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

In this article, we examine the reactions between methane\nmolecules\nas a starting point for hydrocarbon growth in space and assess the\neffectiveness of the ion–ion reaction between CH<sub>4</sub><sup>+</sup> and CH<sub>4</sub><sup>+</sup> using quantum mechanical\nand molecular dynamics methods. We modeled the reaction starting from\nthe dicationically ionized [CH<sub>4</sub>···CH<sub>4</sub>]<sup>2+</sup> cluster. Initially, attractive interactions\noccur between the facing C–H bonds of the tetrahedral structures,\nwhich are electron-deficient. As the structure transitions to a trigonal\npyramid, a bond begins to form between two carbon atoms with unpaired\nelectrons, resulting in a metastable configuration due to the balance\nbetween Coulombic repulsion and attractive forces. The stabilization\nenergy for C–C bond formation was 176.8 kcal/mol, with a bond\nformation efficiency of 32.6%, and the corresponding rate coefficient\nwas 1.394 × 10<sup>–2</sup> fs<sup>–1</sup>. This\nstabilization by C–C bond formation generates kinetic energy,\nand if sufficient energy is redistributed to the vibrational mode\nof the reaction, the reaction can proceed. Reactions involving C–C\nbond formation produced precursors of ethane, ethylene, and acetylene,\nsuch as C<sub>2</sub>H<sub>6</sub><sup>2+</sup>, C<sub>2</sub>H<sub>5</sub><sup>+</sup>, C<sub>2</sub>H<sub>4</sub><sup>+</sup>, and\nC<sub>2</sub>H<sub>3</sub><sup>+</sup>, as well as CH<sub>3</sub><sup>+</sup>, a key species in ion–molecule reactions in space.\nEven without C–C bond formation, a significant amount of CH<sub>3</sub><sup>+</sup> was produced. Our findings underscore the importance\nof exploring novel ion–ion reactions to deepen our understanding\nof molecular growth in space.