FT-IR and Raman study of lithium salts solutions in diethylsulfoxide

Abstract

Solutions of lithium salts LiCl, LiClO 4 and LiNO 3 in diethylsulfoxide (DESO) have been studied by means of both FT-IR and Raman spectroscopy. The curve fitting procedure has been performed in the ν SO and ν CH stretching regions. The broad S O stretching bands of DESO (1100–950 cm −1) were resolved in seven or eight components by deconvolution. On the basis of the previous studies of DESO in aqueous and non-aqueous solutions, the various components are assigned to free and associated species of DESO. The low-frequency component at ∼1007 cm −1 in FT-IR spectra of LiCl/DESO and LiClO 4/DESO solutions is assigned to Li +–DESO complex formation. Moreover, the spectral changes of the SO stretching bands suggest that interaction between Li + ion and DESO breaks partially down the self-associated structure of the solvent. In both IR and Raman spectra the curve fitting procedure in the CH stretching region (3000–2700 cm −1) gives five components, caused by antisymmetric and symmetric stretching vibrations of CH 3 and CH 2 groups of DESO. The low-frequency shift in solutions of lithium chloride indicates the existence of interactions between the CH 2 group of DESO and the chloride anion. The curve fitting procedure performed for the out-of-plane bending of NO 3 − anion (840–820 cm −1) in IR spectra, and for the totally symmetric stretching vibration of ClO 4 − anion (950–920 cm −1) in Raman spectra has shown the coexistence of free ions and contact ion pairs. The comparison of experimental and theoretical calculated frequencies shows that the coordination of ClO 4 − in the ion pairs Li +ClO 4 − is monodentate. Density function theory (DFT) (B3LYP) methods have been used to determine the structure and energies of stable conformers. The optimised geometry corresponding to isolated molecules is the most stable conformation. With the basis sets of the 6-31+G(d) quality, the DFT calculated bond parameters and harmonic vibrations are predicted in a very good agreement with experimental data.