First detection in space of the high-energy isomer of cyanomethanimine: H2CNCN

Abstract

We report the first detection in the interstellar medium of $N$-cyanomethanimine (H${}_2$CNCN), the stable dimer of HCN of highest energy, and the most complex organic molecule identified in space containing the prebiotically relevant NCN backbone. We have identified a plethora of $a$-type rotational transitions with 3 $\le J_{\text{up}}\le$ 11 and $K_{\text{a}}\le$ 2 that belong to this species towards the Galactic Center G+0.693-0.027 molecular cloud, the only interstellar source showing the three cyanomethanimine isomers (including the $Z$- and $E$- isomers of $C$-cyanomethanimine, HNCHCN). We have derived a total column density for H${}_2$CNCN of (2.9$\pm$0.1)$\times$10${}^{12}$ cm${}^{-2}$, which translates into a total molecular abundance with respect to H${}_2$ of (2.1$\pm$0.3)$\times$10${}^{-11}$. We have also revisited the previous detection of $E$- and $Z$-HNCHCN, and found a total $C/N$-cyanomethanimine abundance ratio of 31.8$\pm$1.8 and a $Z/E$-HNCHCN ratio of 4.5$\pm$0.2. While the latter can be explained on the basis of thermodynamic equilibrium, chemical kinetics are more likely responsible for the observed $C/N$-cyanomethanimine abundance ratio, where the gas-phase reaction between methanimine (CH${}_2$NH) and the cyanogen radical (CN) arises as the primary formation route.

22 pages, 8 figures, 3 tables, 2 appendix - Accepted for publication in The Astrophysical Journal