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

We report the first detection in the interstellar medium of N-cyanomethanimine (H₂CNCN), 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 ≤ Jᵤp ≤ 11 and Kₐ ≤ 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 H₂CNCN of (2.9\, ± \,0.1)×10¹² cm⁻², which translates into a total molecular abundance with respect to H₂ of (2.1\, ± \,0.3)×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\, ± \,1.8 and a $Z/E$-HNCHCN ratio of 4.5\, ± \,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 (CH₂NH) and the cyanogen radical (CN) arises as the primary formation route.