Development of cation/anion "interaction" scales for ionic liquids through ESI-MS measurements

Electrospray ionization mass spectrometry applied to ionic liquids allowed the study of loosely bonded supramolecules, originating from these organic salts. Based on the observation that ionic liquids formed cationic [Cq+1Xq](+) and anionic [CqXq+1](-) supramolecular aggregates, we have investigated mixed networks, formed by different cations coordinated to a selected anion or by different anions bonded to a given cation, i.e., [C-1 center dot center dot center dot X center dot center dot center dot C-2](+) and [X-1 center dot center dot center dot C center dot center dot center dot X-2](-), with the aim to build a scale of the cation-anion interaction strength. The qualitative order of intrinsic bond strength to Br- was found to be the following: [emim](+) > [bmim](+) > [mor1,2](+) > [hmim](+) > [omim](+) > [mor1,4](+) > [bupy](+) > [bpyrr](+) > [picol](+) > [bm(2)im](+) > [TBA](+). Similarly, the interaction energies to 1-butyl-3-methylimidazolium (bmim) species envisaged two classes of anions: species tightly coordinated to the cationic moiety that include CF3COO-, Br-, N(CN)(2)(-), and BF4- and anions loosely interacting with the alkylimidazolium species such as OTf-, PF6-, and Tf2N-.