  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.
