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Aiming to constrain the surface formation of HCN and HNC in the dense interstellar medium on ice-covered dust grains, we investigate the interaction of CN radicals with H 2 O and CO ices and their subsequent reactivity with H and H 2 . CN radicals can physisorb on both ices. However, on H 2 O ice, a hemibond formation is the most common binding mode, while on CO ice, the CN-CO van der Waals complex can form NCCO with a small energy barrier. We show low barrier or barrierless pathways to the formation of HCN and HNC for the reaction H + CN on both ices. Reactivity with H 2 involves activation energy barriers to form HCN, which may be overcome by quantum tunneling, while HNC formation is unlikely. The formation of HCN and HNC competes with the formation of NH 2 CHO on H 2 O and HCOCN on CO.

The chemical network governing interstellar sulfur has been the topic of unrelenting discussion for the past few decades due to the conspicuous discrepancy between its expected and observed abundances in different interstellar environments. More recently, the astronomical detections of CH 3 CH 2 SH and CH 2 CS highlighted the importance of interstellar formation routes for sulfur-bearing organic molecules with two carbon atoms. In this work, we perform a laboratory investigation of the solid-state chemistry resulting from the interaction between C 2 H 2 molecules and SH radicals-both thought to be present in interstellar icy mantles-at 10 K. Reflection absorption infrared spectroscopy and quadrupole mass spectrometry combined with temperature-programmed desorption experiments are employed as analytical techniques. We confirm that SH radicals can kick-start a sulfur reaction network under interstellar cloud conditions and identify at least six sulfurated products: CH 3 CH 2 SH, CH 2 CHSH, HSCH 2 CH 2 SH, H 2 S 2 , and tentatively CH 3 CHS and CH 2 CS. Complementarily, we utilize computational calculations to pinpoint the reaction routes that play a role in the chemical network behind our experimental results. The main sulfur-bearing organic molecule formed under our experimental conditions is CH 3 CH 2 SH, and its formation yield increases with the ratios of H to other reactants. It serves as a sink to the sulfur budget within the network, being formed at the expense of the other unsaturated products. The astrophysical implications of the chemical network proposed here are discussed., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star-forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). However, the formation route of this species remains under debate. In the present work, we study the formation of ethanol through the reaction of CCH with one H 2 O molecule belonging to the ice as a test case to investigate the viability of chemical reactions based on a "radical + ice component" scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical-radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH 3 CH 2 OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H 2 O on the water ice clusters can be barrierless (because of the help of boundary icy water molecules acting as proton-transfer assistants), leading to the formation of vinyl alcohol precursors (H 2 CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)