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

The different solvation states of SO 2 in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid were probed by the detailed analysis of the SO 2 symmetric stretching mode, ν s (SO 2 ). The band position and linewidth were monitored in function of gas molar fraction, excitation wavelength, and temperature. As expected, the decreasing of the SO 2 molar fraction, X SO2 , from approximately 0.8 to 0.2, leads to a gradual downshift of the ν s (SO 2 ) band, attributed to a more effective charge transfer interaction from thiocyanate anion to SO 2 . Interestingly, at high gas concentrations (X SO2  = 0.8), a gradual downshift of the ν s (SO 2 ) 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 ν s (SO 2 ) Raman dispersion can be understood as a consequence of the distinct charge transfer electronic transition energies associated to different SO 2 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 SO 2 solvation states distribution. The vibrational spectroscopic data regarding the SO 2 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 SO 2 capture.