Studying the Mechanochemistry of Bimolecular Reactions Using Quantum Chemical Simulations: Addition Reactions to Carbon-Carbon Double Bonds
Carver, Benjamin Samuel
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Chemical reactions usually involve the conversion of reactants to products by overcoming an energetic barrier. Most commonly, this process can be assisted by adding energy through heat (thermochemistry), light (photochemistry) or electric current (electrochemistry). The fourth option is to overcome the reaction barrier through application of mechanical work, termed mechanochemistry. This method has received much attention from the scientific community in the last decade. Both theoretical and experimental studies have been performed, demonstrating the ability of mechanochemistry to activate reactions, with a strong focus on ringopening reactions. The vast majority of studies have focused on unimolecular reactions involving bond-rupture, which is very intuitively activated by the application of tensile stress. However, bimolecular reactions, which often involve bond formation as well as rupture, have received much less attention. In this thesis, we seek to change this by undertaking an in-depth study of mechanochemical activation of addition reactions to carbon-carbon double bonds, which involve the formation of two single bonds while the double bond becomes a single bond. We observe that large barrier changes can be induced by applying external force to reactions of this type, and the magnitude of these changes can be controlled by the choice of alkene substrate. By studying the changes induced in the geometry of the substrate, we are able to begin explaining the origins of the barrier reduction effect. In addition, by studying the contributions to the barrier change from mechanical work and the contributions from geometry changes, we discover that steric hindrance to a reaction can play a very significant role in the mechanochemical activation of the reaction.