A density functional theory study on the interaction between UO22+ and the carbamoylphosphoramidic acid ligand for uranium extraction from seawater

• Published in: Nuclear science and techniques 2018-06, Vol.29 (6), p.128-140, Article 90
• Main Authors: Guo, Xiao-Jing, Li, Cheng, Hu, Jiang-Tao, Ma, Hong-Juan, Qian, Hong-Liang
• Format: Article
• Language: English
• Subjects: Energy; Hadrons; Heavy Ions; Nuclear Energy; Nuclear Physics
• ISSN: 1001-8042; 2210-3147
• DOI: 10.1007/s41365-018-0422-0

Phosphorylurea molecules, which contain both phosphoryl and carbonyl groups, are considered efficient extractants for UO 2 2+ . This study aims to explain the complexation of UO 2 2+ with carbamoylphosphoramidic acid (CPO), a simple model for phosphorylurea, for ligand design for uranium recovery from seawater using density functional theory calculations, natural bond order analysis, and the quantum theory of atoms in molecules. The results showed that, when CPO acts as a monodentate ligand, the affinity of phosphoryl for UO 2 2+ is stronger than that of carbonyl, and CPO coordinates with UO 2 2+ through the phosphoryl oxygen atom. When CPO serves as a bidentate ligand, both the phosphoryl and carbonyl oxygen atoms connect to UO 2 2+ , and the U–O(carbonyl) bond plays a more important role than the U–O(phosphoryl) bond in the interaction between UO 2 2+ and CPO. This paradox may be caused by the significant charge transfer from the U–O(carbonyl) π bond orbital to the C–N σ antibond orbital of the bidentate CPO. The NH spacer between the phosphoryl and carbonyl groups could ensure the delocalization of the electron system of the molecule. The bidentate binding motif is favored by entropy and opposed by enthalpy, while the monodentate binding motif is favored by enthalpy and opposed by entropy. Ultimately, the bidentate binding motif is more favorable than the monodentate one. As expected, the interaction between UO 2 2+ and the deprotonated CPO is stronger than that between UO 2 2+ and the neutral CPO. Comparing the interaction between UO 2 2+ and CPO with that between UO 2 2+ and N -phenylcarbamoylphosphoramidic acid (PhCPO), formed by replacing one hydrogen atom from the terminal nitrogen atom of CPO with a phenyl group, the phenyl substituent at the terminal nitrogen atom of PhCPO shows a slightly negative effect on the interaction between UO 2 2+ and PhCPO.