Molecular dynamic studies of π +-π − stacked imidazolium/imidazolate ion pairs in chloroform solution

Molecular dynamic simulations were carried out to analyze the structure of contact ion pairs containing the imidazolate anion and a variety of methyl substituted 1,3-dimethyl-imidazolium cations in chloroform solution. These ionic liquids were studied under the condition of infinite dilution. The study focused on determining the importance and the effect of different methylations at the cation's imidazolium ring. The structural analysis by radial and spatial distribution functions demonstrated that a stable contact pair is formed in chloroform along the entire simulation period. The cations are localized preferentially above or below the anion's plane ring, reflecting contributions of π +-π − stacking interaction between the two aromatic planar ions. The most acidic hydrogen at position C2 of the imidazolium cation exhibits the strongest structural correlations, at distances within the range of hydrogen bonding, with the imidazolate anion. Methylations at all the cation's ring positions in general weakened the structural correlations. For the methylation at the cation's C2, the difference in the free energy of association has been determined from potential of mean-force calculations in the order of 13 kJ/mol favoring the ion pair containing the protonated C2 atom.