Relationship between stabilization energy and thermophysical properties of different imidazolium ionic liquids: DFT studies

Theoretical studies of thermophysical and thermodynamic properties of ionic liquid (ILs) have attracted considerable attention in recent times due to the possibility of prediction, which will immensely help in designing this ‘designer’s solvent’. Here, a new model is proposed to correlate an important thermophysical properties like melting point and density of the most popular class of ionic liquids, alkyl imidazolium halide with the calculated second order perturbation stabilization energy, E(2). This E(2) has been obtained from Natural Bond Orbital (NBO) analysis at B3LYP level of Density Functional Theory (DFT) calculation. This stabilization energy found to increase with decreasing alkyl chain length for chloride and iodide, though no straightforward trend was established for bromide containing imidazolium ILs. Interesting trends have been observed depending on the types of anion. While melting temperatures of chloride ILs were found to vary linearly with the E(2) values, corresponding iodide anion found to be fitted on an exponential curve. Further, alkyl chain length dependent E(2) values found to corresponds well with experimental density for chloride and iodide ILs. Density of chloride as well as iodide derivatives ILs found to be fitted well in an exponential curve with the E(2) values. The lack of correspondence between stabilization energy with experimental melting temperature or density for bromide derivatives might be due to the structural polymorphism.