Influence of the First Coordination of Uranyl on Its Luminescence Properties: A Study of Uranyl Binitrate with N,N-Dialkyl Amide DEHiBA and Water

Uranyl bi-nitrate complexes have a particular interest in the nuclear industry, especially in the reprocessing of spent nuclear fuel. The modified PUREX extraction process is designed to extract U(VI) in the form of UO₂(NO₃)₂(L)₂ as has been confirmed by extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and time-resolved laser-induced fluorescence spectroscopy (TRLFS) measurements. In this study, the L ligands are two molecules of N,N-di-(ethyl-2-hexyl)isobutyramide (DEHiBA) mono-amide used to bind uranyl in its first coordination sphere. DEHiBA ligands can coordinate uranyl in either trans- or cis-position with respect to the nitrate ligands, and these two conformers may coexist in solution. To use luminescence spectroscopy as a speciation technique, it is important to determine whether or not these conformers can be discriminated by their spectroscopic properties. To answer this question, the spectra of trans- and cis-UO₂(NO₃)₂(DEiBA)₂ conformers were modeled with ab initio methods and compared to the experimental time-resolved luminescence spectra on UO₂(NO₃)₂(DEHiBA)₂ systems. Moreover, the hydrated uranyl bi-nitrate UO₂(NO₃)₂(H₂O)₂ complexes in the same trans and cis configurations were modeled to quantify the impact of organic DEHiBA on the luminescence properties. (authors)