Description
Recent astronomical detections of sulphur-bearing carbon-chain molecules, including HCSCN and HCSCCH, in cold interstellar environments have renewed interest in the gas-phase chemistry responsible for their formation. Despite their astrophysical relevance, the underlying reaction mechanisms and product branching remain poorly constrained, limiting the reliability of chemical models of sulphur chemistry in molecular clouds.
In this work, we present a detailed theoretical investigation of two barrierless radical–molecule reactions involving thioformaldehyde,
\mathrm{CN + H_2CS}
and
\mathrm{CCH + H_2CS},
as possible formation routes to HCSCN and HCSCCH, respectively. The potential energy surfaces were explored using high-level electronic structure calculations, with particular attention to the entrance region, the competition between multiple addition channels, and the subsequent evolution of the intermediate complexes toward product formation or alternative dissociation pathways.
For both systems, several low-energy entrance configurations are identified, leading to distinct reactive wells and competing product channels. These features highlight the importance of accurately describing long-range capture and channel-specific branching at low temperatures. Rate coefficients and product distributions are then evaluated within a master-equation/RRKM framework over temperatures relevant to cold interstellar clouds. The calculations provide mechanistic insight into how structural orientation at capture and internal rearrangement within the energized complexes influence the relative efficiencies of the HCSCN and HCSCCH formation pathways.
The present results offer new molecular-level constraints for astrochemical networks involving sulphur-bearing species and address the broader issue of predicting branching ratios in barrierless reactions under low-temperature interstellar conditions.
| I am the presenting author | Yes |
|---|