Combined crossed molecular beams and computational study on the N(2D) + HCCCN(X1Σ+) reaction and implications for extra-terrestrial environments

The reaction of the nitrogen atom (N) in its first electronically excited state (²D) with cyanoacetylene (HC₃N) has been investigated under single-collision conditions by using the crossed molecular beam method with mass spectrometric detection at a collision energy of 31 kJ mol⁻1. With the support of electronic structure calculations, we found that this reaction proceeds via the barrierless addition of the N(²D) atom to the carbon-carbon triple bond of HC₃N, followed by the formation of a cyclic intermediate adduct HC(N)CCN, which dissociates to C(N)CCN+Hproducts or isomerises to a more stable intermediate HNCCCN by H-migration and ring-opening processes. The long-lived HNCCCN complex produces the linear 3∑_g^-ground state dicyanocarbene (NCCCN) radical plus atomic hydrogen through a barrierless unimolecular dissociation accompanied by a negligible competitive channel forming the NCCCN radical (¹A₁) with a bent C_2v structure plus H. The main product of this neutral-neutral reaction is the ³NCCCN radical that could be a potential precursor to form other nitriles (C₂N₂, C₃N) or more complex organic species in planetary atmospheres, such as that of Titan and Pluto, in cometary comas, and in UV irradiated interstellar environments.