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.