Ab Initio Molecular Dynamics Study on the Electron Capture Processes of Protonated Methane (CH₅⁺)

Electron capture dynamics of protonated methane (CH₅⁺) have been investigated by means of a direct ab initio molecular dynamics (MD) method. First, the ground and two low-lying state structures of CH₅⁺ with eclipsed C_s, staggered C_s and C_2v symmetries were examined as initial geometries in the dynamics calculation. Next, the initial structures of CH₅⁺ in the Franck−Condon (FC) region were generated by inclusion of zero point energy and then trajectories were run from the selected points on the assumption of vertical electron capture. Two competing reaction channels were observed: CH₅⁺ + e⁻ → CH₄ + H (I) and CH₅⁺ + e⁻ → CH₃ + H₂ (II). Channel II occurred only from structures very close to the s-C_s geometry for which two protons with longer C−H distances are electronically equivalent in CH₅⁺. These protons have the highest spin density as hydrogen atoms following vertical electron capture of CH₅⁺ and are lost as H₂. On the other hand, channel I was formed from a wide structural region of CH₅⁺. The mechanism of the electron capture dynamics of CH₅ is discussed on the basis of the theoretical results.