A hydrogen bond accepting (HBA) ability scale for anions of room temperature ionic liquids (RTILs) has been determined by means of 1 H NMR spectroscopy and a solvatochromic UV/vis probe.Scheme 1 Structure of dye 1 used in this work to determine the b values of RTILs.

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Hydrogen-bond-accepting (HBA) and -donating (HBD) parameters of ionic liquids (ILs) with halogeno complex anions are determined by means of (1)H NMR spectroscopy. Thereby, the imidazolium cation serves both as part of the IL and as (1)H NMR probe. The HBA and HBD strength are calculated in terms of the empirical polarity parameters β and α according to the Kamlet-Taft equation. The study includes 1-butyl-3-methylimidazolium ILs with chloroaluminates of various mole fractions from 0 to 0.67 of AlCl(3), anions of the general chemical formula MtX(n)(-) (Mt=metal, X=halide) as well as tri- and pentaiodide. The (1)H NMR chemical shift of the hydrogen atom in the two position of the imidazolium cation is a function of the HBA ability of the corresponding anion. For the chloroaluminates the HBA ability decreases and the HBD ability increases with increasing mole fraction of the Lewis acid. In general, the HBA strength of the studied ILs increases in the following order: MoCl(6)(-) < Al(2)Cl(7)(-) < AlCl(4)(-) < WCl(7)(-) < I(5)(-) < SnCl(5)(-) < I(3)(-) < SbCl(6)(-) < TiCl(5)(-) < BBr(4)(-) < SnCl(3)(-). The corresponding HBD ability shows a reverse trend.