Multinuclear Residual Quadrupolar Couplings in Chemistry

• Applicant: Dr. Michael John
• Subject Area: Analytical Chemistry ; Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428856821

Nuclear magnetic resonance (NMR) spectroscopy is the key analytical method for the determination of molecular structure and dynamics in solution: it is both applicable to a wide range of nuclei and compound classes, and sensitive to small geometric variations and dynamics at multiple timescales. The main drawback of classical solution NMR structure determination is a rather tedious collection of distances or dihedral angle information, while important geometric probes such as dipolar or quadrupolar couplings are principally unobservable. On the other hand, molecular structures of solids are nowadays quickly available from X-ray crystallography, but these often represent “snapshots” that may be affected by crystal packing effects and not report on true association, solvation and dynamics prevailing in solution. The general aim of our project is to bridge the two worlds of solids (where quadrupolar couplings in known structure motifs are experimentally accessible by solid-state NMR), and solutions (where these motifs may be altered or subject to dynamic averaging), using residual quadrupolar couplings (RQCs) in weakly-aligned sample conditions. In the preceding DFG project, we could show that cross-linked polystyrene (PS) - swollen in tetrahydrofuran (THF) or toluene - is an excellent alignment medium even for sensitive organometallic compounds. We also identified 7Li and 11B as highly promising nuclei for RQC measurements and - in combination with calculated electric field gradients (EFGs) - demonstrated their use as valuable structural probe. While 7Li RQCs were used to determine the Li solvation states in a series of relatively rigid chelate complexes, we used 11B RQCs for discrimination and resonance assignment in (car)borane clusters. The aim of the project proposed as renewal here is to make 7Li and 11B RQCs amenable to more “real”, dynamic and complex systems with multiple 7Li / 11B sites but utmost chemical relevance. First, we aim at structure determination of synthetically important lithium amides that in solution exist as dynamic but not yet fully characterised mixture of multiple cyclic oligomers. To overcome excessive broadening of 7Li quadrupolar triplets at low temperatures, we will also switch to 6Li, where resolved quadrupolar doublets for the individual oligomers are expected. Second, we will extend our 11B RQC resonance assignment method to more complex and application-tailored (car)borane clusters, where we aim to reduce peak overlap by both slice-selective and two-dimensional 11B NMR methods. Finally, we aim to explore the use of 11B RQCs and 11B quadrupole / 11B-19F dipole-dipole cross-correlation as quick electronic indicators in the highly important BODIPY fluorescence dyes and Lewis acid-base adducts.

• DFG Programme: Research Grants