A magnetic ionic liquid, which shows a strong response to magnetic field, has been discovered. We synthesized an ionic liquid by mixing 1-butyl-3-methylimidazolium chloride ([bmim]Cl) and FeCl3, in anticipation of unique magnetic properties due to the possible local ordering of the magnetic anions. The visible absorption spectroscopy has shown that this liquid contains the high spin FeCl4− as the anion. Raman spectroscopy has indicated that it contains bmim+ as the cation. It has thus been confirmed that the prepared liquid is [bmim]FeCl4. The liquid [bmim]FeCl4 responded strongly to magnetic field. The SQUID measurements have shown that it is paramagnetic having a large magnetic susceptibility of 40.6 × 10−6 emu g−1. The present discovery of a magnetic ionic liquid has opened up a new research area of the magnetism of liquids.

The crystal structures of 1-butyl-3-methylimidazolium chloride [bmim]Cl and 1-butyl-3-methylimidazolium bromide [bmim]Br show that two rotational isomers, the TT form and the GT form, of the [bmim]+ cation exist in the crystalline state. A vibrational analysis based on a DFT calculation indicates that two characterstic Raman bands of crystalline [bmim]Cl and three of crystalline [bmim]Br can be used as marker bands of the rotational isomerism around the C7–C8 bond of the n-butyl group. The Raman spectra of liquid [bmim]BF4, in which both sets of marker bands are simultaneously observed, then prove that at least two rotational isomers of the [bmim]+ cation coexist in the ionic liquid state.

A single crystal of 1-butyl-3-methylimidazolium chloride [bmim]Cl, a prototype ionic liquid, has successfully been prepared and an X-ray crystallographic analysis has been performed. It reveals the presence of a hydrogen bonding network involving the chloride anion and the ring as well as the n-butyl hydrogen atoms, a strong hydrophobic interaction between the two n-butyl groups of the [bmim] cations, and a unique corrugated sheets structure of the imidazolium rings. Considering the similarity of the Raman spectra of crystalline and liquid states, we suspect that the three dimensional structure determined in the present study also exist, at least partially, in the ionic liquid state.

A prototype ionic-liquid [bmim]Cl shows crystal polymorphism, having two crystal forms, Crystal (1) and Crystal (2), characterized by distinct X-ray powder patterns and Raman spectra. Two sets of characteristic Raman bands of Crystals (1) and (2), which are mutually exclusive with each other, do coexist in the spectrum of liquid [bmim]Cl. It seems that two distinct structures of the [bmim]+ ion, one corresponding to that in Crystal (1) and the other to that in Crystal (2), exist simultaneously in the ionic liquid state.

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