Locality and fluctuation effects in ionic liquids

Abstract

Amino acid (AA) based imidazolium ionic liquids (ILs) are promising materials in biocatalysis, electrochemistry, drug delivery, and green solvents due to their low volatility, high thermal stability, and good solubility. However, the microstructure and interactions of ILs are complex, involving charge transfer, polarization, hydrogen bonds (HB), and π-π interactions. Accurately characterizing their physical and chemical properties is crucial for optimizing their performance. While classical molecular dynamics (MD) methods have limitations in describing polarization and charge transfer, <em>ab initio</em> molecular dynamics (AIMD) based on first principles is a powerful tool for studying these properties. This thesis systematically investigates the application of AIMD to study AA-ILs, focusing on polarization behavior, vibrational spectra, and intermolecular interactions. AA-ILs exhibit a more complex charge distribution than traditional imidazolium ILs due to AA side chains. Using AIMD with different charge distribution schemes (Wannier, Blöchl, Löwdin, Mulliken, and Voronoi), we analyze electronic polarization effects. The Wannier method most accurately describes charge transfer, while Mulliken and Voronoi methods perform differently across ion combinations. The study shows that the π-electron cloud of the imidazolium ring plays a key role in polarization, while anions contribute less due to their lower polarizability. The Wannier and Voronoi methods showed good agreement in predicting relative intensities and overall profiles. The Voronoi method reduced the instability during the time evolution and improved the spectral accuracy. We analyzed the C-H stretching vibration of the imidazole ring, the CO vibration and the HB vibration of the AA side chain in ILs, providing insights into their molecular recognition and environmental response. In addition, the data of the theoretical spectra and the experimental data also showed good agreement. The physicochemical properties of ILs are influenced by their intermolecular interactions. AIMD calculations of radial distribution functions (RDFs) and combined distribution functions (CDFs) reveal short- and long-range interaction patterns. Hydrogen bonding between cations and anions is crucial for system stability, while π-π stacking of the imidazole ring affects structural order and fluidity of ILs. Different AA side chains alter the local structure: ILs with carboxyl groups form stable HB networks, while those with phenyl side chains show strong intermolecular stacking. These structural features influence viscosity, diffusivity, and solvation ability, and play a key role in protein stabilization and catalysis. <br/> This thesis provides a comprehensive study of the polarization effects, vibrational spectra, and intermolecular interactions in AA-ILs using AIMD, offering insights into their microscopic mechanisms. The findings contribute to the theoretical foundation of ILs and their application in biochemistry, electrochemistry, and materials science.