Molecular dynamics of the two-stage mechanism of cyclopentadiene dimerization: concerted or stepwise?

Abstract We report dynamics studies of cyclopentadiene dimerization. Transition structures were re-optimized with different density functionals. All but M06-2X give the ambimodal transition state. With ZPE, M06-2X also gives an ambimodal structure. Dynamics results show that 70% of the trajectories are dynamically-concerted, possessing a time gap between formation of two bonds smaller than 60 fs, which is the lifetime of transition state at room temperature from Eyring’s transition state theory. Thirty percent are dynamically stepwise. A “Two-stage” mechanism was found in 13% of the total trajectories. These are part of the dynamically stepwise trajectories and follow pathways through the Cope transition state hypothesized by Woodward and Katz in 1959. These are categorized as dynamically-stepwise trajectories since there are multiple vibrations before the second bond formation is completed. Dynamically-stepwise trajectories increase to 72% when the temperature of the simulation is set to 1000 K. More deep-recrossing trajectories are detected when trajectories were studied at this temperature.