Correlation of molecular structure and polarity of ionic liquids

Abstract

The empirical Kamlet–Taft solvent parameters for hydrogen bond donating (HBD) ability (α), hydrogen bond accepting (HBA) ability (β), and dipolarity/polarisability (π*) have been determined for 102 ionic liquids (ILs). α, β, and π* have been measured by means of the solvatochromic methods utilizing three specific dyes Fe(phen)2(CN)2 (Fe), 3-(4-amino-3-methyl-phenyl)-7-phenyl-benzo-[1,2-b:4,5-b′]-difuran-2,6-dione (ABF) and 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-Δ3-thiazoline (Th) each solely sensitive to one of the Kamlet–Taft parameter.Different types of ILs with various cation structure such as 1,3-dialkyl imidazolim and 2- and 4-,5-methyl substituted derivatives, pyridinium, tetraalkylammonium, trialkylsulphonium, and tetraalkylphosphonium were used. In conjunction to the [C4mim] cation, more than 20 various established anions, such as halides, tetrafluoroborate, dicyanamide, were considered in this study. The frontiers of HBA ability strength of the anions are marked by the specific anions azide (N3−) (large β) and FAP PF3(C2F5)3− (lowest β).The linear intercorrelation between α and β is clearly evidenced, which shows that anion and cation interact with each other, for each type of IL used. Also correlation of molecular structure of the cation and the HBD ability can be recognized and supported by results from NMR spectroscopy. The interpretation of π* is still not satisfied because dipolarity and polarisability of each IL obviously contribute in different proportions to the value.