On the Electronic Structure of Molecular UO₂ in the Presence of Ar Atoms:  Evidence for Direct U−Ar Bonding

Abstract

Calculations via scalar-relativistic density functional theory (DFT) and ab initio CCSD(T) methodologies are used to explore the possibility of direct interactions between molecular UO2 and Ar atoms. The 3Hg electronic state of UO2, which is an excited state of the isolated molecule, exhibits significant bonding to Ar in the model complexes UO2(Ar) and UO2(Ar)5. The calculated vibrational frequencies of ground-state 3Phiu UO2 and UO2(Ar)5 with an (fphi)1(fdelta)1 electron configuration agree well with the observed frequencies of UO2 in solid neon and solid argon, respectively. The results strongly suggest that the ground electron configuration of UO2 changes from 5f17s1 to 5f2 when the matrix host is changed from neon to argon.

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

“…Bonding of the noble gas atoms to the 3 H g state would not only be favored by the longer bond length of that state but also by the lack of repulsive interaction with the electron in the 7s orbital. This picture is corroborated by DFT calculations 15 of vibrational frequencies for gas phase UO 2 . The 5f 1 7s 1 3 ⌽ u and 5f 2 3 H g states do indeed match the experimental frequencies in the neon and argon matrices, respectively.…”

“…The bond distance is 1.770 Å. puted by DFT ͑856 cm −1 ͒, 15 large basis set CASPT2 ͑948 cm −1 ͒, 18 and DC-FSCC ͑961 cm −1 ͒. In the paper of Li et al 15 the symmetric stretch of the 4 g state is reported at 779 cm −1 , which does fit the experimental band found in the fluorescence experiment. This interpretation is, however, not corroborated by our calculations, as we compute a symmetric stretch vibration of 911 cm −1 for this state.…”

“…I. Li et al 15 have shown that at the ECP-CCSD͑T͒ level of theory on UO 2 ͑Ar͒, the 3 H g is sufficiently low in energy that the effect of spin-orbit coupling could be strong enough to change the ground state. Gagliardi et al 18 do not disregard this possibility, but point out that this would make it very difficult to interpret the fluorescence and REMPI data of Heaven and co-workers.…”

“…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.…”

“…Li et al suggested that this is due to a change in the electronic ground state, and presented density functional theory ͑DFT͒ calculations indicating that a weak bond arises by donation of electron density of the noble gas into the empty uranium 6d orbitals in CUO and UO 2 . 9,15 In argon and heavier noble gas matrices, this bonding interaction is strong enough to change the ordering of the ground and first excited states, leading to the observed strong redshifts. A convincing argument was the very good agreement between the calculated and observed asymmetric stretch frequencies.…”