Microscopic structures of ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate in water probed by the relative chemical shift

Abstract

The relative chemical shifts (Δδ) were put forward to investigate the microscopic structure of 1-ethyl-3-methyl-imidazolium tetrafluoroborate (EmimBF4) during the dilution process with water. The concentration-dependent Δδ (C2)H–(C4)H, Δδ (C2)H–(C5)H and Δδ (C4)H–(C5)H were analyzed. The results reveal that the variations of the microscopic structures of three aromatic protons are inconsistent. The strength of the H-bond between water and three aromatic protons follows the order: (C2)H…O > (C4)H…O > (C5)H…O. The concentration-dependent Δδ (C6)H–(C7)H and Δδ (C6)H–(C8)H indicate the formation of the H-bonds of (Calkyl)H…O is impossible, and more water is located around (C6)H than around (C7)H or (C8)H. The concentration-dependent Δδ (C2)H–(C4)H and Δδ (C2)H–(C5)H both increase rapidly when x water > 0.9 or so, suggesting the ionic pairs of EmimBF4 are dissociated rapidly. The turning points of concentration-dependent Δδ (C2)H–(C4)H and Δδ (C2)H–(C5)H indicate that some physical properties of the EmimBF4/water mixtures also change at the corresponding concentration point. The microscopic structures of EmimBF4 in water could be clearly detected by the relative chemical shifts.