TRAJECTORY STUDIES OF S(N)2 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 ah initio calculations, are used to study the intramolecular and unimolecular dynamics of the Cl----CH3Br compl ex with initial mode specific excitation. Two distinct patterns are ob served in the dynamics of this complex. When the low-frequency modes a re excited, the complex preferentially dissociates to Cl- + CH3Br. How ever, 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 t o ClCH3 + Br- but remain trapped in the central barrier region of the potential energy surface, with extensive barrier recrossings. The intr amolecular 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 f Br product energies, for ClCH3---Br- compl exes which do dissociate, are in agreement with the previous experimen tal study of Graul and Bowers [J. Am. Chem. Sec. 1991, 113, 9696].