Electronic Properties of UN and UN^– from Photoelectron Spectroscopy and Correlated Molecular Orbital Theory

Abstract

The results of calculations of the properties of the anion UN– including electron detachment are described, which further expand our knowledge of this diatomic molecule. High-level electronic structure calculations were conducted for the UN and UN– diatomic molecules and compared to photoelectron spectroscopy measurements. The low-lying Ω states were obtained using multireference CASPT2 including spin-orbit effects up to ∼20,000 cm–1. At the Feller–Peterson–Dixon (FPD) level, the adiabatic electron affinity (AEA) of UN is estimated to be 1.402 eV and the vertical detachment energy (VDE) is 1.423 eV. The assignment of the UN excited states shows good agreement with the experimental results with a VDE of 1.424 eV. An Ω = 4 ground state was obtained for UN– which is mainly associated with the 3H ΛS state. Thermochemical calculations estimate a bond dissociation energy (BDE) for UN– (U– + N) of 665.9 kJ/mol, ∼15% larger than that of UN and UN+. The NBO analysis reveals U–N triple bonds for the UN, UN–, and UN+ species.

Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC+/0/–

“…73 In none of these molecules does the uranium configuration in the ground state contain an unpaired 6d electron. Our NBO calculations 15 on UN 0/+/− predicted one electron (spin paired) in the 6d orbital due to backbonding from N to U and some d contribution to the bonding orbital.…”

“…This number is slightly larger than the 50 states obtained for UN within ∼2.0 eV. 15 The ground state of UC − , Ω = 2.5, is well described by the 4 Γ state with small contributions from the 4 Φ and 2 Φ states. At the SO-CASPT2 level, the values of r e = 1.886 Å and ω e = 957.3 cm −1 for UC − are in reasonable agreement with the values of r e = 1.891 Å and ω e = 934.6 cm −1 calculated at the CCSD(T)/awQ-DK level.…”

“…In our prior work on UN 0/–/+ , we showed that the ground states of UN (5f 2 7s 1 ), UN – (5f 2 7s 2 ), and UN + (5f 2 7s 0 ) arise from changes in the oxidation state of U in terms of the 7s orbital occupations, and the nature of the ligand is not significantly affected. The UN triple bond was found to have mostly f and d characters, so the 7s orbital where the oxidation is occurring acts like a spectator orbital.…”

“…The basic electron configuration of UC (7s 1 ), UC − (7s 2 ) and UC + (7s 0 ) molecules varies in terms of the occupation of the 7s orbital as found for UN, UN + , and UN − . 15,16 This shows that the C ligand is not affected by electron attachment or detachment processes as the 7s is a nonbonding orbital.…”

“…The choice of the (4e/9o) active space used in this work is consistent with the 3 electrons in 8 orbitals used previously for UN, which had also dominant U 5f and 7s characters and yielded results in very good agreement with the photoelectron spectroscopy. 15,16 The state-averaged CASSCF included a total of 11 ΛS quintet states and 18 ΛS triplet states for UC. For UC − , 46 ΛS states were accounted for, namely, 10 sextets, 17 quartets, and 19 doublets.…”