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.