Ionic Structure and Interactions in 1-Methyl-3-Ethylimidazoliurn Chloride/AlCl3 Molten Salts

Abstract

Room temperature chloroaluminate molten salts formed by mixing 1-methyl-3-ethylimidazolium chloride (MEICl) with AlCl3 are of interest as electrolytes and non-aqueous reaction media. We have done NMR, IR, and theoretical studies on this system to elucidate the still unresolved picture of ionic structure and interactions in these melts. NMR studies of MEI+ in melts of various composition (determined by the mole fraction, N, of AlCl3 used in forming a melt) were originally explained by a stack model. In this model, the anions (Cl− and AlCl4− in basic melts and AlCl4− and Al2Cl7− in acidic melts) are located between stacked "parallel" MEI+ ring planes. However, IR and Raman spectra were interpreted as evidence that Cl− formed ion pairs with MEI+ by a H-bonding interaction through the H on the C-2 in the ring, disputing the stack model. Our IR studies now indicate that while the hydrogen on the C-2 does interact with the Cl−, it is clearly not the only point of interaction. It appears that the Cl− affects the hydrogens at C-4 and C-5 members of the ring as well. Semi-empirical molecular orbital calculations support a model in which Cl− interacts with two MEI+'s while occupying an equilibrium position approximately equidistant between the C-2's of adjacent near parallel MEI+'s.