Doubly ionic hydrogen bond interactions within the choline chloride–urea deep eutectic solvent

Abstract

Deep eutectic solvents (DESs) are exemplars of systems with the ability to form neutral, ionic and doubly ionic H-bonds. Herein, the pairwise interactions of the constituent components of the choline chloride–urea DES are examined. Evidence is found for a tripodal CH⋯Cl doubly ionic H-bond motif. Moreover it is found that the covalency of doubly ionic H-bonds can be greater than, or comparable with, neutral and ionic examples. In contrast to many traditional solvents, an “alphabet soup” of many different types of H-bond (OH⋯OC, NH⋯OC, OH⋯Cl, NH⋯Cl, OH⋯NH, CH⋯Cl, CH⋯OC, NH⋯OH and NH⋯NH) can form. These H-bonds exhibit substantial flexibility in terms of number and strength. It is anticipated that H-bonding will have a significant impact on the entropy of the system and thus could play an important role in the formation of the eutectic. The 2 : 1 urea : choline–chloride eutectic point of this DES is often associated with the formation of a [Cl(urea)₂]⁻ complexed anion. However, urea is found to form a H-bonded urea[choline]⁺ complexed cation that is energetically competitive with [Cl(urea)₂]⁻. The negative charge on [Cl(urea)₂]⁻ is found to remain localised on the chloride, moreover, the urea[choline]⁺ complexed cation forms the strongest H-bond studied here. Thus, there is potential to consider a urea[choline]⁺·urea[Cl]⁻ interaction.