Vibration and Frequency Generation Spectroscopy Study of the Effect of Water on the Cation Orientation of Room Temperature Ionic Liquid

Sum frequency generation vibrational spectroscopy was used to determine the orientation of the cations in a room-temperature ionic liquid at the air-liquid interface. The ionic liquid that was studied was 1-butyl-3- methylimidazolium bis-trifluoromethylsulfonimide, [BMIM] + [imide]-. The orientation of the cation, [BMIM] +, was studied as a function of water pressure in the vapor over the range of 10-5 to 200 Torr. At water pressures below 10-4 Torr, the imidazolium ring was oriented parallel to the surface plane. On increasing the water pressure to greater than 10-4 Torr, the imidazolium ring tipped up from the surface with an angle along the surface normal of 40-55 °. No water signal was observed in the spectra obtained at the water pressures studied here.

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