Theoretical Study of the Excited States and Luminescent Properties of (H 2 O) n UO 2 Cl 2 ( n = 1-3).

The low-lying excited-state properties of the water-solvated UO 2 Cl 2 complexes, i.e., (H 2 O) n UO 2 Cl 2 ( n = 1-3), below 33,000 cm -1 , are investigated based on the ab initio NEVPT2 and CCSD(T) with inclusion of scalar relativistic and spin-orbit coupling effects. The simulated luminescence spectral curves agree well with the experimental spectrum in aqueous solution at -120 °C. Water coordination is found to significantly affect the character of luminescent state, which is changed from the 3 Φ g state in UO 2 Cl 2 to the 3 Δ g state in (H 2 O) 2,3 UO 2 Cl 2 . This is distinctly different from the observed unchanged nature of luminescent state in the cases of Ar coordination to UO 2 Cl 2 and H 2 O coordination to UO 2 F 2 in the previous work. Furthermore, by combining with the theoretical results for the solvated UO 2 F 2 system, the reason why water coordination does not remarkably change the spectral shape of UO 2 Cl 2 , as opposed to UO 2 F 2 , was explained based on the analysis of two key spectral parameters, O-U-O symmetrical vibrational frequency and U-O bond length elongation. The roles of ligand field and spin-orbit coupling in the determination of luminescent state character and spectral shape in uranyl dihalide complexes are deeply discussed and summarized. These results deepen our understanding of the luminescent properties of uranyl complexes in aqueous solution.