Structural characterization of the 1-butyl-3-methylimidazolium chloride ion pair using a initio methods

Ab initio theoretical methods are used to investigate the gas-phase ion pairs of the ionic liquid 1-butyl-3-methylimidazolium chloride. Multiple stable conformers with the chloride anion positioned (in-plane) around the imidazolium ring or above the C-2-H bond are determined. The relative energy ordering of the conformers is examined at the B3LYP, MP2, and CCSD(T) levels. Zero-point energies, BSSE, and basis set effects are examined. For accurate results, correlation (dispersion) effects must be included. The most stable conformers are essentially degenerate and have the chloride H-bonding to, or lying above, the C-2-H bond. Other conformers are found to lie similar to 30 and similar to 60 kJ mol(-1) higher in energy. Results are compared with those from recent simulations and experimental studies. The effect of the chloride anion on rotation of the butyl chain is investigated and found to lower some rotational barriers while enhancing others. The origin of the rotational barriers is determined. The number and type of hydrogen bonds formed between the imidazolium cation and chloride anion is found to vary significantly among conformers. No evidence of a possible intra C-alkyl-H center dot center dot center dot pi interaction is obtained; however, hints of a Cl center dot center dot center dot pi interaction are found. The vibrational spectrum of each conformer is examined, and the origin of multiple (H-bonding) features in the vibrational spectrum of the ionic liquid explained.