Spectroscopic Analysis of 1-Butyl-3-methylimidazolium Ionic Liquids: Selection of the Charge Reference and the Electronic Environment

Seven 1-butyl-3-methylimidazolium ionic liquids are analyzed by X-ray photoelectron spectroscopy. A reliable fitting model is developed for the C 1s region. The electronic environment of nitrogen and sulfur atoms is demonstrated. The selection of the internal charge reference is discussed in detail. It is found that when the Kamlet-Taft hydrogen bond acceptor ability value of the anion is larger than 0.75, the aliphatic carbon component can be used as a reliable reference; otherwise, in other cases, F 1s peak is applicable for the charge correction.