A Systematic Theoretical Study on the Acidities for Cations of Ionic Liquids in Dimethyl Sulfoxide

Abstract

The acidities of 40 commonly seen cations of ionic liquids (CILs) in dimethyl sulfoxide (DMSO) were investigated by a well-established theoretical method, SMD/M06-2X/6-311++G (2df,2p)//B3LYP/6-31+G(d). The calculated p K_a's agree excellently with the available experimental data and range from 20.0 to 45.8 with an acidity order of 1,3-dialkylimidazolium > amidinium > pyridinium > tetraalkylphosphonium > morpholinium > pyrrolidinium ≈ piperidinium > guanadinim cations. The established acidity scale in this work provides a useful tool, as verified by the acidity comparisons, to assess the stability of ILs under various extent basic conditions, and also reveals the relative basicity of several classical N-heterocyclic carbenes and olefins as well as ylides.