Theoretical Studies On The Interaction Of Picolinic-type Ligands With NpO₂⁺

Neptunium(Np-237)is of great concern in the spent nuclear fuel reprocessing due to it is the most abundant element of the "minor actinide"(Np,Am,and Cm).It becomes a major hazard because of its long half-life(t1/2=2.144×106 years),high radioactivity and strong mobility.Np has an electronic configuration of[Rn]5f57s2 with multiple oxidation states(i.e.?,?,? and ?),which has been a challenging issue among the separation of Np in the reprocessing of spent nuclear fuel.The penta-valent state of Np is the most stable one and predominantly exist as neptunyl ion(NpO2+)in the aqueous solutions.Owing to the low ionic charge and large ionic radius of Np02+,it does not form strong complexes with many traditional extractants that leading to its low extractability in solvent extraction separation of Np.In recent years,there are so many studies provide a wealth of information about the physical and chemical properties of Np to contribute to the rational design of new types of ligand with enhanced ability to extract neptunyl.Rao et al.and other groups conducted a series of experimental studies show that the deprotonated picolinic acid anion(PA-),the deprotonated dipicolinic acid anion(DPA2-)and the 1,10-phenanthrolin-2,9-dicarboxylic acid anion(PADA2-)have good ability to complex with Np02?,while the details of their complexation reactions at the molecular scale remainned to be identified.In this thesis,a density functional theory(DFT)study has been carried out to investigate the geometric structures,relative energies and bonding nature of the complexes of NpO2+ with the above-mentioned ligands(L).The Quantum Theory of Atoms-in-Molecules(QTAIM)analysis,Charge decomposition analysis(CDA)and Natural atomic orbital analysis(NAO)were performed to characterize the nature of the coordination bonds in the complexes.The main conclusions are as follows:1)The averaged bond lengths d(A)of Np-Oy",Np-Npy,Np-Ocarb,and Np-Owat bonds were suggested that the coordination of ligands to neptunyl in its equatorial plane weakens the Np-Oy1 bonds and compared to Np-Npy and Np-Owat bonds,Np-Ocarb bonds have a stronger electrostatic interaction.The thermodynamics calculations show that the binding mode with combined "hard-soft" donors brings more stabilization energy to the complexes than that with only "hard" donors.2)The calculations show that the coordination of NpO2+to tetradentate ligand is more stable than that to tridentate and bidentate ones,and the coordination ability of three deprotonated ligands follows the order:PADA2-(-55.10 kcal mol-1)>DPA2-(-53.32 kcal mol-1)>PA-(-41.24 kcal mol-1).3)The Quantum Theory of Atoms-in-Molecules(QTAIM)topological analysis suggests the metal-ligand dative bond was identified to be mainly ionic.The charge decomposition analysis(CDA)and the extended charge decomposition analysis(ECDA)calculated that for the Np:L ?1:1 type complexes,the net charge transfer from the ligands to neptunyl decreases in the order of PAD A2">DPA2">PA->H2O,which coincides the sequence of the relative stabilities.The natural atomic orbital(NAO)analysis reveals that the MOs are mainly contributed by the 2p atomic orbital of the O/N atoms and 5f or 6d atomic orbital of neptunium,implying participation of the 5f orbital in covalent interaction between the neptunyl and the ligands.In summary,we conclude that the denticity of ligand and the combined hard-soft donor strategy work cooperatively in the coordination of NpO2+ with ligands.This work is expected to contribute to the rational design of new type of ligands with enhanced ability to extract neptunyl.