The trans/cis ratio of formic (HCOOH) and thioformic (HC(O)SH) acids in the interstellar medium

Context. Observations of the different isomers of molecules in the interstellar medium (ISM) have revealed that both low- and high-energy isomers can be present in space despite the low temperature conditions. It has been shown that the presence of these isomers may be due to tunneling effects.
Aims: We carried out a theoretical study of the cis-trans isomerization reactions of two astrophysically relevant acids, formic acid (HCOOH) and thioformic acid (HC(O)SH), where the latter has recently been discovered in space. We also searched for these molecules towards the hot core G31.41+0.31 to compare their abundances with the expected theoretical isomerization results.
Methods: We employed high-level ab initio calculations to study the reaction rate constants of the isomerization reactions. We used the canonical variational transition state theory with the multidimensional small curvature tunneling approximation in the temperature range of 10-400 K. Moreover, we used the spectrum obtained from the ALMA 3mm spectral survey GUAPOS (GUAPOS: G31 Unbiased ALMA sPectral Observational Survey), with a spectral resolution of ~0.488 MHz and an angular resolution of 1.''2×1.''2 (~4500 au), to derive column densities of HCOOH and HC(O)SH towards G31.41+0.31.
Results: Our results demonstrate that these isomerizations are viable in the conditions of the ISM due to ground-state tunneling effects, which allow the system to reach the thermodynamic equilibrium at moderately low temperatures. At very low temperatures (T_kin ~ 10 K), the reaction rate constants for the cis-to-trans isomerizations are very small, which implies that the cis isomers should not be formed under cold ISM conditions. This is in disagreement with observations of the cis/trans isomers of HCOOH in cold cores where the cis isomer is found to be ~5-6% the trans isomer. At high temperatures (~150-300 K), our theoretical data not only match the observed behavior of the trans/cis abundance ratios for HCOOH (the cis form is undetected), but they support our tentative detection of the trans and - for the first time in the insterstellar medium - the cis isomer of HC(O)SH towards the hot molecular core G31.41+0.31 (with a measured trans/cis abundance ratio of ~3.7).
Conclusions: While the trans/cis ratio for HC(O)SH in the ISM depends on the relative stability of the isomers, the trans/cis ratio for HCOOH cannot be explained by isomerization, and is determined by other competitive chemical processes.