Structure and dynamics of piperidinium based amino acid ionic liquids: a computational investigation

Abstract

Context: There has been growing interest in amino acid ionic liquids because of their low-cost synthesis and superior biodegradability and biocompatibility compared to traditional ionic liquids. In this study, we have investigated the structure and dynamics of three ionic liquids consisting of N-butyl N-methyl piperidinium [Pip] cation with amino acid (lysine [Lys], histidine [His], and arginine [Arg]) anions. The radial distribution functions, the spatial distribution functions, and the coordination numbers have been used to analyze the structure in the bulk phase. The time-correlation functions for hydrogen bonds, ion pairs, and ion cage formation have been calculated to analyze the dynamic properties. The hydrogen bonds found between the ion pairs are mostly electrostatically dominant with moderate to weaker strengths. The [Pip][His] system showed the strongest interaction energy between the ion pairs, while the [Pip][Lys] system demonstrated faster dynamics consistent with its higher diffusion and ion conductivity.

Method: The density functional theory at M06-2X/6-311 + + G(d,p) level was employed for geometry optimization and wave function calculations. The theory of atoms-in-molecule was used for the topological analysis of electron density. The classical molecular dynamics simulations with OPLS-AA force field were employed to study the dynamics of the systems.

Keywords: DFT; H-bond dynamics; Ion cage; Ion pair; MD simulation; RDF.