Energetic and Electronic Properties of UX+/0/– for X = Li and Be and Comparison of the Properties of the Uranium Atom Binding to 2nd Row Elements Li–F

Abstract

The bonding and spectroscopic properties of ULi^+/0/– and UBe^+/0/– to complete the series for UX^+/0/– for X = Li to F were investigated by high-level ab initio SO-CASPT2 and CCSD(T) electronic structure calculations. The low-lying spin–orbit states were obtained at the SA-CASPT2/aQ-PP level; bond dissociation energies (BDEs), ionization energies (IEs), adiabatic electronic affinities (AEAs), and vertical detachment energies (VDEs) were calculated at the Feller-Peterson-Dixon (FPD) level. A dense manifold of low-lying states was predicted for ULi^+/0/– and UBe^+/0/–. The calculated BDEs for ULi (37.7 kJ/mol) and UBe (8.0 kJ/mol) show that UBe is weakly bound. For redox processes, the BDEs increased for ULi⁺ (109.3 kJ/mol), ULi^– (47.4 kJ/mol), UBe⁺ (35.6 kJ/mol), and UBe^– (72.3 kJ/mol). The IE(ULi) = 4.650 eV is lower than IE(Li); the IE(UBe) = 5.901 eV is close to the IE(U) and to the IEs of UB, UC, UN, UO, and UF. The AEAs of ULi (0.708 eV) and UBe (0.989 eV) are lower than those for UB, UC, UN, and UO but higher than that for EA(UF). Natural bond orbital (NBO) calculations show that ULi has the 5f³6d¹7s² configuration for U and 2s¹ for Li, with a small partial negative charge slightly delocalized on U. UBe arises from the U(5f³6d¹7s²) and Be(2s²) electron configurations with no charge separation. The same calculations were made for WX (X = Li, Be, C–F) to enable detailed comparisons of the properties for UX with WX (X = Li–F). For WX, BDE(WX) is higher than that for UX for X = Li to N and lower than BDE(UX) for X = O and F, mostly due to the higher IE of W than U as ionic character becomes more important going from Li to F.