Investigation of the Gas-Phase N₂⁺ + CH₃CN Reaction at Low Temperatures

Rate coefficients for ion-polar-molecule reactions between acetonitrile molecules (CH₃CN) and nitrogen molecular ions (N₂⁺), which are of importance to the upper atmospheric chemistry of Saturn's moon Titan, were measured for the first time at low translational temperatures. In the experiments, the reaction between sympathetically cooled N₂⁺ ions embedded in laser-cooled Ca⁺ Coulomb crystals and velocity-selected acetonitrile molecules generated using a wavy Stark velocity filter was studied to determine the reaction rate coefficients. Capture rate coefficients calculated by the Su-Chesnavich approach and by the perturbed rotational state theory considering the rotational state distribution of CH₃CN were compared to the experimental rate coefficients. The results indicate that the present reaction is barrierless and that the rate coefficients are consistent with the capture rate coefficients. The potential energy surface of the reaction product CH₃CN⁺ was calculated by the CCSD(T)/aug-cc-pVQZ level theory to explore possible isomerization and dissociation pathways. Several locally stable structures leading to H₂CCN⁺, HCCNH⁺, HCNCH⁺, and H₂CNC⁺ were found, while no intrinsic reaction coordinate leading to the CHCN⁺ formation pathway accompanied by H₂ abstraction has been identified. The H₂CCN⁺ + H dissociation channel is a major pathway of the reaction product, though theoretical calculations suggest the CHCN⁺ + H₂ dissociation channel is energetically feasible. The present experimental and theoretical studies will contribute to the accurate modeling of nitrile chemistry in interstellar matter and in the upper atmosphere of Titan.