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 B-11 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 C-13 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. (C) 2001 Elsevier Science B.V. All rights reserved.