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 dotPF6 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 dotA H-bonds follows the order: [PF6] (2.4 kcal.mol(-1)) < [BF4] (3.3 kcal.mol(-1)) < [Tf2N] (3.4 kcal-mol(-1))