Examining Interstellar Ion-Neutral Reactions Using the Unified Reaction Valley Approach: The Case of the CCl+ + CH3CN Reaction

Nitriles are a notable subset of nitrogen-bearing molecules detected within the interstellar medium (ISM). The cyano group (C≡N) enables these species to serve as potential chemical precursors for the formation of prebiotic molecules (e.g., amino acids) among other astrochemically relevant compounds. Acetonitrile, CH₃CN, is one of the simplest detected nitriles that has garnered the attention of the astrochemical community. In this study, a computational mechanistic investigation of the reaction between CH₃CN and the carbon monochloride cation, CCl⁺, was conducted. One of the reaction’s primary products, protonated acetylene (C₂H₃⁺), was recently detected at the z = 0.89 molecular absorber in front of the quasar PKS 1830-211 [ Astron. Astrophys. 2024, 683, A62], where CH₃CN [ Astron. Astrophys. 2011, 535, A103] and HCl, the proposed chemical precursor of CCl⁺ [ Astron. Astrophys. 2019, 629, A128; Astrophys. J. 2009, 706, 1594], have been previously detected. This detection points to the possibility that the reaction occurs within this molecular absorber, and C₂H₃⁺ could serve as a proxy for detecting CCl⁺. The Unified Reaction Valley Approach (URVA) method, developed by our group, was utilized to acquire precise insights into the reaction’s mechanism. Among other mechanistic insights, we find that the dissociation of the acetonitrile’s CN bond is critical to the formation of the C₂H₃⁺ molecular ion, whose exceptionally mobile hydrogen atoms are heavily involved in the four reaction pathways which produce the primary products of the reaction. This study demonstrates the utility of URVA for the in-depth mechanistic analysis of ion-neutral gas-phase reactions in the ISM.