The additional nitrogen atom breaks the uranyl structure: a combined photoelectron spectroscopy and theoretical study of NUO 2 .

We report a joint photoelectron spectroscopic and relativistic quantum chemistry study on gaseous NUO 2 - . The electron affinity (EA) of the neutral NUO 2 molecule is reported for the first time with a value of 2.602(28) eV. The U-O and U-N stretching vibrational modes for the ground state and the first excited state are observed for NUO 2 . The geometric and electronic structures of both the anions and the corresponding neutrals are investigated by relativistic quantum chemistry calculations to interpret the photoelectron spectra and to provide insights into the nature of the chemical bonding. Both the ground state of the anion and neutral are calculated to be planar structures with C 2v symmetry. Unlike the "T"-shape structure of UO 3 which has a quasi-linear O-U-O angle, both the ground-state geometries of the anion and neutral have O-U-O bond angles of around 90°. The significant contraction of the O-U-O bond angle indicates the strong interaction between the U and N atoms compared with the "additional" oxygen in UO 3 . The chemical bonding calculation indicates that multiple bonding of U(VI) can occur in NUO 2 - and NUO 2 , and the U VI -N bond is significantly more covalent than the U-O bond. The current experimental and theoretical results reveal the difference between the U-N and U-O bond in the unified molecular system, and expand our understanding of the bonding capacities of actinide elements with the nitrogen atom.