Ab Initio Prediction of Proton NMR Chemical Shifts in Imidazolium Ionic Liquids

The family of ionic liquids (ILs) containing 1-alkyl-3-methyl-imidazolium cations have been broadly investigated due to the simplicity of their synthesis and relatively low viscosity. Nuclear magnetic resonance (NMR) studies indicate that the proton chemical shift of the H2 proton is very sensitive to interactions between the imidazolium ring and surrounding anions and can be used as a probe of the IL structure and solute interactions that perturb this structure. In this work, we have calculated proton NMR chemical shifts of a number of ionic liquids incorporating the 1-ethyl-3-methyl-imidazolium cation and anions such as chloride, tetrafluoroborate, hexafluorophosphate, acetate, trifluoroacetate, dicyanamide, ethylsulfate, and tosylate using a number of wavefunction-based methods such as HF and MP2 as well as density functional theory. Proton NMR chemical shifts in the cation were calculated for single ion pairs with the view of (1) determining the best quantum chemical method for the accurate prediction of H-1 NMR chemical shifts and (2) understanding the influence on the H2 proton chemical shift of the two energetically preferable ion-pair configurations of the anion interacting with the cation, either from above the imidazolium ring or in the plane of the C2-H bond.