Structural and Spectroscopic Evidence of Hydrogen Bonded Anionic Wires in Protic Ionic Liquids

Hydrogen Bonding (H-bonds) between ions of like charge challenges the fundamental principle of electrostatics that opposite charges attract and like charges repel. This study provides direct structural and spectroscopic evidence of H-bonded anionic wires in protic ionic liquids (PILs), which consist of triethylammonium cations and hydrogen dicarboxylate anions. The anionic wires were identified by X-ray crystallography in the solid state, wherein the anions are linked by ^-OH···O^- type H-bonds. The H-bonds within the anionic wire are significantly stronger than those between cation and anion despite repulsive Coulomb interaction in the first and attractive ones in the latter case. Analysis by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy suggests that the binding motifs of the solid structure are mainly preserved in the liquid state of the supercooled PILs. This conclusion is drawn from density functional theory (DFT) calculations of the same structural motifs as observed in the solid state. The calculated ¹H NMR chemical shifts and IR C═O frequencies for these H-bond structures correspond well with the measured spectroscopic signatures. The anionic chains remain intact even when gradually removing the counterions. Our findings highlight the fundamental role of hydrogen bonding for the unusual attractive interaction between ions of like charge.