Characterization of Halogen Bonds with Different Solid-State Nuclear Magnetic Resonance Methods
Creators: name: Xu, Yijue
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2021
Subjects: nuclear magnetic resonance
halogen bonding interaction
single-crystal NMR

description: The nature and potential application of halogen bonding are currently topics of great interest among various research fields, including biochemistry, functional materials, organocatalysis, etc. Solid-state nuclear magnetic resonance (SSNMR) has been recognized as one of the important tools for characterizing different types of non-covalent interactions. This dissertation demonstrates how different SSNMR methods can be used to characterize the changes in electronic structure associated with halogen bonding interactions and investigates the processes leading to the formation of halogen bonds. A series of cocrystals featuring halogen bonds between phosphine oxides and different diiodoperfluorobenzenes (1,4-diiodotetrafluorobenzene, 1,2-diiodotetrafluorobenzene, and 1,3,5-trifluoro-2,4,6-triiodobenzene) have been synthesized through slow evaporation from solvent or by mechanochemistry. We began by characterizing the naturally abundant and 17O-labeled phosphine oxides and their halogen-bonded cocrystals by a combination of theoretical and experimental multinuclear magnetic resonance spectroscopy (31P and 17O), and X-ray diffraction (XRD) methods. This first study of 17O NMR parameters in halogen-bonded solids has provided key insights into the correlation between NMR observables and the nature of the halogen bond. Then a rare example of a cocrystal featuring a phosphine as a halogen bond acceptor was synthesized and characterized by both solution and solid-state NMR experiments, and the 31P NMR experiment acted as a direct probe for the formation of halogen bonds in both solution and solid states. In addition to the conventional polycrystalline solid-state NMR experiments, single-crystal NMR studies were performed on 17O labeled triphenylphosphine oxide and its halogen-bonded cocrystals with diiodoperfluorobenzenes, which allowed for the measurement of not only the magnitude but also the orientation of the NMR tensors. The work helped investigate the changes in NMR tensor orientation caused by the formation of halogen bonds and thereby provides a novel connection between the electronic structure and the NMR interactions. Novel single-crystal NMR analysis software was developed to fit the data in two ways: either through an unconstrained fitting process or through a constrained fitting process, which can improve the precision of the resulting tensor orientations in some cases. The mechanochemical production of halogen bonds inspired us to investigate the mechanism and kinetics behind this solid-state reaction process in situ. In situ 31P solid-state NMR studies of halogen bond formation between triphenylphosphine oxide and 1,4-diiodotetrafluorobenzene were carried out under different conditions (temperature, spinning speed, and different amounts of liquid added) to provide insight into the crystallization process and provide an estimation of the activation energy. Overall, this thesis provides several new applications of NMR spectroscopy to the study of halogen bonds and provides various novel insights into the connection between the NMR observables and the nature of this non-covalent interaction.