A model multidimensional analytic potential energy function for the Cl^- + CH₃Br → ClCH₃ + Br^- reaction

Hartree-Fock calculations, utilizing the SV4PP split valence contracted Gaussian basis set for Cl and Br and the 6-31G* basis set for CH₃, are used to determine potential energy surface properties for the Cl^- + CH₃Br → ClCH₃ + Br^- S_N2 nucleophilic substitution reaction. This ab initio information is used in concert with experimental data to develop a multidimensional analytic potential energy function for the Cl^- + CH₃Br reaction system, by modifying the analytic potential energy function developed previously for Cl_a^- + CH₃Cl_b → Cl_aCH₃ + Cl_b^-. Each atom and degree of freedom is treated explicitly in the analytic function. Transition-state theory calculations are performed with the derived analytic potential to determine rate constants for Cl^- + CH₃Br → Cl^-⋯CH₃Br association, and for dissociation of the Cl^-⋯CH³Br prereaction complex to reactants Cl^- + CH₃Br, and isomerization of this complex to the postreaction complex ClCH₃⋯Br^-. RRKM calculations are also performed to determine rate constants for dissociation and isomerization of the complexes versus energy and angular momentum. The analytic function developed here for the Cl^- + CH₃Br → ClCH₃ + Br^- system has been incorporated into the general chemical dynamics computer program VENUS.