Analytical Sciences

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

Satoshi KITAOKA,* Kaoru NOBUOKA,* Yuichi ISHIKAWA,* and Akihiro WAKISAKA**

*Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan
**National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

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.

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