Structure and ion-ion interactions in trifluoroacetate-based ionic liquids: Quantum chemical and molecular dynamics simulation studies

Abstract

Ionic liquids (ILs) raise quite interesting but complicated questions for computational study due to the fact that different interactions, e.g., electrostatic interactions, charge-transfer interactions, steric repulsion, dispersion and hydrogen bonding that determine structural organization of ILs, need to be described properly. In this work, we present a comprehensive understanding of structural properties and interactions between ions in trifluoroacetate-based ILs with ammonium cations containing ethyl groups with the general formula EtnNH4-n (n = 0–4). This study includes both gas-phase and implicit solvent calculations of single ionic pairs (DFT, QTAIM and NBO methods) and molecular dynamics (MD) simulations of the ILs themselves. Based on the gas-phase calculations we show that the ionic and molecular pairs with very strong hydrogen bonds are formed by diethylamine and triethylamine with trifluoroacetic acid, while only molecular pairs are generated from ammonia and ethylamine. Unlike this, the use of implicit solvent calculations for all compounds leads to the result that formation of ionic pairs becomes energetically more favorable which, in turn, improves the agreement with experiment. The trend in geometric parameters of hydrogen bonds obtained from MD simulations of the ILs is similar to that which is observed for the implicit solvent calculations of the ion pairs. The increase in the number of alkyl groups in the cation would decrease interaction between the ions as in the ionic pair, as well in the ionic liquid itself. Moreover, we suggest that the change in the ion-ion interaction is one of the most important factors that affects the melting points variation of the ILs for this series.