Cation[formula omitted]anion bonding interactions in 1–Ethyl–3–Methylimidazolium based ionic liquids

Abstract

The potential energy surfaces for the interactions between the [EMIM]+ cation (1–ethyl–3–methylimidazolium) with the series of acetate, bromide, and tetrafluoroborate anions are exhaustively explored under simulated annealing conditions. Rich structural spaces are obtained, with several ionic pairs having energies very close to the putative global minima. All protons in the bare cation show unusually high affinities for the anions, with the central C–H bond in the (C2H5) N▪CH▪N(CH3) fragment acting as the preferred site for explicit cation⋯anion bonding, which occurs mostly in the form of uncharacteristically strong charge assisted secondary hydrogen bonds or direct attack of the anion to the aromatic system, affording strongly bound ion pairs according to highly correlated DLPNO–CCSD(T) binding energies. Formal dissection of intermolecular bonding interactions reveals a wide range of strengths, affording a picture of cation⋯anion bonding with complex contributions from long range closed shell ionic character, with surprisingly high covalent contributions. Computed proton NMR chemical shifts are directly related to the strength of hydrogen bonds in the ion pairs. The structural variety, aided by the complex mixture of interactions, yields multiple states accessible at room temperature, which is a significant result that leads to a lack of structure in the liquid state and thus partially helps explaining, from a molecular perspective, the ionic liquids difficulties to crystallize.