Successive H-atom addition to solid OCS on compact amorphous solid water

TL;DR

This study combines experimental and computational methods to explore how hydrogen atoms react with solid OCS on icy surfaces at low temperatures, revealing pathways to form various sulfur- and carbon-containing molecules relevant to interstellar chemistry.

Contribution

It provides new insights into the reaction mechanisms of solid OCS with H atoms, highlighting quantum tunneling and radical formation on icy grains, which were not previously well understood.

Findings

H addition to OCS proceeds via quantum tunneling.

Formation of CO, H2S, H2CO, CH3OH, and HC(O)SH from H-OCS reactions.

Initial H addition predominantly occurs at the S atom, forming radicals.

Abstract

Carbonyl sulfide (OCS) is an abundant sulfur (S)-bearing species in the interstellar medium. It is present not only in the gas phase, but also on interstellar grains as a solid; therefore, OCS very likely undergoes physicochemical processes on icy surfaces at very low temperatures. The present study experimentally and computationally investigates the reaction of solid OCS with hydrogen (H) atoms on amorphous solid water at low temperatures. The results show that the addition of H to OCS proceeds via quantum tunneling, and further addition of H leads to the formation of carbon monoxide (CO), hydrogen sulfide (H2S), formaldehyde (H2CO), methanol (CH3OH) and thioformic acid (HC(O)SH). These experimental results are explained by our quantum chemical calculations, which demonstrate that the initial addition of H to the S atom of OCS is the most predominant, leading to the formation of OCS-H radicals. Once the formed OCS-H radical is stabilized on ice, further addition of H to the S atom yields CO and H2S, while that to the C atom yields HC(O)SH. We have also confirmed, in a separate experiment, the HC(O)SH formation by the HCO reactions with the SH radicals. The present results would have an important implication for the recent detection of HC(O)SH toward G+0.693-0.027.