Locating nitrate ions in the extraction and separation of trivalent Am and Eu by NTAamide: Quantum chemistry and molecular dynamics probe

Our recent experimentation has realized that actinide/lanthanide are efficiently extracted by nitrilotriacetamide (NTAamide) in the 1:2 complexation (Sep. Purif. Technol. 2024, 330, 125545). However, accessing their atomlevel structures remain extremely challenging due to crystallization difficulties caused by ligand's lipophilicity and nitrate ions's disturbance as well as the scarcity of americium. In this work, we combine density functional theory (DFT) and molecular dynamics (MD) calculations to elucidate the coordination behavior of americium and europium ions in the presence of NTAamide and nitrate counterions. For the first time, the neutral complex is identified as [M(NTAamide)2(NO3)](NO3)2 (M = Am and Eu). One nitrate ion chelates the metal center in the first coordination sphere, while two additional nitrates are weakly coupling in the outer sphere. Totally, the metal in the complex exhibits a tenfold coordination, encompassing six carbonyl oxygen and two nitrogen atoms from NTAamides. This configuration is further confirmed by MD simulations, where a large number of nitric acid and water molecules are explicitly considered. Further elaboration is conducted by DFT calculations on systems of [M(NTAamide)2]3+ with varying nitrate ions in terms of thermodynamics, bondings and geometric/electronic structures.