Interstellar formation of thioethanal (CH

Context. The formation pathways of sulfur-bearing species in the interstellar medium (ISM) are crucial for our understanding of astrochemical processes in cold molecular clouds and gaining new insights into the sulfur budget in these regions. Aims. We aim to explore the recently detected thioethanal (CH3CHS) formation mechanisms from thioethanol (CH3CH2SH) as a precursor, in addition to secondary sulfur products. Methods. We employed electronic structure methods and density functional theory for both gas-phase and ice-grain surface environments. To mimic interstellar ice-mantles, we used both medium (W6) and large amorphous (W22) water clusters, as implemented in Binding Energy Evaluation Platform (BEEP). Results. We identified a formation mechanism that is effectively barrierless with respect to the reactant asymptote for CH3CHS, which remains kinetically feasible under low-temperature interstellar conditions, 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 (CH3CH(SH)SH) on the ice-grain surface, potentially reducing CH3CHS yields. The first accurate binding energy for thioethanol on water ice is also reported here, confirming its greater volatility in comparison to ethanol. Conclusions. The proposed mechanism offers a tentative hypothesis for the apparent mutual exclusive detections of CH3CH2SH and CH3 CHS in TMC-1, Orion, and Sgr B2(N), which requires further 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 ISM and support CH3CH(SH)SH as a promising astrochemical target.