Choline Hydrogen Dicarboxylate Ionic Liquids by X-ray Scattering, Vibrational Spectroscopy and Molecular Dynamics: H-Fumarate and H-Maleate and Their Conformations

We explore the structure of two ionic liquids based on the choline cation and the monoanion of the maleic acid. We consider two isomers of the anion (H-maleate, the cis-isomer and H-fumarate, the trans-isomer) having different physical chemical properties. H-maleate assumes a closed structure and forms a strong intramolecular hydrogen bond whereas H-fumarate has an open structure. X-ray diffraction, infrared and Raman spectroscopy and molecular dynamics have been used to provide a reliable picture of the interactions which characterize the structure of the fluids. All calculations indicate that the choline cation prefers to connect mainly to the carboxylate group through OH⋯O interactions in both the compounds and orient the charged head N(CH3)3+ toward the negative portion of the anion. However, the different structure of the two anions affects the distribution of the ionic components in the fluid. The trans conformation of H-fumarate allows further interactions between anions through COOH and CO2- groups whereas intramolecular hydrogen bonding in H-maleate prevents this association. Our theoretical findings have been validated by comparing them with experimental X-ray data and infrared and Raman spectra.