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 _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 ≤ J _up ≤ 11 and K _a ≤ 2 that belong to this species toward 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 ± 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 _2 NH) and the cyanogen radical (CN) arises as the primary formation route.