Theoretical Insights into a Novel Ion–Ion Reaction of Methane in the Initial Stages of Hydrocarbon Growth in Space

In this article, we examine the reactions between methane molecules as a starting point for hydrocarbon growth in space and assess the effectiveness of the ion–ion reaction between CH₄⁺ and CH₄⁺ using quantum mechanical and molecular dynamics methods. We modeled the reaction starting from the dicationically ionized [CH₄···CH₄]²⁺ cluster. Initially, attractive interactions occur between the facing C–H bonds of the tetrahedral structures, which are electron-deficient. As the structure transitions to a trigonal pyramid, a bond begins to form between two carbon atoms with unpaired electrons, resulting in a metastable configuration due to the balance between Coulombic repulsion and attractive forces. The stabilization energy for C–C bond formation was 176.8 kcal/mol, with a bond formation efficiency of 32.6%, and the corresponding rate coefficient was 1.394 × 10^–2 fs^–1. This stabilization by C–C bond formation generates kinetic energy, and if sufficient energy is redistributed to the vibrational mode of the reaction, the reaction can proceed. Reactions involving C–C bond formation produced precursors of ethane, ethylene, and acetylene, such as C₂H₆²⁺, C₂H₅⁺, C₂H₄⁺, and C₂H₃⁺, as well as CH₃⁺, a key species in ion–molecule reactions in space. Even without C–C bond formation, a significant amount of CH₃⁺ was produced. Our findings underscore the importance of exploring novel ion–ion reactions to deepen our understanding of molecular growth in space.