Cluster Structure of Imidazolium Salts in Methanol Controlled by the Balance of Interactions: Cation-Anion, Cation-Solvent, and Anion-Solvent

We have studied the cluster structure of 1-butyl-3-methylimidazolium halide, bmimX (X = Cl, Br, I), in methanol solution by means of an electrospray mass spectrometer, which is specially designed for analysis of clusters isolated from solution. In positive ion mode experiments, the ratio of solvated bmim+, bmim+(MeOH)n and ion-pair clusters, bmim(bmim+X-)m was dependent on the counter anion. As for bmimCl solutions, few solvated bmim+ clusters were observed, and the ion- pair clusters were clearly observed. On the other hand, bmimBr and bmimI with large anions, the solvated bmim+ clusters increased obviously, and the ion-pair clusters were in turn remarkably decreased. In negative ion-mode experiments, the solvation for Br- by the methanol is found to be the most prominent among those for Cl-, Br-, and I-. These results were reasonably explained in consideration of the balance between ion-solvent and ion-counterion interactions.