Solvent Effects on the H-1-NMR Chemical Shifts of Imidazolium-Based Ionic Liquids

The H-1 nuclear magnetic resonance (H-1-NMR) spectrum is a useful tool for characterizing the hydrogen bonding (H-bonding) interactions in ionic liquids (ILs). As the main hydrogen bond (H-bond) donor of imidazolium-based ILs, the chemical shift (& delta;(H2)) of the proton in the 2-position of the imidazolium ring (H2) exhibits significant and complex solvents, concentrations and anions dependence. In the present work, based on the dielectric constants (& epsilon;) and Kamlet-Taft (KT) parameters of solvents, we identified that the & delta;(H2) are dominated by the solvents polarity and the competitive H-bonding interactions between cations and anions or solvents. Besides, the solvents effects on & delta;(H2) are understood by the structure of ILs in solvents: 1) In diluted solutions of inoizable solvents, ILs exist as free ions and the cations will form H-bond with solvents, resulting in & delta;(H2) being independent with anions but positively correlated with & beta;(S). 2) In diluted solutions of non-ionzable solvents, ILs exist as contact ion-pairs (CIPs) and H2 will form H-bond with anions. Since non-ionizable solvents hardly influence the H-bonding interactions between H2 and anions, the & delta;(H2) are not related to & beta;(S) but positively correlated with & beta;(IL).