An insight into microscopic properties of aprotic ionic liquids: A DFT study

The relationship between the structure of counter-ions and inter-ionic interaction in ion-pairs is systematically studied for 15 aprotic ionic liquids (AILs) using density functional theory. Five different substituted imidazolium cations and three different polyatomic anions (PA) are taken into account. Theoretical calculations show that the inter-ionic interaction decreases as the vdW volume of the counter-ions increases. By means of a new decomposition scheme called ETS-NOCV [33], which decomposes the total interaction energy into different parts, it can be shown that there is a clear linear relationship between the electrostatic part of the interaction energy and the total interaction energy. Further, for a given PA the electrostatic interaction decreases with increase of the vdW volume of the cation as one would expect. A similar decrease is seen in the electrostatic interaction between a given imidazolium cation on the PAs as the PA vdW volume increases.