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