The trans/cis ratio of formic, HCOOH, and thioformic, HC(O)SH, acids in
  the Interstellar Medium
  We studied theoretically 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. 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 (Tkin 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 behaviour 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, of
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). 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 it is
determined by other competitive chemical processes
