NMR Studies of the Proton Equilibrium in Basic Ambient-Temperature Chloroaluminate Ionic Liquids

In basic mixtures of the molten salt composed of aluminum chloride and 1-ethyl-3-methylimidazolium chloride, proton, in the absence of oxides, is partitioned between hydrogen chloride and hydrogen dichloride ion. The equilibrium between these two species has been examined by ²H and ¹H NMR spectroscopies. The magnitude of the hydrogen chloride/dichloride equilibrium constant demonstrates that the equilibrium strongly favors the formation of the hydrogen dichloride ion. The ratio of ²H and ¹H equilibrium constants at 90 °C gave an equilibrium isotope effect of 0.67 ± 0.26, which indicates that deuterium accumulates in the hydrogen chloride molecule. An equilibrium isotope effect of 0.712 was calculated from hydrogen chloride and hydrogen dichloride vibrational frequencies. Analysis of the temperature dependence of the hydrogen chloride/dichloride equilibrium constant gave values for the enthalpy of reaction, ΔH_RXN, and the entropy of reaction, ΔS_RXN, of −9.8 ± 0.8 kJ mol−¹ and 4.8 ± 2.5 J mol−¹ K−¹, respectively. Comparison of the ΔH_RXN for the hydrogen chloride/dichloride equilibrium in the melts with values obtained for other solvents suggests that the melt interacts more strongly with chloride ion than with hydrogen dichloride ion.