How Strong Is Hydrogen Bonding in Ionic Liquids? Combined X-ray Crystallographic, Infrared/Raman Spectroscopic, and Density Functional Theory Study
Hydrogen bonding in ionic liquids based on the 1-(2'-hydroxylethyl)-3-methylimidazolium cation ([C(2)OHmim](+)) 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 [C(2)OHmim][PF6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (E-HB) and the enthalpy (-Delta H-HB) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C(2)OHmim][PF6] and an analysis of the IR spectra in crystal and liquid samples. E-HB for OH center dot center dot center dot[PF6](-) H-bonds amounts to similar 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 center dot center dot center dot[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)) < [TEA] (6.2 kcal-mol(-1)). The formation of aggregates of self-associated [C(2)OHmim](+) cations is present in liquid [C(2)OHmim][PF6], [C(2)OHmim] [BF4], and [C(2)OHmim] [Tf2N], with the energy of the OH center dot center dot center dot OH H-bonds amounting to similar 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 center dot center dot center dot[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.