Ionic liquids (IL) and hydrogen bonding (H-bonding) are two diverse fields for which there is a developing recognition of significant overlap. Doubly ionic H-bonds occur when a H-bond forms between a cation and anion, and are a key feature of ILs. Doubly ionic H-bonds represent a wide area of H-bonding which has yet to be fully recognised, characterised or explored. H-bonds in ILs (both protic and aprotic) are bifurcated and chelating, and unlike many molecular liquids a significant variety of distinct H-bonds are formed between different types and numbers of donor and acceptor sites within a given IL. Traditional more neutral H-bonds can also be formed in functionalised ILs, adding a further level of complexity. Ab initio computed parameters; association energies, partial charges, density descriptors as encompassed by the QTAIM methodology (ρ_BCP), qualitative molecular orbital theory and NBO analysis provide established and robust mechanisms for understanding and interpreting traditional neutral and ionic H-bonds. In this review the applicability and extension of these parameters to describe and quantify the doubly ionic H-bond has been explored. Estimating the H-bonding energy is difficult because at a fundamental level the H-bond and ionic interaction are coupled. The NBO and QTAIM methodologies, unlike the total energy, are local descriptors and therefore can be used to directly compare neutral, ionic and doubly ionic H-bonds. The charged nature of the ions influences the ionic characteristics of the H-bond and vice versa, in addition the close association of the ions leads to enhanced orbital overlap and covalent contributions. The charge on the ions raises the energy of the Y_lp and lowers the energy of the X–H σ* NBOs resulting in greater charge transfer, strengthening the H-bond. Using this range of parameters and comparing doubly ionic H-bonds to more traditional neutral and ionic H-bonds it is clear that doubly ionic H-bonds cover the full range of weak through to very strong H-bonds.

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.

In this paper we have explored the structural and energetic landscape of potential π⁺–π⁺ stacked motifs, hydrogen-bonding arrangements and anion–π⁺ interactions for gas-phase ion pair (IP) conformers and IP-dimers of 1,3-dimethylimidazolium chloride, [C₁C₁im]Cl. We classify cation–cation ring stacking as an electron deficient π⁺–π⁺ interaction, and a competitive anion on-top IP motif as an anion–donor π⁺–acceptor interaction. 21 stable IP-dimers have been obtained within an energy range of 0–126 kJ mol⁻¹. The structures have been found to exhibit a complex interplay of structural features. We have found that low energy IP-dimers are not necessarily formed from the lowest energy IP conformers. The sampled range of IP-dimers exhibits new structural forms that cannot be recovered by examining the ion-pairs alone, moreover the IP-dimers are recovering additional key features of the local liquid structure. Including dispersion is shown to impact both the relative energy ordering and the geometry of the IPs and IP-dimers, however the impact is found to be subtle and dependent on the underlying functional.

The importance of 1° and 2° hydrogen-bonding and anion–π⁺ interactions for ionic liquid structuring.

The role of hydrogen bonding, π ⁺ –π ⁺ stacking and anion–π ⁺ interactions on the structure of ionic liquid mixtures has been elucidated through a combined theoretical and experimental approach.