Direct Dynamics Trajectory Study of Vibrational Effects:  Can Polanyi Rules Be Generalized to a Polyatomic System?

We present an ab initio direct dynamics trajectory study of the hydrogen abstraction reaction:  H₂CO⁺ + CD₄ → H₂COD⁺ + CD₃, with methane excited in two different distortion modes (ν₄ and ν₂). The trajectory simulations were able to reproduce experimental results and for the first time show how vibrational enhancement originates in reaction of small polyatomic species. Roughly equal contributions from two vibrational enhancement mechanisms were found. The “distortion” mechanism correlates the vibrational effects with vibration-induced reactant distortions, and the “velocity” mechanism correlates vibrational effects with vibrational velocities of the constituent atoms. This reaction has a reactant-like transition state and, thus, would correspond to an “early” barrier system in the context of the well-known Polanyi rules for predicting effects of vibration and collision energy. Straightforward application of these rules would predict that vibration should be ineffective in driving reaction, in disagreement with both experiment and trajectory results. Using the trajectories for guidance, we were able to construct a two-dimensional cut through the reaction potential energy surface that does suggest a predictive, Polanyi-type rule.