Interstellar Formation of Thioethanal (CH$_{3}$CHS). Gas-Phase and Ice-Surface Mechanisms involving Secondary Sulfur Products

The formation pathways of sulfur-bearing species in the interstellar medium are crucial to understand astrochemical processes in cold molecular clouds and to gain new insights about the sulfur budget in these regions. We aim to explore the recently detected, thioethanal (CH${3}$CHS) formation mechanisms from thioethanol (CH${3}$CH${2}$SH) as a precursor in addition to secondary sulfur products. The electronic structure methods and density functional theory for both gas-phase and ice-grain surface environments is employed. To mimic interstellar ice-mantles, we use medium (W6) and large amorphized (W22) water clusters as implemented in Binding Energy Evaluation protocol. A barrierless formation mechanism for CH${3}$CHS under low-temperature interstellar conditions is identified, in the gas phase. Surface environments modulate activation barriers in a site-specific manner, elucidated through both Langmuir-Hinshelwood and Eley-Rideal initiated surface reaction pathways. Compared to oxygen analogs, sulfur chemistry enables alternate pathways due to weaker S-H bonding, with a competing route forming ethane-1,1-di-thiol (CH${3}$CH(SH)SH), on the ice-grain surface, potentially reducing CH${3}$CHS yields. The first accurate binding energy for thioethanol on water ice is also reported, confirming its greater volatility than ethanol. The proposed mechanism offers a tentative hypothesis for the apparent mutual exclusive detections of the CH${3}$CH${2}$SH and CH${3}$CHS in TMC-1, Orion, and Sgr B2(N), that further requires validation through quantitative astrochemical modeling and also to distinguish this chemical differentiation from observational sensitivity limitations. These qualitative findings highlight the multifaceted chemical behavior of sulfur-bearing organics in the interstellar medium and support CH${3}$CH(SH)SH as promising astro-chemical targets.

💡 Research Summary

The paper investigates the formation of thioacetaldehyde (CH₃CHS) in interstellar environments, focusing on both gas‑phase and ice‑surface pathways that originate from thioethanol (CH₃CH₂SH). Using a hierarchy of quantum‑chemical methods—initial geometry optimizations with BHandHLYP/def2‑TZVP, refinement with M06‑2X/def2‑TZVP, and high‑level single‑point energies at CCSD(T)/CBS—the authors map out the potential energy surface for reactions involving OH radicals, atomic sulfur, and hydrogen atoms.

In the gas phase, three possible hydrogen‑abstraction routes from thioethanol by OH are examined: abstraction from the methylene (‑CH₂) group, the thiol (‑SH) group, and the methyl (‑CH₃) group. The methylene abstraction is found to be the most favorable, proceeding through a weakly bound pre‑reactive complex (‑11.7 kJ mol⁻¹ relative to reactants) and a submerged transition state (‑0.8 kJ mol⁻¹) that yields a modest effective barrier of ~10.9 kJ mol⁻¹. The reaction is highly exothermic (products at ‑102.7 kJ mol⁻¹). The thiol abstraction also shows a submerged transition state but is less kinetically competitive, while methyl abstraction is disfavored both kinetically and thermodynamically. The resulting CH₃CHSH radical can then react with atomic sulfur to form a CH₃CHSSH intermediate, which either loses SH to give CH₃CHS or captures an H atom to produce ethane‑1,1‑di‑thiol (CH₃CH(SH)SH).

To model ice‑grain chemistry, the authors employ the Binding Energy Evaluation Platform (BEEP) with two families of amorphous solid water (ASW) clusters: medium‑size W6 (six water molecules) and larger W22 (twenty‑two water molecules). A systematic sampling of 130 distinct adsorption sites for CH₃CH₂SH on the W22 set yields a distribution of binding energies, computed at MPWB1K‑D3(BJ)/def2‑TZVPD with zero‑point and counterpoise corrections. The average binding energy (~4.2 kJ mol⁻¹) is lower than that of ethanol, indicating that thioethanol is more volatile and desorbs more readily from icy mantles.

Surface reaction mechanisms are explored through two paradigms: Langmuir‑Hinshelwood (LH), where both OH and thioethanol are co‑adsorbed on the ice, and Eley‑Rideal (ER), where a gas‑phase OH collides with an adsorbed thioethanol. In LH pathways, the water network stabilizes the transition state, reducing the effective barrier for methylene H‑abstraction to ~5 kJ mol⁻¹. In ER pathways, the direct impact of OH further lowers the barrier, making the step essentially barrierless. Subsequent reactions of the CH₃CHSH radical with atomic S on the surface can follow two competing routes: (i) formation of CH₃CHS via SH loss, or (ii) hydrogen addition to give CH₃CH(SH)SH. Because the S–H bond is relatively weak, the di‑thiol pathway has a lower activation energy (≈8–12 kJ mol⁻¹) and can dominate under certain site‑specific conditions, potentially suppressing CH₃CHS yields on grains.

The authors connect these mechanistic insights to astronomical observations. Recent detections of CH₃CHS in the cold dark cloud TMC‑1, contrasted with the presence of CH₃CH₂SH in Orion KL and Sgr B2(N), suggest a mutual exclusivity that may stem from the competition between gas‑phase formation (favoring CH₃CHS) and grain‑surface pathways (favoring the di‑thiol). The computed binding energy distribution supports the notion that thioethanol can desorb efficiently, feeding the gas‑phase route, whereas on ice surfaces the alternative di‑thiol channel can reduce the net production of CH₃CHS.

In conclusion, the study provides (1) a robust gas‑phase pathway for thioacetaldehyde that is viable at the low temperatures of dense clouds, (2) a detailed picture of how amorphous water ice modulates reaction barriers via LH and ER mechanisms, and (3) a plausible chemical explanation for the observed anti‑correlation of CH₃CH₂SH and CH₃CHS in various interstellar sources. The authors recommend incorporating the derived rate constants and binding‑energy distributions into astrochemical networks for quantitative modeling, and they highlight the need for laboratory kinetic measurements and extended surface studies (including mixed ices with CO, CH₃OH, etc.) to further validate the proposed mechanisms.