Electronic spectra of uranyl chloride complexes in acetone: a CASSCF/CASPT2 investigation

Abstract

A theoretical study is presented of the electronic spectra of the complexes UO(2)Cl(2)ac(4), UO(2)Cl(2)ac(3), [UO(2)Cl(3)ac(2)](-) and [UO(2)Cl(3)ac](-) (ac = acetone) using perturbation theory based on a complete-active-space type wavefunction (CASSCF/CASPT2). Both scalar relativistic effects and spin-orbit coupling were included in the calculations. The calculated excitation energies and oscillator strength values have been compared to the experimental absorption spectrum for uranyl chloride complexes in acetone solution, for chloride-to-uranyl ratios between two and three. The main purpose of this work was to investigate the origin of the remarkable intensity increase observed in the lower part of the experimental absorption spectra, upon addition of chloride to uranyl complexes in acetone. The calculated excitation energies for the different complexes are similar and closely correspond to the experimental data. However, in none of the theoretical spectra, the high intensities observed in the lower part of the experimental spectrum are reproduced.

Electronic spectrum of UO22+ and [UO2Cl4]2− calculated with time-dependent density functional theory

“…For a more detailed discussion of available ground state structural data in the literature we would like to refer to our previous work. 30,40 For ͓UO 2 Cl 4 ͔ 2− excellent agreement is found between the DFT excitation energies and the experimental data. All energies are slightly too low, but the error never exceeds 1000 cm −1 , and are even smaller for the lowest two excited states: 37-500 cm −1 .…”

“…At first sight this may seem unlikely, given that experimental data so far have rather indicated that the position of the bands between 20 000-30 000 cm −1 is quite indifferent to the nature of the equatorial field. However, these spectra have always been recorded in an equatorial surrounding that is saturated with ligands ͑be it four chorines, three nitrates, or a combination of ligands in solution spectra͒ 24,25,[41][42][43] A recent CASPT2 investigation of the electronic spectra of uranyl chloride complexes in acetone 40 has confirmed that the excitation energies in such complexes indeed differ by no more than a few hundreds cm −1 between complexes with different numbers of equatorial chlorine/ acetone ligands. The answer to the question whether the complete removal of all ligands from the equatorial plane would indeed introduce a shift in the electronic spectra predicted by the present SAOP-TDDFT or previous CASPT2 calculations can only come from an experimental spectrum of the bare uranyl ion.…”

Investigation of charge-transfer absorptions in the uranyl UO22+(VI) ion and related chemical reduction of UO22+(VI) to UO2+(V) by UV–Vis and electron paramagnetic resonance spectroscopies

“…Of these oxidation states, U 4+ can readily precipitate out as U(OH) 4 in neutral or weakly basic aqueous media [10], facilitating removal of radioactive waste (uranium) from the water system. Studying chemical speciation of UO 2 2+ in both aqueous and non-aqueous solutions by electronic and molecular spectroscopies and theoretical calculations has received much attention [11][12][13][14][15][16][17][18][19]. As part of the efforts made in this area of (VI) reduction.…”

Photoelectron spectroscopy and the electronic structure of the uranyl tetrachloride dianion: UO2Cl42−

“…[11][12][13][14][15][20][21][22][23][24][25][26] However, accurate interpretations of the optical spectra of UO 2 Cl 4 2− in the condensed environments remain a challenging task in computational actinide chemistry. 16,18,[26][27][28] For instance, the explanation of the spectroscopic data of crystallized Cs 2 UO 2 Cl 4 still relies on empirical models or approximate qualitative analyses. 25,29 Furthermore, due to prohibitive computational cost, highlevel ab initio quantum chemistry calculations can only be applied to isolated actinyl species, such as UO 2 2+ , UO 2 Cl 4 2− , and NpO 2 Cl 4 2− .…”