Reactions of laser-ablated uranium atoms with H₂O in excess argon: A matrix infrared and relativistic DFT investigation of uranium oxyhydrides

Laser-ablated U atoms react with H₂O during condensation in excess argon. Infrared absorptions at 1416.3, 1377.1, and 859.4 cm^-1 are assigned to symmetric H-U-H, antisymmetric H-U-H, and U=O stretching vibrations of the primary reaction product H₂UO. Uranium monoxide, UO, also formed in the reaction, inserts into H₂O to produce HUO(OH), which absorbs at 1370.5, 834.3, and 575.7 cm^-1. The HUO(OH) uranium(IV) product undergoes ultraviolet photoisomerization to a more stable H₂UO₂ uranium(VI) molecule, which absorbs at 1406.4 and 885.9 cm^-1. Several of these species, particularly H ₂UO₂, appear to form weak Ar-coordinated complexes. The predicted vibrational frequencies, relative absorption intensities, and isotopic shifts from relativistic DFT calculations are in good agreement with observed spectra, which further supports the identification of novel uranium oxyhydrides from matrix infrared spectra.