High-energy interstellar isomers: cis-N-methylformamide in the G+0.693-0.027 molecular cloud

Context. Isomerism in complex organic molecules provides key insights into the formation mechanisms and physical conditions of the interstellar medium (ISM). Among the C2H5NO isomers, only acetamide and trans-N-methylformamide (trans-NMF) have been detected in space. The recent detection of higher-energy isomers in other chemical families raises questions about the formation and abundance of less stable isomers. Aims. We aim to search for cis-N-methylformamide (cis-NMF), the next higher-energy conformer in the C2H5NO family, and investigate its possible formation pathways. Methods. We used ultra-sensitive wide-band spectral surveys obtained with the Yebes 40 m and IRAM 30 m telescopes to search for cis-NMF towards the Galactic centre molecular cloud, G+0.693-0.027. A spectroscopic catalogue was extrapolated from literature data to aid the search. Results. We present the first detection of cis-NMF in the ISM, with 55 unblended or slightly blended transitions, 44 of which were new transitions identified based on extrapolated spectroscopic data. Due to the lack of collisional rate coefficients, a quasi-non-LTE analysis, which separated the transitions into different Ka ladders, was used to determine the excitation conditions. The derived column density is (1.5±0.1) × 10^13 cm−2, corresponding to a molecular abundance of (1.1 ±0.2) × 10^−10 relative to H2. The resulting trans/cis-NMF isomeric ratio of 2.9±0.6 deviates significantly from thermodynamic expectations, suggesting that kinetic non-equilibrium processes and stereospecific chemical pathways are responsible for the formation of cis-NMF in this environment. Conclusions. The detection of cis-NMF expands the known inventory of interstellar C2H5NO isomers and challenges the assumption that isomer abundances strictly correlate with thermodynamic stability. Laboratory and theoretical studies propose formation via CH3NCO hydrogenation or spin-forbidden reactions involving CH2 and NH2CHO, though these may not reflect typical ISM conditions. This finding highlights the need for further investigation into isomerisation mechanisms and constrains astrochemical models of complex organic molecules.