Post-transition state dynamics and product energy partitioning following thermal excitation of the F⋯HCH₂CN transition state: Disagreement with experiment

Born-Oppenheimer direct dynamics simulations were performed to study atomistic details of the F + CH₃CN → HF + CH₂CN H-atom abstraction reaction. The simulation trajectories were calculated with a combined M06-2X/MP2 algorithm utilizing the 6-311++G∗∗ basis set. The experiments were performed at 300 K, and assuming the accuracy of transition state theory (TST), the trajectories were initiated at the F⋯HCH₂CN abstraction TS with a 300 K Boltzmann distribution of energy and directed towards products. Recrossing of the TS was negligible, confirming the accuracy of TST. HF formation was rapid, occurring within 0.014 ps of the trajectory initiation. The intrinsic reaction coordinate (IRC) for reaction involves rotation of HF about CH₂CN and then trapping in the CH₂CN⋯HF post-reaction potential energy well of ∼10 kcal/mol with respect to the HF + CH₂CN products. In contrast to this IRC, five different trajectory types were observed: the majority proceeded by direct H-atom transfer and only 11% approximately following the IRC. The HF vibrational and rotational quantum numbers, n and J, were calculated when HF was initially formed and they increase as potential energy is released in forming the HF + CH₂CN products. The population of the HF product vibrational states is only in qualitative agreement with experiment, with the simulations showing depressed and enhanced populations of the n = 1 and 2 states as compared to experiment. Simulations with an anharmonic zero-point energy constraint gave product distributions for relative translation, HF rotation, HF vibration, CH₂CN rotation, and CH₂CN vibration as 5%, 11%, 60%, 7%, and 16%, respectively. In contrast, the experimental energy partitioning percentages to HF rotation and vibration are 6% and 41%. Comparisons are made between the current simulation and those for other F + H-atom abstraction reactions. The simulation product energy partitioning and HF vibrational population for F + CH₃CN → HF + CH₂CN resemble those for other reactions. A detailed discussion is given of possible origins of the difference between the simulation and experimental energy partitioning dynamics for F + CH₃CN → HF + CH₂CN. The F + CH₃CN reaction also forms the CH₃C(F)N intermediate, in which the F-atom adds to the C≡N bond. However, this intermediate and F⋯CH₃CN and CH₃CN⋯F van der Waals complexes are not expected to affect the F + CH₃CN → HF + CH₂CN product energy partitioning.