Infrared Spectra of UO₂, UO₂⁺, and UO₂^- in Solid Neon

Abstract

Reactions of laser-ablated uranium atoms, cations, and electrons with O2 during condensation with excess neon produce UO, UO2, UO3, UO2 +, and UO2 - as characterized by infrared spectra with oxygen isotopic substitution and B3LYP/pseudopotential calculations. Differences in low-lying states for UO2 give rise to substantial shifts and ground state reversal between argon and neon matrices. A series of B3LYP/pseudopotential calculations has been undertaken on oxide species related to the uranyl dication by the addition of one, two, or three electrons. Several electronic states have been characterized for each species. These simple, low-cost calculations predict vibrational frequencies which match those observed in neon matrices extremely well (typically 3−5% too high). The ground state of neutral UO2 appears to have 3Φu symmetry, while 2Φu ground states are implied for UO2 + and UO2 -.

A Fock space coupled cluster study on the electronic structure of the UO2, UO2+, U4+, and U5+ species

“…An example is the ionization potential ͑IP͒ of the UO 2 molecule, measured as 5.4 eV by Capone et al 2 using the electron impact technique. Theoretical calculations 3 consistently gave a higher value. Gagliardi et al, 4 who had done accurate complete active space second order perturbation theory ͑CASPT2͒ calculations that gave an IP of 6.27 eV, proposed that the experimental data were in error.…”

“…In argon, this interaction should then be sufficiently strong to reverse the order of the two states, while in the more weakly interacting neon matrix, the ordering is the same as in the gas phase. This explanation of the large matrix effect is currently generally accepted for CUO, even though the CASPT2 calculations of Roos et al 17 gave the triplet 3 ⌽ at slightly lower energy in the gas phase. Our previous work 19 using the Dirac-Coulomb coupled cluster method, DC-CCSD͑T͒, gives the closed shell state as ground state in the gas phase, in agreement with the original picture.…”

“…A particularly interesting aspect is the interaction of small actinide molecules with noble gas matrices. Laser ablation spectroscopy has been used by Andrews and co-workers to trap UO, UO 2 , and CUO in noble gas matrices 3,[6][7][8][9][10][11][12][13][14][15] and measure vibrational frequencies as a function of the matrix composition ͑Ne, Ar, Kr, Xe, or mixtures thereof͒. An intriguing feature of both CUO and UO 2 is the large redshift ͑about 130 cm −1 ͒ in the antisymmetric stretch found when replacing a neon matrix by an argon matrix.…”

On the reaction of a uranium atom with a nitrogen molecule: a theoretical attempt

“…[1][2][3][4][5] The vibrational frequencies of the linear triatomics OUO, OUO + , OUO 2+ , NUN, and CUO have recently been measured in solid neon and argon matrices using infrared spectroscopy, using laser ablation techniques to prepare the species. 6 From the theoretical point of view, the pioneer in this area was Pyykkö ,7 who already in 1994, presented, together with Runeberg, a Hartree-Fock study on the trends of geometries and vibrational frequencies of several uranyl iso-electronic species like NUN, CUO, CUN À , and NUO + . 8 More recently, theoretical studies, using the density functional approach (DFT/B3LYP), have been performed.…”

“…Experimentally, laser-ablated uranium atoms react with the diatomic XY during condensation and the reaction products include the insertion product XUY. 6 It seems that this reaction occurs rather easily indicating a low barrier. One would conclude that uranium atoms can easily break the strong XY bond, for example in the N 2 molecule.…”