Formation, spectroscopy, photochemistry, and quantum chemistry of the (S₂)(O₂) complex in solid argon

Samples formed by codepositing S₂ from a superheater source with Ar/O₂ on a 10 K substrate exhibit very strong 1403.0-cm^-1 and weaker 725.5-cm^-1 infrared absorptions. Photolysis decreases these bands slightly and produces SO₂ and S₂O. The infrared bands show ¹⁸O₂ and ³⁴S₂ shifts appropriate for O-O and S-S fundamental vibrations, and triplet absorptions in mixed isotopic experiments suggesting two equivalent O and two equivalent S atoms in the product complex. Similar results were obtained for Se₂ and O₂; new absorptions appeared at 1404.5 and 391 cm^-1. Ab initio calculations at the SCF, CISD, and CCSD levels of theory failed to find a complex with the observed spectroscopic properties. However, calculations with the BP density functional characterized a singlet (S₂)(O₂) parallelogram structure bound by 15.6 kJ/mol relative to triplet S₂ and O₂ with the O-O stretching frequency red-shifted 187 cm^-1 and the S-S fundamental blue-shifted 20 cm^-1. This weakly bound (S₂)(O₂) complex is chemically intermediate between the unstable O₄ and stable S₄ molecules. The argon matrix has made possible the formation of the weak (S₂)(O₂) complex and DFT with the BP functional has characterized this weak charge-transfer interaction.