Residual Quadrupolar Couplings as a Tool for Structural Analysis of Molecules Containing Quadrupolar Nuclei

by Franziska Traeger
Doctoral thesis
Date of Examination:2024-12-03
Date of issue:2025-07-14
Advisor:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Ricardo Mata
Persistent Address: http://dx.doi.org/10.53846/goediss-11380

 

 

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Abstract

English

The quadrupolar nature of approximately 75% of all nuclei is often perceived as troubling for nuclear magnetic resonance (NMR) spectroscopists, since the quadrupolar interaction between the nuclear quadrupole moment and the electric field gradient (EFG) leads to broad lines in the NMR spectrum. However, this interaction contains valuable information about the closest surroundings of a quadrupolar nucleus and thus the structure of a molecule. Using weak alignment methods, the quadrupolar interaction can be visualized with NMR spectroscopy in the form of residual quadrupolar couplings (RQCs). The solution-like conditions allow to extract quadrupolar couplings while maintaining the good resolution of solution state NMR. In the first part of this work, the solvation state of lithium complexes containing bidentate ligands based on bis(benzoxazol-2-yl)methanide was studied using 7Li RQCs. In weakly aligned tetrahydrofuran (THF) solution, experimentally determined 7Li RQCs were compared to theoretical values calculated for all possible solvation states. The second part focuses on the use of 11B RQCs in the resonance assignment of borane and carborane clusters. Here, Cornilescu´s quality factor (Q-factor) was calculated for all assignment possibilities and the correct assignment was determined by the lowest Q-factor. Using a slightly modified version of the 1H,11B HMQC spectrum, it was possible to experimentally extract the sign of the 11B RQCs. The third and last part deals with the influence of the ligand substitution pattern of boron dipyrromethene (BODIPY) and bis(benzoxazol-2-yl) complexes on the size and sign of the boron EFG and the resulting 11B RQC.
Keywords: NMR spectroscopy; Quadrupolar Interactions; Anisotropic NMR; Bis(benzoxazolyl)methanes; Borane and Carborane Clusters; Boron Dipyrromethene
 
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