A new sulfur oxide, OSOSO, and its cation, likely present in the Io’s atmosphere: detection and characterization by mass spectrometric and theoretical methods

The gas-phase reaction of SO+ with SO2, relevant to the atmospheric chemistry of Io, was investigated with mass spectrometric and computational methods. Consistent with previous reports, no net chemical change was observed at 10-8−10-7 Torr by FT-ICR spectrometry. However, formation of a transient S2O3+ adduct of OSOSO connectivity, a, was suggested by the fast (k = 6.0 ± 1.5 × 10-10 cm-3 s-1 molec-1) 34SO+/32SO2 isotope exchange. The adduct was directly observed in SO2/CI experiments, and structurally probed by MIKE and CAD spectrometry, the results of which are also consistent with connectivity a. Computational results at the B3LYP/6-311+G(2d) level of theory, complemented by single-point CCSD(T) calculations, confirmed that a planar S2O3+ ion of connectivity a is more stable at 298 K than the isomer b of connectivity OSSO2 by 23.4 kcal mol-1 at the CCSD(T) level of theory. NR spectrometry of S2O3+ allowed detection of a hitherto unknown sulfur oxide, S2O3, also of OSOSO connectivity, characterized as a metastable triplet whose dissociation into SO2 and SO (X3Σ-), exothermic by 16.8 kcal mol-1 at 298 K, requires overcoming a barrier of 6.1 kcal mol-1 at the CCSD(T) level. The atmospheric implications of the results are briefly discussed.