Ionic liquids are chemical compounds with low symmetry, which is manifested by the existence of the liquid phase below room temperature. A common class of ionic liquids is based on the imidazolium cation and an inorganic anion. The specific structure gives rise to some peculiar properties, including low vapour pressure, thermal and chemical stability, electrical conductivity, catalytic activity, and good solvation ability for both polar and non-polar compounds. The complex non-covalent interactions between the ions give rise to an internal structure with specific distribution of the polar and non-polar moieties. Of particular interest is the cage-like structure suggested by 129Xe NMR spectroscopy, and confirmed by molecular dynamics simulations, as small molecules or noble gas atoms can be embedded in these cavities.
Computational studies on ionic liquids can be performed at different levels of theory using a multiscale approach. Molecular dynamics can give the distribution of ion pairs in the bulk structure. Density functional theory allows evaluations of the intermolecular interactions in small clusters. High-level ab initio methods are suitable for calculating thermodynamic properties and interaction energies. In this work, the ionic liquid 1-butyl-3-methylimidazolium chloride and its interactions with xenon have been investigated using density functional theory calculations. Studies on an isolated pair provided geometrical parameters, and revealed a favourable interaction with a xenon atom. The calculation on a system consisting of four ion pairs showed that the properties of ionic liquids have to be investigated on larger systems in order to avoid artificial interactions. A cluster consisting of 32 ion pairs was optimized at the PBEh-3c/def2-mSVP level of theory. The interaction energy with xenon was found to be 5.4 kcal/mol, which confirms the experimentally observed ability of imidazolium-based ionic liquids to dissolve the noble gas.
