Theoretical Study on Ionic Liquid Based on 1-Ethyl-3-Methyl- Imidazolium Cation and Hexafluorophosphate or Tetrafluoroborate

The Hartree-Fock and DFT/B3LYP methods have been employed to investigate the electronic structures of 1-ethyl-3-methyl-imidazolium cation (EMIM+), BF−4, PF−6, EMIM+-BF–4, and EMIM+-PF−6 using the Gaussian-94 soft-package at 6-31+G(d,p) basis set level for hydrogen, carbon, nitrogen, boron, phosphorus, and fluorine atoms. Comparison of the electronic structures of the lowest energy of EMIM+- BF−4 and EMIM+-PF−6 pairs, and single EMIM+, BF−4 and PF−6 showed that the optimized EMIM+-BF−4 and EMIM+-PF–6 pair conformers were BF−4 and PF−6 outside the 5-ring plane between the ethyl group and the methyl group. The cohesion of C—H… F hydrogen bond between cation and anion is reinforced by charge assistance. The interaction energy between EMIM+ and PF–6 is 328.8 kJ/mol at the B3LYP level and 326.6 kJ/mol at the Hartree-Fock level, whereas that between EMIM+ and BF−4 is 353.5 kJ/mol at the B3LYP level and 350.5 kJ/mol at the Hartree-Fock level. The low energy interactions caused by bulky asymmetric EMIM+, and charge dispersion of cation and anion give rise to the low melting point of ionic liquid EMIM+-BF–4 and EMIM+-PF−6. The two hydrogen bonding models of single hydrogen bond formation, and the hydrogen transfer between C2 in EMIM+ and F in BF−4 or PF−6 were principally depicted.