Effects of non-specific and specific solvation on structure of tripropylammonium-based protic ionic liquids: Insight from computational modeling

Abstract

To explore the structure of three protic ionic liquids based on tripropylammonium cation and anions of strong acids – methanesulfonic, sulfuric, and triflic acids – we employed different computational methods, including quantum chemical calculations and molecular dynamics simulations. Solvation effects (non-specific and specific) were considered in order to evaluate the applicability of the methods used for the description of the ionic liquid’s structure. In each of the ionic liquids, we identified the cation–anion pair where a strong hydrogen bond and coulombic interactions kept the ions together. The results of both gas phase and implicit solvation calculations show an increase in the strength of the hydrogen bond and the magnitude of the ion-ion interaction energy in the ion pairs with tripropylammonium cation in the order of anions: triflate < hydrogen sulfate < mesylate, but the absolute values become smaller when going from the gas phase to the continuum medium. The molecular dynamics simulations reveal that the cation–anion pairs are preserved in the bulk phase of these liquids, but besides this, the formation of hydrogen bonds between anions in the tripropylammonium hydrogen sulfate is also possible.