Anion and cation effects on imidazolium salt melting points: a descriptor modelling study

The challenge of predicting the melting point of ionic liquids is outlined. A descriptor modelling approach for two separate sets of ionic liquids is presented. In each case, the cations and the anions are modelled separately, using quantitative structure-property relationships. Both models include constitutional, topological and geometric descriptors as well as quantum mechanical ones. This approach gives access to (nxm) ionic liquids using only (n+m) calculations. The protocol is tested and validated for predicting the melting points of two sets, comprised of 22 and 62 imidazolium-based ionic liquids, respectively. Good correlations and predictions are obtained in both cases. Within the data set selected (only monopositive and mononegative ions are studied, and so total charge was not a factor), the degree of sphericity is the most important variable for the anion, while for the cation the main descriptors pertained to three radial distribution functions that describe three different sections in the cation. These characterise the ionic interactions, the symmetry-breaking region, and the length of the side chains.