A theoretical investigation of uranyl covalency via symmetry-preserving excited state structures

Time dependent density functional theory (TDDFT) calculations have been performed on a series of symmetry-preserving excited states of the uranyl dication, UO₂²⁺. The simulated excited state electronic structures are compared to that of the ground state at both ground and excited state-optimised geometries. For the first time, the Quantum Theory of Atoms in Molecules (QTAIM) has been applied to the excited states electronic structures of uranyl in order to quantify the variation in bond covalency upon electronic excitation. QTAIM analysis of vertical excitations at the ground state geometry demonstrated an inverse relationship between the orbital mixing coefficient, λ, and the excitation energy. Furthermore, it was found that, for MOs with U 5f character, λ was more dependent on the metal-ligand Hamiltonian matrix element H_ML, whereas for those with U 6d character, λ became increasingly dependent on the difference in fragment orbital energy levels, ΔE_ML. Charge transfer from O to U reduced as the excitation energy increased, as did the degree of electron sharing between the centres. When considering the relaxed excited state geometries, a relationship between excitation energy and bond elongation was established, commensurate with the large magnitude of λ and its dependence on H_ML for MOs with U 5f character, and enhanced charge transfer otherwise.