Quantitative structure-property relationships for melting points and densities of ionic liquids

Although innumerable different ionic liquids are possible, even basic physical-property data, such as the density and melting point, exist only for relatively few. Derivation of melting point quantitative structure-property relationships (QSPRs) for energetic ionic liquids would therefore greatly aid in the molecular design of new compounds. A new class of ionic liquids, based on 1-substituted 4-amino-1,2,4-triazolium bromide and nitrate salts, were recently synthesized and their melting points and densities measured. We optimized the molecular geometries of the cations of the ionic liquids using ab initio quantum chemical methods. Melting point QSPRs were then derived from molecular orbital, thermodynamic, and electrostatic descriptors. Good correlations with the experimental data were found. The correlation coefficients for three-parameter melting point QSPRs and for one-parameter density QSPRs exceed 0.9. Although some of the descriptors that appear in our QSPRs were designed to describe chemical reactions, we infer that they serve in this study to quantify interactions between the cation and anion.