Theoretical Study of Ion Pairing in Protic Ionic Liquids of ΔpK_arange 6 to 18

Studies on “poor” protic ionic liquids, having proton-transfer strength ΔpK a = 0-6, have shown the need to make corrections to the Fuoss equation for prediction of ΔG ioniz and K ioniz for ion-pairing equilibria in these systems. Recent work built empirical functions to replace the Fuoss equation for this ΔpK a region, based on ab initio ion-pairing energies, but the functions did not perform well at the higher end of this range. Here, we extend the study to the range ΔpKa = 6-18, examining hypothetical mixtures of trimethylamine (TEA) with acids from weak (CH 3 COOH) to quite strong (CF 3 SO 3 H). Ab initio pairing energies were obtained for these systems, followed by refitting. Preliminary comparisons with experimental ionicities appear to show inaccuracies in the ab initio dataset, not in the fitting function form. Future work is planned to include dataset shifts, empirical or theoretical, to allow the goal of a predictive function for ionicity as a function of ΔpKa and acid:base mixing ratio.