Liquid structure and conformation of a low-viscosity ionic liquid, N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl) imide studied by high-energy X-ray scattering

Liquid structure of a low-viscosity ionic liquid, N-methyl-N-propyl- pyrrolidinium bis(fluorosulfonyl) imide (P13+FSI−) has been studied by a high-energy X-ray scattering technique at 298 K. The radial distribution function (RDF) indicates that the ionic liquid involves an ordered structure with significant intermolecular interactions at around 6, 10 and 16 Å. In order to obtain information of intermolecular interactions in the range r < 6 Å, where intra- and intermolecular interactions are overlapped, the RDF in terms of ginter(r) for the intermolecular interaction was extracted by subtracting the intramolecular gintra(r) from the observed g(r). Here, the gintra(r) was evaluated by taking into account the distribution of conformers, i.e., the e1, e4 and e6 conformers for P13+ and the C1 and C2 conformers for FSI−, and by knowing their crystal structures. The distribution of the P13+ conformers has been established in a previous work. The distribution of the FSI− conformers was determined by Raman spectra of the ionic liquid in the range 270-400 cm− 1 at various temperatures. The ginter(r) thus extracted shows a peak at 3.6 Å, which may be ascribed to the specific contact ion-pair interaction of P13+FSI−. The Gibbs energy, enthalpy and entropy of conformational change from C2 to C1 were evaluated to be − 3(2) kJ mol− 1, 6.8(6) kJ mol− 1 and 33(3) J K− 1 mol− 1, respectively.