Ion⁻molecule reaction between atomic oxygen anion (O-) and methane (CH₄) has been systematically investigated employing the on-the-fly ab initio molecular dynamics simulations. Besides the major H-abstraction process as the exothermic reaction studied widely, an endothermic pathway to produce OCH₃- and H is also observed in this study. Three typical O- attack modes with reference to the pyramid structure of CH₄ fixed in space have been considered. It was found that the internal motions of the radical products are significantly dependent on the O- attack modes. As for the reaction between O- and the thermally vibrating CH₄, the major pathway to produce OH- and CH₃ is preferred by the direct H-abstraction and the minor pathway to produce H and OCH₃- is the roaming reaction via the transient negative ion [HO-CH₃]-.
Keywords: Ab initio molecular dynamics simulation; ion–molecule reaction; roaming reaction; stereodynamics.