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Abundant Z-cyanomethanimine in the interstellar medium: Paving the way to the synthesis of adenine

Abstract:

We report the first detection in the interstellar medium (ISM) 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 Centre quiescent molecular cloud G + 0.693. The Z-isomer has a column density of (2.0 ± 0.6) × 10 14 cm -2 and an abundance of 1.5 × 10 -9. The relative abundance ratio between the isomers is [Z/E]1/46. This value cannot be explained by the two chemical formation routes previously proposed (gas phase and grain surface), which pbkp_redicts 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.