Measurement of the asymmetric UO₂²+ stretching frequency for [U^VIO₂(F)₃]- using IRMPD spectroscopy

In a previous study [Int. J. Mass Spectrom. 2010; 297: 67–75], the asymmetric O=U=O stretch (ν₃) was measured for anionic uranyl complexes with composition [UO₂(X)₃]^-, X = Cl^-, Br^- and ^I-. Within this group of complexes, the ν₃ frequency red-shifts following the trend I > Br > Cl, suggesting concomitant weakening of the U=O bonds. However, a value for [UO₂(F)₃]^- was not measured, which prevented a comprehensive comparison of measured ν₃ positions to computed frequencies from density functional theory (DFT) calculations. Because the shift in ν₃ is predicted to be most dramatic when X = F, we revisited these species using infrared multiple-photon photodissociation spectroscopy. As in our earlier study, a modest red-shift to the ν₃ vibration of ∼ 6 cm^-1 was observed for X = I^-, Br-, and Cl^-, and the position of the frequency follows the trend I^- > Br- > Cl^-. The value measured for [UO₂(F)₃]^- is ∼43 cm^-1 lower than the one measured for [UO₂(Cl)₃]-. Overall, the trend with respect to ν₃ position is reproduced well by computed frequencies from DFT.