Spectroscopic Signatures and Photochemical Stability of the ClSO 3 and ClOSO 2 Radicals: New Benchmarks for Atmospheric and Astrochemical Models

Abstract Chlorine–sulfur oxides represent potentially important yet largely uncharacterized intermediates in atmospheric and astrochemical environments, from explosive volcanic plumes to irradiated exoplanet atmospheres and the sulfur-rich exospheres of Io and icy moons. Here, we present the first comprehensive structural, spectroscopic, and energetic characterization of the ClSO 3 and ClOSO 2 radicals. Using a hierarchy of high-accuracy composite quantum-chemical approaches—including CCSD(T)-F12, core–valence corrections, dual-level anharmonic force fields, and multireference excited-state treatments—we determine equilibrium geometries, rotational and vibrational spectra, vertical excitation energies, and photodissociation thresholds. ClSO 3 is found to be substantially more stable than the ClOSO 2 isomer, whose dissociation into ClO + SO 2 is nearly thermoneutral, indicating that it may act as a short-lived but chemically relevant intermediate. Strong IR features in the 600–1200 cm −1 region, together with broad electronic absorption, show that both radicals can affect UV/visible opacity and photochemistry in chlorine- and sulfur-bearing environments. The predicted dipole moments and rotational constants place several intense transitions within Atacama Large Millimeter/submillimeter Array and NOEMA frequency ranges, making these species promising targets for future astronomical searches. The benchmark dataset reported here provides the spectroscopic foundation required for laboratory identification and enables the incorporation of halogen–sulfur intermediates into atmospheric, planetary, and interstellar chemical networks.