Deuteron quadrupole coupling constants and rotational correlation times of N-D and O-D bonds involved in doubly ionic hydrogen bonds in ionic liquids and their mixtures by means of NMR liquid and solid state experiments

• Applicant: Professor Dr. Ralf Ludwig
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 401427621

We want to determine NMR deuteron quadrupole coupling constants (DQCCs) and rotational correlation times of N-D and O-D molecular vectors involved in doubly ionic hydrogen bonds (DIHBs) of ionic liquids (ILs) and their mixtures. Usually, due to fast proton exchange on the NMR time scale, expensive 15N- or 17O-enhanced proton relaxation time experiments are needed to determine accurate DQCCs for the liquid phase. To avoid the demanding and costly synthesis of isotopically substituted compounds, we recently developed a method for deriving DQCCs from a relation between density functional theory (DFT) calculated N-D DQCCs and N-H proton chemical shifts within clusters of ionic liquids. With simple measurements of the proton chemical shifts, we have access to accurate DQCCs in the liquid phase. This approach is valid regardless of temperature and solvent effects. As proof of concept, we could show that this relation also holds for molecular liquids, such as ammonia or amides. In this project, a similar relation will be calculated for O-D/O-H bonds of cations. Both relations are valid for all N-D or O-D bonds in the investigated liquids either being ionic or molecular. For further judgement of the predicted DQCCs, we will also measure these coupling parameters in the solid phase by means of solid state NMR. Reliable DQCCs are a prerequisite for the determination of rotational correlation times from NMR deuteron quadrupole relaxation rates. Using this approach, the first reliable single particle rotational correlation times can be determined for ionic liquids and their mixtures. They will describe the rotational dynamics for N-D and O-D bonds which are involved in DIHBs in different molecular configurations, such as ion pairs or ionic clusters. The correlation times will be determined as a function of temperature, providing important information about the heterogeneity in protic ionic liquids (PILs). Moreover, the applicability of hydrodynamic models will be tested for the liquid systems including different neutral or ionic aggregates. The first reliable rotational correlation times are also of importance for the force field development of ionic liquids. This has been demonstrated earlier for molecular liquids, such as water and alcohols.

• DFG Programme: Research Grants