Vibrational and low temperature optical spectra of the uranyl tricarbonato complex [UO2(CO3)3]4−

Abstract

Abstract Diffuse reflectance (20,000–30,000 cm −1 ) and luminescence (18,000–21,000 cm −1 ) spectra of polycrystalline Na 4 [UO 2 (CO 3 ) 3 ] have been measured in the temperature range of 4.2–300 K; the i.r. (50–2000 cm −1 ) and Raman (50–2000 cm −1 ) spectra have been recorded at room temperature. The vibrational spectra exhibit all frequencies expected for a UO 2 L 3 complex of D 3 h symmetry with bidentate ligands L. The symmetric and asymmetric stretching vibrations of the uranyl ion occur at wave numbers as low as 808 and 843 cm −1 , respectively, indicating strong uranium ← carbonate bonding. The diffuse reflectance spectra consist of four overlapping electronic transitions, the lowest one with origin at 20,680 cm −1 is also seen in emission. The absorption bands are assigned to the spin and parity forbidden components of the π u →δ u and π u →φ u one-electron excitations. From the splitting of the excited configurations π 3 u δ 1 u and π 3 u φ 1 u the spin-orbit coupling constant is calculated to be ζ 5ƒ = 1750 cm −1 . Possible relationships between measurable spectroscopic properties of the uranyl ion and the strength of bonding in the equatorial plane are discussed.