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UBC Theses and Dissertations

Defining the role of partially covalent weak interactions in chemical reactions Tong, Xing


Weak interactions, commonly known as secondary bond interactions (SBIs), have received far less attention than their stronger counterparts (i.e., primary bonds, e.g., ionic, metallic, and covalent bonds). The most mainstream perspective is that those interactions are induced by the attraction between partially positive and partially negative areas on molecular surfaces. This electrostatics-driven model has been widely accepted as it is straightforward, intuitive, and widely applicable; however, more and more computational and experimental discoveries have thrown this “simple and practical” theory into question. Other quantum chemical effects like charge transfer cannot be ignored if a generalized picture of weak interactions is required. Due to the relatively weak bond strengths, it is often difficult to study weak interactions exclusively and accurately. With synchrotron X-ray absorption spectroscopy (XAS) in combination with different computational methods, the nature of two common weak interactions (i.e., halogen bonding XB and chalcogen bonding ChB) has been comprehensively investigated. To begin with, two types of model systems (i.e., hypervalent halogen salts and bis-triazole-pyridinium series) were chosen to study the degree of charge transfer in XB and ChB. These studies have given evidence for a substantial degree of covalent contribution in both XB and ChB interactions. The relevance of such interactions in modulating chemical processes has also been explored. For example, the significance of the charge transfer component in XB was confirmed in a catalyzed Ritter-like process (carbon-halogen bond cleavage). We have further demonstrated the important role of ChB in processes ranging from petroleum-coke modification to pharmaceutical drug synthesis. The obtained data has shown that both XBs and ChBs should be considered as partially covalent interactions and the covalent contributions can be tuned by the properties of involved electron donor/acceptor pairs. Those results are intended for a more rational design of the weak-interaction-assisted systems in many fields, such as catalysis, synthesis, anion recognition, and molecular electronics.

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