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 of 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 (ASW) 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 apparent absence of the cis isomer in 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 ASW.

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms. Insights on isomerism and deuteration

“…Some directions may promote or inhibit the reactions in specific places within the molecule. As a matter of fact, we recently found orientation effects during the formation of TFA from OCS due to the formation of hydrogen bonds of the OCSH radical with the surface (Molpeceres et al 2021a).…”

“…where the isomer form of the products is deliberately left ambiguous. In this second step the parent molecule can be formed again as different isomer, depending on the orientation of the radical with the surface (Molpeceres et al 2021a).…”

“…In the core of the origin of isomerism in space, there is a rich interplay between different chemical and physical processes. Among those we can find: specific formation routes both in the gas phase and on grains (Vazart et al 2015;Loomis et al 2015;Quan et al 2016;Melli et al 2018;Bizzocchi et al 2020;Molpeceres et al 2021a), quantum-tunneling mediated isomerization (García de la García de la Concepción et al 2022), photoisomerization (Cuadrado et al 2016) or chemical conversion after formation on grains (Shingledecker et al 2020).…”

“…In this work, we put the focus on H abstraction reactions from FA and TFA acids on grains, continuing our recent efforts to understand their chemistry and isomerism. In addition to the formation routes of TFA (Nguyen et al 2021;Molpeceres et al 2021a) we have very recently tackled the tunneling-mediated isomerization in the gas phase of both FA and TFA, finding that, according to this mechanism, t-FA should be the only observable isomer (García de la Concepción et al 2022). At the same time, the situation is more complex in the case of TFA, where the isomerization is indeed possible in the interstellar medium.…”

“…We employed theoretical methods with an accurate treatment of quantum tunneling (Meisner et al 2017;Oba et al 2018;Álvarez-Barcia et al 2018;Miksch et al 2021;Molpeceres et al 2021a) to investigate H abstraction reactions from FA and TFA, both by H and D atoms. Moreover, the effect of the binding of the acids with the surface in the context of their chemistry was elucidated.…”