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.