Dependence of the conformational isomerism in 1-n-butyl-3-methylimidazolium ionic liquids on the nature of the halide anion.

The conformational isomerism of the 1-n-butyl-3-methylimidazolium cation, C(4)mim, in halide-based ionic liquids--[C(4)mim]Cl, [C(4)mim]Br, and [C(4)mim]I--was explored by Raman spectroscopy. The C(4)mim cation exhibits trans-gauche conformational isomerism with respect to the N1-C7-C8-C9 dihedral angle of its butyl chain. The thermodynamics of trans-gauche conversion were analyzed through the successful evaluation of the corresponding Gibbs free energy, Δ(iso)G°, enthalpy, Δ(iso)H°, and entropy, Δ(iso)S°, of conformational isomerization. The values of Δ(iso)G° obtained are small (a few units of kJ/mol) and show a slight negative variation with the decrease of the size of the halide anion. On the other hand, Δ(iso)H° and Δ(iso)S° values are positive for [C(4)mim]I and decrease with the anion size to yield negative values for [C(4)mim]Cl and [C(4)mim]Br. This suggests that the negative electrostatic field around the halide anions stabilizes the gauche isomer from an enthalpic point of view. In order to study the structure and ion-ion interactions in this type of ionic liquids, high-energy X-ray diffraction experiments were performed for [C(4)mim]Cl at different temperatures and for supercooled [C(4)mim][Br] at ambient temperature. Molecular dynamics (MD) simulations for these systems were also carried out at several temperatures. Δ(iso)G° and Δ(iso)H° values derived from the simulations qualitatively agree with the experimental ones. Experimental X-ray structure factors are also well reproduced by the simulations. The MD results also allowed the calculation of different spatial distribution functions (SDFs) for the three ionic liquids. Although all SDFs exhibit similar trends, [C(4)mim]I shows a reduced anion density facing the C(2)-H atoms of the cation and enhanced anion densities above and below the imidazolium ring plane. This indicates that anions localized near the C(2)-H atoms of the cation can stabilize their gauche conformer, an effect that is stronger with smaller anions. This conclusion is also supported by ab initio calculations at the CCSD(T) level for isolated ion pairs.