Towards reliable description of rotational and translational motion in ionic liquids by means of field cycling and high field NMR relaxometry as well as molecular dynamics simulations

• Applicant: Dr. Anne Strate
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 459405854

In this project we want to study the rotational and translational motion in ionic liquids (ILs) by means of high field (HF) and field cycling (FC) NMR relaxometry as well as molecular dynamics (MD) simulations. In combination with specially synthesized ILs, we are able to measure and simulate over broad frequency and temperature ranges. Our approach allows testing the applicability of commonly used NMR relaxation models and suggesting models for more complex phenomena. This way, we overcome earlier problems, where too narrow temperature ranges and small data sets constrained scientists to the application of the simple Bloembergen-Purcell-Pound (BBP) relation.Here firstly, we study purely intramolecular quadrupolar (2H) relaxation for molecular vectors ND and OD of the cations in the ILs, which are all involved in hydrogen bonding. The broad liquid ranges down to glass transition temperature, provide frequency dependent information and allow for considering more sophisticated models including anisotropic or internal motion. We also determine reliable deuteron quadrupole coupling constants and rotational correlation times for bonds characterized by hydrogen bonds with differently strong interacting anions. Secondly, we determine rotational and translational dynamics from dipolar relaxation rates (1H and 19F) measured by FC relaxometry for broad temperature and frequency ranges to unravel details of the spectral densities. Here, the challenge is to dissect the total relaxation rates of the nuclear magnetic relaxation dispersion (NMRD) profiles into intra- and intermolecular contributions in a reliable way. This procedure will be supported by using partially deuterated ILs for suppressing 1H relaxation. Addressing different molecular vectors within the cations and anions allow validating anisotropic or internal rotation. The resulting translational diffusion coefficients we compare with those obtained from the low frequency dispersion law and pulsed field gradient NMR. Thirdly, for the most relevant ILs we determine the correlation functions, coupling parameters and relaxation rates from classical MD simulations. The challenge here is to address overlapping temperature ranges with experiment. That allows justifying the underlying relaxation models we used for evaluating the measured relaxation data. Beyond validating the relaxation models this combination of experimental and simulation methods provides insight into structure and dynamics of ILs at the molecular level.

• DFG Programme: Research Grants