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

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