Dicyanoacetylene (NC4N) Formation in the CN + Cyanoacetylene (HC3N) Reaction: A Combined Crossed-Molecular Beams and Theoretical Study

Unsaturated nitriles are significant in prebiotic and astrochemistry. Dicyanoacetylene, in particular, is a possible precursor of uracil and was previously detected in Titan’s atmosphere. Its null dipole moment hindered detection through rotational spectroscopy in interstellar clouds, and it escaped identification until recently, when its protonated form NC₄NH⁺ was finally detected toward the Taurus molecular cloud (TMC-1) (Agúndez et al., Astronom. Astrophys. 2023, 669, L1). Given the low-temperature conditions of both Titan and TMC-1, a facile formation route must be available. Low-temperature kinetics experiments and theoretical characterization of the entrance channel demonstrated that the CN + HC₃N reaction is a compelling candidate for NC₄N formation in cold clouds. Here, we report on a combined crossed-molecular beams (CMB) and theoretical study of the reaction mechanism up to product formation, demonstrating that NC₄N + H is the sole open channel from low to high temperatures (collision energies). Indeed, unlike other CN reactions, the formation of the isocyano isomer (3-isocyano-2-propynenitrile) was not seen to occur at the high collision energy (44.8 kJ/mol) of the CMB experiment. Preliminary calculations on the related CN + HC₅N reaction indicate that the reaction channel leading to NC₆N + H is exothermic and occurs via submerged transition states. We therefore expect it to be fast and that the mechanism is generalizable to the entire family of CN +cyanopolyyne reactions. Furthermore, we derive some properties of the related reactions C₂H + CNCN (isocyanogen) and CN + HCCNC (isocyanoacetylene): the C₂H + CNCN reaction leads to the formation of HC₃N + CN, and the main channel of the CN + HCCNC reaction also leads to CN + HC₃N. This last reaction efficiently converts isocyanoacetylene and, by extension, any isocyanopolyyne into their cyano counterparts without a net loss of cyano radicals. Finally, we also characterized the entrance channel of the reaction C₂H + NC₄N and verified that the addition of C₂H to all possible sites of NC₄N is characterized by a significant entrance barrier, thus confirming that, once formed, dicyanoacetylene terminates the growth of cyanopolyynes via the sequence of steps involving polyynes, cyanopolyynes, and C₂H/CN radicals.