DYNAMICS OF ROOM-TEMPERATURE MELTS - NUCLEAR-MAGNETIC-RESONANCE MEASUREMENTS OF DIALKYLIMIDAZOLIUM HALOALUMINATES

The room-temperature molten salts formed by combining AlCl3 with 1-eth yl-3-methylimidazolium chloride (EMIC) have been investigated by Al-27 and Cl-35 NMR spectroscopy at temperatures from 25 to 100-degrees-C. The initial AlCl3 loading was varied from 50 mol%, where AlCl4- is the major complex ion, to 67 mol%, where Al2Cl7- is the predominant alumi nium-containing species. The Al-27 NMR spectra for the melts all exhib it a single major resonance line with minimal variation in the chemica l shift, 103.3-103.4 ppm. However, it is possible to distinguish the A lCl4- and Al2Cl7- complexes by their characteristic linewidths. Thus, the Al-27 linewidth for the symmetrically substituted AlCl4- complex i s two orders of magnitude smaller than the linewidth for Al2Cl7- (full peak width at half height, 22.8 vs. 2624 Hz). In addition, a third mi nor species was observed at 97 ppm in the Al-27 NMR spectra for melts with initial AlCl3 loadings of 55-64 mol%. This third species (which e arlier work attributed to the AlCl4- ion) was normally present at trac e concentration levels, 0.3 mol%, and was found to increase with the a ddition of water to the melt. For melt compositions with the initial A lCl3 loadings intermediate between 50 and 67 mol%, the apparent Al-27 NMR linewidth varies with the pre-acquisition delay time, consistent w ith a mixture of the AlCl4- and Al2Cl7-complexes. The resulting non-Lo rentzian NMR lineshapes were analysed on the basis of a standard two-s ite exchange process between AlCl4- and Al2Cl7-. Qualitatively, the ex change between the aluminium centres in AlCl4- and Al2Cl7- is fast on the Al-27 NMR timescale. Thus, the lifetime of an aluminium centre in the AlCl4- complex varies from roughly 30 mus at 25-degrees-C to 70 mu s at 100-degrees-C and exhibits an activation energy of ca. 2 kcal mol -1.