Systematic Raman spectroscopic study of the complexation of uranyl with fluoride

A simple aqueous complexing system of UO₂²⁺ with F^- is selected to systematically illustrate the application of Raman spectroscopy in exploring uranyl(VI) chemistry. Five successive complexes, UO₂F⁺, UO₂F₂(aq), UO₂F₃^-, UO₂F₄^2-, and UO₂F₅^3-, are identified, as well as the formation constants except for the 1 : 5 species UO₂F₅^3-, which was experimentally observed here for the first time. The standard relative molar Raman scattering intensity for each species is obtained by deconvolution of the spectra collected during titrations. The results of relativistic quantum chemical first-principles and ab initio calculations are presented for the complete set of [UO₂(H₂O)F]^2- complexes (n = 0-5), both for the gas phase as well as for aqueous solution modelling bulk water using the conductor-like screening model. Electronic structure calculations at the Møller-Plesset second-order perturbation theory level provide accurate geometrical parameters and in particular reveal that k water molecules in the second coordination sphere coordinating to the F^- ligands in the resulting [UO₂(H₂O)F]^2-(H₂O) complexes need to be treated explicitly in order to obtain vibrational frequencies in very good agreement with experimental data. The thermodynamics and structural information obtained in this work and the developed methodology could be instructive for the future experimental and computational research on the complexation of the uranyl ion.