A hydrogen bond accepting (HBA) ability scale for anions of room temperature ionic liquids (RTILs) has been determined by means of ¹H NMR spectroscopy and a solvatochromic UV/vis probe.

Improved hydrogen bond donor (HBD) strength parameters of 1-butyl-3-methylimidazolium room temperature ionic liquids (RTILs), acceptor number (AN) of Gutmann and α of Kamlet–Taft, have been determined using [Fe(phen)₂(CN)₂]ClO₄ as an original solvatochromic probe. Each of the parameters shows an excellent correlation with the independently determined HBA strength β (Kamlet–Taft) of the anion which demonstrates that empirical polarity parameters of RTILs can be utilized independently of each other.

Based on the developed tool to measure Kamlet–Taft polarity parameters α (hydrogen bond donating ability), β (hydrogen bond accepting ability), and π* (dipolarity/polarisability) for ionic liquids (ILs), it is now possible to precisely determine the influence of both cation and anion structure on π*. α, β, and π* values of 38 1-alkyl-3-methylimidazolium ILs ([Rmim]X) are presented in this work to give a solid background for the discussion. The dipolarity/polarisability of [Rmim]X determined by means of the established π*-sensitive solvatochromic probe 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-Δ³-thiazoline is a blend of dipolarity and polarisability in equal parts. The large π* parameters of ionic liquids are attributed to two cumulative effects: the ion pairing strength and the individual polarisabilities of both the cation and anion. This is shown by the influence of anion and cation structures (alkyl chain length of R) on the dipolarity/polarisability. For all studied ionic liquids we observed a general trend. The stronger the ion pairing effect, the greater is the determined π* value.

Kamlet–Taft solvatochromic polarity parameters β and π* of ionic liquids with 1-alkyl-3-methylimidazolium cations are determined in two ways by the counter anion X⁻: the β value represents the hydrogen bond basicity and the π* value depends on the polarizability of X⁻.