Gas-phase reactions between interstellar molecules C2 and CH3CN: insights from reactive molecular dynamics simulations

The cold dark cloud TMC-1 hosts an extraordinary diversity of unsaturated carbon-chain molecules and nitrile-bearing species, which challenges the long standing view that complex organic synthesis requires warm or energetic environments. Among these species, acetonitrile (CH3CN) is one of the most abundant and widely detected nitriles, and the dicarbon molecule (C2) is a highly reactive transient species known to drive carbon-chain growth. However, the detailed reaction dynamics between C2 and CH3CN, and their potential to form larger cyanated hydrocarbons such as cyanopropagyl radicals, remain poorly characterised. Here we use reactive molecular dynamics simulations based on the ReaxFF potential to investigate the gas-phase reaction C2 + CH3CN over a broad temperature range. Our simulations reveal a rich synthetic network that yields 36 distinct products, including the interstellar species cyanoallene, propargyl cyanide and methylcyanoacetylene. Mechanistic analysis identifies the dissociation of CN from CH3CN as a critical enabling step, liberating CN to react with C2 and form C3N, while concurrent hydrogen migration drives the formation of CH2C2HCN and CHCCH2CN. These pathways demonstrate that complex cyanopolyynes can arise from a single precursor pair under non-equilibrium energetic conditions. The detection of the same products and their radical fragments in TMC-1 suggests that the elementary steps identified here, CN dissociation, hydrogen transfer and carbon-chain elongation, may operate under interstellar conditions, providing a plausible gas-phase route to the molecular complexity observed in cold molecular clouds.