Paving the way to the synthesis of adenine in the ISM: abundant Z-cyanomethanimine .

We report the first detection in the interstellar medium of the Z-isomer of cyanomethanimine (HNCHCN), an HCN dimer proposed as precursor of adenine. We identified six transitions of Z-cyanomethanimine, along with five transitions of E-cyanomethanimine, using IRAM 30m observations towards the Galactic Center quiescent molecular cloud G+0.693. The Z-isomer has a column density of (2.0±0.6)×10¹⁴ cm^-2 and an abundance of 1.5×10^-9. The relative abundance ratio between the isomers is [Z/E]∼6. This value cannot be explained by the two chemical formation routes previously proposed (gas-phase and grain surface), which predict abundances ratios between 0.9 and 1.5. The observed [Z/E] ratio is in good agreement with thermodynamic equilibrium at the gas kinetic temperature (130-210 K). Since isomerization is not possible in the ISM, the two species may be formed at high temperature. New chemical models, including surface chemistry on dust grains and gas-phase reactions, should be explored to explain our findings. Whatever the formation mechanism, the high abundance of Z-HNCHCN shows that precursors of adenine are efficiently formed in the ISM.