The complex formation of uranyl with chloride ions in non-aqueous solvents (acetone, tributylphosphate and methyl-i-butylketone) was studied by absorption, luminescence and MCD spectroscopy. The maximal coordination of the uranyl ion by chloride ions in these organic solvents was found to be four, which is in agreement with single crystal data. The MCD spectra unambiguously exclude the existence of [UO 2 Cl 3 ] À with D 3h coordination symmetry. For the intermediately formed complexes we propose a structure of the type: UO 2 Cl 2 (solv) 4 , with an orthorhombic symmetry. For these intermediate complexes, the 19 500-22 000 cm À1 region of the absorption spectrum exhibits an intensity enhancement with respect to that of the solvated uranyl ion. This intensity enhancement as well as the increase in total luminescence intensity is explained in terms of vibronic coupling of the first electronic state (P g ) with the asymmetric stretching (n a :a 2u ) and the bending (n b :e u ) vibrations of the uranyl ion and the out-of-plane bending mode of the equatorial ligands (n 10 :b 1u ). The spectra are dominated by transitions arising from the s u þ d u configuration. The spectroscopic properties of the UO 2 Cl 2 (TBP) x complexes are compared with those of the UO 2 (NO 3 ) 2 (TBP) 2 complex (TBP ¼ tributylphosphate).
The spectroscopic properties of the complexes of uranyl with oxacrowns, azacrowns, thiacrowns of different ring size, polyethyleneglycols , and [2.2.1] and [2.2.2] cryptands were studied in acetonitrile and propylene carbonate solutions by absorption and luminescence spectroscopy. Depending on the ligand, marked differences in the fine structure of the spectra were observed. The crown ethers with six oxygen atoms (18-crown-6, dicyclohexano-18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6 and [2,4]-dibenzo-18crown-6) form inclusion complexes with the uranyl ion in the central cavity. In the case of 18-crown-6 and dibenzo-18-crown-6, a complex with a high coordination symmetry is formed. Outer-sphere complexes are formed with the crown ethers with small ring size (12-crown-4 and 15-crown-5) and with PEG-200. Only minor changes in the spectra were observed when changing the ligand. The spectra of the large crown ethers ([3,4]-dibenzo-21crown-7, dibenzo-24-crown-8 and dibenzo-30-crown-10), together with those of PEG-300 and PEG-400 show that these ligands coordinate in the equatorial plane to the uranyl ion. In the presence of competing inorganic ligands like chloride and nitrate ions, the crown ether is replaced in the first coordination sphere. Azacrowns and cryptands coordinate externally to the uranyl ion, and the corresponding spectra do not show fine structure. In the case of the thiacrowns, no coordination of the ligand to the uranyl ion was observed.
The spectroscopic properties of two uranyl crown ether complexes, [UO 2 (18-crown-6)] 2+ and [UO 2 (dicyclohexano-18-crown-6)] 2+ , dissolved in acetonitrile and propylene carbonate were studied. UV/VIS and magnetic circular dichroism (MCD) spectra were recorded, as well as emission and excitation spectra. The inclusion complexes formed, have a distorted D 3d symmetry, which is common for a crown ether ring, coordinated with a central metal ion. The electronic transitions and vibrational progressions are identified in a D 1h , D 3d and D 3 distortion scheme. Relaxation of the selection rules by vibronic coupling with the fundamental vibrations of uranyl is discussed.
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