A missing oxidation node in chlorine–sulfur chemical networks: Accurate spectroscopic signatures of Cl2S2O isomers

Sulfur-chlorine oxidation chemistry governs key processes in planetary atmospheres and irradiated astrophysical environments, yet the corresponding reaction networks remain molecularly incomplete because critical intermediates are highly reactive, spectroscopically unassigned, and experimentally elusive. Here, we identify Cl2S2O as a plausible missing oxidation node in sulfur-chlorine chemistry. We show that this molecular formula supports a compact but chemically diverse manifold of low-energy isomers that are viable products of chlorosulfane oxidation and compelling targets for spectroscopic detection. For such intrinsically transient species, predictive theoretical spectroscopy is not ancillary but essential, as it provides the only viable route to molecular-level characterization. Using a high-accuracy composite quantum-chemical framework combined with state-of-the-art anharmonic treatments, we deliver a complete multispectroscopic characterization of the three lowest-energy Cl2S2O isomers, encompassing rotational, vibrational, and electronic spectroscopic signatures. The resulting spectroscopic parameters reach near-observational accuracy and enable direct, vis-à-vis comparison with laboratory measurements and astronomical observations, thereby defining concrete molecular targets for radioastronomical searches and planetary atmospheric studies. More broadly, this work demonstrates how modern computational spectroscopy can actively complete chemical reaction networks by coupling multispectroscopic predictions with experiment and observation, advancing a circular astrochemical paradigm in which theory, laboratory spectroscopy, and astronomical detection jointly drive molecular discovery.