Complexation of the Carbonate, Nitrate, and Acetate Anions with the Uranyl Dication:  Density Functional Studies with Relativistic Effective Core Potentials^†

The structures and vibrational frequencies of uranyl carbonates, [UO₂(CO₃)_n]^(2-2n) and [(UO₂)₃(CO₃)₆],- uranyl nitrates, [UO₂(NO₃)_n]^(2-n), and uranyl acetates, [UO₂(CH₃COO)_n]^(2-n) (n = 1,2,3) have been calculated by using local density functional theory (LDFT). Only bidentate ligand coordination modes to the uranyl dication have been modeled. The calculated structures and frequencies are compared to available experimental data, including IR, Raman, X-ray diffraction, and EXAFS solution and crystal structure data. The energetics of ligand binding have been calculated using the B3LYP hybrid functional. In general, the structural and vibrational results at the LDFT level are in good agreement with experimental results and provide realistic pictures of solution phase and solid-state behavior. For the [UO₂(CO₃)₃]^6- anion, calculations suggest that complexity in the CO₃^2- stretching signature upon complexation is due to the formation of CO and C−O domains, the latter of which can split by as much as 300 cm^-1. Assessment of the binding energies indicate that the [UO₂(CO₃)₂]^2- anion is more stable than the [UO₂(CO₃)₃]^4- anion due to the accumulation of excess charge, whereas the tri-ligand species are the most stable in the nitrate and acetate anions.