CO₂-Induced Anion Exchange and Diffusion Behavior in Imidazolium-Based Ionic Liquids Studied by DOSY NMR

Abstract

Understanding the molecular mechanisms underlying CO2 capture in ionic liquids is essential for the rational design of efficient and selective sorbents. Here, we investigate the effect of CO₂ absorption on the molecular organization, ion dynamics, and speciation of imidazolium-based ionic liquids using diffusion-ordered spectroscopy (DOSY). The behavior of 1-n-butyl-2,3-dimethylimidazolium imidazolate (BMMIm.Im) and 1-n-butyl-2,3-dimethylimidazolium 2-methylimidazolate (BMMIm.2-MeIm) was studied in DMSO-d₆/D₂O mixtures to elucidate the relationship between ion pairing and reactivity. Prior to CO2 exposure, both systems exhibit strongly correlated cation-anion diffusion, consistent with contact ion pairing. After CO₂ uptake, ¹H-DOSY reveals changes in diffusion coefficients indicative of a physical reorganization of the ionic microenvironment, whereas ¹³C-DOSY provides direct evidence of bicarbonate formation. The convergence of cation and bicarbonate diffusion coefficients indicates the formation of strongly associated ion pairs that replace the original imidazolate-based aggregates. These results show that CO₂ capture is governed by a CO₂-water-anion equilibrium and highlight the central role of anion basicity and solvation in controlling reactivity and reversibility in ionic liquids.