Structural Arrangements of Guanidinium-Based Dicarboxylic Acid Ionic Liquids and Insights into Carbon Dioxide Uptake through Structural Voids

Abstract

Crystallization of ionic liquids (ILs) is essential for determining the crystal structure and molecular interactions between cations and anions; however, studies on crystallization of viscous ILs are scarce. This study investigates the network formation and intermolecular interactions present in the ILs to understand and enhance their CO2 absorption capability. In this context, five 1,1,3,3-tetramethylguanidine [TMG]-based ILs, with odd and even anionic dicarboxylic acids such as, succinate [Suc], glutarate [Glu], adipate [Adp], pimelate [Pim], and suberate [Sub], were synthesized. The in-situ cryo-crystallization technique was used to determine the structure of two viscous ILs, [TMG][Glu] and [TMG][Pim], at low temperatures (173 K). The dominance of intermolecular atomic contacts in studied ILs was visualized by performing a Hirshfeld surface analysis. The CIF obtained from the crystal data was used to optimize the ILs, and the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energies were calculated using the DFT at the B3LYP/6-311G++ (d, p) level. Among the several interesting properties of [TMG]-based ILs, the CO2 absorption capacity of [TMG][Glu] and [TMG][Pim] has been investigated through structural voids to establish the increase in CO2 absorption with the addition of −CH2 group on the anion.