2025A&A...698A..36S

Context. In recent astrochemical studies it has become essential to study not only the most stable conformers, but also all the structures within the conformational panorama of the molecule, some of which are potentially detectable in the interstellar medium (ISM). In this context, the isomeric ratio can be used as a powerful tool to distinguish between different formation routes of molecules with increasing levels of complexity.
Aims. While the most stable cis-conformer of methyl formate (CH₃OCHO) is ubiquitous in the ISM, there is just one tentative detection of the higher-energy trans form (ΔE = 3000 K) toward the envelope of the star-forming region Sgr B2(N). For this work, our aim was to search for trans-methyl formate in additional sources and to perform new theoretical computations to better understand its conformational isomerism.
Methods. We used an ultradeep molecular line survey of the Galactic Center molecular cloud G+0.693-0.027, carried out with the Yebes 40m and IRAM 30m telescopes, as well as publicly available data from the Large Program ASAI observed toward the prototypical protostellar shock L1157-B1. The observational results are compared with predictions based on new grain-surface theoretical computations, which are sensitive to the stereochemistry of the molecule.
Results. We present the detections of trans-methyl formate in both astronomical regions, providing conclusive observational evidence of its presence in the ISM. Numerous unblended or slightly blended a-type K_a = 0, 1 transitions belonging to the A-substate of trans-methyl formate have been identified in both sources, many of which have been directly observed in radio astronomical data for the first time and remain unmeasured in the laboratory. We derive a molecular column density for trans-methyl formate of N = (8.2 ± 0.4) ×10¹² cm^–2 and N = (1.6 ± 0.3) ×10¹² cm^–2, respectively, for each source, yielding a molecular abundance with respect to H₂ of ∼6 ×10^–11 and ∼8 ×10^–10, respectively. Therefore, we obtain cis/trans isomeric ratios of ∼72 and ∼34 toward G+0.693 and L1157-B1, respectively, which are about seven and three times higher than that found in the Sgr B2(N) region. The results from our theoretical computations suggest that a stereospecific formation of trans-methyl formate via the CH₃O + HCO route on grain surfaces can qualitatively explain the observed cis/trans abundance ratio. Nevertheless, we show that additional stereospecific gas-phase routes could also play a crucial role in maintaining the intricate balance between formation and destruction of trans-MF, ultimately leading to its detection.