Infrared Multiphoton Dissociation Spectroscopy of a Gas-Phase Complex of Uranyl and 3-Oxa-Glutaramide: An Extreme Red-Shift of the [O=U=O](2+) Asymmetric Stretch

The gas-phase complex UO2(TMOGA)(2)(2+) (TMOGA = tetramethyl-3-oxa-glutaramide) prepared by electrospray ionization was characterized by infrared multiphoton dissociation (IRMPD) spectroscopy. The IRMPD spectrum from 700-1800 cm(-1) was interpreted using a computational study based on density functional theory. The predicted vibrational frequencies are in good agreement with the measured values, with an average deviation of only 8 cm(-1) (<1%) and a maximum deviation of 21 cm(-1) (<2%). The only IR peak assigned to the linear uranyl moiety was the asymmetric v(3) mode, which appeared at 965 cm(-1) and was predicted by DFT as 953 cm(-1). This v(3) frequency is red-shifted relative to bare uranyl, UO22+, by ca. 150 cm(-1) due to electron donation from the TMOGA ligands. Based on the degree of red-shifting, it is inferred that two TMOGA oxygen-donor ligands have a greater effective gas basicity than the four monodentate acetone ligands in UO2(acetone)(4)(2+). The uranyl v(3) frequency was also computed for uranyl coordinated by two TMGA ligands, in which the central O-ether, of TMOGA has been replaced by CH2. The computed v(3) for UO2(TMGA)(2)(2+), 950 cm(-1), is essentially the same as that for UO2(TMOGA)(2)(2+), suggesting that electron donation to uranyl from the ether of TMOGA is minor. The computed v(3) asymmetric stretching frequencies for the three actinyl complexes, UO2(TMOGA)(2)(2+), NpO2(TMOGA)(2)(2+) and PuO2(TMOGA)(2)(2+), are comparable. This similarity is discussed in the context of the relationship between v(3) and intrinsic actinide-oxygen bond energies in actinyl complexes.