CO₂-induced anion exchange and diffusion behavior in imidazolium-based ionic liquids studied by DOSY NMR

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.