Trajectory Studies of S_N2 Nucleophilic Substitution. 4. Intramolecular and Unimolecular Dynamics of the Cl^----CH₃Br and ClCH₃---Br^- Complexes

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].