Dynamics of room-temperature melts: nuclear magnetic resonance measurements of dialkylimidazolium haloaluminates

Abstract

The room-temperature molten salts formed by combining AlCl₃ with 1-ethyl-3-methylimidazolium chloride (EMIC) have been investigated by ²⁷Al and ³⁵Cl NMR spectroscopy at temperatures from 25 to 100 °C. The initial AlCl₃ loading was varied from 50 mol%, where AlCl^–₄ is the major complex ion, to 67 mol%, where Al₂Cl^–₇ is the predominant aluminium-containing species. The ²⁷Al NMR spectra for the melts all exhibit a single major resonance line with minimal variation in the chemical shift, 103.3–103.4 ppm. However, it is possible to distinguish the AlCl^–₄ and Al₂Cl^–₇ complexes by their characteristic linewidths. Thus, the ²⁷Al linewidth for the symmetrically substituted AlCl^–₄ complex is two orders of magnitude smaller than the linewidth for Al₂Cl^–₇(full peak width at half height, 22.8 vs. 2624 Hz). In addition, a third minor species was observed at 97 ppm in the ²⁷Al NMR spectra for melts with initial AlCl₃ loadings of 55–64 mol%. This third species (which earlier work attributed to the AlCl^–₄ ion) was normally present at trace concentration levels, 0.3 mol%, and was found to increase with the addition of water to the melt.

For melt compositions with the initial AlCl₃ loadings intermediate between 50 and 67 mol%, the apparent ²⁷Al NMR linewidth varies with the pre-acquisition delay time, consistent with a mixture of the AlCl^–₄ and Al₂Cl^–₇ complexes. The resulting non-Lorentzian NMR lineshapes were analysed on the basis of a standard two-site exchange process between AlCl^–₄ and Al₂Cl^–₇. Qualitatively, the exchange between the aluminium centres in AlCl^–₄ and Al₂Cl^–₇ is fast on the ²⁷Al NMR timescale. Thus, the lifetime of an aluminium centre in the AlCl^–₄ complex varies from roughly 30 µs at 25 °C to 70 µs at 100 °C and exhibits an activation energy of ca. 2 kcal mol^–1.