Evidence of Conformational Equilibrium of 1-Ethyl-3-methylimidazolium in Its Ionic Liquid Salts:  Raman Spectroscopic Study and Quantum Chemical Calculations

Abstract

Raman spectra of liquid 1-ethyl-3-methylimidazolium (EMI+) salts, EMI(+)BF4-, EMI(+)PF6-, EMI(+)CF3SO3-, and EMI(+)N(CF3SO2)2-, were measured over the frequency range 200-1600 cm(-1). In the range 200-500 cm(-1), we found five bands originating from the EMI+ ion at 241, 297, 387, 430, and 448 cm(-1). However, the 448 cm(-1) band could hardly be reproduced by theoretical calculations in terms of a given EMI+ conformer, implying that the band originates from another conformer. This is expected because the EMI+ involves an ethyl group bound to the N atom of the imidazolium ring, and the ethyl group can rotate along the C-N bond to yield conformers. The torsion energy for the rotation was then theoretically calculated. Two local minima with an energy difference of ca. 2 kJ mol(-1) were found, suggesting that two conformers are present in equilibrium. Full geometry optimizations followed by normal frequency analyses indicate that the two conformers are those with planar and nonplanar ethyl groups against the imidazolium ring plane, and the nonplanar conformer is favorable. It elucidates that bands at 241, 297, 387, and 430 cm(-1) mainly originate from the nonplanar conformer, whereas the 448 cm(-1) band does originate from the planar conformer. Indeed, the enthalpy for conformational change from nonplanar to planar EMI+ experimentally obtained by analyzing band intensities of the conformers at varying temperatures is practically the same as that evaluated by theoretical calculations. We thus conclude that the EMI+ ion exists as either a nonplanar or planar conformer in equilibrium in its liquid salts.

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…Indeed, the enthalpy for conformational change from nonplanar to planar [C 2 mim] þ , obtained experimentally by analyzing band intensities of the conformers at various temperatures, was practically the same as the enthalpy evaluated by the theoretical calculations. We thus conclude that the [C 2 mim] þ ion exists as planar or nonplanar conformers in equilibrium in its liquid salts [137], and this was confirmed by X-ray diffraction [127]. For the longer chain [C n mim] þ systems also non-planar forms are most stable, e.g., compare with the AA and GA conformers of [C 4 mim] þ in Fig.…”

“…Similarly, studies were done on [C 2 mim][BF 4 ] and other 1-ethyl-3-methylimidazolium liquid salts [67,127,137]. In the Raman spectral range 200-500 cm À1 , Umebayashi et al [137] for liquids contain- [127].…”

“…In the Raman spectral range 200-500 cm À1 , Umebayashi et al [137] for liquids contain- [127]. Figure adopted from Umebayashi et al [137] ing [BF 4 þ conformer.…”

“…In the Raman spectral range 200-500 cm À1 , Umebayashi et al [137] for liquids contain- [127]. Figure adopted from Umebayashi et al [137] ing [BF 4 þ conformer. The ethyl group bound to one N atom of the imidazolium ring is able to rotate around the C-N bond to yield two different torsional conformers, see Fig.…”

“…15. The energy barrier of this rotation was calculated with an energy amplitude of $2 KJ mol À1 [137]. Two local minima were found, suggesting that the two conformers can be present in equilibrium.…”