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

TL;DR

This study models the hydrogenation of OCS on interstellar dust grains, revealing a diastereoselective pathway to trans-HC(O)SH, a prebiotic molecule, and explaining the absence of its cis form in space observations.

Contribution

It provides a theoretical mechanism for the formation of trans-HC(O)SH on interstellar grains, highlighting diastereoselectivity and radical intermediates involved.

Findings

Trans-HC(O)SH is the main product of OCS hydrogenation.

The reaction proceeds via a localized radical intermediate, cis-OCSH.

The diastereoselectivity explains the absence of cis-HC(O)SH in observations.

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.