2025A&A...703A.164M

The presence of molecular isomers in interstellar environments has become a topic of growing interest within the astrochemical community. Contrary to predictions based on thermodynamic equilibrium, recent observations reveal a diverse array of high-energy isomers and conformers. One of the most iconic molecular isomers detected in space, formic acid (HCOOH, FA), has been the focus of extensive theoretical research aimed at understanding its speciation into cis and trans conformers in dark clouds and photodissociation regions. In this work, we report the detection of c-FA, the higher energy conformer, using ultrasensitive observations of TMC-1. This detection builds on previous findings related to the Barnard-5 and L483 dark clouds. The derived trans-to-cis isomer ratio (trans/cis) in TMC-1, 17.5, closely matches those observed in other sources, suggesting that the same chemical processes are at play across these environments. To investigate this, we conducted detailed astrochemical gas-grain models tailored to formic acid isomerism to explain the observed ratios. Our models successfully reproduce the observed trans/cis values and indicate that the presence of cis-formic acid can be attributed to the release of c-FA from grains. This is followed by an isomerization process driven by the excess energy released during desorption, which we have named isomerization-upon-desorption (IUD). The models also show that the isomerization of t-FA to c-FA in the gas phase is negligible at 10 K, meaning the observed ratios are a direct consequence of the formation pathways of both isomers on the surface of dust grains. However, at higher temperatures, quantum tunneling mediated direct isomerization in the gas becomes significant and the ratios converge toward the thermodynamic equilibrium value.