Understanding the molecular level chemistry of f-element compounds (actinides (An) and lanthanides (Ln)) is critical for predicting behavior in complex solutions relevant to spent nuclear fuel reprocessing or nuclear waste repositories.  While binary compounds (i.e., OH^- or CO₃^2- complexes) are fairly well understood, the understanding of f-element behavior in systems with multiple chelating ligands remains incomplete.  Radiolysis products such as H₂O₂, formate, or oxalate are also strong complexing agents and can compete with carbonate and hydroxide to bind to the An and Ln ions, often forming multi-ligand solution species.  The thermodynamic properties of these new complexes are often surprising (stability, solubility, etc.), and can be significantly different from the binary species.  For example, H₂O₂ strongly complexes with U(VI) in solution to form UO₂(O₂)(CO₃)₂^4-, which is more than an order of magnitude more soluble than the previously thought solubility limiting species UO₂(CO₃)₃^4- and is several orders of magnitude more thermodynamically stable.   

Extraordinary advances have been made during the last two decades in quantum chemical studies of actinide hydroxo and carbonato complexes using DFT-based models, but the 5f-electronic structure still represents a challenge for quantum theoretical calculations.  In this work we expand the application of computational chemistry to systems with multiple complexing ligands and show that hybrid DFT calculations show good agreement with experimental data (crystal structures, x-ray diffraction (XRD), UV-vis, Raman and IR vibrational frequencies) for a number of actinide solution complexes, such as UO₂(CO₃)₃^4-, UO₂(O₂)(CO₃)₂^4-, and UO₂(O₂)₂(OH)₂^4-.  The remarkable agreement of experimental data with the calculated results gives good confidence in the models and allows us to extend the calculations to results that cannot be validated experimentally.  For example, spectroscopic evidence (¹³C-NMR and UV-vis) indicates the formation of a dimeric uranium carbonate compound with one peroxide, but this species has not been successfully isolated for single crystal XRD studies.  DFT calculations have been successfully employed to show that the conformation of (UO₂)₂(O₂)(CO₃)₄^6- with side-on coordinated peroxide is favored by 26 kcal/mol over an end on coordinated peroxide in aqueous solution.