Destruction rates of interstellar methyl cyanide (CH3CN) by collisions with He+ ions

Methyl cyanide (a.k.a. acetonitrile) is a molecule of great astrochemical interest as it is one of the simplest interstellar complex organic molecules (iCOMs) routinely detected in young solar analogues. It has been observed in Class 0 and I hot-corinos (e.g. Taquet et al. 2015, Yang et al. 2021, Bianchi et al. 2022, Ceccarelli et al. 2024), in shocked regions (e.g., Codella et al. 2009), as well as in planet-forming disks (Öberg et al. 2015, Bergner et al. 2018, Loomis et al. 2018, 2020, Ilee et al. 2021). Moreover, CH₃CN has been detected in comets, including towards 67/P Churyumov-Gerasimenko in the context of the ESA-Rosetta mission (Le Roy et al. 2015; Altwegg et al. 2019). Nitriles have a strong prebiotic relevance, as they act as intermediates in the formation of biomolecules, by reacting with water and participating in multi-step synthesis of amino acids/RNA precursors (e.g. Sutherland 2017). Hence the presence of nitriles and water in comets, with CH₃CN ranging from ~0.008 to 0.054% with respect to water (Biver & Bockelée-Morvan 2019), is particularly interesting, and makes CH₃CN a key species to explore the "chemical" connections between disks and comets. However, for such comparison to be meaningful, reaction rates and branching fractions for CH₃CN formation and destruction pathways should be updated.

The network of gas-phase formation routes of CH₃CN has been revised and extended recently (Giani et al. 2024), and here we focus on its destruction routes by collisions with energetic ions H⁺, H₃⁺, HCO⁺ and He⁺. While the reactions of H₃⁺ and HCO⁺ lead to non-dissociative proton transfer (see experiments cited in KIDA and UMIST databases) giving CH₃CNH⁺ (from which methyl cyanide can be regenerated by recombination with electrons or proton transfer to NH₃), reactions with H⁺ and He⁺ are mostly destructive, due to the large exothermicity of the charge exchange process, equal to 12.39 eV in the He⁺ case. Therefore, collisions with He⁺ are an important pathway for the decomposition of iCOMs, as demonstrated previously for CH₃OCH₃, HCOOCH₃ and CH₃OH (Cernuto et al. 2017, Cernuto et al. 2018, Ascenzi et al 2019, Richardson et al. 2022). For CH₃CN, while the reaction with H⁺ has been experimentally studied (Smith et al. 1992, see the KIDA and UMIST databases), no previous experimental or theoretical studies have been carried out for He⁺, and the rates reported in the astrochemical databases refer to expectations from capture theories.

By applying a combined experimental and theoretical approach, we have developed the Potential Energy Surface for He⁺ plus CH₃CN (L. Mancini et al. 2024) and we have used it to model the dynamics of the charge exchange process, by determining cross sections and branching ratios in a wide range of collision energies, from which rate constants at varying temperatures can be obtained. The electron capture is expected to occur from an inner valence orbital of CH₃CN, leaving the radical cation in a highly excited electronic state leading to complete fragmentation. The main detected ionic fragments are HC₂N⁺/C₂NH⁺, CH₂⁺ and HCNH⁺, quite different from those proposed in KIDA/UMIST databases (CH₃⁺ and CN⁺), and the rate constant at T=10 K is 4.6x10^-9 cm³molec^-1s^-1, to be compared with values of 4.5x10^-8 and 1.3x10^-8 cm³molec^-1s^-1 from KIDA and UMIST databases, respectively.

Previous astrochemical models suggested that CH₃CN in disks is mostly formed on grains, because gas-phase chemistry cannot account for the large observed abundances (Öberg et al. 2015, Loomis et al. 2018). These studies compared the abundance ratios of CH₃CN with smaller N-bearing compounds (e.g. HCN, HC₃N) in disks and in comets to test the inheritance of disk material into the forming planets and small bodies. Our experiments, however, indicate that the rate coefficient for the destruction of CH₃CN through collisions with He⁺ might be almost one order of magnitude lower than what previously reported. Combined with the revised chemical network for formation of CH₃CN (Giani et al. 2024), this may suggest that CH₃CN in disks is mostly a gas-phase product. The formation of abundant CH₃CN in gas-phase would explain why this molecule is routinely detected in planet-forming disks out to large radii (a few hundred of au), in contrast with CH₃OH, which is only formed on grains, hence only detected where grains are sublimated, i.e. in the inner disk region. Our findings are thus expected to have important implications on the origin of methyl cyanide in planet-forming disks, as well as on the comparison of the abundance ratios of N-bearing molecules with cometary abundance ratios.

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