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

TL;DR

This study reports the first cis-formic acid detection in TMC-1 and shows a trans/cis ratio of $\approx17.5$, consistent with other dark clouds. Using a Rokko three-phase gas-grain framework, the authors develop an isomer-resolved HOCO/HCOOH network and introduce an isomerization upon desorption (IUD) mechanism, along with non-thermal desorption processes, to reproduce observed FA isomer ratios. A sensitivity analysis finds that a high IUD probability ($P_{\rm IUD} \approx 90\%$) best matches the data, with microcanonical calculations supporting favored c-FA formation after desorption at 10 K. At elevated temperatures, gas-phase tunneling and destruction promote approach toward thermodynamic equilibrium, reducing cis-FA detectability; these findings explain why c-FA is observed in cold dark clouds but not in warmer regions. The work underscores the role of non-thermal, energy-driven processes on grain surfaces in shaping the ISM isomer inventory and offers a framework applicable to other molecules exhibiting high-energy isomerism in space.

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.

Figures (7)

• Figure 1: Lines of t-FA and c-FA (left and right panels, respectively) observed in TMC-1. Blanked channels correspond to negative artifacts resulting from the frequency-switching technique. The red lines correspond to the computed line profiles adopting the parameters given in Sect. \ref{['sec:observations']}. The lines in top and bottom panels of c-FA are only marginally detected (See text)
• Figure 2: t-FA / c-FA ratio over time as a function of the isomerization upon desorption (IUD) ratio. In the legend t and c indicate the isomeric form and the numbers the percentage of desorption of each of them in the t-HOCO + H -> (t/c)-HCOOH reaction. Horizontal lines represent the gas-phase observational abundances of both isomers in the Barnard-5 molecular cloud Taquet2017, L483 Agundez2019 and TMC-1 (This work).
• Figure 3: Microcanonical rate constant ratio for c-FA and t-FA isomerization upon desorption. The limit on the x-axis is determined from the reaction energy for the HOCO radical hydrogenation on ices Molpeceres2025 in between 90-105 kcal mol$^{-1}$ depending on factors as the HOCO isomer being hydrogenated or the ice binding site. The energy of c-FA is set as the origin of energies.
• Figure 4: Left panel - Chemical model using the best performing model for c-FA and t-FA in the gas. Horizontal bands represent the range of observational abundances with respect to H2 of both isomers in the three clouds where they have been detected, the Barnard-5 molecular cloud Taquet2017, L483 Agundez2019 and TMC-1 (This work). Middle panel - Equivalent to above but for ice abundances (comparing with H2O) and comparing with the upper and lower bounds provided in Boogert2015 and with the well constrained narrow band of Rocha2024. We note that HCOOH and HCOO- are summed in establishing the observational abundances Right - t-FA/c-FA gas phase ratios for the best performing model in linear scale. The gray area represents an approximated characteristic timescale of TMC-1 Pratap1997.
• Figure 5: Gas phase t-FA / c-FA for different $T_{\rm g}$. $T_{\rm d}$ is kept constant (10 K). Horizontal lines indicate that chemical equilibrium is reached. The equilibrium constants are, according to GarciadelaConcepcion2022, K$_{\rm t/c}$(10 K)=2.1(88), K$_{\rm t/c}$(20 K)=1.5(44), K$_{\rm t/c}$(50 K)=4.7(17), K$_{\rm t/c}$(70 K)=4.3(12), K$_{\rm t/c}$(90 K)=6.7(9), K$_{\rm t/c}$(100 K)=7.0(8). A(B) is equivalent to A$\times$10$^{B}$.