Dicyanoacetylene (NC4N) formation in the CN + cyanoacetylene (HC3N) reaction: a combined crossed molecular beams and theoretical study

Due to their potential role in prebiotic chemistry, unsaturated dinitriles are an intriguing class of molecules in astrochemistry and cosmochemistry. In particular, dicyanoacetylene has attracted considerable interest because it is a precursor of uracil (via its hydrolysis to acetylenedicarboxylic acid and subsequent reaction with urea) and was identified in the atmosphere of Titan long ago. Having a null dipole moment, its detection via rotational spectroscopy in interstellar clouds can not be achieved. For this reason, it escaped identification until recently, when its protonated form NC4NH+ was finally detected toward the Taurus molecular cloud (TMC-1) (Agundez et al., Astronom. Astrophys. 2023, 669, L1). Given the low temperature conditions of both Titan and, especially, TMC-1, a facile formation route must be available. Following low-temperature kinetics experiments and theoretical characterization of the entrance channel, the CN + HC3N reaction has emerged as a compelling candidate for NC4N formation in cold clouds. Here, we report on a 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. High-temperature conditions are relevant in hotter environments, such as the circumstellar envelopes of asymptotic giant branch stars or photodissociation regions of the interstellar medium. 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. The effects of this new family of reactions should be tested in astrochemical models. 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.