Binding Affinity Of Pyridines With Am~?/Cm~? Elucidated By Density Functional Theory Calcuations

With the development of clean energy(nuclear energy),the disposal of high-level radioactive waste(spent nuclear fuel)generated by nuclear fuel cycle also arises.The main components of spent nuclear fuel are uranium U,nuclear fission products(such as strontium Sr,cesium Cs and Tc technetium),plutonium Pu and minor actinides(americium Am,curium Cm).Among them,minor actinides(Am and Cm)have strong radioactivity and high toxicity,in order to carry out the nuclear fuel cycle safely,people must look for ways to transform or separate them.At the same time,due to the high application value of Am and Cm,the separation between Am and Cm is also very important,but the similarity of chemical properties between Am and Cm makes the separation quite challenging.At present,the separation of lanthanides and actinides have been studied mainly by ion exchange and solution extraction.Based on this reference,the separation between Am and Cm has also been studied.Solution extraction separation method has many advantages such as wide application and less pollution.There are many studies on extractants(ligands),mainly focusing on ligands containing O and N.Among them,N-heterocyclic ligands are widely used for the following reasons:(1)N-heterocyclic ligands such as bipyridine and phenanthroline have high charge density in structure and are easy to bind to metal;(2)in terms of the softness and hardness of ligands,ligands containing N atoms are softer than ligands containing O atoms,combining with hard-soft acid-base theory and 4f/5f orbital difference,N-heterocyclic ligands are easy to bind to softer actinides;(3)By modifying or recombining N-heterocyclic ligands with substituents,a recombinant ligand containing N and O(dipyridinamide)can be obtained,in which N/O has different binding capacity with lanthanide actinides,and thus lanthanide actinides or actinides can be separated.Based on these factors,we designed a simple N-heterocyclic pyridine ligand to study its application in the separation of Am and Cm,and tried to find out the factors affecting the reaction strength and selectivity between ligand and metal.DFT calculation of density functional theory provides great convenience for chemical research.It can be combined with experiments to simulate the microscopic changes of many reactions.By comparing the experimental and computational results,the process of reaction occurrence,changes in energy,bond length and charge distribution can be intuitively explained.It can also simulate the experimental process and use theoretical calculation to screen out better reacti(on conditions for the experiment.In recent years.,many theoretical calculations on Am and Cm separation have attempted to reveal the intrinsic relationship between ligand and metal coordination and explain the selective source of ligand.Firstly,a series of ligand exchange reactions between 9-water complexes[M(H2O)9]3+ of Am?/Cm? and pyridine in aqueous solution system were designed.Thermodynamic correlation,space structure and charge distribution of metal pyridine complexes were obtained by theoretical calculation.The binding affinity differences between a series of simulated pyridine ligands(Ln=1-7)and Am?/Cm?,were discussed.At the same time,we introduced nitrate ions into the simulated system to obtain[M(H2O)9-2x(NO3)x]3-x.Combined with the calculation results of ligand exchange reaction,we discussed the influence of nitrate ion concentration on the structure of the complex,the binding ability of the ligand to the metal and the selectivity of the ligand.In addition,we also calculated the kinetic study of[M(H2O)9]3+ and pyridine,and considered the ligand exchange reaction in the presence of DGA(diglycolamide)ligands and counterions in order to better assist the experiment.According to the calculated data,we found that the substituent effect on the pyridine ligand would affect its binding ability to the metal,but did not affect the selectivity,and the concentration of nitrate would affect the selectivity of the ligand.These results are expected to help design more efficient ligands and suitable reaction conditions.