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

• Main Authors: Simone Di Muzio, Fabio Ramondo, Lorenzo Gontrani, Francesco Ferella, Michele Nardone, Paola Benassi
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Molecules
• Subjects: ionic liquids; structure; molecular dynamics; X-ray diffraction; vibrational spectroscopy
• Online Access: https://www.mdpi.com/1420-3049/25/21/4990

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 <i>cis</i>-isomer and H-fumarate, the <i>trans</i>-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.