NMR EVIDENCE OF HYDROGEN BOND IN 1-ETHYL-3-METHYLIMIDAZOLIUM-TETRAFLUOROBORATE ROOM TEMPERATURE IONIC LIQUID

Abstract

The 1-ethyl-3-methylimidazolium-tetrafluoroborate (EMI-BF₄) room-temperature molten salt was investigated with NMR techniques. Diffusion coefficients measured at temperatures ranging from 300 to 360 K indicate phase-change occurred in the vicinity of 333 K, which is supported by ¹¹B quadrupolar relaxation rates. Combined with the sizes of diffusion particles obtained form the Stokes-Einstein equation, this phase change is ascribed to that the diffusion particle is transformed 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 counter ions, EMI⁺ and BF₄ ⁻. That the values of viscosity in EMI-BF₄ are higher than those in EMI-AlCl₄ at corresponding temperatures is therefore understood according to the viscosity approach employed to investigate the H-bonding in molten salts

Keywords:

• Room temperature ionic liquid
• Hydrogen bond
• Diffusion coefficients
• Phase transition
• NMR relaxation rates

ACKNOWLEDGMENTS

The authors thank the National Science Council of the Republic of China, Taiwan for supporting of this work (Grant NSC89-2113-M006-016).