Piecing together formic acid isomerism in dark clouds. Detection of cis-formic acid in TMC-1 and astrochemical modeling

Abstract

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 adds to previous findings in the Barnard-5 and L483 dark clouds. The derived trans-to-cis isomer ratio 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 ratios and indicate that the presence of cis-formic acid can be attributed to the release of c-FA from grains, followed by isomerization driven by the excess energy released during the desorption process, a process that we name as isomerization upon desorption. 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.

Accepted for publication in A&A

References in corpus (40)

• Non-thermal desorption from interstellar dust grains via exothermic surface reactions
• An Ice Age JWST inventory of dense molecular cloud ices
• Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH${}_3$CHCH${}_2$O)
• Simulation of the Formation and Morphology of Ice Mantles on Interstellar Grains
• Discovery of benzyne, o-C6H4, in TMC-1 with the QUIJOTE line survey
• Formation of Complex Organic Molecules in Cold Interstellar Environments through non-diffusive grain-surface and ice-mantle chemistry
• Discovery of Fulminic Acid, HCNO, in Dark Clouds
• Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry
• Discovery of two isomers of ethynyl cyclopentadiene in TMC-1: Abundances of CCH and CN derivatives of hydrocarbon cycles
• Discovery of two cyano derivatives of acenaphthylene (C${}_{12}$H${}_8$) in TMC-1 with the QUIJOTE line survey
• Gas phase Elemental abundances in Molecular cloudS (GEMS) VII. Sulfur elemental abundance
• Efficiency of non-thermal desorptions in cold-core conditions. Testing the sputtering of grain mantles induced by cosmic rays
• H2 formation on interstellar grains and the fate of reaction energy
• Investigating the Minimum Energy Principle in Searches for New Molecular Species - the Case of H${}_2$C${}_3$O Isomers
• Chemical complexity induced by efficient ice evaporation in the Barnard 5 molecular cloud
• Trans-cis molecular photoswitching in interstellar Space
• Discovery of the elusive carbonic acid (HOCOOH) in space
• Carbon Atom Reactivity with Amorphous Solid Water: H${}_2$O Catalyzed Formation of H${}_2$CO
• The Case of H${}_2$C${}_3$O Isomers, Revisited: Solving the Mystery of the Missing Propadienone
• Where does the energy go during the interstellar NH${}_3$ formation on water ice? A computational study
• A study of C4H3N isomers in TMC-1: line by line detection of HCCCH2CN
• Isomers in Interstellar Environments (I): The Case of Z- and E-Cyanomethanimine
• Diffusion activation energy and desorption activation energy for astrochemically relevant species on water ice show no clear relation
• The trans/cis ratio of formic, HCOOH, and thioformic, HC(O)SH, acids in the Interstellar Medium
• First detection in space of the high-energy isomer of cyanomethanimine: H2CNCN
• The origin of the E/Z isomer ratio of imines in the interstellar medium
• Surface Diffusion of Carbon Atoms as a Driver of Interstellar Organic Chemistry
• The spatial distribution of an aromatic molecule, C6H5CN, in the cold dark cloud TMC-1
• Reaction dynamics on amorphous solid water surfaces using interatomic machine learned potentials. Microscopic energy partition revealed from the P + H -> PH reaction
• Interstellar detection of the simplest aminocarbyne, H2NC: an ignored but abundant molecule
• A New "Non-energetic" Route to Complex Organic Molecules in Astrophysical Environments: The C + H${}_2$O $\to$ H${}_2$CO Solid-state Reaction
• Discovery of five cyano derivatives of propene with the QUIJOTE line survey
• Cosmic ray sputtering yield of interstellar ice mantles: CO and CO2 ice thickness dependence
• Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms. Insights on isomerism and deuteration
• Cracking the Puzzle of CO2 Formation on Interstellar Ices. Quantum Chemical and Kinetic Study of the CO + OH -> CO2 + H Reaction
• Modelling methanol and hydride formation in the JWST Ice Age era
• Proposed importance of HOCO chemistry: Inefficient formation of CO${}_2$ from CO and OH reactions on ice dust
• Conformational isomerism of methyl formate: new detections of the higher-energy trans conformer and theoretical insights
• A sequential acid-base (SAB) mechanism in the interstellar medium: The emergence of cis formic acid in dark molecular clouds
• The chemistry of H2NC in the interstellar medium and the role of the C + NH3 reaction