Dialkylimidazolium hydrogen bond

Stereochemical and kinetic studies have confirmed the enhancement of the hydrogen bond ability of the imidazolium cation on going from l,2-dimethyl-3-alkylimidazolium salts to 1,3-dialkylimidazolium cations. However, deprotonation of the site between nitrogens is not particularly simple, it requires strong bases and depends on the ionic liquid counter-anion (Scheme 4.1). ... 

Multinuclear NMR spectroscopy experiments of various 1,3-dialkylimidazolium ILs dissolved in organic solvents have also pointed to the formation of floating aggregates through hydrogen bonds [81-84]. In particular, it has been demonstrated by heteronuclear NMR experiments on [C4CjIm]BF4 that contact ion pairs exist in the presence of small amounts of water and even in dimethyl sulfoxide (DMSO) solution [85]. 

Fig. 5.M Ability ofthe 1,3-dialkylimidazolium cation to hydrogen bond methyl acrylate in the course of its reaction with cyclopentadiene.

ILs can act as both hydrogen bond acceptors (anions) and donors (cations), and as expected, they interact with substances with both acceptor and donor sites. In particular, 1,3-dialkylimidazolium ILs can dissolve a plethora of classical polar and non-polar compounds. All 1,3-dialkylimidazolium ILs reported to date are hygroscopic, and their miscibility with water is largely controlled by the nature of the anion. While salts containing nitrate, chloride, and perchlorate anions are usually miscible with water in all compositions, those associated with hexafluorophosphate and bis(trifluoromethane) sulfonylamidate anions are almost completely immiscible with water.It is also known that an increase in the A-alkyl chain length increases the hydrophobicity for a series of l-alkyl-3-methylimidazolium hexafluorophosphate ILs." The miscibility of water in ILs can be increased by the addition of short-chain alcohols or lowered by the addition of salts (salting-out effect). ... 

In COlL-1, all of the main aspects of the liquid-state structure of room-temperature ionic liquids were already laid out in contributions by several groups, although sometimes still in an incipient way. Hardacre and co-workers reported neutron diffraction studies of short-chain dialkylimidazolium ionic liquids, perfectly illustrating the charge ordering and n-interactions [17]. They also used different spectroscopic and simulation methods to study how the solvation of aromatic and polar molecules (benzene and ethanenitrile, respectively) in the ionic liquids modifies the structure of the media [46] and the balance between the different terms in the interactions coulombic, van der Waals (dispersive), hydrogen bonds and multipolar. 

---