Chlorine–sulfur oxides in the Venusian atmosphere: Benchmark data from computational spectroscopy

Chlorine-sulfur oxides (Cl2SOn, n = 1-4) are key intermediates in the oxidative and photochemical cycles that couple sulfur and chlorine chemistry in the atmosphere of Venus. These compounds are thought to participate in catalytic processes converting SO2 to SO3 and, ultimately, to H2SO4, thereby contributing to the formation and maintenance of the planet's characteristic cloud system. While thionyl and sulfuryl chloride have been experimentally characterized, the higher oxidation members of the series remain largely unobserved and their structures and spectroscopic signatures are essentially unknown. In this work, high-accuracy equilibrium geometries, rotational constants, and anharmonic vibrational frequencies are determined for the Cl2SOn series using explicitly correlated ab initio methods combined with second-order vibrational perturbation theory. This approach enables a balanced description of electron correlation and anharmonic effects, providing spectroscopic parameters that closely reproduce available experimental data and extend predictive accuracy to the uncharacterized higher oxides. The resulting dataset furnishes the first consistent reference for these reactive species, facilitating their potential identification in laboratory spectra and planetary observations. The predicted rotational and vibrational features are expected to assist in the interpretation of forthcoming high-resolution measurements from ESA's EnVision and NASA's VERITAS missions, offering new insight into the complex chlorine-sulfur-oxygen chemistry shaping the Venusian atmosphere.