Collision-induced and complex-mediated roaming dynamics in the H + C₂H₄ → H₂ + C₂H₃ reaction

Roaming is a novel mechanism in reaction dynamics. It describes an unusual pathway, which can be quite different from the conventional minimum-energy path, leading to products. While roaming has been reported or suggested in a number of unimolecular reactions, it has been rarely reported for bimolecular reactions. Here, we report a high-level computational study of roaming dynamics in the important bimolecular combustion reaction H + C₂H₄ → H₂ + C₂H₃, using a new, high-level machine learning-based potential energy surface. In addition to the complex-mediated roaming mechanism, a non-complex forming roaming mechanism is found. It can be described as a direct inelastic collision where the departing H atom roams and then abstracts an H atom. We denoted this as "collision-induced" roaming. These two roaming mechanisms have different angular distributions; however, both produce highly internally excited C₂H₃. The roaming pathway leads to remarkably different dynamics as compared with the direct abstraction pathway. A clear signature of the roaming mechanism is highly internally excited C₂H₃, which could be observed experimentally.