The chemistry of H₂NC in the interstellar medium and the role of the C + NH₃ reaction^★

¹ Instituto de Física Fundamental, CSIC, C/ Serrano 123, 28006 Madrid, Spain
e-mail: marcelino.agundez@csic.es; octavio.roncero@csic.es
² Observatorio Astronómico Nacional, IGN, Calle Alfonso XII 3, 28014 Madrid, Spain
³ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

Received: 28 February 2023
Accepted: 21 March 2023

Abstract

We carried out an observational search for the recently discovered molecule H₂NC, and its more stable isomer H₂CN, toward eight cold dense clouds (L1544, L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH₃, L1498, and L1641N) and two diffuse clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in different types of interstellar regions and shed light on its formation mechanism. We detected H₂NC in all but one of the cold dense clouds targeted, while H₂CN was only detected in five out of the eight clouds. The column densities derived for both H₂NC and H₂CN are in the range 10¹¹–10¹² cm⁻², and the abundance ratio H₂NC/H₂CN varies between 0.51 and >2.7. The metastable isomer H₂NC is therefore widespread in cold dense clouds, where it is present with an abundance similar to that of H₂CN. We did not detect H₂NC or H₂CN in any of the two diffuse clouds targeted, meaning we can make no conclusions regarding how the chemistry of H₂NC and H₂CN varies between dense and diffuse clouds. We find that the column density of H₂NC is correlated with that of NH₃, which strongly suggests that these two molecules are chemically linked, ammonia most likely being a precursor of H₂NC through the C + NH₃ reaction. We performed electronic structure and statistical calculations that show that both H₂CN and H₂NC can be formed in the C + NH₃ reaction through two different channels involving two different transition states that are very close in energy. The predicted product branching ratio H₂NC/H₂CN is very method dependent, but values between 0.5 and 0.8 are the most likely. Therefore, both the astronomical observations and the theoretical calculations support the reaction C + NH₃ being the main source of H₂NC in interstellar clouds.