NMR evidence of hydrogen bonding in 1-ethyl-3-methylimidazolium-tetrafluoroborate room temperature ionic liquid

The 1-ethyl-3-methylimidazolium-tetrafluoroborate (EMI–BF4) room temperature ionic liquid was investigated with NMR techniques. Diffusion coefficients measured at temperatures ranging from 300 to 360 K indicate that phase-change occurred in the vicinity of 333 K, which is supported by 11B quadrupolar relaxation rates. This phase change is ascribed to the transformation of the diffusion particle from ‘discrete ion-pair’ to ‘individual ion’ at temperatures above 335 K due to decomposition of the EMI–BF4 ion pair. Analysis of the 13C dipole–dipole relaxation rates identifies the formation of hydrogen bond (C2H⋯F) between the counterions, EMI+ and BF4 −. This hydrogen bonding may have significant contribution to the higher viscosity of this ionic liquid in comparison with the EMI–AlCl4 ionic liquid at corresponding temperatures.