In situ NMR spectroscopy as a tool to investigate inter-ionic distances, molecular interactions and dynamics in ionic liquids
Final Report Abstract
The goal of our project was the application of modern NMR spectroscopic techniques in ionic liquids (or „ILs" for short) to study distances, interactions and dynamics on a fundamental level. To that end, we have first focussed on NOE based NMR methods. Since the NOE only develops between nuclei that are spatially close to each other, this technique delivers information on proximity around a given nucleus. In a NOESY type of experiment we were able to scan the surroundings of ionic liquid cations this way. To be able to measure real distances or preferred probabilities of presence around IL ions in the liquid phase, it was necessary to switch to the heteronuclear version of the NOE experiment (HOESY) due to strong spin diffusion effects in the NOESY. We established H,F HOESY NMR experiments routinely and investigated their mathematical evaluation much in detail. It turned out that for distance measurements, the common mathematical formulae for the NOE that can be found in textbooks were not precise enough. In the following we applied a more complex mathematical treatment of the data that included the individual relaxation rates of each single nucleus in the system. To be able to really "measure” distances between anions and cations, an internal standard was needed. We developed a synthetic procedure for a monofluorinated imidazolium cation that we could use for this purpose. This way, it was finally possible to determine distances and preferred probabilities of presence in neat ionic liquids solely by NMR measurements. On the road to routine measurement of interactions in ionic liquids, substantial speedup of the process was needed; standard HOESY experiments took three days on average to finish. We tackled this challenge by implementing a 1D transient NOE experiment, as proposed earlier by Gerig et al. This experiment was able to produce the same results as before, but in three hours instead of three days, including a substantial improvement in sensitivity. Finally, we exploited a rarely-used NMR experiment called CRAZED (together with Martin Brehm from the Kirchner group in Leipzig). In the CRAZED experiment, the suprameolecular surrounding of a particle can be probed much further into space by applying strong external magnetic field gradients. CRAZED signals commonly do not stem from spatially next neighbors, so this method is complementary to NOE based experiments. Within this project, we were able to produce the very first CRAZED NMR spectra of non-artificial real-world systems.
