Structural Motifs in Cold Ternary Ion Complexes of Hydroxyl-Functionalized Ionic Liquids: Isolating the Role of Cation–Cation Interactions

We address the competition between intermolecular forces underlying the recent observation that ionic liquids (ILs) with a hydroxyl-functionalized cation can form domains with attractive interactions between the nominally repulsive positively charged constituents. Here we show that this behavior is present even in the isolated ternary (HEMIm+)2NTf2– complex (HEMIm+ = 1-(2-hydroxyethyl)-3-methylimidazolium) cooled to about 35 K in a photodissociation mass spectrometer. Of the three isomers isolated by double resonance techniques, one is identified to exhibit direct contact between the cations. This linkage involves a cooperative H-bond wherein the OH group on one cation binds to the OH group on the other, which then attaches to the basic N atom of the anion. Formation of this motif comes at the expense of the usually dominant interaction of the acidic C(2)H group on the Im ring with molecular anions, as evidenced by isomer-dependent shifts in the C(2)H vibrational fundamentals.