How Strong Is Hydrogen Bonding in Ionic Liquids? Combined X-ray Crystallographic, Infrared/Raman Spectroscopic, and Density Functional Theory Study

Abstract

Hydrogen bonding in ionic liquids based on the 1-(2'-hydroxylethyl)-3-methylimidazolium cation ([C₂OHmim](+)) and various anions ([A](-)) of differing H-bond acceptor strength, viz. hexafluorophosphate [PF6](-), tetrafluoroborate [BF₄](-), bis(trifluoromethanesulfonimide) [Tf₂N](-), trifluoromethylsulfonate [OTf](-), and trifluoroacetate [TFA](-), was studied by a range of spectroscopic and computational techniques and, in the case of [C₂OHmim][PF6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (E(HB)) and the enthalpy (-ΔH(HB)) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C₂OHmim][PF6] and an analysis of the IR spectra in crystal and liquid samples. E(HB) for OH···[PF6](-) H-bonds amounts to ~3.4-3.8 kcal·mol(-1), whereas weaker H-bonds (2.8-3.1 kcal·mol(-1)) are formed between aromatic C2H group of imidazolium ring and the [PF6](-) anion. The enthalpy of the OH···[A](-) H-bonds follows the order: [PF6] (2.4 kcal·mol(-1)) < [BF₄] (3.3 kcal·mol(-1)) < [Tf₂N] (3.4 kcal·mol(-1)) < [OTf] (4.7 kcal·mol(-1)l) < [TFA] (6.2 kcal·mol(-1)). The formation of aggregates of self-associated [C₂OHmim](+) cations is present in liquid [C₂OHmim][PF6], [C₂OHmim][BF₄], and [C₂OHmim][Tf₂N], with the energy of the OH···OH H-bonds amounting to ~6 kcal·mol(-1). Multiple secondary interactions in the bulk ILs influence their structure, vibrational spectra, and H-bond strength. In particular, these interactions can blue-shift the stretching frequencies of the CH groups of the imidazolium ring in spite of red-shifting CH···[A](-) H-bonds. They also weaken the H-bonding in the IL relative to the isolated ion pairs, with these anticooperative effects amounting to ca. 50% of the E(HB) value.

Determination of three estrogens and bisphenol A by functional ionic liquid dispersive liquid-phase microextraction coupled with ultra-high performance liquid chromatography and ultraviolet detection

“…Previous studies pointed out that the IL containing hydroxyl functionality can act as an additional H-bond acceptor in comparison with nonfunctionalized ILs [30]. Therefore, HEMIM-Tf 2 N should have potential advantages in the extraction of those four chemicals.…”

Structure of protic (HCnImNTf2, n=0–12) and aprotic (C1CnImNTf2, n=1–12) imidazolium ionic liquids: A vibrational spectroscopic study

“…Furthermore, its overall intensity is decreased compared to the C 4,5 -H. We have to note here that the assignment of this band is a matter of debate, since it is well established that there is Fermi resonance with the ring stretching vibrational mode. This make the interpretation of this spectral area quite complicated, as we already mentioned in a recent publication and we will not offer any new discussion here [36][37][38][39][40][41][42][43]. No more shifts are observed with increasing n from 2 to 12.…”

Solid, Solution and Gas Phase Interactions of an Imidazolium-Based Task-Specific Ionic Liquid Derived from Natural Kojic Acid

“…2 Besides ionic interactions, whether supramolecular interactions also govern the organization of imidazoliumbased ILs is still hotly debated with a vast number of important contributions. [24][25][26][27][28][29] Despite their importance, the number of significant contributions specifically discussing the supramolecular organization and interactions of TSILs is much scarcer [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] when compared to nonfunctionalized ILs. TSILs have become one of the most important strategies for furthering IL chemistry, 1 as noted for the large and increasing number of innovative applications described for these ionic compounds.…”