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

We report the first detection in the interstellar medium of N-cyanomethanimine (H_2CNCN), 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 ≤ J_up≤ 11 and K_a≤ 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_2CNCN of (2.9±0.1)×10^12 cm^-2, which translates into a total molecular abundance with respect to H_2 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 (CH_2NH) and the cyanogen radical (CN) arises as the primary formation route.