Direct dynamics calculations with neglect of diatomic differential overlap molecular orbital theory with specific reaction parameters

We have calculated the α-deuterium secondary kinetic isotope effect and the heavy-water solvent kinetic isotope effect for the reaction Cl^-(H₂O)_n + CH₃Cl′ → CH₃Cl + Cl′^-(H₂O)_n with n = 0, 1, and 2. Instead of using an analytic potential energy function, we calculated the energy and gradient whenever we needed them by neglect of diatomic differential overlap (NDDO) molecular orbital theory with parameters adjusted specifically for these individual reactions. The interface of the molecular orbital calculations with the dynamics calculations was accomplished by the use of a new direct dynamics computer program MORATE. The results are compared in detail to previous calculations based on 18-, 27-, and 36-dimensional semiglobal analytic potential energy functions, and the correspondences between the kinetic isotope effects and their interpretation in terms of specific modes are very encouraging. We conclude that use of NDDO molecular orbital theory with specific reaction parameters should be a very useful technique for modeling potential energy surfaces for polyatomic reactions.