Ion-Pair Formation of the Ionic Liquid 1-Ethyl-3-methylimidazolium bis(trifyl)imide in Low Dielectric Media

A proton nuclear magnetic resonance (¹H NMR) study of the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifyl)imide, dilute in low dielectric constant solvent media is presented. Equilibrium species are directly observed in the ¹H NMR spectrum as two sets of resonance features, thus displaying unusually long lifetimes. In mixed solvent systems the species mole fractions are directly proportional to the bulk dielectric constant of the solution. The two sets of resonance features are assigned to the freely dissolved ions and ion-pair aggregates. The assignment is supported by measured spin lattice relaxation rates, and the observed temperature and concentration dependencies of the equilibrium. The thermodynamic quantities G_m, S_m, and H_m are also evaluated from the temperature-dependent data.