Detecting and quantifying doubly hydrogen bonded anionic dimers in ionic liquids

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Dicarboxylate species including both dicarboxylate anions and dicarboxylic acids are mostly relevant in biological, atmospheric and industrial processes. The use for reception, hosting, complexation or sensing strongly depends on non-covalent bonding, which is a paradigm of supramolecular chemistry. In this project, we want to synthesize and characterise ionic liquids, including trialkyl ammonium cations and singly deprotonated dicarboxylic acid anions. The idea is to design hydrogen bonded structural motifs in the liquid phase, strengthened or weakened by attractive or repulsive Coulomb interactions. The key motif includes the intriguing double hydrogen bonded cyclic arrangement between the carboxyl groups of two anions, wherein short-range directional hydrogen bonding overcomes the long-range repulsive Coulomb forces, monitored by varying the distance between the carboxyl and carboxylate groups within each anion. We have charge-assisted +NH…O− and charge-opposed −OH···O− single or double hydrogen bonds that control aggregation and dynamical properties of these model ionic liquids. The non-covalent forces and hydrogen bonding arrangements will be studied by far infrared (FIR) and terahertz (THz) spectroscopy that allow addressing the vibrational signatures of the intermolecular hydrogen bonds of varying strength directly. For understanding the influence of hydrogen bonding on the dynamics, we measure the deuteron NMR relaxation times of cations and anions. We compare the measured spectroscopic properties such as IR frequencies, NMR chemical shifts and quadrupole coupling constants with calculated properties from quantum chemical calculations of complexes and aggregates showing similar structural motifs and bonding. The spectroscopic descriptors will be analysed by means of the natural bond orbital (NBO) approach, revealing the correlation between these properties. A final but ultimate goal is stripping the counter ions from the neutral species for isolating the pure anionic dimers from these ionic liquids, providing the first thermodynamically stable dimers of like-charged ions detected in the gas phase by means of cryogenic ion vibrational (CIV) spectroscopy.