Investigation of cation-anion interaction in 1-(2-hydroxyethyl)-3- methylimidazolium-based ion pairs by density functional theory calculations and experiments

Gas-phase structure, hydrogen bonding, and cation-anion interactions of a series of 1-(2-hydroxyethyl)-3-methylimidazolium ([HOEMIm] ⁺)-based ionic liquids (hereafter called hydroxyl ILs) with different anions (X = [NTf ₂] ^-, [PF ₆] ^-, [ClO ₄] ^-, [BF ₄] ^-, [DCA] ^-, [NO ₃] ^-, [AC] ^- and [Cl] ^-), as well as 1-ethyl-3-methylimizolium ([EMIm] ⁺)-based ionic liquids (hereafter called nonhydroxyl ILs), were investigated by density functional theory calculations and experiments. Electrostatic potential surfaces and optimized structures of isolated ions, and ion pairs of all ILs have been obtained through calculations at the Becke, three-parameter, Lee-Yang-Parr/6-31 + G(d,p) level and their hydrogen bonding behavior was further studied by the polarity and Kamlet-Taft Parameters, and ¹H-NMR analysis. In [EMIm] ⁺-based nonhydroxyl ILs, hydrogen bonding preferred to be formed between anions and C2-H on the imidazolium ring, while in [HOEMIm] ⁺-based hydroxyl ILs, it was replaced by a much stronger one that preferably formed between anions and OH. The O-H⋯X hydrogen bonding is much more anion-dependent than the C2-H⋯X, and it is weakened when the anion is changed from [AC] ^- to [NTf ₂] ^-. The different interaction between [HOEMIm] ⁺ and variable anion involving O-H⋯X hydrogen bonding resulted in significant effect on their bulk phase properties such as ¹H-NMR shift, polarity and hydrogen-bond donor ability (acidity, α).