Theoretical Study Of Nitrogen Heterocyclic In Transplutonium Actinides And Trivalent Lanthanides Actinides Separation

With the rapid development of the nuclear power industry,a large amount of high-level radioactive spent fuel has been produced,which must be properly handled.Extracting radioactive actinides from spent fuel can effectively reduce the harm of spent fuel.At present,the separation method for recovering uranium and plutonium from spent fuel is relatively complete.Due to the similarity of chemical properties,it is very difficult to separate the minor actinides in spent fuel from fission products such as lanthanides.The separation of transplutonium elements from adjacent actinides（in-group separation）is more complicated and difficult.Therefore,studying the electronic structure of transplutonium element complexes is very important to design the separated ligands.Due to the high experimental cost caused by the high radioactivity of actinides,it is very necessary to use computational chemistry methods to study the actinides separation over lanthanides and the separation of transplutonium actinides.In this paper,the relativistic density functional theory（DFT）method is used to conduct a theoretical study on the ability of nitrogen-containing heterocyclic ligands and their derivatives to separate actinides over lanthanides and transplutonium actinides within the group.The main results are as follows:The extraction and separation behavior of the phenanthroline amide ligand Et-Tol-DAPhen Et-Tol-DAPhen（N²,N⁹–diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide,L_a）and its derivatives(5-Bromo-（N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide,L_b）and 5-（4-（λ¹-oxidaneyl）phenyl）-（N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide,L_c）for the transplutonium actinides have been analyzed at the PBE/6-31G（d,p）/RECP level of theory.The electrostatic potential and molecular orbital analysis of the ligand show that the ligand L_c substituted by the electron-donating group phenol is a better electron donor for actinides than L_a and L_b.The extracted products An L（NO₃）₃ and[An L₂（NO₃）]²⁺（L=L_a,L_b,L_c;An=Am,Cm,Bk,Cf）have similar structures.The analysis of bond properties shows that the covalent interaction in the metal-ligand bond is gradually increased from Am to Cf.The 5f orbital energy level of the transplutonium actinide element gradually decreases and gradually approaches the 2p orbital energy level of the ligand coordination atom,resulting in a covalent interaction driven by energy level degeneracy.Compared with the other two ligands,the L_c ligand shows a slightly stronger covalent interaction.At the same time,thermodynamic analysis confirmed that the Cf³⁺complex has a stronger metal-ligand bond,that is,the L_c extraction product has higher stability.Since the extraction ability of the ligands for different actinide ions is all improved,the difference in extraction ability increases or decreases,so the increase in covalent effect does not necessarily lead to the increase in separation ability.The extraction and separation behavior of the phenanthroline N heterocyclic ligand including CyMe₄-BTPhen（L¹）,5-Br-CyMe₄-BTPhen（L²）,5-phenol-CyMe₄-BTPhen（L³）,4-Br-CyMe₄-BTPhen（L⁴）and 4-phenol-CyMe₄-BTPhen（L⁵）for the transplutonium actinides have been analyzed at the PBE/6-31G（d,p）/RECP level of theory.The L³ and L⁵ ligands of the phenol substituent of the electron donating group have stronger coordination ability than the L¹,L² and L⁴ ligands.Bonding analysis shows that the covalent interaction of An³⁺complexes gradually increases from Am to Cf,which is due to the gradual decrease of the energy level of the 5f orbital and the energy level degeneration with the 2p orbital of the ligand.QTAIM analysis shows that the An-L bond in these actinide complexes is a coordination bond and there is a certain covalent interaction.In thermodynamic analysis,the binding energy and extraction reaction energy of the Cf³⁺complex indicate that the strength of the Cf-L bond is higher than that of other elements,and the formed complex has the strongest stability.In addition,the covalent interaction between BTPhen derivatives and transplutonium actinides may also be positively related to the extraction capabilities of these ligands.But in the same way,the increase of covalency did not lead to an increase in separation ability.At the B3LYP/6-31G（d,p）/RECP level of theory,the complexation behavior of the water-soluble ligand Pytri-Diol with lanthanides and actinides and the Ln/An separation ability have been studied.The ligand forms 1:1,1:2 and 1:3（metal:ligand）complexes in aqueous solution.The analyses of the bonding properties show that the triazole nitrogen atom in the ligand has a stronger ability to coordinate metal ions than the pyridine nitrogen atom.Thermodynamic analysis shows that the extraction reaction of the ligand to form three complexes can proceed spontaneously,and the ligand has a certain Ln/An separation ability.