Structural evaluation and protium-deuterium exchange in 1-ethyl-3-methylimidazolium halide-ethylene glycol mixtures

Abstract

A series of equimolar mixtures of 1-ethyl-3-methylimidazolium halide ([C₂C₁im]X: X⁻ = F⁻, Cl⁻, Br and I⁻) and ethylene glycol (EG) are studied by ¹H NMR and IR spectroscopies. The chemical shifts for the protons in EG and imidazolium ring shift towards the downfield in the order of F⁻ >> Cl⁻ > Br⁻ > I⁻, owing to the strength of their respective X…H interactions. Amongst all the studied systems, the fluoride complex ([C₂C₁im]F·EG) shows extremely strong interactions between F⁻ and OH hydrogen of EG, resulting in no “free” EG in the mixture as reflected in the infrared spectra. Quantum chemical calculations suggest several possible geometries for all the halide systems, where the geometry with the EG molecule forming a chelate of halide ion gives the most stable structure. The calculated interaction energies of these geometries also confirm that the fluoride complex has a significantly higher interaction energy than those of the other halide systems. Furthermore, the halide anion affects the selectivity of protium-deuterium exchange site in these systems, and some imidazolium ring hydrogen atoms are activated only in the presence of F⁻.