Uranium oxo and superoxo cations revealed using infrared spectroscopy in the gas phase

The UO₄⁺ and UO₆⁺ cations are produced in a supersonic molecular beam by laser vaporization and studied with infrared laser photodissociation spectroscopy using rare gas atom predissociation. The argon complexes UO₄⁺Ar₂ and UO₆⁺Ar₂ are mass-selected in a reflectron time-of-flight spectrometer and excited with an IR-OPO laser system in the range of the O-U-O and O-O stretching vibrations. These same systems are studied with computational quantum chemistry. UO₄⁺ is found to have a central UO₂ core, with an additional ?² coordinated oxygen molecule. Charge transfer/oxidation gives the system the character of a UO ₂²⁺, O₂^- ion pair. UO ₆⁺ has this same core structure, with an additional weakly bound oxygen molecule in an η¹ coordination configuration. The O-U-O stretch is sensitive to the local environment and approximates the vibration of the isolated uranyl cation in these systems.