Sulfur Dioxide Sulfide (SSO2): The Elusive Isovalent Analogue of Sulfur Trioxide (SO3)

Abstract Sulfur dioxide sulfide (SSO2) has remained experimentally undetected despite repeated theoretical predictions of its stability. As a missing sulfur oxide isovalent to sulfur trioxide (SO3), SSO2 occupies a distinctive position at the boundary of formally hypervalent bonding, where replacing oxygen with sulfur perturbs a delocalized chalcogen-oxide framework. Here, we close this long-standing gap by identifying SSO2 in cryogenic SO2 ices processed by energetic electrons using tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and high-level quantum chemical computations. These studies reveal a broader sulfur oxide network including sulfur oxide peroxide (OSOO), 1,2,3-dioxathiirane 3-oxide (cyclic-S(O)OO), and disulfur monoxide (S2O). Our results establish sulfur-atom incorporation into SO2 as a viable condensed-phase route to a previously elusive sulfur oxide─SSO2─and highlight its unique molecular structure in sulfur-rich planetary and astrochemical environments.