Theoretical Elucidation of Am(III)/Cm(III) Separation Mechanism with Diamide-type Ligands Using Relativistic Density Functional Theory Calculation

We
elucidated the separation mechanism between Am­(III) and Cm­(III)
ions by using two different types of diamide ligands, diglycolamide
(DGA) and alkylated diamide amine (ADAAM), by means of the density
functional theory technique and electron density analysis. The molecular
geometries and formation reactions of the metal–ligand complexes
were modeled by using [M­(DGA)3]3+ and [M­(ADAAM)­(NO3)3(H2O)]. We successfully reproduced
Cm­(III) selectivity over Am­(III) with DGA and Am­(III) selectivity
over Cm­(III) with ADAAM. Furthermore, we analyzed the bonding properties
between the metal ion and the diamide-type ligands by using model
complexes, [M­(DGA)3]3+ and [M­(ADAAM)­(NO3)3(H2O)], and revealed the differences
in terms of the bond dissociation energy and the metal 5f orbital
participation in the covalency between the Am­(III) and the Cm­(III)
complexes. It was suggested that the differences were key factors
to understand the Am­(III)/Cm­(III) selectivity.