Ab Initio Calculation of Room Temperature Ionic Liquid 1-Ethyl-3-Methyl-Imidazolium Chlorocuprate (I)

The Hartree-Fock method has been employed to investigate the electronic structures of EMIM+(1-ethyl-3-methyl-imidazolium+), CuCl−2, Cu2Cl−3, CuCl2−, EMIM+-CuCl−2, EMIM+-Cu2Cl−3, and EMIM+-CuCl2−3 pairs. Full optimization and frequency analyses of EMIM+, CuCl−2, Cu2Cl−3, CuCl2−3, eight initial EMIM+-CuCl−2, six EMIM+-Cu2Cl−3, and four EMIM+-CuCl2−3 geometries have been carried out using Gaussian-94 soft-package at 6-31+G(d,p) basis set level for hydrogen, carbon, nitrogen, chlorine atoms and Hay-Wadt effective core potential for copper atom. The electronic structures of lowest energy of EMIM+-CuCl−2, EMIM+-Cu2Cl−3, EMIM+-CuCl2−3, single EMIM+, CuCl−2, Cu2Cl−3, and CuCl23− have been comparatively studied. The calculated results showed that EMIM+-CuCl−3 pair conformer of lowest energy was five ring parallel to Cl—Cu—Cl with 3.2 Å distance, EMIM+-CuCl2−3 pair conformer of lowest energy was five ring parallel to CuCl23−plane with 3.4 Å distance, and the optimized EMIM+-Cu2Cl−3 pair conformer of lowest energy was five ring perpendicular to Cl—Cu—Cl—Cu—Cl plane with 3.0 Å distance between the terminal Cl atoms and the 5-ring of EMIM+. The cohesion between cations and anions is brought about by C—H … Cl hydrogen bonds that are reinforced by charge assistance. The frequency analyses suggested that all stationary points are minimum because of no appearing of imaginary frequency. The assigned frequencies were in agreement with the experimental report. The low energy of interaction because of the bulky asymmetry of EMIM+ and the charge dispersion of cation and anion leads to the low melting point of the ionic liquids, EMIM+-CuCl−2, EMIM+-Cu2Cl−3, and EMIM+-CuCl2−3. The interaction energy of EMIM+-CuCl−2, EMIM+-Cu2Cl−3, and EMIM+-CuCl2−3 is 309.0 kJ/mol, 316.8 kJ/mol, and 320.2 kJ/mol, respectively. The relationship of interaction energy via distance between cations and anions was also investigated by single point energy scan.