Influence of Alkaline Earth Metal Ions on Structures and Luminescent Properties of NamMnUO2(CO3)3(4-m-2n)- (M = Mg, Ca; m, n = 0-2): Time-Resolved Fluorescence Spectroscopy and Ab Initio Studies.

Abstract

The luminescence spectra of triscarbonatouranyl complexes were determined by experimental and theoretical methods. Time-resolved laser-induced fluorescence spectroscopy was used to monitor spectra of uranyl and bicarbonate solutions at 0.1 mol kgw-1 ionic strength and pH ca. 8. The concentrations of Mg2+ and Ca2+ in the samples were chosen in order to vary the proportions of the alkaline earth ternary uranyl complexes MgUO2(CO3)32-, CaUO2(CO3)32-, and Ca2UO2(CO3)3. The luminescence spectrum of each complex was determined by decomposition in order to compare it with the simulated spectra of model structures NamMnUO2(CO3)3(4-m-2n)- (M = Mg, Ca; m, n = 0-2) obtained by quantum chemical methods. The density functional theory (DFT) and time-dependent (TD)-DFT methods were used with the PBE0 functional to optimize the structures in the ground and excited states, respectively, including relativistic effects at the spin-free level, and water solvent effects using a continuum polarizable conductor model. The changes in the structural parameters were quantified with respect to the nature and the amount of alkaline earth counterions to explain the luminescence spectra behavior. The first low-lying excited state was successfully computed, together with the vibrational harmonic frequencies. The DFT calculations confirmed that uranyl luminescence originates from electronic transitions from one of the four nonbonding 5f orbitals of uranium to an orbital that has a uranyl-σ (5f, 6d) character mixed with the 2p atomic orbitals of the carbonate oxygens. Additional single-point calculations using the more accurate TD-DFT/CAM-B3LYP allow one to determine the position of the luminescence "hot band" for each structure in the range 467-476 nm and compared fairly well with experimental reports at around 465 nm. The complete luminescence spectra were built from theoretical results with the corresponding assignment of the electronic transitions and vibronic modes involved, mainly the U-Oax stretching mode. The resulting calculated spectra showed a very good agreement with experimental band positions and band spacing attributed to MgUO2(CO3)32-, CaUO2(CO3)32-, and Ca2UO2(CO3)3. The evolution of luminescence intensities with the number of alkaline earth metal ions in the structure was also correctly reproduced.