Density functional modeling of Am3+/Eu3+ selectivity with diethylenetriaminepentaacetic acid and its bisamide chelates.

Density-functional theory calculations were applied to mo lecular structure and complex formation reaction modelings of metal ion complexes with diethylenetriaminepentaacetic acid (DTPA) and its bisamide (DTPABA) chelates to understand the metal ion selectivity between Am3+ and Eu3+. The calculated complexes with DTPA and DTPABA chelates reproduced the coordination geometries of experimental crystal structures. Calculated Gibbs free energies of the complex formation reactions indicated that Am3+ ion forms higher stable complexes with both chelates than Eu3+ ion, being consistent with the experimental results. The higher Am3+ selectivity over Eu3+ was suggested to originate in the larger bond overlap between Am3+ 5 f-orbital and N 2s, 2p-orbital. This means that the covalent contribution between the metal ion and donor atoms differentiates the complex formation stabilities, leading to the Am3+/Eu3+ selectivity. We expect that this study contributes to systematize the origin of metal ion selectivity and to accelerate novel ligand exploration.