The resonance Raman effect of uranyl formate in dimethyl sulfoxide

Abstract

The resonance Raman scattering spectra of uranyl formate (UO 2(HCOO) 2) in dimethyl sulfoxide ((CH 3) 2SO, DMSO) have been measured under laser excitation of the uranyl ion in resonance with the 1Σ g +→ 1Φ g Laport forbidden f–f electronic transitions (ranging from 510 to 450 nm) by using ten output lines with wavelength ranging from 528.7 to 454.5 nm of the argon-ion laser at room temperature. The observed resonance excitation profile resembles the vibronic structure of the electronic absorption spectrum (ABS) but does not completely superimpose on it. Such a discrepancy is quantitatively explained by the interference effect, which occurs noticeably in the UO 2L 2 (L=NO 3, CH 3COO, Cl or HCOO)–DMSO system. Transform theory that makes use of the electronic ABS of the resonant electronic state has been applied to predict the Raman excitation profile (REP) of the uranyl totally symmetric stretching vibrational mode. Comparing the experimental REP with the transform theory prediction, it is found that the resonance Raman intensities of this stretching mode depend mainly on the vibronic interaction (non-Condon effect) in excited electronic states. Reliable value of the nuclear displacement on going the 1Σ g +→ 1Φ g electronic transition and the amount of charge transferred from the ligand to uranium of uranyl ion both in the ground and excited states are obtained. Elongation of the UO equilibrium bond length due to the electronic transition is related to the magnitude of the change in the excitation profile, and has linear relation to the change in the amount of charge transferred from the ligand to uranium of uranyl ion in UO 2L 2 type uranyl compounds in DMSO.