The solvation inhomogeneity of sulfur dioxide in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid probed by Raman spectroscopy

Abstract

The different solvation states of SO2 in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid were probed by the detailed analysis of the SO2 symmetric stretching mode, νs(SO2). The band position and linewidth were monitored in function of gas molar fraction, excitation wavelength, and temperature. As expected, the decreasing of the SO2 molar fraction, XSO2, from approximately 0.8 to 0.2, leads to a gradual downshift of the νs(SO2) band, attributed to a more effective charge transfer interaction from thiocyanate anion to SO2. Interestingly, at high gas concentrations (XSO2 = 0.8), a gradual downshift of the νs(SO2) band was noticed as the Raman excitation energy increases, an unexpected behavior because the change in the excitation wavelength in Raman spectroscopy should affect only the band intensities. Such νs(SO2) Raman dispersion can be understood as a consequence of the distinct charge transfer electronic transition energies associated to different SO2 solvation states, whose relative band intensities are modulated by resonance Raman conditions. Moreover, the lowering from room temperature to 100 K showed an increasing in the Raman dispersion effect. The temperature dependence was attributed to the strengthening of the cation–anion interaction, responsible for the increasing of SO2 solvation states distribution. The vibrational spectroscopic data regarding the SO2 solvation inhomogeneity in ionic liquids are showed for the first time, and it may contribute in the understanding of why such liquids are promising solvents for SO2 capture.