---Br- complexes which do dissociate, are in agreement with the previous experimental study of Graul and Bowers [J. Am. Chem. Soc. 1991, 113, 9696].

Original languageEnglish
Pages (from-to)9644-9651
Number of pages8
JournalJournal of the American Chemical Society
Issue number21
StatePublished - Oct 1 1994


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