Photodissociation and Infrared Spectroscopy of Uranium–Nitrogen Cation Complexes

Abstract

Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U + (N 2 ) n (n = 1−8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U + (N 2 ) n indicate that N 2 ligands are intact molecules and that there is no insertion chemistry resulting in UN + or NUN + . Fixed frequency photodissociation at 532 and 355 nm indicate that the U + −N 2 bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U + −N 2 dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N−N stretch vibration via elimination of N 2 molecules. These resonances are observed to be shifted about 130 cm −1 to the red from the free-N 2 frequency for complexes with n = 3−8. Density functional theory indicates that U + is most stable in the sextet state in these complexes and that N 2 molecules bind in end-on configurations. The fully coordinated complex is predicted to be U + (N 2 ) 8 , which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U + −N 2 dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.

Conversion of a UO22+ Precursor to UH+ and U+ Using Tandem Mass Spectrometry to Remove Both “yl” Oxo Ligands

“…While spin–orbit effects were not included in our computed reaction energetics, a previous experimental and computational study of the U–H bond dissociation energy have shown that spin–orbit corrections can lower the energies of [UH] + and U + by 0.779 eV (75.2 kJ/mol) and 0.852 eV (85.2 kJ/mol), respectively. Based on previous reports, − we expect that spin–orbit effects for the U 5f orbitals could enhance the coupling between the singlet, triplet, and quintet potential energy surfaces …”

Cation Complexes of Uranium and Thorium with Cyclooctatetraene: Photochemistry and Decomposition Products

“…As shown, many of the same structures occur for both Th + and U + complexes, with similar relative energies. The significant challenges of theory on actinide molecules are well-known. ,− We use DFT with a functional and basis set known to give reasonable accuracy at a reasonable cost for actinide systems. , Because of this DFT method, these energetics are not expected to be highly accurate, but they should be reliable enough to establish qualitative trends.…”

“…21,47−56 We use DFT with a functional and basis set known to give reasonable accuracy at a reasonable cost for actinide systems. 36,37 Because of this DFT method, these energetics are not expected to be highly accurate, but they should be reliable enough to establish qualitative trends. Both metals are predicted to produce acetylene complexes in a cation−π configuration, with the hydrogen atoms bent away from the C−C axis and the metal.…”

“…Ions are analyzed and selected in a reflectron time-of-flight mass spectrometer. , After mass selection, UV laser excitation takes place in the turning region of the reflectron field, and fragment ions are analyzed by their flight times through a second field-free drift section. The methods employed were described in our recent studies of the spectroscopy and photochemistry of other uranium-containing cluster ions. − Laser radiation for fixed-frequency photodissociation at 532 or 355 nm was produced by a Spectra Physics GCR-150 Nd:YAG laser.…”