Solvation structure of lanthanide(iii) ions in solvent mixtures of N,N-dimethylformamide and N,N-dimethylacetamide studied by titration Raman spectroscopy

Abstract

The Raman spectra of neodymium(III), gadolinium(III) and thulium(III) perchlorate solutions of varying salt molalities were measured in N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and their mixtures by titration Raman spectroscopy at 298 K. The in-plane OC–N bending vibration at 660 cm⁻¹ of DMF and the stretching N–CH₃ vibration at 740 cm⁻¹ of DMA show an appreciable shift to a higher frequency upon coordination of the solvent molecules to the metal ion. In DMF–DMA mixtures, the magnitude of the shift for the δ(OC–N) and ν(N–CH₃) vibrations of bound solvent molecules, Δν_DMF, and Δν_DMA, respectively, depends on the solvent composition, and the variation profile of Δν_DMA is in parallel with that of the Ln–O(solvent) bond length. The number of solvent molecules bound to the metal ion or the individual solvation number in a solvent mixture, n_DMF and n_DMA for DMF and DMA, respectively, were evaluated by analyzing the intensity decrease of the free solvent bands with increasing molality of the metal ion. It is indicated that a strong solvation steric effect operates among bound solvent molecules, i.e., the total solvation number decreases with increasing DMA content, and the variation profile depends on the metal ion. The individual solvation number of DMA was found to be 3 at around x_DMA = 0.4 for all the metal systems examined.