Diastereoselective Formation of trans-HC(O)SH Through Hydrogenation of OCS on Interstellar Dust Grains

Abstract

With the presence of evermore complex S-bearing molecules being detected lately, studies on their chemical formation routes need to keep up the pace to rationalize observations, suggest new candidates for detection, and provide input for chemical evolution models. In this paper, we theoretically characterize the hydrogenation channels of OCS on top of amorphous solid water as an interstellar dust grain analog in molecular clouds. Our results show that the significant reaction outcome is trans−HC(O)SH, a recently detected prebiotic molecule toward G+0.693. The reaction is diastereoselective, explaining the seemingly absence of the cis isomer in the astronomical observations. We found that the reaction proceeds through a highly localized radical intermediate (cis−OCSH), which could be essential in the formation of other sulfur-bearing complex organic molecules due to its slow isomerization dynamics on top of amorphous solid water.

The trans/cis ratio of formic, HCOOH, and thioformic, HC(O)SH, acids in the Interstellar Medium

“…This species was found toward the giant molecular cloud G+0.693-0.027 located in the Galactic center (with a kinetic temperature of T kin ∼150 K), where only the trans isomer could be detected (the derived upper limit abundance of the cis isomer, c-HC(O)SH, is ≤0.2×10 −10 ; Rodríguez-Almeida et al 2021). The selective formation of t-HC(O)SH on dust grains has been found to be very efficient through the hydrogenation of OCS in ices (Molpeceres et al 2021). However, this is the only detection of this species in the ISM so far (Rodríguez-Almeida et al 2021), and it remains unknown whether it could be present toward other sources, either in its trans isomeric form or in both trans and cis forms.…”