Matrix infrared spectroscopy and a theoretical investigation of SUO and US₂

US₂ and SUO molecules have been prepared by laser ablation of the solid materials and reaction of the elements during condensation in solid argon, which give the same absorptions as earlier U atom reactions with sulfur vapor and sulfur dioxide. The antisymmetric stretching mode of US₂ shifts from 438.7 cm^-1 in solid argon to 442.3 cm^-1 in solid neon, which shows that the same electronic state is trapped in both matrix environments. Density functional calculations find a bent (³B ₂) ground state for the US₂ molecule, and CASSCF/CASPT2 calculations reveal a multiconfigurational mixture of (5fI) ¹(5fδ)¹-type states, whereas the most stable state for UO₂ is a linear structure of the (5fI)(7s) type. The bent triplet ground state SUO molecule exhibits similar multireference character with the U-O stretching mode at 857.1 cm^-1 in solid argon. The linear SUO molecule computed at the CASPT2 level is only 2 kcal/mol above the bent structure. A detailed analysis of the bonding in US₂ and SUO is provided and compared to the better known UO₂ molecule.